Thermal stress effects on grain yield in Brachypodium distachyon occur via H2A. Z-nucleosomes

Background:: Crop plants are highly sensitive to ambient temperature, with a 1 degrees C difference in temperature sufficient to affect development and yield. Monocot crop plants are particularly vulnerable to higher temperatures during the reproductive and grain-filling phases. The molecular mechanisms by which temperature influences grain development are, however, unknown. In Arabidopsis thaliana, H2A. Z-nucleosomes coordinate transcriptional responses to higher temperature. We therefore investigated whether the effects of high temperature on grain development are mediated by H2A. Z-nucleosomes. Results:: We have analyzed the thermal responses of the Pooid grass, Brachypodium distachyon, a model system for crops. We find that H2A. Z-nucleosome occupancy is more responsive to increases in ambient temperature in the reproductive tissue of developing grains compared withvegetative seedlings. This difference correlates with strong phenotypic responses of developing grain to increased temperature, including early maturity and reduced yield. Conversely, temperature has limited impact on the timing of transition from the vegetative to generative stage, with increased temperature unable to substitute for long photoperiod induction of flowering. RNAi silencing of components necessary for H2A. Z-nucleosome deposition is sufficient to phenocopythe effects of warmer temperature on grain development. Conclusions:: H2A. Z-nucleosomes are important in coordinating the sensitivity of temperate grasses to increased temperature during grain development. Perturbing H2A.Z occupancy, through higher temperature or genetically, strongly reduces yield. Thus, we provide a molecular understanding of the pathways through which high temperature impacts on yield. These findings may be useful for breeding crops resilient to thermal stress.