Sense in sensitivity: difference in the meaning of photoperiod insensitivity between wheat and barley.

The description of long photoperiod sensitivity in wheat and barley is a cause of confusion for researchers working with these crops, usually accustomed to free exchange of physiological and genetic knowledge of such similar crops. Indeed, wheat and barley scientists customarily quote studies of either crop species when researching one of them. Among their numerous similarities, the main gene controlling the long photoperiod sensitivity is the same in both crops (PPD1; PPD-H1 in barley and PPD-D1 in hexaploid wheat). However, the photoperiod responses are different: (i) the main dominant allele inducing shorter time to anthesis is the insensitive allele in wheat (Ppd-D1a) but the sensitive allele in barley (Ppd-H1) (i.e. sensitivity to photoperiod produces opposite effects on time to heading in wheat and barley); (ii) the main 'insensitive' allele in wheat, Ppd-D1a, does confer insensitivity, whilst that of barley reduces the sensitivity but still responds to photoperiod. The different behaviour of PPD1 genes in wheat and barley is put in a common framework based on the similarities and differences of the molecular bases of their mutations, which include polymorphism at gene expression levels, copy number variation, and sequence of coding regions. This common perspective sheds light on a source of confusion for cereal researchers, and prompts us to recommend accounting for the photoperiod sensitivity status of the plant materials when conducting research on genetic control of phenology. Finally, we provide advice to facilitate the management of natural PPD1 diversity in breeding programmes and suggest targets for further modification through gene editing, based on mutual knowledge on the two crops.