Though auxin is synthesized in the plant apices of shoots and roots, it is transported towards their respective basal parts. Quantitative estimation of auxins found in the segments of seedlings, by spectrophotometric analysis; show that stem apex possesses 1.5 to 2.0 fold higher amounts than found in the root apices. The lowest concentration is found in the region of the stem where cotyledons are attached. Even leaves contain some amount of Auxins.
Looking at the structure, functional responses, plants are no different from animals’ system neuronal network. Holistically one can imagine plants having a system are no different from animal systems.
Auxins exist either in bound form or in a free state. The non diffusible form is considered as bound form and it is active, but the freely diffusible form is referred to as an inactive form. The bound state of the auxin has been explained as due to the binding of receptor proteins or binding proteins to auxins. Such proteins are found in the plasma membranes, cytoplasm and chromosomes. The mol. wt. of auxin binding proteins has been determined as 10,000 and 31, 5000 Daltons. Such proteins have been isolated from the free nuclei of coconut liquid endosperm. These proteins in the presence of IAA are found to be active in inducing gene expression. In living cells, the
relative concentrations of bound form and free forms of auxin is not constant but depends upon the environmental conditions or the functional state of cells.
Auxin is transported from the site of synthesis to the site of action, which is not far away. Nevertheless auxin is also translocated to other regions of the plant body. The transportation of auxin is polar, i.e., from apex to the base, which is called basipetal movement, but acropetal movement i.e. from the base to apex has also been observed but the amount transported is almost negligible. The ratio between basipetal and acropetal movement is approximately 3:1.
Furthermore, the transportation is more or less an active process. The long distance transportation appears to take place through sieve tubes and it is a facilitated mechanism. The rate of auxin movement is about 6.4-20 mm/hr, which is many times faster than the rate of diffusion. It is now clear that the transportation is through carriers. Auxin exists in ionized form called A (-) and uncharged form called AH. Transportation across the membranes is through carriers and in ioized form. But transportation in free space is diffusion and it is in AH form. One of the proteins that is responsible for efflux transport of auxins is PIN; there are several forms of these proteins.
SITE OF SYNTHESIS:
The natural auxin found in plants is called Indole Acetic Acid (IAA) and it is the first of the phytohormones to be discovered. Indole acetic acid is mostly synthesized in the stem and root apexes. Using radioactive 14C as the tracer, it has been demonstrated that the meristematic cells, just above differentiating into vascular tissues, are found to be active in synthesizing IAA. Shoot apexes synthesize more auxin than the root species.
Tryptophan has been found to be the precursor for IAA synthesis. In some cases Indole acetonitril has also been found to be used in the synthesis of IAA. The enzymes responsible for the biosynthesis of IAA have been identified. There are two pathways which converge in the production of IAA. To start with, tryptophan is converted to Indole pyruvate by deamination reaction catalyzed by specific deaminase enzymes. The indole 3D pyruvate is then subjected to decarboxylation to produce indole acetaldehyde, which is then oxidized by alde-hydrases to indole 3- acetic acid. On the other hand, tryptophan may be first subjected to decarboxylation step. The tryptamine synthesized in this reaction is deaminated to indole 3- acetaldehyde then it is oxidized to IAA.
Indole aceto nitrile is also found in plants and the same is used in the synthesis of IAA by converting IAN to Indole acetamide then to IAA or IAN to IAA directly.
Once the native auxin has been identified as indole acetic acid, plant biochemists started looking for similar compounds in nature as well as in the laboratory. As a result, a host of synthetic auxins have been discovered and their effects as growth promoting hormones have been characterized, ex. Indole propionic acid Indole butyric acid Naphthalene acetic acid, phenyl acetic acid, 2,4 Dichlorophenoxy acetic acid are just few of the known synthetic auxins.
The hormonal activity of IAA and the other synthetic compounds has been attributed to the presence of a ring system as the nucleus, a side chain possessing a carboxyl group and the presence of at least one carbon atom between the ring and the carboxyl end. The length of the side chain, the number of substituents in the nucleus and the side chain and the basic structure of the nuclear ring have been found to exert profound influence in eliciting physiological responses.
