The Pea Photoperiod Response Gene STERILE NODES Is an Ortholog of LUX ARRHYTHMO      

Abstract

The STERILE NODES (SN) locus in pea (Pisum sativum) was one of the first photoperiod response genes to be described and provided early evidence for the genetic control of long-distance signaling in flowering-time regulation. Lines homozygous for recessive sn mutations are early flowering and photoperiod insensitive, with an increased ability to promote flowering across a graft union in short-day conditions. Here, we show that SN controls developmental regulation of genes in the FT family and rhythmic regulation of genes related to circadian clock function. Using a positional and functional candidate approach, we identify SN as the pea ortholog of LUX ARRHYTHMO, a GARP transcription factor from Arabidopsis (Arabidopsis thaliana) with an important role in circadian clock function. In addition to induced mutants, sequence analysis demonstrates the presence of at least three other independent, naturally occurring loss-of-function mutations among known sn cultivars. Examination of genetic and regulatory interactions between SN and two other circadian clock genes, HIGH RESPONSE TO PHOTOPERIOD (HR) and DIE NEUTRALIS (DNE), suggests a complex relationship in which HR regulates expression of SN and the role of DNE and HR in control of flowering is dependent on SN. These results extend previous work to show that pea orthologs of all three Arabidopsis evening complex genes regulate clock function and photoperiod-responsive flowering and suggest that the function of these genes may be widely conserved.

EARLY FLOWERING3 Redundancy Fine-Tunes Photoperiod Sensitivity

“…Trough expression levels for the evening genes LATE1 and PRR59a were also significantly higher in ppd-3 than in the wild type, a difference also apparent under SDs. In contrast, there was no clear evidence of any effect of ppd-3 on expression of PRR37a, consistent with earlier reports on sn and dne mutants (Liew et al, 2009(Liew et al, , 2014.…”

“…Both of these parental lines carry the same hr mutation (Weller et al, 2012). Figure 1 and Supplemental Figure S1 confirm that in contrast to the photoperiod-responsive wild type (P , 0.001), the ppd-3 allele confers early flowering (P , 0.001) regardless of photoperiod, equivalent to ppd-1 and ppd-2 (Taylor and Murfet, 1996) and to sn and dne mutants (Murfet, 1971;King and Murfet, 1985;Liew et al, 2009Liew et al, , 2014. As all three ppd mutations appeared similar in their phenotypic effects, we used the ppd-3 mutant to explore the roles of PPD, as it was generated in the same genetic background as other relevant mutants, including sn and dne.…”

“…Previous studies have shown that the early flowering phenotypes of sn and dne mutants under short-day (SD) conditions are associated with elevated expression levels of several FT homologs, similar to the inductive effect of long days (LDs) observed for the wild type (Hecht et al, 2011;Liew et al, 2014). We used qRT-PCR to compare the expression of pea FT genes in ppd-3 and these other mutants over the course of development under SDs.…”

“…Interestingly, in both crop groups, mutations in ELF3 genes have been shown to contribute to the expansion in range of species through alteration of photoperiod responsiveness and the associated impact on yield (Faure et al, 2012;Matsubara et al, 2012;Weller et al, 2012;Zakhrabekova et al, 2012;Alvarez et al, 2016;Lu et al, 2017). In crop species, a role for ELF4 function has been clearly established only in the legume species pea (Pisum sativum; Liew et al, 2009), but mutations in LUX orthologs have also been shown to cause early flowering and impaired photoperiod response in pea and in the cereals einkorn wheat (Triticum aestivum) and barley (Hordeum vulgare; Mizuno et al, 2012;Campoli et al, 2013;Liew et al, 2014), suggesting that the EC genes are likely to be intimately linked to the photoperiod response mechanism in both crop groups.…”

“…Naturally occurring and induced mutations in the LUX ortholog SN completely eliminate responsiveness to photoperiod (Liew et al, 2014), whereas a mutant for the ELF4 ortholog DNE enhances the effect of the hr mutation but has little effect when functional HR is present (Liew et al, 2009(Liew et al, , 2014. In addition to these three EC homologs, mutations at two other loci confer early, photoperiod-insensitive flowering in pea: a dominant mutation in the PHYTOCHROME A (PHYA) gene (Weller et al, 2004) and recessive mutations at the PHOTOPERIOD (PPD) locus (Arumingtyas and Murfet, 1994;Taylor and Murfet, 1996;Murfet and Taylor, 1999), whose molecular identity is not yet known.…”