Boron and calcium sites involved in indole-3-acetic Acid transport in sunflower hypocotyl segments

Sunflower (Helianthus annuus L. cv Russian Mammoth) hypocotyl segments deficient in either B or Ca exhibited a higher rate of potassium leakage, compared to nondeficient segments. Potassium leakage, used here as an indication of membrane integrity, was completely reversed by the addition of H₃BO₃ or Ca(NO₃)₂ to the incubation medium of the B-deficient or Ca-deficient hypocotyl segments, respectively. This role of B and Ca in membrane integrity, which may be important in the entry and exit of auxin in cells, is identified as the first site of action for each of these two essential elements in the basipetal secretion of auxin. A second site for B is postulated because auxin transport was not restored, even when K⁺ leakage has been completely reversed to the nondeficient level, when B-deficient hypocotyls were incubated in B solution. This lack of reversibility of auxin transport implied that the incubation for 2 h in B solution was not enough to restore the auxin transport process. However, since the transfer of B-deficient seedlings to B solutions prevented further deterioration of auxin transport, these observations suggest that: (a) either an intact seedling, or a longer period of incubation of the hypocotyl in B solution, is required for the synthesis or maintenance of the functional second site for B; (b) B is probably essential in the synthesis of a ligand, which may or may not be needed to bind B, but which is essential in the basipetal transport of auxin. The second site for Ca in auxin transport, is indicated by the complete reversal of its inhibition in Ca-deficient hypocotyl, when incubated in Ca solution. The second site for Ca is thought to be directly involved in the secretion of auxin, in which Ca probably plays the role of a second messenger, as in stimulus-response coupling. The two sites for Ca can be distinguished from each other by their cation specificity. The requirement for Ca in the first site can be substituted by other divalent cations, while the second site is highly specific for Ca.     

RCN1-regulated phosphatase activity and EIN2 modulate hypocotyl gravitropism by a mechanism that does not require ethylene signaling

The roots curl in naphthylphthalamic acid1 (rcn1) mutant of Arabidopsis (Arabidopsis thaliana) has altered auxin transport, gravitropism, and ethylene response, providing an opportunity to analyze the interplay between ethylene and auxin in control of seedling growth. Roots of rcn1 seedlings were previously shown to have altered auxin transport, growth, and gravitropism, while rcn1 hypocotyl elongation exhibited enhanced ethylene response. We have characterized auxin transport and gravitropism phenotypes of rcn1 hypocotyls and have explored the roles of auxin and ethylene in controlling these phenotypes. As in roots, auxin transport is increased in etiolated rcn1 hypocotyls. Hypocotyl gravity response is accelerated, although overall elongation is reduced, in etiolated rcn1 hypocotyls. Etiolated, but not light grown, rcn1 seedlings also overproduce ethylene, and mutations conferring ethylene insensitivity restore normal hypocotyl elongation to rcn1. Auxin transport is unaffected by treatment with the ethylene precursor 1-aminocyclopropane carboxylic acid in etiolated hypocotyls of wild-type and rcn1 seedlings. Surprisingly, the ethylene insensitive2-1 (ein2-1) and ein2-5 mutations dramatically reduce gravitropic bending in hypocotyls. However, the ethylene resistant1-3 (etr1-3) mutation does not significantly affect hypocotyl gravity response. Furthermore, neither the etr1 nor the ein2 mutation abrogates the accelerated gravitropism observed in rcn1 hypocotyls, indicating that both wild-type gravity response and enhanced gravity response in rcn1 do not require an intact ethylene-signaling pathway. We therefore conclude that the RCN1 protein affects overall hypocotyl elongation via negative regulation of ethylene synthesis in etiolated seedlings, and that RCN1 and EIN2 modulate hypocotyl gravitropism and ethylene responses through independent pathways.     