Isolated tissues, hypocotyls segments, epicotyl segments, leaves, excised roots and even whole plants have been used to monitor various biochemical and physiological responses to hormone treatment. Auxin has been found to accelerate cellular metabolism in treated tissues. Particularly respiratory rate increases by at least by 20%.
The general activation of metabolic processes in terms of turnover is all pervading. Even mitochondria and plastids show increased activity. Many of the increased metabolic activities in response to auxin treatment have been attributed to the changes in membrane permeability and activation of some membrane factors. As a consequence of increased respiratory activities, photosynthetic activity, amino acid metabolism, nucleic acid synthesis and protein synthesis and others, cells build up the required materials for their growth.
Auxin binding protein
Plant growth involves interaction between metabolites such as sugars, phytohormones and their action on gene expression. Auxin as a signaling molecule has various effects depending upon the tissue where it acts.
Auxins show some dramatic effects on growth and development of plants. Though the immediate effect of auxin is known to be at the plasma membrane and cytosolic level, with time its effect on gene activation and protein synthesis is over bearing. The effectiveness of Auxin’s activity is due to the presence of auxin binding proteins. They act as receptors and after complexing with the auxins, they are rendered highly active.
The effect of auxins induced cell elongation does not require DNA synthesis, but in long term effect, DNA synthesis is always accompanied with cell division. The interaction between auxin and cytokinin has been interpreted as at the auxin plays permissive role in DNA synthesis and cytokinin stimulates it.
Auxins do not induce transcription immediately after application of the hormone. But auxin induced transcription sustained for 49-50 minutes. But auxin after effect on increased translational activity has been explained as due to the activation of translational machinery through the activation of translational factors. However, transcriptional activity in response to auxin at later stages results in the synthesis of all species of RNAs, such as mRNAs, tRNAs and rRNAs. Among the population of mRNAs induced by auxins, some specific mRNAs for cellulase, tubulin and others, are found is higher concentrations.
As mentioned earlier, auxin, in many systems enhances the rate of protein synthesis very early without the concomitant increase in RNA synthesis; which suggests that auxins earlier effect is on translational activity. This may be achieved either through the activation of inactive mRNPs, or through the activation of translational machinery and translational factors. Increase in polysome content in response to auxin treatment in many plant cells is good evidence in support of auxins’ promotive effect on protein synthesis. However, the increased level of protein synthesis at later stages is actually due to the auxin induced transcriptional activity, which really sustains the translational activity for a longer period of time. Though auxin enhances the synthesis of almost all house keeping enzymes, it also induces the synthesis of specific proteins like cellulase, cellulose synthase, tubulins and other specific factors required for cell elongation. It should be remembered that the specific effect of auxin on the expression of a particular protein product depends upon kind of tissue involved. Auxin induced activation of polymerization of tubulins into microtubules is another interesting feature of auxins effect.
Among all the hormonal effects the effect of auxin on cell elongation has been studied in detail. For a long time, the exact mechanism of auxin induced cell elongation has remained unsolved. For historical interests it is essential to understand a few theories which where proposed in the past to explain the mechanism. In this text recent views have also been provided.
This is probably the oldest of the theories that have been proposed so far. This theory is based on the assumption, that auxin stimulates respiratory and other metabolic activities.
As a result, the osmotically inactive components of cells are degraded to osmotically active molecules. This in turn affects the diffusion pressure deficit of the cell and water potential gradient is created between the growing cells. So water from neighboring other cells, particularly xylem diffuses into the activated cell passively. As a consequence of this turgour pressure that builds up within the cell, the cell is stretched from within, thus the cell elongates. According to this concept the cell elongation is a passive phenomenon.
