Epistatic interactions between PHOTOPERIOD1, CONSTANS1 and CONSTANS2 modulate the photoperiodic response in wheat

Author summary An understanding of the mechanisms involved in the regulation of wheat heading time is required to engineer more productive varieties better adapted to new or changing environments. A large proportion of wheat's natural variation in heading time is determined by differences in genes controlling the photoperiodic response. In this study, we show that the wheatPHOTOPERIOD1(PPD1) gene has a stronger effect on heading time thanCONSTANS1(CO1) andCO2in the regulation of the photoperiodic response, and that complex genetic interactions among these genes are important to fine-tune heading time. Using loss-of-function mutants for bothCO1andCO2, we demonstrate that these genes are not required forPPD1to perceive differences in photoperiod and regulate heading time. Similarly, we show that in the absence ofPPD1,CO1can accelerate heading time more than 60 days in response to long days. Our results indicate that each of these two wheat photoperiod pathways can respond to differences in photoperiod even in the absence of the other one. Differences in the relative importance of these two pathways and in their epistatic interactions have contributed to the diversity of photoperiodic responses observed in different grass species. In Arabidopsis,CONSTANS(CO) integrates light and circadian clock signals to promote flowering under long days (LD). In the grasses, a duplication generated two paralogs designated asCONSTANS1(CO1) andCONSTANS2(CO2). Here we show that in tetraploid wheat plants grown under LD, combined loss-of-function mutations in the A and B-genome homeologs ofCO1andCO2(co1 co2) result in a small (3 d) but significant (P<0.0001) acceleration of heading time both inPHOTOPERIOD1(PPD1) sensitive (Ppd-A1b, functional ancestral allele) and insensitive (Ppd-A1a, functional dominant allele) backgrounds. Under short days (SD),co1 co2mutants headed 13 d earlier than the wild type (P<0.0001) in the presence ofPpd-A1a. However, in the presence ofPpd-A1b, spikes from both genotypes failed to emerge by 180 d. These results indicate thatCO1andCO2operate mainly as weak heading time repressors in both LD and SD. By contrast, inppd1mutants with loss-of-function mutations in bothPPD1homeologs, the wild typeCo1allele accelerated heading time >60 d relative to theco1mutant allele under LD. We detected significant genetic interactions amongCO1,CO2andPPD1genes on heading time, which were reflected in complex interactions at the transcriptional and protein levels. Loss-of-function mutations inPPD1delayed heading more than combinedco1 co2mutations and, more importantly,PPD1was able to perceive and respond to differences in photoperiod in the absence of functionalCO1andCO2genes. Similarly,CO1was able to accelerate heading time in response to LD in the absence of a functionalPPD1. Taken together, these results indicate thatPPD1andCO1are able to respond to photoperiod in the absence of each other, and that interactions between these two photoperiod pathways at the transcriptional and protein levels are important to fine-tune the flowering response in wheat.