Effect of IAA on Growth and Soluble Cell Wall Polysaccharides Centrifuged from Pine Hypocotyl Sections

Auxin-induced elongation and cell wall polysaccharide metabolism were studied in excised hypocotyl sections of ponderosa pine (Pinus ponderosa) seedlings. Sections excised from hypocotyls of ponderosa pine elongate in response to the addition of auxin. The neutral sugar composition of the extracellular solution removed from hypocotyl sections by centrifugation was examined. In cell wall solution from freshly excised sections, glucose, galactose, xylose, and arabinose make up more than 90% of the neutral sugars, while rhamnose, fucose, and mannose are relatively minor components. The neutral sugar composition of the polysaccharides of the pine cell wall solution is both qualitatively and quantitatively similar to that of pea. Following auxin treatment of pine hypocotyls, the neutral sugar composition of the cell wall changes; glucose, xylose, rhamnose, and fucose increase by nearly 2-fold relative to controls in buffer without auxin. These changes in neutral sugars in response to auxin treatment are similar to those found in pea, with the exception that in pea, rhamnose levels decline in response to auxin treatment.     

The growth of tomato (Lycopersicon esculentum Mill.) hypocotyls in the light and in darkness differentially involves auxin.

Light and auxin antagonistically regulate hypocotyl elongation. We have investigated the physiological interactions of light and auxin in the control of tomato (Lycopersicon esculentum Mill.) hypocotyl elongation by studying the auxin-insensitive mutant diageotropica (dgt). The length of the hypocotyls of the dgt mutant is significantly reduced when compared to the wild type line Ailsa Craig (AC) in the dark and under red light, but not under the other light conditions tested, indicating that auxin sensitivity is involved in the elongation of hypocotyls only in these conditions. Similarly, the auxin transport inhibitor naphthylphthalamic [correction of naphtylphtalamic] acid (NPA) differentially affects elongation of dark- or light-grown hypocotyls of the MoneyMaker (MM) tomato wild type. Using different photomorphogenic mutants, we demonstrate that at least phytochrome A, phytochrome B1 and, to a much lesser extent [correction of extend], cryptochrome 1, are necessary for a switch from an auxin transport-dependent elongation of hypocotyls in the dark to an auxin transport-independent elongation in the light. Interestingly, the dgt mutant and NPA-treated seedlings exhibit a looped phenotype only under red light, indicating that the negative gravitropism of hypocotyls also differentially involves auxin in the various light conditions.     

The Arabidopsis mutant alh1 illustrates a cross talk between ethylene and auxin

Ethylene or its precursor 1-aminocyclopropane-1-carboxylic acid (ACC) can stimulate hypocotyl elongation in light-grown Arabidopsis seedlings. A mutant, designated ACC-related long hypocotyl 1 (alh1), that displayed a long hypocotyl in the light in the absence of the hormone was characterized. Etiolated alh1 seedlings overproduced ethylene and had an exaggerated apical hook and a thicker hypocotyl, although no difference in hypocotyl length was observed when compared with wild type. Alh1 plants were less sensitive to ethylene, as reflected by reduction of ACC-mediated inhibition of hypocotyl growth in the dark and delay in flowering and leaf senescence. Alh1 also had an altered response to auxin, whereas auxin levels in whole alh1 seedlings remained unaffected. In contrast to wild type, alh1 seedlings showed a limited hypocotyl elongation when treated with indole-3-acetic acid. Alh1 roots had a faster response to gravity. Furthermore, the hypocotyl elongation of alh1 and of ACC-treated wild type was reverted by auxin transport inhibitors. In addition, auxin up-regulated genes were ectopically expressed in hypocotyls upon ACC treatment, suggesting that the ethylene response is mediated by auxins. Together, these data indicate that alh1 is altered in the cross talk between ethylene and auxins, probably at the level of auxin transport.     