It is true that auxins enhances the respiratory activity, but for the cell to elongate, due to the turgour pressure developed inside, the cell also requires loosening of the cell wall, without which whatever turgour pressure that develops, cannot force the cell to elongate or expand. In recent years, with the use of refined techniques, it has been demonstrated that in many plant systems the auxin induced cell elongation takes place under negative turgour pressure. Furthermore, respiratory inhibitors like DNP, cyanide inhibit auxin mediated cell elongation, which suggests that cell elongation is an active process. Because of these the turgour theory is not favored. One of the proteins involved in cell expansion is expansin
With new discoveries in the field of molecular biology, plant scientists also thought, that auxin brings about the cell elongation through gene activation. Bonneret.al. suggested that as auxins are acidic in nature, they can easily bind to basic proteins found associated with the chromatin material. The binding of auxins to histones, certain segments of DNA’s are freed from the surrounding histone proteins for transcriptional activity. As a result, the required mRNAs for cellulase are produced. As cellulase is required for degradation and loosening of the cell wall, they proposed gene activation. The combined effect of gene activity and auxin induced turgour pressure has believed to be mechanism of cell elongation. Further elongation stops at later stages and the synthesis of cell wall materials continues which again is due to the activation of gene expression.
This theory however falls short of its expectations because transcriptional activity in response to auxin increases only after 15-30 minutes later, but the apparent growth of cell starts as early as 10-15 minutes after auxin treatment. So this theory also fails to explain the early phase of growth, moreover, the binding of auxin to the histone in bringing about gene expression is no more tenable.
Cleland and others have demonstrated that stem segments on exposure to auxin, secrete protons into external medium and render it acidic. Sharp fall in pH of the medium due to auxin treatment has been demonstrated by many workers. Proton secretion has been attributed to the activation of H/ATPase found in the plasma membrane. As the protons diffuse through the cell wall, the pH in that region falls and acidic form cellulase will be activated. As a result, the cell wall fibrils are cut and loosened which greatly facilitates the elongation of cells.
Acid theory explains the early effect of auxin on cell elongation even before the auxin induced transcription starts. This theory also assumes that the internal pressure responsible for the elongation as turgour pressure. It is now known that not in all cases turgour pressure has been demonstrated as the cause for cell elongation. However, this theory explains the early effect of auxin on cell elongation.
Auxin also induces cellulase enzymes which act on cellulose fibers and loosen the cell wall and facilitate the expansion due to increase in turgid pressure.
This theory is the most recent theory and it is based on many experimental evidences. It is very important to know that auxin induced cell elongation or growth exhibits two phases. The first phase is initiated as early as 5-15 minutes after treatment. It is rapid and lasts for 30-45 minutes. This phase is insensitive to the inhibitors of transcription and translation. But it is highly sensitive to colchicine treatment. The above features suggest that the early rapid growth does not require transcription or translation products. But it requires microtubule formation because colchicine inhibits polymerization of tubulin monomers into microtubules. On the other hand, the second phase of growth is slow but steady. It is sensitive to Actinomycin-D, CHI and also colchicine. It means the second phase requires transcriptional products, translational products and polymerization of tubulins into microtubules which act as cytoskeletons.
In the first phase, auxin first enhances the rate of respiration, thereby ATP production increases. At the same time, it also activates nucleating centers and H/ATPase pump located in the plasma membrane. Nucleating centers are a group of aggregated tubulin monomers which on activation start assembling available tubulin monomers from the cytosolic pool. Thereby, a large number of microtubules grow and elongate from the nucleating centers. Polymerization of tubules requires GTP and ATPs as energy source. The hydrolysis of them generates significant amount of H+. Meanwhile, the activated H/ATPase pumps excrete H+ into the exterior of the plasma membrane by active process. As the pH in the cell was becomes acidic, the acidic form of cellulase enzyme which are already present become active glucanase activity they hydrolyze the cellulose fibrils, thus the cell walls fibrils loosened. In this process, certain amount of Ca2+ ions are also removed from the middle wall, which renders the middle wall more labile and plastic. As more and more microtubules grow and elongate and build up, they build up a kind of mechanical force within the cell and the cell is stretched. MTs contribute to the deposition of cell wall materials to be transported and deposited out side the plasma membranes. The elongation is further facilitated by the loosened cell wall. Thus the cells grow in length.
