Ehd4 Encodes a Novel and Oryza-Genus-Specific Regulator of Photoperiodic Flowering in Rice

Abstract

Land plants have evolved increasingly complex regulatory modes of their flowering time (or heading date in crops). Rice (Oryza sativa L.) is a short-day plant that flowers more rapidly in short-day but delays under long-day conditions. Previous studies have shown that the CO-FT module initially identified in long-day plants (Arabidopsis) is evolutionary conserved in short-day plants (Hd1-Hd3a in rice). However, in rice, there is a unique Ehd1-dependent flowering pathway that is Hd1-independent. Here, we report isolation and characterization of a positive regulator of Ehd1, Early heading date 4 (Ehd4). ehd4 mutants showed a never flowering phenotype under natural long-day conditions. Map-based cloning revealed that Ehd4 encodes a novel CCCH-type zinc finger protein, which is localized to the nucleus and is able to bind to nucleic acids in vitro and transactivate transcription in yeast, suggesting that it likely functions as a transcriptional regulator. Ehd4 expression is most active in young leaves with a diurnal expression pattern similar to that of Ehd1 under both short-day and long-day conditions. We show that Ehd4 up-regulates the expression of the “florigen” genes Hd3a and RFT1 through Ehd1, but it acts independently of other known Ehd1 regulators. Strikingly, Ehd4 is highly conserved in the Oryza genus including wild and cultivated rice, but has no homologs in other species, suggesting that Ehd4 is originated along with the diversification of the Oryza genus from the grass family during evolution. We conclude that Ehd4 is a novel Oryza-genus-specific regulator of Ehd1, and it plays an essential role in photoperiodic control of flowering time 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).…”

“…The CCT domain has been shown to be involved in protein localization and probably has an important role in mediating protein-protein interaction (22)(23)(24). Interestingly, OsPRR37 has been reported to be the same allele with a major QTL, Hd2 (also known as Ghd7.1), that was firstly identified in a photoperiod-sensitive variety Nipponbare background (3,(25)(26)(27). In addition, homologous genes, such as Ppd-H1 in barley, SbPRR37 in sorghum, and Arabidopsis PRR1, 3, 5, 7, and 9 (SI Appendix, Fig.…”

“…Generally its flowering is delayed when days are long and nights are short but accelerated when days are getting shorter. Cultivars with reduced PS response are characterized with early flowering and were developed to suit for growing at higher latitudes in the temperate zone (the northern limit is ∼53°N) or for two (even three) successive rounds of planting in the long growing season areas (e.g., some provinces in South China) (3,4). By contrast, cultivars with enhanced PS (late flowering) have been developed for increased grain yield in most rice planting regions (5,6).…”

“…Molecular genetic studies in the past few decades have uncovered a number of flowering-time loci (3)(4)(5)(6)(7)(8)(9). Among them, a major quantitative trait locus (QTL), Heading date 3a (Hd3a), which encodes a rice ortholog of the Arabidopsis Flowering locus T (FT), promotes floral transition under short day conditions (SDs) (9).…”

Overexpression of OsMYB1R1–VP64 fusion protein increases grain yield in rice by delaying flowering time

“…To determine if OsMYB1R1 and the OsMYB1R1-VP64 fusion function as transcriptional activators, we fused OsMYB1R1 and OsMYB1R1-VP64 fusion with the GAL4 DNA-binding domain and transformed them into yeast. Ehd4 was used as a positive control [42]. Compared to the empty control, OsMYB1R1 and OsMYB1R1-VP64 fusion both had transcriptional activation activity, but the activity of OsMYB1R1 was relatively weak compared to OsMYB1R1-VP64 fusion (Fig.…”