Side Of Hypocotyls Research Articles

Hydrogen peroxide mediates high-intensity blue light-induced hypocotyl phototropism of cotton seedlings

Phototropism is a classic adaptive growth response that helps plants to enhance light capture for photosynthesis. It was shown that hydrogen peroxide (H2O2) participates in the regulation of blue light-induced hypocotyl phototropism; however, the underlying mechanism is unclear. In this study, we demonstrate that the unilateral high-intensity blue light (HBL) could induce asymmetric distribution of H2O2 in cotton hypocotyls. Disruption of the HBL-induced asymmetric distribution of H2O2 by applying either H2O2 itself evenly on the hypocotyls or H2O2 scavengers on the lit side of hypocotyls could efficiently inhibit hypocotyl phototropic growth. Consistently, application of H2O2 on the shaded and lit sides of the hypocotyls led to reduced and enhanced hypocotyl phototropism, respectively. Further, we show that H2O2 inhibits hypocotyl elongation of cotton seedlings, thus supporting the repressive role of H2O2 in HBL-induced hypocotyl phototropism. Moreover, our results show that H2O2 interferes with HBL-induced asymmetric distribution of auxin in the cotton hypocotyls. Taken together, our study uncovers that H2O2 changes the asymmetric accumulation of auxin and inhibits hypocotyl cell elongation, thus mediating HBL-induced hypocotyl phototropism.

External Mechanical Cues Reveal a Katanin-Independent Mechanism behind Auxin-Mediated Tissue Bending in Plants.

Tissue folding is a central building block of plant and animal morphogenesis. In dicotyledonous plants, hypocotyl folds to form hooks after seedling germination that protects their aerial stem cell niche during emergence from soil. Auxin response factors and auxin transport are reported to play a key role in this process. Here, we show that the microtubule-severing enzyme katanin contributes to hook formation. However, by exposing hypocotyls to external mechanical cues mimicking the natural soil environment, we reveal that auxin response factors ARF7/ARF19, auxin influx carriers, and katanin are dispensable for apical hook formation, indicating that these factors primarily play the role of catalyzers of tissue bending in the absence of external mechanical cues. Instead, our results reveal the key roles of the non-canonical TMK-mediated auxin pathway, PIN efflux carriers, and cellulose microfibrils as components of the core pathway behind hook formation in the presence or absence of external mechanical cues.

Cryptochrome-mediated hypocotyl phototropism was regulated antagonistically by gibberellic acid and sucrose in Arabidopsis.

Both phototropins (phot1 and phot2) and cryptochromes (cry1 and cry2) were proven as the Arabidopsis thaliana blue light receptors. Phototropins predominately function in photomovement, and cryptochromes play a role in photomorphogenesis. Although cryptochromes have been proposed to serve as positive modulators of phototropic responses, the underlying mechanism remains unknown. Here, we report that depleting sucrose from the medium or adding gibberellic acids (GAs) can partially restore the defects in phototropic curvature of the phot1 phot2 double mutants under high‐intensity blue light; this restoration does not occur in phot1 phot2 cry1 cry2 quadruple mutants and nph3 (nonphototropic hypocotyl 3) mutants which were impaired phototropic response in sucrose‐containing medium. These results indicate that GAs and sucrose antagonistically regulate hypocotyl phototropism in a cryptochromes dependent manner, but it showed a crosstalk with phototropin signaling on NPH3. Furthermore, cryptochromes activation by blue light inhibit GAs synthesis, thus stabilizing DELLAs to block hypocotyl growth, which result in the higher GAs content in the shade side than the lit side of hypocotyl to support the asymmetric growth of hypocotyl. Through modulation of the abundance of DELLAs by sucrose depletion or added GAs, it revealed that cryptochromes have a function in mediating phototropic curvature.

Rethinking the Origin of Auxin Biosynthesis in Plants.

Rethinking the Origin of Auxin Biosynthesis in Plants.

Polarized by the Light

Light-dependent phosphorylation of the auxin efflux carrier PIN3 determines its trafficking pattern in the hypocotyl endodermis.

Differential expression of the auxin primary response gene MASSUGU2/IAA19during tropic responses of Arabidopsis hypocotyls

We have examined the expression pattern of an auxin primary response gene, MSG2/IAA19, during photo‐ and gravitropic responses of hypocotyls using a transgenic Arabidopsis harboring MSG2/IAA19 promoter::GUS. The upper portion of most etiolated hypocotyls showed uniform β‐glucuronidase (GUS) staining with the strongest activity in the pericycle. When hypocotyls were irradiated with unilateral blue light, GUS activity on the concave side of hypocotyls was decreased, resulting in differential GUS staining with a stronger signal on the convex side. The number of differentially stained hypocotyls peaked at 24 h after the onset of the phototropic stimuli, while hypocotyl curvature continued to increase for the entire 36‐h experimental period. This result suggests that the MSG2/IAA19expression precedes the phototropic responses. When seedlings were grown under dim white light, their hypocotyls displayed almost no GUS activity. The light‐grown hypocotyls also showed differential GUS staining after phototropic stimuli as result of the increase in GUS activity on the convex side of hypocotyls, especially in the epidermis, the outer cortex and pericycle, although GUS activity was much weaker than that observed in etiolated hypocotyls. Similar but less obvious differential staining was obtained for gravitropic response of hypocotyls. Considering the recent finding that Aux/IAA proteins are immediate targets of the auxin F box receptors, MSG2/IAA19is likely to act as one of master genes for tropic responses.