The second phase of cell elongation however required more and more of tubulin pool because most of it is depleted in the first phase. So it also requires fresh synthesis of tubulins, cellulase enzymes and other required factors for the sustained cell elongation as per the demand transcriptional activity increases and the RNAs produced are used by the translational machinery and more of proteins are synthesized. Though there is an increase in the synthesis of most of the house keeping proteins, the increased synthesis of tubulin, cellulase and cellulose synthesizing enzymes is significant. Utilizing tubulin monomers continue to polymerize and more of microtubules generate at the same time loosening of cell wall continues. There by the second phase of cell elongation proceeds slowly but steadily. That is why the second phase is so sensitive to the inhibitory actions of actinomycin D, CHI and colchicine.
This theory appears to explain most of the observed properties of cell elongation. Furthermore, it also explains how cells elongate even in the absence of sufficient turgour pressure within the cell.
Roots developing from any part of the plant body other than the radicle are called adventitious roots. It is not an uncommon phenomenon to see the plant parts are propagated by inducing new root formation. But the exogenous supply of auxins to stem and leaf cuttings readily induces the new root formation, which ensures vegetative propagation. The synthetic hormones like IBA and NAA are more effective in new root formation than the native IAA.
If defoliated stem cuttings are continuously washed in water to leach out the endogenous auxins, they do not produce any adventitious roots in a nutrient media. But if such washed segments are treated Indole Butyric Acid for about 10-20 minutes and placed in a nutrient medium, stem cuttings produce a large number of roots. These roots normally develop from the terminally differentiated pericyclic tissue found around phloem tissue. Within 36-72 hours after treatment some of the founder cells in the said pericyclic region undergo transformation and organize into root primordia, which later grow through the cortex and emerge out of the stem.
The transformation and organization of pericyclic cells into root primordia is a phenomenon of dedifferentiation. Studies on molecular aspects of IBA induced new root formation in the hypocotyls of phaseolus vulgaris reveal that the new root formation is due to the differential gene expression. Using techniques like translation of isolated mRNAs from treated and untreated segments at different time periods in a cell free system and the analysis of invitro and in vivo protein products by polyacrylamide SDS gel electrophoresis and autoradiography and immunoprecipitation of labeled proteins by using specific antibodies, show that among the many proteins synthesized, the synthesis of 55-58 KD proteins and 105 kd proteins in the IBA treated segments increases significantly. The 55-58% proteins have been identified as α and β tubulin, which are the precursor for microtubules. Involvement of microtubular assembly and orientation during the differentiation and organization of pericyclic cells into root primordia is confirmed by the use of colchicine and cytochalasin B. The said drugs prevents the root initiation in the IBA treated hypocotyl segments within 36
hours after hormonal treatment, but the drugs have no effect if the hypocotyls that have already passed through 36 hours after IBA treatment. The above results indicate how auxins can bring about new root formation by inducing differential gene expression. Some of the concepts of auxin action on gene expression has been shown below in the form consolidated figures which are self explaining.
Lateral root develop from preexisting roots and new roots develop from stems. The root initials start from cambial founder cells. It is important to remember that even plant tissue, as in the case of animal systems, contain stem cells, called them as founder cells. Proper signals can induce organ formation from such stem cells. Auxin activates founder cells in pericycle to enter into G1-S transition. It is now clear that auxin induces certain cyclins and CDKs (B) types. Auxin induced transcriptional mechanism perhaps go through activators, Auxin binds to auxin response factors TIR1, which in turn translocates into the nucleus and bind to auxin response elements of genes. However the degradation ARF/IAA/Auxin protein complex by SCF –TIR is important for the ARFs to bind to auxin response elements properly. It is also possible several other factors are involved in induction of lateral roots; probably auxin binding protein (ABP) is one among them.
