Stable early heading in photoperiod-insensitive rice varieties results from an extremely short photoperiod-sensitive phase and weak temperature sensitivity

The transition from vegetative to reproductive growth in rice (Oryza sativa) is mainly regulated by photoperiod and temperature; however, it remains unclear how photoperiod-insensitive (photo-In) early heading rice varieties make this transition. Vegetative growth consists of a basic vegetative phase (BVP) and a photoperiod-sensitive phase (PSP). A certain duration of the BVP is required prior to the PSP, which varies according to daylength conditions. PSP duration is conventionally used as a parameter of photoperiod sensitivity, but this is not applicable to photo-In rice varieties. Here, we aimed to characterize the PSP in photo-In rice varieties. We examined four photo-In varieties and photoperiod-sensitive (photo-Se) varieties grown in four controlled environments, evaluating the lengths of their growth phases based on the turning point of the interval of leaf emergence and panicle initiation (PI). The photo-In varieties had an extremely short PSP, regardless of daylength, compared with the photo-Se varieties. Low temperature uniformly prolonged all growth stages in the photo-In varieties but led to a much longer PSP in the photo-Se varieties. The photo-In varieties, each carrying a non-functional allele of Ghd7, lost the ability to suppress PI, resulting in early heading. Hemi-knockout plants at the Ghd7 locus demonstrated the earlier heading than the wild-type plant due to shortening of PSP. Our results suggest that the photo-In varieties begin PI immediately after the end of the BVP, but their flowering is not a temperature-dependent process.