Lateral redistribution of auxin is not the means for gravitropic differential growth of coleoptiles: A new model

Plant root development: is the classical theory for auxin-regulated root growth false?

One of the longest standing theories and, therein-based, regulation-model of plant root development, posits the inhibitory action of auxin (IAA, indolylacetic acid) on elongation growth of root cells. This effect, as induced by exogenously supplied IAA, served as the foundation stone for root growth regulation. For decades, auxin ruled the day and only allowed hormonal side players to be somehow involved, or in some way affected. However, this copiously reiterated, apparent cardinal role of auxin only applies in roots immersed in solutions; it vanishes as soon as IAA-supplied roots are not surrounded by liquid. When roots grow in humid air, exogenous IAA has no inhibitory effect on elongation growth of maize roots, regardless of whether it is applied basipetally from the top of the root or to the entire residual seedling immersed in IAA solution. Nevertheless, such treatment leads to pronounced root-borne ethylene emission and lateral rooting, illustrating and confirming thereby induced auxin presence and its effect on the root - yet, not on root cell elongation. Based on these findings, a new root growth regulatory model is proposed. In this model, it is not IAA, but IAA-triggered ethylene which plays the cardinal regulatory role - taking effect, or not - depending on the external circumstances. In this model, in water- or solution-incubated roots, IAA-dependent ethylene acts due to its accumulation within the root proper by inhibited/restrained diffusion into the liquid phase. In roots exposed to moist air or gas, there is no effect on cell elongation, since IAA-triggered ethylene diffuses out of the root without an impact on growth.

Characterization of a Novel Creeping Tartary Buckwheat (Fagopyrum tataricum) Mutant lazy1

Gravity is known as an important environmental factor involved in the regulation of plant architecture. To identify genes related to the gravitropism of Tartary buckwheat, a creeping line was obtained and designated as lazy1 from the mutant bank by ⁶⁰Co-γ ray radiation. Genetic analysis indicated that the creeping phenotype of lazy1 was attributed to a single recessive locus. As revealed by the horizontal and inverted suspension tests, lazy1 was completely lacking in shoot negative gravitropism. The creeping growth of lazy1 occurred at the early seedling stage, which could not be recovered by exogenous heteroauxin, hormodin, α-rhodofix, or gibberellin. Different from the well-organized and equivalent cell elongation of wild type (WT), lazy1 exhibited dilated, distorted, and abnormally arranged cells in the bending stem. However, no statistical difference of indole-3-acetic acid (IAA) levels was found between the far- and near-ground bending sides in lazy1, which suggests that the asymmetric cell elongation of lazy1 was not induced by auxin gradient. Whereas, lazy1 showed up-expressed gibberellin-regulated genes by quantitative real-time PCR (qRT-PCR) as well as significantly higher levels of gibberellin, suggesting that gibberellin might be partly involved in the regulation of creeping growth in lazy1. RNA sequencing (RNA-seq) identified a number of differentially expressed genes (DEGs) related to gravitropism at stages I (before bending), II (bending), and III (after bending) between WT and lazy1. Venn diagram indicated that only Pectate lyase 5 was down-expressed at stages I [Log₂ fold change (Log₂FC): -3.20], II (Log₂FC: -4.97), and III (Log₂FC: -1.23) in lazy1, compared with WT. Gene sequencing revealed that a fragment deletion occurred in the coding region of Pectate lyase 5, which induced the destruction of a pbH domain in Pectate lyase 5 of lazy1. qRT-PCR indicated that Pectate lyase 5 was extremely down-expressed in lazy1 at stage II (0.02-fold of WT). Meanwhile, lazy1 showed the affected expression of lignin- and cellulose-related genes and cumulatively abnormal levels of pectin, lignin, and cellulose. These results demonstrate the possibility that Pectate lyase 5 functions as the key gene that could mediate primary cell wall metabolism and get involved in the asymmetric cell elongation regulation of lazy1.