Once auxin is transported into cells, it binds to its receptor protein TIR, which inturn binds ARFs /IAA/Auxin protein complex which are acting as repressors. IAA-TIR binding leads to activation of SCF mediated degradation of IAA/Aux binding protein which frees ARFs and now ARFs bind to ARE promoter elements. Aux/IAA binding proteins were acting as repressors. The auxin-TIR1 now binds to Aux/IAA proteins and using SCF complex ubiquitinate AUX/IIA proteins and feed to proteosomes for degradation. Thus the ARFs become active and activate specific genes to which they are bound. This is general mechanism for auxin induced gene activation.
Auxin binding protein (ABP)
TIR1 protein bound to auxin activates Auxin response factors and thus gene expression.
The above figure proposed by Thomas Guiylfoyle provides the input how auxin mediated specific genes are activated. This proposal is considered to be the most famous among plant molecular biologists.
EFFECT ON APICAL DOMINANCE
Plants like pines and other conifers exhibit a growth pattern, which is quite distinct from a banyan tree or a tamarind tree. The conical growth of the pine plant is due to the predominant growth activity of the apical meristems where the growths of lateral buds are more or less suppressed. On the other hand, the growth pattern in banyan tree is diffused, where lateral branches grow as vigorously as the apical branches. The suppressive effect of apical buds on the growth of lateral buds is often called apical dominance. Such a phenomenon is not just restricted to only conifers, but it is also found in other plants.
Apical dominance has been explained as due to the action of auxin present in the main apex. This view is amply supported by an experiment, where if the stem tip is cut off, the axillary buds found below sprout immediately. Instead, if an agar block containing auxin placed over the decapitated stem, the axillary buds remain suppressed. It is clear from the above experiment, that auxin present in main apex some how inhibits the growth of the axillary bud. The severity of apical dominance is greater on the axillary buds present nearer to the main apex. Nevertheless, apical dominance exerted by the auxins can be overcome by the application of cytokinins to axillary buds. This is because cytokinin induces cytokinesis. Probably, the mitotic block that is operating in axillary buds may be due to the inadequate supply of cytokinins. But how the supply of cytokinin to axillary buds is made inadequate by the apical bud is not clear.
The idea of nutritional inadequacy to the axillary buds due to the stronger influence of apical meristems has been considered by many scientists. It is assumed that because of apical dominance, most of the nutrition is drawn towards the apex than to the axillary bud. This explanation for apical dominance has no conclusive evidences to prove the claim. On the other hand, studies have shown that higher concentrative of IAA induces the synthesis of ethylene. As apical buds contain more of auxins, it may induce the synthesis more of ethylene which in turn may inhibit the normal growth of the axillary bud. But by removing the apical bud, the concentration of auxin drops, so also ethylene hence axillary buds sprout.
The growth curvature in stem apex in response to light is called phototropic movement. Such growth curvature in shoot tips can be induced even in the absence of light by placing auxin containing agar blocks asymmetrically on decapitated stem tips. This indicates that unequal concentration of auxin is responsible for unequal growth there by the curvature. But in light induced curvatures, how does light brings about unequal concentration of auxin and which part of the light spectrum is responsible for the curvature are the few questions that needs explanation. Using monochromatic light, it has been determined that the most effective spectrum is 445 mm. But the answer to the first question is still elusive and various theories have been proposed from time to time to explain this phenomenon.
This theory was proposed by Cholodney & F.W. Went. Accordingly, the concentration of auxin in the stem apex is uniform all-round. Once the light rays fall on the stem from one side, it induces the movement of auxin from the illuminated region towards the darker region. By such movement, auxins accumulate in greater amounts in the darker region. The motive force for the movement of the auxin in response to light is attributed to the differential electrical charges on the said surfaces. Then he tips were illuminated from one side and the basipetally diffused auxin was collected and the amount of auxin found in the agar blocks was determined. According to the authors in the control tips they detected more of auxin in the far side the surface that was in the dark than from the surface that was illuminated. But in the tip with a mica plate the amount of auxin found in both the blocks was same. So it was deduced that the difference in the amount of the auxin detected in the blocks on the darker side is actually was due to lateral movement. Accordingly resultant differential concentration is responsible for differential growth, hence the phototropic curvature. The growth is dependent auxin concentration.