Growth and stomata development of Arabidopsis hypocotyls are controlled by gibberellins and modulated by ethylene and auxins

The plant hormones gibberellin (GA), ethylene and auxin can promote hypocotyl elongation of Arabidopsis seedlings grown in the light on a low nutrient medium (LNM). In this study, we used hypocotyl elongation as a system to investigate interactions between GA and ethylene or auxin and analysed their influence on the development of stomata in the hypocotyl. When applied together, GA and ethylene or auxin exerted a synergistic effect on hypocotyl elongation. Stimulated cell elongation is the main cause of hypocotyl elongation. Furthermore, hypocotyls treated with GA plus either ethylene or auxin show an increased endoreduplication. In addition, a small but significant increase in cell number was observed in the cortical cell files of hypocotyls treated with ethylene and GA together. However, studies with transgenic seedlings expressing CycB1::uidA genes revealed that cell division in the hypocotyl occurs only in the epidermis and mainly to form stomata, a process strictly regulated by hormones. Stomata formation in the hypocotyl is induced by the treatment with either GA or ethylene. The effect of GA could be strongly enhanced by the simultaneous addition of ethylene or auxin to the growth medium. Gibberellin is the main signal inducing stomata formation in the hypocotyl. In addition, this signal regulates hypocotyl elongation and is modulated by ethylene and auxin. The implication of these three hormones in relation to cell division and stomata formation is discussed.     

Variation in indole-3-acetic acid transport and its relationship with growth in etiolated lupin hypocotyls

The relationship between the variation in polar auxin transport (PAT) and elongating growth in etiolated Lupinus albus hypocotyls was investigated. Parameters of auxin transport, such as the amount transported, intensity of the transport and sensitivity to 1-N-naphthylphthalamic acid (NPA) inhibition were measured in isolated sections from different sites (apical, middle and basal) along the hypocotyls in seedlings of different ages. Auxin transport was studied by applying radioactive indole-3-acetic acid (IAA) to upright and inverted sections. Basipetal transport was much higher than acropetal and very sensitive to NPA inhibition, which indicates that transport is polarized. Polarity was expressed as the NPA-induced inhibition and the basipetal/acropetal ratio. As a rule, both the amount of IAA transported and the polarity varied with the age of the seedlings, with values increasing from 3 to 5d and then decreasing. Both parameters were higher in apical (where most growth is localized) than in middle and basal regions, although this longitudinal gradient tended to disappear with aging as hypocotyl growth slowed and finally ceased. The application of NPA did not modify hypocotyl elongation in 5-d-old intact seedlings. Derooting of the seedlings drastically reduced elongation in the control, while NPA partially restored the growth, which suggests that NPA induces an increase in auxin in the elongation region. These results suggest that a basipetally decreasing gradient in PAT along the hypocotyl, which changes with age, may be responsible for auxin distribution pattern controlling growth.     

ABCB19‐mediated polar auxin transport modulates Arabidopsis hypocotyl elongation and the endoreplication variant of the cell cycle

Elongation of the Arabidopsis hypocotyl pushes the shoot‐producing meristem out of the soil by rapid expansion of cells already present in the embryo. This elongation process is shown here to be impaired by as much as 35% in mutants lacking ABCB19, an ATP‐binding cassette membrane protein required for polar auxin transport, during a limited time of fast growth in dim white light beginning 2.5 days after germination. The discovery of high ectopic expression of a cyclin B1;1‐based reporter of mitosis throughout abcb19 hypocotyls without an equivalent effect on mitosis prompted investigations of the endoreplication variant of the cell cycle. Flow cytometry performed on nuclei isolated from upper (growing) regions of 3‐day‐old hypocotyls showed ploidy levels to be lower in abcb19 mutants compared with wild type. CCS52A2 messenger RNA encoding a nuclear protein that promotes a shift from mitosis to endoreplication was lower in abcb19 hypocotyls, and fluorescence microscopy showed the CCS52A2 protein to be lower in the nuclei of abcb19 hypocotyls compared with wild type. Providing abcb19 seedlings with nanomolar auxin rescued their low CCS52A2 levels, endocycle defects, aberrant cyclin B1;1 expression, and growth rate defect. The abcb19‐like growth rate of ccs52a2 mutants was not rescued by auxin, placing CCS52A2 after ABCB19‐dependent polar auxin transport in a pathway responsible for a component of ploidy‐related hypocotyl growth. A ccs52A2 mutation did not affect the level or pattern of cyclin B1;1 expression, indicating that CCS52A2 does not mediate the effect of auxin on cyclin B1;1.     

