Drought impairs growth and spike development, and is therefore a major cause of yield losses in the temperate cereals barley and wheat. Here, we show that the photoperiod response gene PHOTOPERIOD-H1 (Ppd-H1) interacts with drought stress signals to modulate spike development. We tested the effects of a continuous mild and a transient severe drought stress on developmental timing and spike development in spring barley cultivars with a natural mutation in ppd-H1 and derived introgression lines carrying the wild-type Ppd-H1 allele from wild barley. Mild drought reduced the spikelet number and delayed floral development in spring cultivars but not in the introgression lines with a wild-type Ppd-H1 allele. Similarly, drought-triggered reductions in plant height, and tiller and spike number were more pronounced in the parental lines compared with the introgression lines. Transient severe stress halted growth and floral development; upon rewatering, introgression lines, but not the spring cultivars, accelerated development so that control and stressed plants flowered almost simultaneously. These genetic differences in development were correlated with a differential down-regulation of the flowering promotors FLOWERING LOCUS T1 and the BARLEY MADS-box genes BM3 and BM8. Our findings therefore demonstrate that Ppd-H1 affects developmental plasticity in response to drought in barley.

Highlight 19 We show that Ppd-H1 integrates photoperiod and drought stress signals via 20 FLOWERING LOCUS T 1 (FT1) and the downstream MADS-box genes BM3 and BM8 21 to modulate reproductive development, and shoot and spike morphology in barley. 22Abstract 23 Drought impairs growth and spike development and is therefore a major cause of yield 24 losses in the temperate cereals barley and wheat. Here, we show that the photoperiod 25 response gene PHOTOPERIOD-H1 (Ppd-H1) interacts with drought stress signals to 26 modulate spike development. We tested the effects of a continuous mild and a transient 27 severe drought stress on developmental timing and spike development in spring barley 28 cultivars with a natural mutation in ppd-H1 and derived introgression lines carrying the 29 wild-type Ppd-H1 allele from wild barley. Mild drought reduced the spikelet number and 30 delayed floral development in spring cultivars but not the introgression lines with a wild-31 type Ppd-H1 allele. Similarly, drought-triggered reductions in plant height, tiller and 32 spike number were more pronounced in the parental lines compared to the 33 introgression lines. Transient severe stress halted growth and floral development, upon 34 rewatering introgression lines, but not the spring cultivars, accelerated development so 35 that control and stressed plants flowered almost simultaneously. These genetic 36 differences in development were correlated with a differential downregulation of the 37 flowering promotors FLOWERING LOCUS T1 and the BARLEY MADS-box genes 38 BM3 and BM8. Our findings, therefore, demonstrate that Ppd-H1 affects 39 developmental plasticity in response to drought in barley. 40 41