Recombinant GUS gene expression of a gene that is responsive to auxin shows the distribution of Auxin in response to light
Their claim was disputed by other workers, who used radioactive C-labeled auxin for exact quantitative determination of the transported auxin from one region to another. They did not find any substantial lateral transportation. So the lateral movement theory has remained unconvinced.
Inhibition of basipetal Transport Theory:
Gordon and others proposed that the unequal distribution of auxins in response to light treatment is not due to lateral movement, but due to the inhibition of basipetal movement from the illuminated side, which causes unequal concentration of auxins and growth curvature is due to it. Slow growth of the stems in day times and steady growth of the plants in intense sun light has been attributed to this effect. However this theory has not been tested with covincing experiments.
Photo inactivation Theory:
When blue light at 445 nm has been detected as the action spectrum for the induction of phototropic curvature, plant physiologists started looking for pigments that absorb blue light at 445 nm. This logic was based on the fact that known photo responses like photosynthesis and photoperiodic responses are due to specific pigments. The search for such pigments revealed the presence of B carotenes and riboflavin in the stem tips. As some plants, which respond to light induced phototropic movement, did not possess B carotenes it was deduced that riboflavin as the causative pigment. The photo inactivation was explained on the assumption that riboflavin after absorbing light gets activated and the same inactivates the auxin directly or destroys the auxin through certain IAA oxidase. This result in unequal concentration of auxin in the stem tip which in turn is believed to be responsible for the phototropic curvature again this theory has never been proved unequivocally.
In the past, many experimental results were based on very crude extraction methods. Such experimental results are not very convincing today. Use of radioactive isotopes i.e. (14C) IAA, solvent extraction methods combined with GLC analysis for qualitative and quantitative estimations of auxins have cast grave doubts about previous theories. In the light of recent work, it has been suggested that light has profound effect on inducing the synthesis or releasing the growth inhibitors like ABA. The release of ABA in the region where it is illuminated causes inhibition of growth in the said region. The effect of ABA on osmotic changes by creating effluxes of ions is well known. Moreover, it is now known that auxins could be made inactive or active by auxin binding proteins. If binding protein complexes with IAA, it is rendered physiologically active, if it is free, it remains inactive. So light induced changes in the concentration of ABA and other auxin binding factors are believed to bring about variations in the endogenous levels of active auxins. Such changes are ultimately responsible for phototropic growth curvatures,
For a long time the above concepts were accepted as facts, however, finding of phototropin, a blue light absorbing protein, has role in phototropic curvature is also an accepted fact. Phototropin exists in two forms; both are flavin binding proteins of mol.wt 110 Kda and 124Kda. Interestingly these proteins contain at least 11 kinase (PAS) domains; these domains are celled LOVE domains (Relation to Light, Oxygen and Voltage). They also contain different domains such as BTB and D1-to D15 domains. Phototropins act as light receptors and absorption leads to autophosphorylation at serine/threonine sites and they are involved in phosphorylation of many other membrane bound proteins. Membrane bound proteins have many roles in transportation of ions, transportation of Auxin/auxin receptor.
However the exact mechanism by which they bring about photrophic curvature movement is yet to be discerned.
Conformational changes of Love domain proteins called Phototropin in the presence of light and in the absence of light.
Plants regulate their
growth directions in response to light direction, and its response is called
phototropism. Phototropism is a model research to understand the regulatory
mechanisms of auxin metabolisms in response to environmental stimuli, and we
are studying on this topic by a molecular genetic approach using Arabidopsis
mutants. We identified a signal transducer RPT2 and a novel blue-light
photoreceptor phot2, both of which are required for the phototropic responses of
Arabidopsis. We indicated that phot1 and phot2 show partially overlapping
functions in two different responses, phototropic response and chloroplast
relocation, in a fluence rate-dependent manner, as blue light receptors. To
reveal the phototropin-signaling pathways, We studied on the regulation of
cytosolic Ca2+ concentrations by phototropins, involvements of RPT2 and another
phototropin-signaling factor NPH3 in chloroplast relocation and stomatal
opening, and functional sharing between phototropins and other blue-light
receptors cryptochromes. Now, We are studying on a dephosphorylation mechanism
of NPH3 and transcriptional and post-transcriptional regulations of RPT2 in
response to blue light irradiation.
