Light and Abscisic Acid Coordinately Regulate Greening of Seedlings

Abstract

The greening of etiolated seedlings is crucial for the growth and survival of plants. After reaching the soil surface and sunlight, etiolated seedlings integrate numerous environmental signals and internal cues to control the initiation and rate of greening thus to improve their survival and adaption. However, the underlying regulatory mechanisms by which light and phytohormones, such as abscisic acid (ABA), coordinately regulate greening of the etiolated seedlings is still unknown. In this study, we showed that Arabidopsis (Arabidopsis thaliana) DE-ETIOLATED1 (DET1), a key negative regulator of photomorphogenesis, positively regulated light-induced greening by repressing ABA responses. Upon irradiating etiolated seedlings with light, DET1 physically interacts with FAR-RED ELONGATED HYPOCOTYL3 (FHY3) and subsequently associates to the promoter region of the FHY3 direct downstream target ABA INSENSITIVE5 (ABI5). Further, DET1 recruits HISTONE DEACETYLASE6 to the locus of the ABI5 promoter and reduces the enrichments of H3K27ac and H3K4me3 modification, thus subsequently repressing ABI5 expression and promoting the greening of etiolated seedlings. This study reveals the physiological and molecular function of DET1 and FHY3 in the greening of seedlings and provides insights into the regulatory mechanism by which plants integrate light and ABA signals to fine-tune early seedling establishment.

Light and abscisic acid interplay in early seedling development

“…ABA accumulates in the cotyledons and the elongation zone of the hypocotyl of dark‐grown seedlings, helping to maintain etiolated development (Humplík et al ., 2015). During dark to light transition, the greening rate of etiolated seedlings is reduced in the presence of exogenously provided ABA, suggesting that ABA negatively regulates this process (D. Xu et al ., 2020).…”

“…DE‐ETIOLATED1 (DET1), a key negative regulator of photomorphogenesis, suppresses the ABA‐mediated inhibition of greening by negatively regulating ABI5 transcription (D. Xu et al ., 2020). DET1 physically interacts with FAR‐RED ELONGATED HYPOCOTYL3 (FHY3), a transcription factor participating in both light and ABA signalling pathways.…”

“…FHY3 positively regulates the phyA‐mediated light signalling pathway, and promotes ABA signalling by directly activating transcription of ABI5 (Wang & Deng, 2002; Tang et al ., 2013). Interaction with DET1 reduces the ability of FHY3 to activate ABI5 transcription, dampening the inhibitory action of ABA on greening of seedlings (D. Xu et al ., 2020). DET1 further represses ABI5 by associating with its promoter and recruiting histone deacetylase HDA6 to induce chromatin modifications that inhibit transcription (Fig.…”

“…DET1 further represses ABI5 by associating with its promoter and recruiting histone deacetylase HDA6 to induce chromatin modifications that inhibit transcription (Fig. 1) (D. Xu et al ., 2020). In addition to this module, DET1 regulates ABI5 expression through multiple other mechanisms.…”

“…Therefore, DET1 might play a contrasting role in ABA‐mediated inhibition of greening depending on the light conditions, either suppressing greening in the dark by promoting ABI5 expression through the stabilisation of PIFs or also promoting greening in the light by inhibiting ABI5 expression through FHY3 and HDA6. Furthermore, DET1‐mediated degradation of HY5 can have a complex consequence on the ABA‐mediated inhibition of greening, as HY5 promotes the expression of chlorophyll biosynthetic genes, whereas it positively regulates ABI5 expression (Toledo‐Ortiz et al ., 2014; D. Xu et al ., 2020).…”