Hormesis in Plants: The Role of Oxidative Stress, Auxins and Photosynthesis in Corn Treated with Cd or Pb

Hormesis, which describes the stimulatory effect of low doses of toxic substances on growth, is a well-known phenomenon in the plant and animal kingdoms. However, the mechanisms that are involved in this phenomenon are still poorly understood. We performed preliminary studies on corn coleoptile sections, which showed a positive correlation between the stimulation of growth by Cd or Pb and an increase in the auxin and H₂O₂ content in the coleoptile sections. Subsequently, we grew corn seedlings in hydroponic culture and tested a wide range of Cd or Pb concentrations in order to determine hormetic growth stimulation. In these seedlings the gas exchange and the chlorophyll a fluorescence, as well as the content of chlorophyll, flavonol, auxin and hydrogen peroxide, were measured. We found that during the hormetic stimulation of growth, the response of the photosynthetic apparatus to Cd and Pb differed significantly. While the application of Cd mostly caused a decrease in various photosynthetic parameters, the application of Pb stimulated some of them. Nevertheless, we discovered that the common features of the hormetic stimulation of shoot growth by heavy metals are an increase in the auxin and flavonol content and the maintenance of hydrogen peroxide at the same level as the control plants.

Auxin and physical constraint exerted by the perianth promote androgynophore bending in Passiflora mucronata L. (Passifloraceae)

The androgynophore column, a distinctive floral feature in passion flowers, is strongly crooked or bent in many Passiflora species pollinated by bats. This is a floral feature that facilitates the adaptation to bat pollination. Crooking or bending of plant organs are generally caused by environmental stimulus (e.g. mechanical barriers) and might involve the differential distribution of auxin. Our aim was to study the role of the perianth organs and the effect of auxin in bending of the androgynophore of the bat-pollinated species Passiflora mucronata. Morpho-anatomical characterisation of the androgynophore, including measurements of curvature angles and cell sizes both at the dorsal (convex) and ventral (concave) sides of the androgynophore, was performed on control flowers, flowers from which perianth organs were partially removed and flowers treated either with auxin (2,4-dichlorophenoxyacetic acid; 2,4-D) or with an inhibitor of auxin polar transport (naphthylphthalamic acid; NPA). Asymmetric growth of the androgynophore column, leading to bending, occurs at a late stage of flower development. Removing the physical constraint exerted by perianth organs or treatment with NPA significantly reduced androgynophore bending. Additionally, the androgynophores of plants treated with 2,4-D were more curved when compared to controls. There was a larger cellular expansion at the dorsal side of the androgynophores of plants treated with 2,4-D and in both sides of the androgynophores of plants treated with NPA. This study suggests that the physical constraint exerted by perianth and auxin redistribution promotes androgynophore bending in P. mucronata and might be related to the evolution of chiropterophily in the genus Passiflora.

Role of xyloglucan in gravitropic bending of azuki bean epicotyl

The mechanism of the gravitropic bending was studied in azuki bean epicotyls. The cell wall extensibility of the lower side became higher than that of the upper side in the epicotyl bending upward. The contents of matrix polysaccharides of the cell wall (pectin and xyloglucan in hemicellulose-II) in the lower side became smaller than those in the upper side. The molecular mass of xyloglucans in the lower side decreased. After an epicotyl was fixed to a metal rod to prevent the bending, gravistimulation was applied. Fundamentally the same results were obtained with respect to rheological and chemical characteristics of the cell wall as those of epicotyls showing gravitropic bending. The present results suggested that the initial gravitropic bending was caused by the increase in extensibility of the lower side and the decrease in extensibility of the upper side via the change of the cell wall matrix, especially xyloglucans.