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.

Growth Distribution during Phototropism of Arabidopsis thaliana Seedlings.

The elongation rates of two opposite sides of hypocotyls of Arabidopsis thaliana seedlings were measured during phototropism by using an infrared imaging system. In first positive phototropism, second positive phototropism, and red light-enhanced first positive phototropism, curvature toward the light source was the result of an increase in the rate of elongation of the shaded side and a decrease in the rate of elongation of the lighted side of the seedlings. The phase of straightening that followed maximum curvature resulted from a decrease in the elongation rate of the shaded side and an increase in the elongation rate of the lighted side. These data for the three types of blue light-induced phototropism tested in this study and for the phase of straightening are all clearly consistent with the growth rate changes predicted by the Cholodny-Went theory.

Expression of the Arabidopsis AtAux2-11 auxin-responsive gene in transgenic plants.

Five constructions containing deletions of the promoter from an auxin-inducible gene of Arabidopsis thaliana, AtAux2-11, were fused to the coding region of the reporter gene LacZ, which encodes beta-galactosidase, and a polyadenylation 3'-untranslated nopaline synthase sequence from Agrobacterium. These chimeric genes were introduced into Arabidopsis by Agrobacterium tumefaciens-mediated transformation, and expression of the gene was examined by spectrophotometric and histochemical analyses. A 600 bp fragment from the AtAux2-11 promoter conferred histochemical patterns of staining similar to the longest 5' promoter tested, a 3.0 kb fragment. Localization of AtAux2-11/LacZ activity in the transgenic plants revealed spatial and temporal expression patterns that correlated with tissues and cells undergoing physiological processes modulated by auxin. LacZ activity was expressed in the elongating region of roots, etiolated hypocotyls, and anther filaments. Expression was detected in the vascular cylinder of the root and the vascular tissue, epidermis, and cortex of the hypocotyl, and filament. The AtAux2-11/LacZ gene was preferentially expressed in cells on the elongating side of hypocotyls undergoing gravitropic curvature. Expression of the chimeric gene in the hypocotyls of light-grown seedlings was less than that in etiolated seedling hypocotyls. The AtAux2-11/LacZ gene was active in the root cap, and expression in the root stele increased at sites of lateral root initiation. Staining was evident in cell types that develop lignified cell walls, e.g. trichomes, anther endothecial cells, and especially developing xylem. The chimeric gene was not expressed in primary meristems. While the magnitude of expression increased after application of exogenous auxin (2,4-D), the histochemical localization of AtAux2-11/LacZ remained unchanged. Transgenic plants with a 600 bp promoter construct (-0.6 kb AtAux2-11/LacZ) had higher levels of basal and auxin-inducible expression than plants with a 3.0 kb promoter construct. Transgenic plants with a -500 bp promoter had levels of expression similar to the -3.0 kb construct. The -0.6 kb AtAux2-11/LacZ gene responded maximally to a concentration of 5 x 10(-6) to 5 x 10(-5) M 2,4-D and was responsive to as little as 5 x 10(-8) M. The evidence presented here suggests that this gene may play a role in several auxin-mediated developmental and physiological processes.

Tissue-specific and organ-specific expression of soybean auxin-responsive transcripts GH3 and SAURs.

We used in situ hybridization to localize two classes of auxin-regulated transcripts, GH3 and SAURs, within organs and tissues of soybean seedlings and flowers. GH3 transcripts occurred in the inner cortex and protoxylem ridges of roots and were expressed transiently during flower and pod development. SAUR transcripts were expressed in the epidermis, cortex, and starch sheath of epicotyls and immature hypocotyls. SAUR transcripts became more abundant on the bottom side of hypocotyls that were undergoing gravitropic curvature. SAURs were also expressed in developing xylem elements of the hypocotyl hook. When soybean organ sections were treated with 50 micromolar 2,4-dichlorophenoxyacetic acid (2,4-D), GH3 transcripts became more abundant in the vascular regions of all organs analyzed. High levels of GH3 transcripts were also found in developing palisade mesophyll cells of leaves, cotyledons, and flowers treated with 2,4-D. SAUR transcripts became more abundant in the epidermis, cortex, starch sheath, and pith of epicotyls and hypocotyls after 2,4-D treatment. Our results showed that a variety of tissues and cell types express auxin-responsive transcripts and that different tissues respond rapidly to exogenous auxin by expressing different hormone-responsive genes.

