Loss-of-Function Mutations in Three Homoeologous PHYTOCLOCK 1 Genes in Common Wheat Are Associated with the Extra-Early Flowering Phenotype

Triticum aestivum L. cv 'Chogokuwase' is an extra-early flowering common wheat cultivar that is insensitive to photoperiod conferred by the photoperiod insensitive alleles at the Photoperiod-B1 (Ppd-B1) and Ppd-D1loci, and does not require vernalization for flowering. This reduced vernalization requirement is likely due to the spring habitat allele Vrn-D1 at the VERNALIZATION-D1 locus. Genotypes of the Ppd-1 loci that determine photoperiod sensitivity do not fully explain the insensitivity to photoperiod seen in 'Chogokuwase'. We detected altered expression patterns of clock and clock-output genes including Ppd-1 in 'Chogokuwase' that were similar to those in an einkorn wheat mutant that lacks the clock-gene homologue, wheat PHYTOCLOCK 1 (WPCL1). Presumptive loss-of-function mutations in all WPCL1 homoeologous genes were found in 'Chogokuwase' and 'Geurumil', one of the parental cultivars. Segregation analysis of the two intervarietal F2 populations revealed that all the examined F2 plants that headed as early as 'Chogokuwase' had the loss-of-function wpcl1 alleles at all three homoeoloci. Some F2 plants carrying the wpcl1 alleles at three homoeoloci headed later than 'Chogokuwase', suggesting the presence of other loci influencing heading date. Flowering repressor Vrn-2 was up-regulated in 'Chogokuwase' and 'Geurumil' that had the triple recessive wpcl1 alleles. An elevated transcript abundance of Vrn-2 could explain the observation that 'Geurumil' and some F2 plants carrying the three recessive wpcl1 homeoealleles headed later than 'Chogokuwase'. In spite of the up-regulation of Vrn-2, 'Chogokuwase' may have headed earlier due to unidentified earliness genes. Our observations indicated that loss-of-function mutations in the clock gene wpcl1 are necessary but are not sufficient to explain the extra-early heading of 'Chogokuwase'.