Ehd3, encoding a plant homeodomain finger‐containing protein, is a critical promoter of rice flowering

Abstract

SUMMARYOryza sativa (rice) flowers in response to photoperiod, and is a facultative short-day (SD) plant. Under SD conditions, flowering is promoted through the activation of FT-like genes (rice florigens) by Heading date 1 (Hd1, a rice CONSTANS homolog) and Early heading date 1 (Ehd1, with no ortholog in the Arabidopsis genome). On the other hand, under long-day (LD) conditions, flowering is delayed by the repressive function of Hd1 on FT-like genes and by downregulation of Ehd1 by the flowering repressor Ghd7 -a unique pathway in rice. We report here that an early heading date 3 (ehd3) mutant flowered later than wild-type plants, particularly under LD conditions, regardless of the Hd1-deficient background. Map-based cloning revealed that Ehd3 encodes a nuclear protein that contains a putative transcriptional regulator with two plant homeodomain (PHD) finger motifs. To identify the role of Ehd3 within the gene regulatory network for rice flowering, we compared the transcript levels of genes related to rice flowering in wild-type plants and ehd3 mutants. Increased transcription of Ghd7 under LD conditions and reduced transcription of downstream Ehd1 and FTlike genes in the ehd3 mutants suggested that Ehd3 normally functions as an LD downregulator of Ghd7 in floral induction. Furthermore, Ehd3 ghd7 plants flowered earlier and show higher Ehd1 transcript levels than ehd3 ghd7 plants, suggesting a Ghd7-independent role of Ehd3 in the upregulation of Ehd1. Our results demonstrate that the PHD-finger gene Ehd3 acts as a promoter in the unique genetic pathway responsible for photoperiodic flowering in rice.

Days to heading 7 , a major quantitative locus determining photoperiod sensitivity and regional adaptation in rice

“…On the other hand, several positive Ehd1 regulators have also been cloned by isolating mutants with extreme late flowering. It was shown that Early heading date 2 (Ehd2)/Rice Indeterminate 1 (RID1)/Oryza sativa Indeterminate 1 (OsId1) (referred to as Ehd2 hereafter), encoding a Cs2/His2-type zinc finger protein; Early heading date 3 (Ehd3), encoding a putative plant homeodomain finger containing protein; and Early heading date 4 (Ehd4), encoding a CCCHtype zinc finger protein, independently promote Ehd1 expression under both SDs and LDs (3,17,18). Therefore, it appears that Ehd1 is a pivotal convergence point that integrates multiple signaling pathways, including "SD-activation pathway," "LD-repression pathway," and "LD-activation pathway," to regulate the flowering time of rice under diverse environmental conditions (19).…”

SIP1 participates in regulation of flowering time in rice by recruiting OsTrx1 to Ehd1

“…In addition to being activated by MADs‐box proteins, Ehd1 is suppressed by Grain Number Plant Height and Heading date7 ( Ghd7 ), which encodes a CCT domain protein (Xue et al ., 2008). Ghd7 is activated by EL1 ( Early flowering 1 )/ Hd16 , which encodes a casein kinase I protein (Dai & Xue, 2010), and is suppressed by Ehd3 (Matsubara et al ., 2011). …”

A Drought-Inducible Transcription Factor Delays Reproductive Timing in Rice

“…Ehd1 encodes a rice-specific B-type response factor that acts in both LDs and SDs to promote Hd3a and RFT1 mRNA expression, thus inducing floral initiation (Doi et al, 2004;Komiya et al, 2009;Sun et al, 2014). Many repressors or activators, including Ehd2, Ehd3, Ehd4, Ghd7, OsMADS51, OsLFL1, and OsPPR37/ DTH7, regulate the expression and activity of Ehd1 itself in day length-dependent or -independent manners (Doi et al, 2004;Kim et al, 2007;Peng et al, 2007;Xue et al, 2008;Wei et al, 2010;Matsubara et al, 2011;Koo et al, 2013;Tsuji et al, 2013;Yan et al, 2013;Kwon et al, 2015). It appears that Ehd1 is under the convergent regulation of SD activators, LD repressors, and photoperiod-independent regulators to control the heading date of rice in response to the internal development "clock" and environmental cues.…”