In addition, we revealed that photoreceptors phytochromes and cryptochromes regulate auxin biosynthesis, metabolism, and transport in response to light irradiation and indicated that a suppression mechanism of expression of auxin transporter PGP19 by phytochromes and cryptochromes is one of mechanisms to enhance the phototropic response by phytochrome and cryptochromes. Other analyses on the light signaling and auxin mechanisms were also published by collaborations with other research groups.
Light activates three kinds of photoreceptor families, phototropins, phytochromes, and cryptochromes, and affects the plant growth directions through changes of auxin biosynthesis/metabolism and transport.
Effect on Growth Curvatures:
Growth movement of any part or the organ of the plant body in response to gravitational pull of the earth is referred to as geotropic movements. The most remarkable feature of the plant body is that, though both shoot and root system are derived from the same embryonic cells, they respond differently to the same gravitational stimulus. This property may be due to unique embryonic developmental programmes dictated by the inbuilt genetic factors, which probably enforce the respective organism to behave differently to different environmental factors.
Growth curvature away from the earth’s gravity is called negative geotropism and the growth of the roots towards earth is called positive geotropism. Most of the roots, with the exception of pneumatophores and coralloid roots which are negatively geotropic, exhibit positive geotropism. However, not all stem show absolute negative geotropism. It is a common observation that some of the underground stems like rhizomes, suckers, etc, grow obliquely in the soil and such growth movements are called dia-geotropic movements.
Mechanism of geotropic for with Curvatures:
Again, Cholodney and went are the pioneers in explaining geotropic movements. They proposed that stem apexes and root apexes require different concentration of auxins to bring about the maximal growth. In the sense, it is said, that the concentration of auxin that is favorable for the growth of shoot apex is inhibitory to the growth of the root apex. On the contrary, the concentration, that is optimal for the growth of the root apexes, is not adequate for the growth of the stem tips. It implies that stem cells require greater amount of auxins for the in maximal growth and root cells require very low concentration of auxins for their optimal growth. Based on these premises the mechanisms of geotropic responses have been explained.
If a straight seedling is placed horizontally on the soil, shoots curl upwards and roots bend towards the soil. This behavior has been attributed to the gravitational force that acts upon the respective organs for they are endowed with different programmes, though both have the same genetic properties. Cholodney and went assumed that the gravitation once acts on both root and shoot. So auxins having certain mass of their own move downwards and accumulate in greater amounts in the lower cells which are in contact with the soil.
As higher concentration of auxin is promotive in stem tips the cells grow faster than the others. So the stem curves and grows upwards. On the contrary, as the higher concentration of auxin is inhibitory for the root cells, the growth of root cells is inhibited, but the cells containing less amount of auxins show maximal growth activity. Hence root tips curve towards soil and grow forwards in to it. Thus stems exhibit negative geotropic growth movements and roots show positive geotropic movements.
The above said theory enjoyed the general acceptance for a long period of time. But people realized that gravitational force, by its mass action, has greater effect on amyloplasts than on auxins found in the plant cells. Because of their greater mass, amyloplasts settle down on the plasma membranes of the cells as shown in the figure. The contact of the amyloplasts with plasma membranes acts as on irritant as a result growth of cells on that side of the membrane is inhibited but the growth on he other side is favored. But this explanation has never been favored because some plants which are lacking in amyloplasts also exhibit geotropic responses.
In recent years, research work on geotropic movements has revealed that the site of geotropic perception is not the root apex, and it requires root cap for its response. If the root cap is cut off, the decapitated root dies not show any geotropic responses. When the cap is replaced he roots respond normally for geotropic stimulus. This observation has been explained as due to the synthesis of ABA in the root caps. The ABA is transported basipetally, and then it also moves downwards due to the gravitational force. As the lower cells receive ABA their growth is inhibited. But the upper cells grow normally and bring about geotropic curvature.