Tracking Down Plant Genes: Paths, Patterns, and Footprints

From November 13 to 18, 1988, approximately 1000 scientists from 32 countries experienced 243 oral presentations and 420 posters from 8:40 in the morning to 10:30 at night at the Second International Congress sponsored by the International Society for Plant Molecular Biology (ISPMB) in Jersualem, Israel. Despite a warm welcome by the organizers, the meeting opened with the coldest day in Jerusalem in 68 years, resulting in the consumption of 6800 shekel's worth ($4000) of coffee. The following dis? cussion touches on only a few of the highlights amidst the many concurrent sessions. The number of nuclear plant genes and promoters that have been cloned, sequenced, and subjected to develop? mental studies has grown greatly since the first ISPMB conference 3 years ago. Striking views of expression in situ by hybridization or the /5-glucuronidase (GUS) reporter method were provided by a number of investigators, who have collected genes expressed specifically in vascular tissue, lateral roots, wounds, petal epidermis or mesophyll, anther parenchyma, stigma, style, receptacle, tapetal layer, etc. One particularly intriguing pattern of expression was reported by Tom Guilfoyle (University of Missouri, Colum? bia), who found hybridization to an auxin-induced cDNA specifically on the faster elongating side of hypocotyls undergoing gravitropic responses (McClure and Guilfoyle, 1989). A word of caution about interpreting hybridization and histochemical stains for reporter gene activity was given by Richard Jefferson (AFRC Institute of Plant Research, Cambridge, United Kingdom). Tissue sections where cells are small and closely packed can give rise to potentially misleading visual signals. For example, phloem cells might contain the same number of GUS enzyme units or autoradiography grains as larger parenchyma cells, but the signal will appear more concentrated per unit area in the smaller phloem cells. This is readily misinterpreted as higher expression in the vascular tissue. Jefferson also discussed the prospect of producing substrates for GUS that could provide nondestructive selection or screening methods for GUS activity. Such a development could eliminate a major drawback to the GUS reporter system, the necessity to destroy the cells under assay. Technical achievements by several laboratories point the way toward simpler and/or more powerful means to analyze plant gene function. Robert Ferl (University of Florida, Gainesville) reported on the successful application of the genomic sequencing method for examining devel? pment changes in chromosomal configuration and protein factors binding to a maize alcohol dehydrogenase gene. Elaine Tobin (University of California, Los Angeles) re? ported some promising experiments aimed at developing the fms2 gene of Agrobacterium, which encodes an indole acetamide hydrolase into a suicide gene. Tobin's postdoctoral associate George Karlin-Neumann has observed that seedling progeny of a plant containing a phytochrome regulated promoter-fms2 fusion were more sensitive than wild-type to 1-napthaleneacetamide (NAM), which is converted to napthaleneacetic acid (NAA) by the fms2 gene product. A remarkable method developed by Smith et al. (1989) for visualizing DNA undergoing electrophoresis in field inversion gels was described by Arnold Bendich (University of Washington, Seattle), who is applying the method to organelle DNAs. A videotape recording of fluorescent microscopy provided extraordinary views of linear DNA mol? ecules weaving through the gel and circular molecules becoming trapped. This method may help establish the true configuration of chloroplast and mitochondrial genomes, which remains unknown. Although circular restric? tion maps containing repeated sequences that can recombine have been produced from organelle DNAs of many different plant species, the large size and potential fragility of many of the predicted circular molecules preclude unambiguous analysis by electron microscopy. Field inver? sion gel electrophoresis of chloroplast DNA molecules has already given a surprising result, reported by Wilhelm Gruissem (University of California, Berkeley) and Pim Zabel (Agricultural University, Wageningen, The Netherlands). Chloroplast DNA molecules with apparent molecular weights ranging from monomer to tetramer were observed in tomato, maize, spinach, and pea. In contrast, no labo? ratory has reported banding plant mitochondrial DNAs following field inversion gel electrophoresis, although yeast mitochondrial genomes have been found as a band of appropriate molecular weight. Whether the multiple circular molecules predicted from current models of the plant mitochondrial genome actually predominate in vivo re? mains to be demonstrated. Those curious about the microscopic method can order a videotape made by Steve Smith entitled Individual DNA Molecules Undergoing Gel Electro-

The Role of Water Distribution in Plant Tropisms.

Dark-grown seedlings of Helianthus annuus, Pisum sativum and Phaseolus vulgaris all showed twofold to threefold increases in the rate of shoot elongation when the relative humidity was increased from 50% to 100%. In experiments with H. annuus, the lower half of the geostimulated hypocotyl had a higher water content and dry weight than the upper half, while the shaded side of hypocotyls subjected to phototropic stimulation had a higher water content and dry weight than the side towards the light. These differences occurred even when tropic curvatures were physically prevented. It was therefore postulated that water is normally the limiting factor in shoot elongation and that changes in its lateral distribution are primarily responsible for the differential growth induced by geotropic and phototropic stimulation. Experiments with P. sativum and Zea mays, which showed that root growth was inhibited by immersion of the root apex in water, lent support to the hypothesis that O2 is normally the limiting factor in root growth and that the positive geotropism of roots is caused by a gravity-induced increase in the water content of the lower half of the apex which inhibits growth in this region by reducing the O2 supply.