Polar transport of kinetin in tissues of radish

Polar transport of kinetin-8-¹⁴C occurred in segments of petioles, hypocotyls, and roots of radish (Raphanus sativus L.). The polarity was basipetal in petioles and hypocotyls and acropetal in roots. In segments excised from seedlings with fully expanded cotyledons, indole-3-acetic acid was required for polarity to develop. In hypocotyl segments isolated at this stage, basipetal and acropetal movements were equal during the first 12 hours of auxin treatment after which time acropetal movement declined. Pretreatment with auxin eliminated this delay in the appearance of polarity. In hypocotyl segments excised from seedlings with expanding cotyledons, exogenous auxin was unnecessary for polarity. Potassium cyanide abolished polarity at both stages of growth by allowing increased acropetal movement. The rate of accumulation of kinetin in receiver blocks was greater than the in vivo increase in cytokinin content of developing radish roots.     

Role of basipetal auxin transport and lateral auxin movement in rooting and growth of etiolated lupin hypocotyls

The involvement of polar auxin transport (PAT) on the growth of light-grown seedlings and rooting is generally accepted, while the role of auxin and PAT on the growth of dark-grown seedlings is subject to controversy. To further investigate this question, we have firstly studied the influence of NPA, a known inhibitor of PAT, on the rooting and growth of etiolated Lupinus albus hypocotyls. Rooting was inhibited when the basal ends of de-rooted seedlings were immersed in 100 micro m NPA but was partially restored after immersion in NPA + auxin. However, NPA applied to de-rooted seedlings or the roots of intact seedlings did not inhibit hypocotyl growth. It was taken up and distributed along the organ, and actually inhibited the basipetal transport of ((3)H)-IAA applied to isolated hypocotyl sections. Since the apex is the presumed auxin source for hypocotyl growth and rooting, and the epidermis is considered the limiting factor in auxin-induced growth, the basipetal and lateral auxin movement (LAM) after application of ((3)H)-IAA to decapitated seedlings were studied, in an attempt to evaluate the role of PAT and LAM in the provision of auxin to competent cells for growth and rooting. Local application of ((3)H)-IAA to the stele led to the basipetal transport of auxin in this tissue, but the process was drastically reduced when roots were immersed in NPA since no radioactivity was detected below the apical elongation region of the hypocotyl. LAM from the stele to the cortex and the epidermis occurred during basipetal transport, since radioactivity in these tissues increased as transport time progressed. Radioactivity on a per FW basis in the epidermis was 2-4 times higher than in the cortex, which suggests that epidermal cells acted as a sink for LAM. NPA did not inhibit LAM along the elongation region. These results suggest that while PAT was essential for rooting, LAM from the PAT pathway to the auxin-sensitive epidermal cells could play a key role in supplying auxin for hypocotyl elongation in etiolated lupin seedlings.     

Promotion of hypocotyl elongation in loblolly pine (Pinus taeda L.) by indole-3-acetic acid

Elongation of excised loblolly pine ( Pinus taeda L.) hypocotyls was promoted by indole-3-acetic acid and the fungal metabolite, fusicoccin. Gibberellic acid, kinetin, zeatin, or zeatin-riboside were either without effect or promoted elongation only slightly. The most auxin-responsive tissue was just below the cotyledonary node, and elongation was confined to sections excised from the upper 2 cm of the hypocotyl. Indole-3-acetic acid induced elongation rates in the hypocotyl sections equal to those of intact hypocotyls when the sections were excised from young seedlings. Elongation rates decreased in intact hypocotyls before the excised tissues lost responsiveness to the auxin. Hypocotyl elongation in loblolly pine is discussed in relation to hypocotyl elongation in angiosperm species.     

