PPD-H1Improves Stress Resistance and Energy Metabolism to Boost Spike Fertility under High Ambient Temperatures

Abstract

High ambient temperature (HT) impairs reproductive development and grain yield in the temperate crops barley and wheat. To ensure reproductive success under HT, plants must maintain developmental stability. However, the mechanisms integrating plant development and temperature resilience are largely unknown. Here we analysed natural and induced barley mutants for the flowering time gene PHOTOPERIOD 1 (PPD-H1) under favourable and high-temperature conditions. We demonstrate that PPD-H1 controls developmental stability, HT delayed inflorescence development and reduced grain number in mutant ppd-h1 lines, while the ancestral wild-type Ppd-H1 genotypes accelerated reproductive development and showed stable grain set under HT. The wild-type Ppd-H1 genotypes maintained the activity of the inflorescence meristem (IM), thereby controlling the rate and duration of spikelet meristem (SM) and floral meristem (FM) induction under HT. Under HT, the mutant ppd-h1 lines were characterised by high abscisic acid levels and strong upregulation of stress response genes in the leaf and inflorescence, while wild-type genotypes displayed relatively stable phytohormone and transcriptome levels. In the inflorescence, reduced auxin and cytokinin levels and the downregulation of photosynthesis and energy metabolism genes were linked to impaired floret, anther, and pollen development only in the ppd-h1 mutants but not in the wild-type plants. Our findings suggest that the wild-type Ppd-H1 allele enhances stress resistance and energy metabolism, thereby controlling reproductive development, floret fertility and grain set under HT.