CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA‐mediated inhibition of post‐germination seedling establishment

Abstract

Summary Under acute stress conditions, precocious seedling development may result in the premature death of young seedlings, before they switch to autotrophic growth. The phytohormone abscisic acid (ABA) inhibits seed germination and post‐germination seedling establishment under unfavorable conditions. Various environmental signals interact with the ABA pathway to optimize these early developmental events under stress. Here, we show that light availability critically influences ABA sensitivity during early seedling development. In dark conditions, the ABA‐mediated inhibition of post‐germination seedling establishment is strongly enhanced. COP1, a central regulator of seedling development in the dark, is necessary for this enhanced post‐germination ABA sensitivity in darkness. Despite their slower germination, cop1 seedlings establish faster than wild type in the presence of ABA in both light and dark. PHY and CRY photoreceptors that inhibit COP1 activity in light modulate ABA‐mediated inhibition of seedling establishment in light. Genetically, COP1 acts downstream to ABI5, a key transcriptional regulator of ABA signaling, and does not influence the transcriptional and protein levels of ABI5 during the early post‐germination stages. COP1 promotes post‐germination growth arrest independent of the antagonistic interaction between ABA and cytokinin signaling pathways. COP1 facilitates the binding of ABI5 on its target promoters and the ABA‐mediated upregulation of these target genes is reduced in cop1‐4. Together, our results suggest that COP1 positively regulates ABA signaling to inhibit post‐germination seedling establishment under stress.

BBX28/BBX29, HY5 and BBX30/31 form a feedback loop to fine‐tune photomorphogenic development

“…The COP1–HY5 module represents a central regulatory hub in the light signal transduction pathway. The nucleus is enriched in COP1, where it ubiquitinates substrates and promotes their degradation in the dark (Huang et al ., 2014; Hoecker, 2017; Han et al ., 2020; Yadukrishnan et al ., 2020), and HY5 directly or indirectly controls the expression of over 3000 genes in the light (Lee et al ., 2007; Zhang et al ., 2011). Multiple BBXs have been shown to act as key components of the COP1–HY5‐mediated signaling network (Gangappa and Botto, 2014; Xu, 2019; Song et al ., 2020).…”

Light and abscisic acid interplay in early seedling development

“…Recent evidence has delineated that environmental light signals substantially influence ABA sensitivity during postgermination seedling development. ABA‐mediated inhibition of postgermination development is significantly enhanced in darkness compared with under light conditions (Yadukrishnan et al ., 2020a). This involves modulation of ABI5 at transcriptional and posttranslational levels by PIFs and CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), respectively (Fig.…”

“…1). These factors promote the ABA signalling pathway by acting upstream and downstream of ABI5, respectively, feeding the darkness cue to ABA‐mediated developmental arrest programme (Qi et al ., 2020; Yadukrishnan et al ., 2020a).…”

“…cop1 mutants show ABA hyposensitivity during postgermination seedling development, although cop1 seeds germinate slower than wildtype seeds in the presence of exogenously supplemented ABA (Fernando & Schroeder, 2015; Yadukrishnan et al ., 2020a). COP1 promotes developmental arrest of seedlings by facilitating the binding of ABI5 to its target promoters (Yadukrishnan et al ., 2020a). However, the mechanism behind this positive effect of COP1 on the downstream function of ABI5 during postgermination arrest remains elusive.…”

“…1) (Xu et al ., 2014; Bursch et al ., 2020). As both HY5 and BBX21 undergo COP1‐mediated degradation, examining the contribution of these two factors in the ABA hyposensitive phenotype of cop1 can be important for deciphering the molecular basis of ABA‐mediated inhibition of postgermination development (Osterlund et al ., 2000; D. Q. Xu et al ., 2016; Yadukrishnan et al ., 2020a). In agreement, it was recently identified that HY5 acts downstream of COP1 to negatively regulate ABA‐mediated inhibition of postgermination seedling development, (Yadukrishnan et al ., 2020b), although the molecular mechanism of this module is yet to be fully understood.…”