Another evidence for inhibitory effect of auxin in adventitious bud formation of decapitated flax (Linum usitatissimum L.) seedlings

Adventitious buds were formed on the hypocotyls of decapitated flax seedlings. Scanning electron and light microscopic examinations of hypocotyls showed that epidermal cells divided to produce meristematic spots from which several leaf primordia were formed. Between leaf primordia and the original vascular tissues of hypocotyls, new xylem cells were formed which connected them. About 10, 30 and 60% of adventitious buds were formed on upper, middle and basal parts of hypocotyls of decapitated seedlings, respectively. Removal of apical meristem together with longer hypocotyl zero to four cm long below the apical meristem) induced higher percentage of adventitious bud formation in the remaining hypocotyl. When the entire hypocotyl was cut into 16 segments (0.25 cm each) and these segments were cultured on MS medium containing 3% sucrose and 0.8% agar, adventitious buds were mainly formed in the lowest five segments. These results suggested that there was a gradient of inhibitory factor(s) from apical to basal part of hypocotyl with respect to adventitious bud formation. Auxin transport inhibitors, morphactin and TIBA induced adventitious bud formation on intact seedlings by suppressing the basipetal movement of auxin.     

Role of calcium in the polar secretion of indoleacetic Acid

The rate of auxin transport in sunflower hypocotyls (Helianthus annuus L. cv `Russian mammoth') or corn coleoptiles (Zea mays L. cv `WF9 × 38') was less in seedlings grown in Ca-deficient medium than in controls. The rate of IAA transport depended on the concentration of Ca in the root medium up to 1 millimolar. Further increases in auxin transport were observed when the isolated segments were incubated in medium containing up to 30 millimolar Ca. We suggest that the rate of auxin transport in plant tissue is dependent on the pool of ionic Ca in the extracellular space.

Segments from Ca-deficient seedlings exhibited a high specific requirement for Ca²⁺ in auxin transport. Magnesium, strontium, and several other divalent cations tested for their ability to replace Ca²⁺ in restoring auxin transport showed no effect; partial replacement by lanthanum was observed.

Auxin transport, or auxin flux through the segment, which is the result of IAA secretion by individual cells, was reduced in the low Ca²⁺ segments due both to lowered velocity and to reduced capacity of transport. The requirement for Ca²⁺ in the secretion of auxin is believed to be equivalent to the phenomenon observed in animal cell secretion, where the influx of Ca²⁺ serves as a link between an external stimulus and the secretion response.

     

Effect of Asymmetric Auxin Application on Helianthus Hypocotyl Curvature

Indole-3-acetic acid was applied asymmetrically to the hypocotyls of sunflower (Helianthus annuus L.) seedlings. After 5 hours on a clinostat, auxin gradients as small as 1 to 1.3 produced substantial (more than 60 degrees) hypocotyl curvature. This result suggests the asymmetric growth underlying hypocotyl gravitropism can be explained by lateral auxin redistribution.     

Arabidopsis SMALL AUXIN UP RNA63 promotes hypocotyl and stamen filament elongation

Auxin regulates plant growth and development in part by activating gene expression. Arabidopsis thaliana SMALL AUXIN UP RNAs (SAURs) are a family of early auxin-responsive genes with unknown functionality. Here, we show that transgenic plant lines expressing artificial microRNA constructs (aMIR-SAUR-A or -B) that target a SAUR subfamily (SAUR61-SAUR68 and SAUR75) had slightly reduced hypocotyl and stamen filament elongation. In contrast, transgenic plants expressing SAUR63:GFP or SAUR63:GUS fusions had long hypocotyls, petals and stamen filaments, suggesting that these protein fusions caused a gain of function. SAUR63:GFP and SAUR63:GUS seedlings also accumulated a higher level of basipetally transported auxin in the hypocotyl than did wild-type seedlings, and had wavy hypocotyls and twisted inflorescence stems. Mutations in auxin efflux carriers could partially suppress some SAUR63:GUS phenotypes. In contrast, SAUR63:HA plants had wild-type elongation and auxin transport. SAUR63:GFP protein had a longer half-life than SAUR63:HA. Fluorescence imaging and microsomal fractionation studies revealed that SAUR63:GFP was localized mainly in the plasma membrane, whereas SAUR63:HA was present in both soluble and membrane fractions. Low light conditions increased SAUR63:HA protein turnover rate. These results indicate that membrane-associated Arabidopsis SAUR63 promotes auxin-stimulated organ elongation.     

