The transcriptional repressor OsPRR73 links circadian clock and photoperiod pathway to control heading date in rice

Abstract

The phase transition from vegetative to reproductive growth is triggered by internal and external signals that participate in circadian clock in plants. We identified a rice floral inhibitor OsPRR73 encoding a CONSTANS protein. Overexpression of OsPRR73 resulted in late heading under both long‐day (LD) and short‐day (SD) conditions. Knockout mutants led to early heading under LD conditions but no change under SD. OsPRR73 mRNA accumulated at noon and exhibited a robust oscillation under constant light (LL) and constant darkness (DD) conditions. OsPRR73 overexpression exerted negative feedback on endogenous OsPRR73 expression and altered diurnal expressions of key flowering genes and circadian clock genes. OsPRR73 bound to the promoters of the floral gene Ehd1 and the circadian gene OsLHY, and significantly suppressed their expression at dawn. In LL and DD, the oscillatory patterns of the circadian genes OsLHY, OsTOC1, OsGI and OsELF3 were varied in OsPRR73OX and osprr73 mutants. OsPRR73 expression was decreased in osphyb mutants, and overexpression of OsPRR73 complemented the early heading date phenotype of osphyb, indicating OsPRR73 works downstream of OsPhyB. Therefore, OsPRR73 is involved in a feedback loop of the rice clock and connects the photoperiod flowering pathway by binding to the Ehd1 promoter in rice.

A clock regulatory module is required for salt tolerance and control of heading date in rice

“…We next investigated whether OsELF4a plays an essential role in regulation of rice circadian clock by conducting time course RT‐qPCR analysis of circadian clock associated genes in rice including OsPRRs , OsCCA1 , OsELF3‐1 , OsELF3‐2 and OsZTL (Izawa et al, 2011; Murakami, Ashikari, Miura, Yamashino, & Mizuno, 2003; Zhao et al, 2012). Importantly, we found that OsPRR73 , was up‐regulated in oself4a CRISPR mutant in the day time in both SD and NLD conditions, which was recently reported to regulate circadian clock, heading date and salt tolerance (Liang et al, 2020; Wei et al, 2021). We found other circadian genes expression patterns in SD and NLD were altered with the changed peak values (Figure S6 and Figure S7), whilst obvious phase advance of OsPRR1 , OsPRR95 , OsPRR59 , OsGI and OsZTL were observed in constant light condition (Figure S8), indicating OsELF4a is a clock associated gene in rice.…”

“…On the other hand, in any of loss of function mutants of OsEC1, the elevated expression of OsGI could delay heading date through OsGI‐Hd1‐Hd3a pathway in both SD and LD condition. In addition, OsEC1 might also positively regulate heading date independent of OsGI through indirectly represses OsPRR73 (Figure 10), as the elevated OsPRR73 could delay heading date irrespective of photoperiods (Liang et al, 2020). Given all of the genes including OsELF4a , OsELF3‐1 , OsLUX and OsGI are core clock components of rice circadian clock, we proposed that the transcriptional module composed by OsEC1 repressing OsGI likely links circadian clock with heading date control and salt stress adaptation in rice, which is benefit for ameliorating crop quality and environment adaptability.…”

“…Recently, overexpression of OsPRR73 was shown to delay heading date in both LD and SD condition. As the transcript level of OsPRR73 was up‐regulated in oself4a CRISPR mutant, it may also partially explained the late heading date phenotype of oself4a CRISPR (Liang et al, 2020). Intriguingly, OsELF4a overexpressing plants also showed the late heading date in SD conditions (Figure 2e,f).…”

“…By contrast, another OsPRRs family member , OsPRR73 is the specific member amongst OsPRR gene family to confer rice salt tolerance by directly repressing OsHKT2;1 transcription via recruiting Histone deacetylase 10 (HDAC10) (Wei et al, 2021). Intriguingly, the null mutation of OsPRR73 showed very subtle heading date phenotype with only flowered about 3 days earlier than wild type in natural long day (NLD) conditions and did not exhibit any of difference with the control plants at natural short day (NSD) conditions (Liang et al, 2020). Hence, the coordinated regulatory networks bridging circadian clock with salt tolerance and heading date control are still largely uncharacterized in rice.…”

Rice functional genomics: decades’ efforts and roads ahead

“…OsPRR37 is a flowering suppressor in LD in the PS network, acting in a heterotrimer with DTH8 and NF-YCs (Koo et al, 2013;Liu et al, 2018b;Yan et al, 2013;Zhang et al, 2019a). OsPRR73 suppresses the expression of OsCCA1 and Ehd1 by binding to their promoter regions in LD (Liang et al, 2021). In addition, OsPRR73 also positively promotes salt tolerance (Wei et al, 2021a).…”