Light‐induced stabilization of ACS contributes to hypocotyl elongation during the dark‐to‐light transition in Arabidopsis seedlings

Hypocotyl growth during seedling emergence is a crucial developmental transition influenced by light and phytohormones such as ethylene. Ethylene and light antagonistically control hypocotyl growth in either continuous light or darkness. However, how ethylene and light regulate hypocotyl growth, including seedling emergence, during the dark‐to‐light transition remains elusive. Here, we show that ethylene and light cooperatively stimulate a transient increase in hypocotyl growth during the dark‐to‐light transition via the light‐mediated stabilization of 1‐aminocyclopropane‐1‐carboxylic acid (ACC) synthases (ACSs), the rate‐limiting enzymes in ethylene biosynthesis. We found that, in contrast to the known inhibitory role of light in hypocotyl growth, light treatment transiently increases hypocotyl growth in wild‐type etiolated seedlings. Moreover, ACC, the direct precursor of ethylene, accentuates the effects of light on hypocotyl elongation during the dark‐to‐light transition. We determined that light leads to the transient elongation of hypocotyls by stabilizing the ACS5 protein during the dark‐to‐light transition. Furthermore, biochemical analysis of an ACS5 mutant protein bearing an alteration in the C‐terminus indicated that light stabilizes ACS5 by inhibiting the degradation mechanism that acts through the C‐terminus of ACS5. Our study reveals that plants regulate hypocotyl elongation during seedling establishment by coordinating light‐induced ethylene biosynthesis at the post‐translational level. Moreover, the stimulatory role of light on hypocotyl growth during the dark‐to‐light transition provides additional insights into the known inhibitory role of light in hypocotyl development.     

Ethylene Promotes Hypocotyl Growth and HY5 Degradation by Enhancing the Movement of COP1 to the Nucleus in the Light

In the dark, etiolated seedlings display a long hypocotyl, the growth of which is rapidly inhibited when the seedlings are exposed to light. In contrast, the phytohormone ethylene prevents hypocotyl elongation in the dark but enhances its growth in the light. However, the mechanism by which light and ethylene signalling oppositely affect this process at the protein level is unclear. Here, we report that ethylene enhances the movement of CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1) to the nucleus where it mediates the degradation of LONG HYPOCOTYL 5 (HY5), contributing to hypocotyl growth in the light. Our results indicate that HY5 is required for ethylene-promoted hypocotyl growth in the light, but not in the dark. Using genetic and biochemical analyses, we found that HY5 functions downstream of ETHYLENE INSENSITIVE 3 (EIN3) for ethylene-promoted hypocotyl growth. Furthermore, the upstream regulation of HY5 stability by ethylene is COP1-dependent, and COP1 is genetically located downstream of EIN3, indicating that the COP1-HY5 complex integrates light and ethylene signalling downstream of EIN3. Importantly, the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) enriched the nuclear localisation of COP1; however, this effect was dependent on EIN3 only in the presence of light, strongly suggesting that ethylene promotes the effects of light on the movement of COP1 from the cytoplasm to the nucleus. Thus, our investigation demonstrates that the COP1-HY5 complex is a novel integrator that plays an essential role in ethylene-promoted hypocotyl growth in the light.     

Nitric oxide regulates DELLA content and PIF expression to promote photomorphogenesis in Arabidopsis

The transition from etiolated to green seedlings involves a shift from hypocotyl growth-promoting conditions to growth restraint. These changes occur through a complex light-driven process involving multiple and tightly coordinated hormonal signaling pathways. Nitric oxide (NO) has been lately characterized as a regulator of plant development interacting with hormone signaling. Here, we show that Arabidopsis (Arabidopsis thaliana) NO-deficient mutant hypocotyls are longer than those from wild-type seedlings under red light but not under blue or far-red light. Accordingly, exogenous treatment with the NO donor sodium nitroprusside and mutant plants with increased endogenous NO levels resulted in reduced hypocotyl length. In addition to increased hypocotyl elongation, NO deficiency led to increased anthocyanin levels and reduced PHYB content under red light, all processes governed by phytochrome-interacting factors (PIFs). NO-deficient plants accordingly showed an enhanced expression of PIF3, PIF1, and PIF4. Moreover, exogenous NO increased the levels of the gibberellin (GA)-regulated DELLA proteins and shortened hypocotyls, likely through the negative regulation of the GA Insensitive Dwarf1 (GID1)-Sleepy1 (SLY1) module. Consequently, NO-deficient seedlings displayed up-regulation of SLY1, defective DELLA accumulation, and altered GA sensitivity, thus resulting in defective deetiolation under red light. Accumulation of NO in wild-type seedlings undergoing red light-triggered deetiolation and elevated levels of NO in the GA-deficient ga1-3 mutant in darkness suggest a mutual NO-GA antagonism in controlling photomorphogenesis. PHYB-dependent NO production promotes photomorphogenesis by a GID1-GA-SLY1-mediated mechanism based on the coordinated repression of growth-promoting PIF genes and the increase in the content of DELLA proteins.     