Boron and blue light reduce responsiveness of <Emphasis Type="Italic">Arabidopsis</Emphasis> hypocotyls to exogenous auxins

We reported earlier that boron stimulates hypocotyl growth in several Arabidopsis ecotypes but not in the boron-deficient mutant bor1-1. Others have shown that boron influences the metabolism and transport of the plant hormone auxin. We investigated how boron, in interaction with light, influences Arabidopsis hypocotyl growth responses to the exogenous auxins 1-NAA, 2,4-D and IAA. In either light condition, 1-NAA similarly inhibited hypocotyl growth in bor1-1 and the corresponding WT (Col-0), while in both genotypes, boron did not essentially affect the extent of the inhibition. Whatever the light conditions and in the absence of boron, 2,4-D inhibited hypocotyl elongation in WT, while in BL seedlings, high responsiveness to 2,4-D vanished when boron was added to the culture medium. Hypocotyl of bor1-1 seedlings in all boron concentrations tested and grown in the dark or RL responded to the auxin similar to WT plants. In BL, the mutant hypocotyls retained full sensitivity to 2,4-D at 0.1 mM H₃BO₃ but lost that sensitivity by 2 mM. In both genotypes tested, in the dark or RL, IAA inhibited hypocotyl growth. Conversely, IAA stimulated hypocotyl elongation in both genotypes developed in BL at 0.1 mM H₃BO₃. That stimulation disappeared when the boron supply increased to 2 mM. Our results suggest that specifically in BL, boron reduces hypocotyl responsiveness to auxins 2,4-D or IAA via the functional transporter BOR1. Our results lead to a discussion of how BL and BOR1 influence the mechanisms of auxin transport into and out of the cell.     

A role for ABCB19‐mediated polar auxin transport in seedling photomorphogenesis mediated by cryptochrome 1 and phytochrome B

During seedling establishment, blue and red light suppress hypocotyl growth through the cryptochrome 1 (cry1) and phytochrome B (phyB) photosensory pathways, respectively. How these photosensory pathways integrate with growth control mechanisms to achieve the appropriate degree of stem elongation was investigated by combining cry1 and phyB photoreceptor mutations with genetic manipulations of a multidrug resistance‐like membrane protein known as ABCB19 that influenced auxin distribution within the plant, as evidenced by a combination of reporter gene assays and direct auxin measurements. Auxin signaling and ABCB19 protein levels, hypocotyl growth rates, and apical hook opening were measured in mutant and wild‐type seedlings exposed to a range of red and blue light conditions. Ectopic/overexpression of ABCB19 (B19OE) greatly increased auxin in the hypocotyl, which reduced the sensitivity of hypocotyl growth specifically to blue light in long‐term assays and red light in high‐resolution, short‐term assays. Loss of ABCB19 partially suppressed the cry1 hypocotyl growth phenotype in blue light. Hypocotyl growth of B19OE seedlings in red light was very similar to phyB mutants. Altered auxin distribution in B19OE seedlings also affected the opening of the apical hook. The cry1 and phyB photoreceptor mutations both increased ABCB19 protein levels at the plasma membrane, as measured by confocal microscopy. The B19OE plant proved to be a useful tool for determining aspects of the mechanism by which light, acting through cry1 or phyB, influences the auxin transport process to control hypocotyl growth during de‐etiolation.     