The epidermal-growth-control theory of stem elongation: an old and a new perspective

The botanist G. Kraus postulated in 1867 that the peripheral cell layers determine the rate of organ elongation based on the observation that the separated outer and inner tissues of growing stems spontaneously change their lengths upon isolation from each other. Here, we summarize the modern version of this classical concept, the "epidermal-growth-control" or "tensile skin" theory of stem elongation. First, we present newly acquired data from sunflower hypocotyls, which demonstrate that the expansion of the isolated inner tissues is not an experimental artefact, as recently claimed, but rather the result of metabolism-independent cell elongation caused by the removal of the growth-controlling peripheral walls. Second, we present data showing that auxin-induced elongation of excised stem segments is attributable to the loosening of the thick epidermal walls, which provides additional evidence for the "epidermal-growth-control concept". Third, we show that the cuticle of aerial organs can be thin and mechanically weak in seedlings raised at high humidity, but thick and mechanically important for organs growing under relatively dry air conditions. Finally, we present a modified model of the "tensile skin-theory" that draws attention to the mechanical and physiological roles of (a) the thickened, helicoidal outer cell walls, (b) the mechanical constraint of a cuticle, and (c) the interactions among outer and inner cell layers as growth is coordinated by hormonal signals.

Gravitropic plant growth regulation and ethylene: an unsought cardinal coordinate for a disused model

According to the Cholodny-Went hypothesis, gravitropic differential growth is brought about by the redistribution of auxin (indolyl-3-acetic acid, IAA). We reinvestigated the relevance of different auxins and studied the role of ethylene in hypocotyls of sunflower and shoots and roots of rye and maize seedlings. Incubation of coleoptiles and of sunflower hypocotyls in solutions of IAA and dichlorophenoxyacetic acid as well as naphthylacetic acid resulted in a two- to threefold length increase compared to water controls. In spite of this pronounced general effect on elongation growth, gravi-curvature was similar to water controls. In contrast to this, inhibition of ethylene synthesis by aminoethoxyvinylglycine prevented differential growth of both hypocotyls and coleoptiles and of roots of maize. In horizontally stimulated maize roots growing on surfaces, inhibition of ethylene perception by methylcyclopropene inhibited roots to adapt growth to the surface, resulting in a lasting vertical orientation of the root tips. This effect is accompanied by up- and down-regulation of a number of proteins as detected by two-dimensional matrix-assisted laser desorption-ionization time-of-flight mass spectrometry. Together the data query the regulatory relevance of IAA redistribution for gravitropic differential growth. They corroborate the crucial regulatory role of ethylene for gravitropic differential growth, both in roots and coleoptiles of maize as well as in hypocotyls.

A major factor in gravitropism in radish hypocotyls is the suppression of growth on the upper side of hypocotyls

The changes in length on the two opposite sides of etiolated radish (Raphanus sativus) hypocotyls prior to, and following gravitropic stimulation, were measured using an infrared-imaging system. It was observed that the growth suppression on the upper side began first at least 10 min after the onset of gravitropic stimulation, and after 30 min the acceleration in growth on the lower side started. The gravitropic curvature was steadily induced from 10 min. When radish hypocotyls were switched from a vertical to horizontal position for different durations and then replaced to the vertical position, the growth suppression on the gravity-stimulated (upper) side was observed in all cases, but the acceleration in growth on the opposite (lower) side appeared only in continuously gravity-stimulated seedlings, although it occurred later than the growth suppression on the upper side. These results suggest that the suppression in growth on the upper side of the hypocotyls is a direct effect of gravitropic stimulation, but not the acceleration on the lower side. When 4-methylthio-3-butenyl isothiocyanate (4-MTBI), which has an inhibitory activity against radish hypocotyl growth, was applied on the one side of radish hypocotyls and then the 4-MTBI-applied side or opposite side was placed in a horizontal position, the former showed greater bending than the control, suggesting that the growth suppression on the upper side is enhanced and maintained with MTBI application there. In the latter case, the seedlings showed less bending than the control, suggesting a decrease in growth on the lower side with MTBI application. All the results suggest that gravitropism of radish hypocotyls may be caused by an increase in growth-inhibiting substance(s) induced with gravitropic stimulation in the upper side, inducing growth inhibition there.