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.     

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.     

CCA1 and ELF3 Interact in the control of hypocotyl length and flowering time in Arabidopsis

The circadian clock is an endogenous oscillator with a period of approximately 24 h that allows organisms to anticipate, and respond to, changes in the environment. In Arabidopsis (Arabidopsis thaliana), the circadian clock regulates a wide variety of physiological processes, including hypocotyl elongation and flowering time. CIRCADIAN CLOCK ASSOCIATED1 (CCA1) is a central clock component, and CCA1 overexpression causes circadian dysfunction, elongated hypocotyls, and late flowering. EARLY FLOWERING3 (ELF3) modulates light input to the clock and is also postulated to be part of the clock mechanism. elf3 mutations cause light-dependent arrhythmicity, elongated hypocotyls, and early flowering. Although both genes affect similar processes, their relationship is not clear. Here, we show that CCA1 represses ELF3 by associating with its promoter, completing a CCA1-ELF3 negative feedback loop that places ELF3 within the oscillator. We also show that ELF3 acts downstream of CCA1, mediating the repression of PHYTOCHROME-INTERACTING FACTOR4 (PIF4) and PIF5 in the control of hypocotyl elongation. In the regulation of flowering, our findings show that ELF3 and CCA1 either cooperate or act in parallel through the CONSTANS/FLOWERING LOCUS T pathway. In addition, we show that CCA1 represses GIGANTEA and SUPPRESSOR OF CONSTANS1 by direct interaction with their promoters, revealing additional connections between the circadian clock and the flowering pathways.     

Control of cell elongation and stress responses by steroid hormones and carbon catabolic repression in plants.

Molecular analysis of Arabidopsis mutants displaying hypocotyl elongation defects in both the dark and light revealed recently that steroids play an essential role as hormones in plants. Deficiencies in brassinosteroid biosynthesis and signalling permit photomorphogenic development and light-regulated gene expression in the dark, and result in severe dwarfism, male sterility and de-repression of stress-induced genes in the light. A cytochrome P450 steroid hydroxylase (CYP90) controls a rate limiting step in brassinosteroid biosynthesis and appears to function as a signalling factor in stress responses. Another key step in steroid biosynthesis is controlled by the Arabidopsis SNF1 kinases that phosphorylate the 3-hydroxy-3methylglutaryl-CoA reductase. The activity of SNF1 kinases is regulated by PRL1, an evolutionarily conserved alpha-importin-binding nuclear WD-protein. The prl1 mutation results in cell elongation defects, de-repression of numerous stress-induced genes, and augments the sensitivity of plants to glucose, cold stress and several hormones, including cytokinin, ethylene, auxin, and abscisic acid.     

Comparison of the effects of white light and the growth retardant paclobutrazol on the ethylene production in bean hypocotyls

At a concentration of 17 µmol·L^–1, paclobutrazol (PP), a triazole plant growth retardant, effectively reduced the elongation and increased the thickness of hypocotyls in 6-day-old Phaseolus vulgaris L. cv. Juliska seedlings, both in the light and in the dark. PP treatment did not increase the cell number in transverse sections of hypocotyls. The diameter of hypocotyls was uniform from the zone of intensive elongation along the whole hypocotyl in etiolated plants, but those grown in the light exhibited an additional lateral expansion at the base. Ethylene evolution was not reduced by PP in etiolated hypocotyls, and did not differ significantly in the elongating apical and fully grown basal zones. PP reduced the ethylene release by the growing zones in green hypocotyls, but not in the basal parts, which resulted in an increasing ethylene gradient towards the hypocotyl base. The level of 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, was much higher in retardant-treated hypocotyls than in the controls, which was due in part to the reduced malonylation. The swelling of the hypocotyl bases could be eliminated by inhibitors of ethylene biosynthesis or action, or could be induced by 10 µmol·L^–1ACC in control plants in the light. None of these treatments had a significant effect on the lateral expansion of hypocotyls in etiolated seedlings. PP treatment induced a similar effect to that of white light in etiolated seedlings, and amplified the effect of light in green plants with respect to the ACC distribution, and consequently, the ethylene production in the hypocotyls of 6-day-old bean seedlings. It can be concluded that the lateral expansion of hypocotyl bases in PP-treated green plants is controlled by ethylene.