From clock to field: Unraveling cytonuclear pleiotropic effects in barley for thermal plasticity

Robustness in face of temperature changes is considered a key feature of circadian clock systems. In this study, we generated new wild barley doubled haploid (DH) population to dissect genetic and developmental basis of phenotypic robustness. That the population originated from a reciprocal cross between two differentially adapted Hordeum vulgare sps. spontaneum accessions belonging to the Barley1K collection, one from southern Ashkelon site and one from northern Mt. Hermon , namely ASHER, allowed testing the effects of plasmotypes (chloroplast and mitochondria). The phenotype was achieved under ambient temperature and high temperature for fitness traits, as well for circadian clock rhythmicity under optimal (22ºC) and high (32ºC) temperatures. The later was using a newly-developed SensyPAM platform to record pulsed amplitude modulation of chlorophyll fluorescence under continuous light. This comparison between two thermal environments pointed to the prevalence of clock acceleration under heat, with a significantly faster clock of the Ashkelon plasmotype carriers. In parallel, analysis of life history fitness traits also uncovered temporal differences in robustness between environments of the two subpopulations; the vegetative dry weight of DH plants carrying the Hermon plasmotype was more robust than those carrying the Ashkelon type. This is in contrary for spikes dry weight in which Ashkelon-type maintained similar values (robustness) under both environments. Notably, some nuclear QTLs are pleiotropic for the clock characteristics and the fitness traits plasticity. These findings within the Barley1K framework pave the way to unravel genetic networks and mechanisms underlying plant robustness and their role in plant adaptation and evolution.