Cholodny-Went revisited: a role for jasmonate in gravitropism of rice coleoptiles

Gravitropism is explained by the Cholodny-Went hypothesis: the basipetal flow of auxin is diverted laterally. The resulting lateral auxin gradient triggers asymmetric growth. However, the Cholodny-Went hypothesis has been questioned repeatedly because the internal auxin gradient is too small to account for the observed growth asymmetry. Therefore, an additional gradient in indolyl-3-acetic acid (IAA) sensitivity has been suggested (Brauner and Hager in Planta 51:115-147, 1958). We challenged the Cholodny-Went hypothesis for gravitropism of rice coleoptiles (Oryza sativa L.) and found it to be essentially true. However, we observed, additionally, that the two halves of gravitropically stimulated coleoptiles responded differentially to the same amount of exogenous auxin: the auxin response is reduced in the upper flank but normal in the lower flank. This indicates that the auxin-gradient is amplified by a gradient of auxin responsiveness. Hormone contents were measured across the coleoptile by a GC-MS/MS technique and a gradient of jasmonate was detected opposing the auxin gradient. Furthermore, the total content of jasmonate increased during the gravitropic response. Jasmonate gradient and increase persist even when the lateral IAA gradient is inhibited by 1-N-naphtylphtalamic acid. Flooding with jasmonate delays the onset of gravitropic bending. Moreover, a jasmonate-deficient rice mutant bends more slowly and later than the wild type. We discuss a role of jasmonate as modulator of auxin responsiveness in gravitropism.

Auxins

Auxin is a multifactorial phytohormone that is required for cell division. Fine gradients determine points of developmental change in time and space. It is associated intimately with the axiality of plant growth, and increasing doses lead to cell expansion or inhibition of cell expansion in different tissues. From embryonic patterning to fruit dehiscence every plant process has some involvement with auxin as a hormonal signal, including responses to wounding. Moreover, synthetic auxins have widespread uses as agrochemicals, particularly as selective herbicides. Despite the importance of auxin as a plant signal the pathways of its biosynthesis are still not clear. Much more is known about auxin perception and the mechanisms through which gene transcription is regulated. One receptor has been identified, and protein crystallography data has explained its auxin-binding capacity, but this is likely to control only a subset of auxin-mediated responses. Little is known of the signal transduction intermediates. A second receptor has been nominated and may be involved in controlling auxin-mediated gene transcription. A complex set of proteins comprising signalosome and proteasome contribute to the regulation of sets of transcription factors to confer regulation by derepression. A set of auxin transport proteins has been described with associated regulatory interactors, and these account for polar auxin flow and the control of auxin movements across cells, tissues, and around the plant. The gradients these transport systems build regulate the responses of growth and differentiation, including the plant's response to gravity. These areas are described and discussed by relating the physiology of the whole plant to the details of genetic and protein activities.

Gravitropic bending and plant hormones

Gravitropism is a complex multistep process that redirects the growth of roots and various above-ground organs in response to changes in the direction of the gravity vector. The anatomy and morphology of these graviresponding organs indicates a certain spatial separation between the sensing region and the responding one, a situation that strongly suggests the requirement of phytohormones as mediators to coordinate the process. The Cholodny-Went hypothesis suggested auxin as the main mediator of gravitropism. So far, ample evidence has been gathered with regard to auxin asymmetrical detection, polar and lateral transport involving influx and efflux carriers, response signaling pathway, and possible modes of action in differential cell elongation, supports its major role in gravitropism at least in roots. However, it is becoming clear that the participation of other hormones, acting in concert with auxin, is necessary as well. Of particular importance is the role of ethylene in shoot gravitropism, possibly associated with the modulation of auxin transport or sensitivity, and the key role implicated for cytokinin as the putative root cap inhibitor that controls early root gravitropism. Therefore, the major advances in the understanding of transport and signaling of auxin, ethylene, and cytokinin may shed light on the possibly tight and complicated interactions between them in gravitropism. Not much convincing evidence has been accumulated regarding the participation of other phytohormones, such as gibberellins, abscisic acid, brassinosteroids, jasmonates, and salicylic acid, in gravitropism. However, the emerging concept of cooperative hormone action opens new possibilities for a better understanding of the complex interactions of all phytohormones and their possible synergistic effects and involvement in the gravitropic bending process.