Effect on Parthenocarpy:
Development of fruits without fertilization is known as parthenogenesis and the fruit is said to be parthenocarpic fruit. Auxins have been found to be effective in inducing parthenocarpic fruits in some plants. It has been demonstrated that the extracts of pollen grains also induce the development of parthenocarpic fruits, discharge of pollen tube contents into embryo sac is believed to be cause for the increase in he content of auxin in embryo sac. Some botanists suspect that pollen tubes carry some enzymes which produce
It is important to note the synthesis of more auxins not lead to fertilization. Whether the act of fertilization has any stimulatory effect on the synthesis of auxin is not clear nonetheless many plants respond to auxin treatment produce parthenocarpic fruits.
Induction of parthenocarpic fruits by auxins has greater application value in cultivating fruit yielding plants. The auxin induced fruits, besides seedless, they are larger in size and sweeter. Commercial production of such fruits brings more income to farmers.
It is common to observe that older leaves, debladed petioles, abortive flowers, and often fruits, fall off from the plants. In the above said cases a distinct and characteristic layer of cells develop at the base of the petiole of the leaf or the pedicel of fruit, which acts as a weak point, hence the said structures fall down. The layer that is responsible for this process is called abscission layer or abscission zone.
It consists of a number of layers of thin walled cells which are rich in cytoplasm and actively dividing. Disappearance of middle lamella from the cells of this layer is a characteristic feature. This is due to the activity of pectinase enzyme. As a result, this part of the stalk becomes weak and the leaf or the fruit falls down by its own weight.
In many plants, just below the abscission zone towards the stem, a layer of actively dividing cells develops. This layer is called protective layer. This layer develops after the fall of leaf at the free surface, but in some cases the protective layers develop even before the fall of the leaves.
These cells by repeated cell divisions produce many layers of cells, which get suberized and protect the inner layer of cells from injury or infection. Thus, this layer acts as wound healing tissue.
Development of Abscission Layer:
Leaves fall with the age and fruits fall down after ripening, but in deciduous plants, onset of winter acts as the signal for the plants to shed their leaves. Sometimes, unseasoned cold waves induce premature falling of fruits. In all these cases, the falling of leaves or falling of fruits is due to the formation of abscission layer at the base of their stalks. The factors that induce the formation of abscission layers vary. In some cases, ageing acts as an important causative factor, in others environmental conditions like winter may act as the factor. In the case of aging the accumulation of senescing factors induce abscission layer formation. One of the senescing factors is believed to be ABA. Reduction in the quantity of auxin n the distal parts of the leaf is also known to be another causative factor. In the initial stages of abscission layer formation, if IAA is applied to distal part of the leaf, the development of the abscission layer is inhibited. Instead, if auxin is applied at a later stage of abscission the process I shortened and the leaf falls quickly.
Once, it was thought that ABA is mainly responsible for leaf abscission, but recent investigations indicate that ethylene is highly effective in inducing abscission layer formation, but the role of ABA is not totally ruled out. Incorporation of radioactive label during the abscission layer formation suggests, differential gene expression in the abscission zone results in the production of pectinases and cellulases, which by their activity breakdown the middle wall and also some cellulose. At the same time the cells in this zone become meristematic.
The application of auxin to leaves and fruits is now known to prevent the development of abscission layer. Many synthetic hormones like NAA, IBA, 2,4-D have been found to very effective in preventing the premature fall of fruits, particularly commercial crops like citrus, apple, oranges, mangoes, grapes. The use of such hormones not only prevents the premature falling of fruits, but also increases the quantum of fruit setting. And fruits thus produced are larger and sweeter. Thus auxins can be used for commercial gains in the field of pomiculture.
Auxin and Cytokinins interactions:
In plants each of the phytohormones, though have independent effects, often they affect the other or they may have synergistic effect. For example Auxin prepares the cell but cytokinins execute cell division. When auxin induces new root formation at a particular concentration, cytokinins inhibit auxin induced new root formation.