Alternative functions of Hd1 in repressing or promoting heading are determined by Ghd7 status under long-day conditions

Abstract

Previous studies suggested that Hd1 promoted heading under short-day conditions (SD) and delayed heading under long-day conditions (LD). However in this study, Hd1 was demonstrated to consistently promote heading date in Zhenshan 97 (ZS97) background by upregulating Ehd1, Hd3a and RFT1 expression under both SD and LD. While the high photoperiod sensitivity of Hd1 was observed in Minghui 63 (MH63) background, with heading being suppressed in LD but promoted in SD. Comparative analysis of two sets of near isogenic lines of Hd1 in MH63 and ZS97 backgrounds indicated that the alternative functions of Hd1 in promoting or suppressing heading under LD are dependent on the previously cloned flowering repressor gene Ghd7. The interaction between proteins Ghd7 and Hd1 occurred through binding of the CCT domain of Ghd7 to the transcription-activating domain of Hd1, resulting in suppression of Ehd1 and florigen gene expression. The involvement of the transcription-activating domain of Hd1 in this protein-protein interaction probably blocked or weakened its transcriptional activity. These findings suggest that Hd1 alone essentially acts as a promoter of heading date, and the protein interaction between Ghd7 and Hd1 determines photoperiod sensitivity and integrated Hd1-mediated and Ehd1-mediated flowering pathways in rice.

Genetic interactions among Ghd7, Ghd7.1, Ghd8 and Hd1 contribute to large variation in heading date in rice

“…However, in this study, Ghd7.1 delayed rice heading in the ghd7ghd8hd1 , ghd7ghd8Hd1 and ghd7Ghd8Hd1 backgrounds but significantly promoted heading in the Ghd7Ghd8hd1 , Ghd7Ghd8Hd1 and Ghd7ghd8Hd1 backgrounds under NSD conditions (Table 4), which clearly demonstrated that Ghd7.1 had alternative functions under NSD conditions. This differed from the results for Hd1 , which had alternative functions under NLD conditions (Zhang et al , 2017). Ghd7.1 suppressed heading date by inhibiting the expression of its downstream genes Ehd1 and Hd3a under LD conditions (Koo et al 2013; Gao et al 2014).…”

“…ZS97 carries a functional allele of Hd1 but a nonfunctional allele of ghd7 , ghd7.1 and ghd8 (Xue et al , 2008; Yan et al , 2011; Yan et al , 2013; Zhang et al , 2017). We previously developed a near-isogenic line (NIL-1) pyramiding functional Ghd7 MH63 and Ghd8 9311 in a ZS97 background (Zhang et al , 2015a).…”

“…Among these genes, several major QTLs, especially those cloned from natural variations, have pleiotropic effects on heading date, plant height and grain yield, which have been widely subjected to artificial selection in the process of rice genetic improvement. For example, Heading date1 ( Hd1 ), the homolog of Arabidopsis CONSTANS ( CO ), encodes a zinc finger CCT (CO, CO-LIKE and TIMING OF CAB1) domain and acts as a major flowering activator in rice (Yano et al , 2000; Zhang et al , 2017). Hd1 delays heading date in some varieties under long-day (LD) conditions by interacting with other flowering genes such as Ghd7, resulting in a taller plant and more grain yield (Nemoto et al , 2016; Zhang et al , 2017).…”

“…For example, Heading date1 ( Hd1 ), the homolog of Arabidopsis CONSTANS ( CO ), encodes a zinc finger CCT (CO, CO-LIKE and TIMING OF CAB1) domain and acts as a major flowering activator in rice (Yano et al , 2000; Zhang et al , 2017). Hd1 delays heading date in some varieties under long-day (LD) conditions by interacting with other flowering genes such as Ghd7, resulting in a taller plant and more grain yield (Nemoto et al , 2016; Zhang et al , 2017). Ghd7 is a rice-specific gene encoding a CCT domain protein and is important for heading date, grain yield, rice adaptation and drought resistance (Xue et al , 2008; Weng et al , 2014).…”

“…One is the OsGI-Hd1-Hd3a pathway, which is conserved with the GI-CO-FT pathway in Arabidopsis (Shrestha et al, 2014). Hd1 is upregulated by OsGI and activates the expression of Hd3a to promote rice heading under both short-day (SD) and LD conditions (Hayama et al , 2003; Zhang et al , 2017). Another is the Ehd1-Hd3a pathway, a unique pathway in rice regulated by many genes (Doi et al , 2004; Tsuji et al , 2011).…”