The dose-response curve of the gravitropic reaction: a re-analysis

The dose-response curve of the gravitropic reaction is often used to evaluate the gravisensing of plant organs. It has been proposed (Larsen 1957) that the response (curvature) varies linearly as a function of the logarithm of the dose of gravistimulus. As this model fitted correctly most of the data obtained in the literature, the presentation time (tp, minimal duration of stimulation in the gravitational field to induce a response) or the presentation dose (dp, minimal quantity in g.s of stimulation to induce a response) were estimated by extrapolating down to zero curvature the straight line representing the response as a function of the logarithm of the stimulus. This method was preferred to a direct measurement of dp or tp with minute stimulations, since very slight gravitropic response cannot be distinguished from the background oscillations of the extremity of the organs. In the present review, it is shown that generally the logarithmic model (L) does not fit the experimental data published in the literature as well as the hyperbolic model (H). The H model in its simplest form is related to a response in which a ligand-receptor system is the limiting phase in the cascade of events leading to the response (Weyers et al. 1987). However, it is demonstrated that the differential growth, responsible for the curvature (and the angle of curvature), would vary as a hyperbolic function of the dose of stimulation, even if several steps involving ligand-receptor systems are responsible for the gravitropic curvature. In the H model, there is theoretically no presentation time (or presentation dose) since the curve passes through the origin. The value of the derivative of the H function equals a/b and represents the slope of the cune at the origin. It could be therefore used to estimate gravisensitivity. This provides a measurement of graviresponsiveness for threshold doses of stimulation. These results imply that the presentation time (or presentation dose) derived from the L model cannot be used anymore as an estimate of gravisensitivity. On the contrary, the perception time (minimal duration of a repeated stimulation which induces a response), which is less than 1 s, should be related to the perception of gravity. The consequences of these results on the mode of action and the nature of graviperception are discussed.

Plant hormones and the control of physiological processes

This review examines contemporary views of the role of plant hormones in the control of physiological processes. Past and present difficulties with nomenclature encapsulate the problems inherent in using the 'classic' hormone concept in plants, with their distinctive multicellular organization. Chemical control may be a more relevant notion. However, control may also reside in the responding tissue via changes in sensitivity, or as combined control, where response is dictated by both sensitivity and concentration. Criteria for demonstrating these modes of action are reviewed, as well as frameworks for deciding whether hormone transport is involved. Problems of measuring relevant hormone concentrations are discussed. Methods for measuring and comparing tissue sensitivity to hormones are outlined and relative control is introduced as a means of assessing the importance of hormonal control against a background of other influences. While animals and plants appear to have coinherited homologueous intracellular signalling systems, at the whole organism level modes of hormone action may diverge. It is postulated that the synthesis-transport-action mechanism of action may be just one of several possible ways that phytohormones could control physiological processes. Twelve separate roles are discussed, and it is suggested that some of these could operate simultaneously to the plant's advantage. Contents Summary 375 I. Introduction 376 II. The history of the hormone concept in plant systems 376 III. Issues of nomenclature 380 IV. The need for sound conceptual frameworks in plant hormone research 382 V. Development of criteria for chemical control 384 VI. Identification and quantitative analysis of plant hormones 387 VII. Hormone transport in plants 389 VIII. Hormone sensitivity and its quantification 390 IX. Roles of receptors, second messengers and signal amplification in hormone sensitivity changes 393 X. Relative control as a pivotal concept 395 XI. Diversity of physiological roles for chemical influences in plants 397 XII. Concluding remarks 400 Acknowledgements 402 References 402.