Phytochrome-mediated phototropism in de-etiolated seedlings : occurrence and ecological significance

Phototropic responses to broadband far red (FR) radiation were investigated in fully de-etiolated seedlings of a long-hypocotyl mutant (lh) of cucumber (Cucumis sativus L.), which is deficient in phytochrome-B, and its near isogenic wild type (WT). Continuous unilateral FR light provided against a background of white light induced negative curvatures (i.e. bending away from the FR light source) in hypocotyls of WT seedlings. This response was fluence-rate dependent and was absent in the lh mutant, even at very high fluence rates of FR. The phototropic effect of FR light on WT seedlings was triggered in the hypocotyls and occurred over a range of fluence rates in which FR was very effective in promoting hypocotyl elongation. FR light had no effect on elongation of lh-mutant hypocotyls. Seedlings grown in the field showed negative phototropic responses to the proximity of neighboring plants that absorbed blue (B) and red light and back-reflected FR radiation. The bending response was significantly larger in WT than in lh seedlings. Responses of WT and lh seedlings to lateral B light were very similar; however, elimination of the lateral B light gradients created by the proximity of plant neighbors abolished the negative curvature only in the case of lh seedlings. More than 40% of the total hypocotyl curvature induced in WT seedlings by the presence of neighboring plants was present after equilibrating the fluence rates of B light received by opposite sides of the hypocotyl. These results suggest that: (a) phytochrome functions as a phototropic sensor in de-etiolated plants, and (b) in patchy canopy environments, young seedlings actively project new leaves into light gaps via stem bending responses elicited by the B-absorbing photoreceptor(s) and phytochrome.     

Gene expression profile of zeitlupe/lov kelch protein1 T-DNA insertion mutants in Arabidopsis thaliana: Downregulation of auxin-inducible genes in hypocotyls

Elongation of hypocotyl cells has been studied as a model for elucidating the contribution of cellular expansion to plant organ growth. ZEITLUPE (ZTL) or LOV KELCH PROTEIN1 (LKP1) is a positive regulator of warmth-induced hypocotyl elongation under white light in Arabidopsis, although the molecular mechanisms by which it promotes hypocotyl cell elongation remain unknown. Microarray analysis showed that 134 genes were upregulated and 204 genes including 15 auxin-inducible genes were downregulated in the seedlings of 2 ztl T-DNA insertion mutants grown under warm conditions with continuous white light. Application of a polar auxin transport inhibitor, an auxin antagonist or an auxin biosynthesis inhibitor inhibited hypocotyl elongation of control seedlings to the level observed with the ztl mutant. Our data suggest the involvement of auxin and auxin-inducible genes in ZTL-mediated hypocotyl elongation.     

Phytochromes and cryptochromes regulate the differential growth of Arabidopsis hypocotyls in both a PGP19-dependent and a PGP19-independent manner

Photoreceptors, phytochromes and cryptochromes regulate hypocotyl growth under specific conditions, by suppressing negative gravitropism, modulating phototropism and inhibiting elongation. Although these effects seem to be partially caused via the regulation of the phytohormone auxin, the molecular mechanisms underlying this process are still poorly understood. In our present study, we demonstrate that the flabby mutation enhances both phytochrome- and cryptochrome-inducible hypocotyl bending in Arabidopsis. The FLABBY gene encodes the ABC-type auxin transporter, PGP19, and its expression is suppressed by the activation of phytochromes and cryptochromes. Our current results therefore indicate that the phytochromes and cryptochromes have at least two effects upon the tropic responses of the hypocotyls in Arabidopsis: the enhancement of hypocotyl bending through the suppression of PGP19, and a PGP19-independent mechanism that induces hypocotyl bending. By the using an auxin polar transport assay and DR5:GUS expression analysis, we further find that the phytochromes inhibit basipetal auxin transport, and induce the asymmetric distribution of auxin in the hypocotyls. These data suggest that the control of auxin transport by phytochromes and cryptochromes is a critical regulatory component of hypocotyl growth in response to light.