How does PAP, a stress-induced metabolite, regulate gene expression?

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Plants are commonly exposed to environmental fluctuations (such as high light or insufficient water) which are sufficient to limit crop yield in both field-grown and glasshouse-grown contexts. Damage induced by such stresses is typically first observed within the chloroplast and mitochondria, where perturbations in metabolism rapidly induce oxidative stress. These perturbations are communicated from organelles to the nucleus via multiple retrograde signaling pathways that alter nuclear gene expression, allowing plants to adjust their metabolism and development to tolerate environmental stress. However, the extent to which retrograde signals can regulate plant homeostasis, and by what mechanism(s), remain enigmatic.Many abiotic and biotic are predominantly associated with specific times of day, driving the evolution of biological timing mechanisms that enable anticipation of biotic and abiotic stresses associated with either day or night. These biological timers (commonly referred to as the circadian system) have subsequently been co-opted to modulate many physiological processes including growth, photosynthesis, and flowering time. In addition to providing an endogenous timing reference, seasonal changes in daylength require that the circadian system is synchronized with environmental factors such as dusk and dawn. This induces a complex interplay between environmental signals, endogenous biological timers, and metabolic changes induced by sub-optimal environmental conditions. If we are to fully exploit the potential yield of crops it is vital that we understand how plants interact with their environment during daily environmental fluctuations.Recently, it has been suggested that the circadian system acts as a metabolic governor (as found on steam engines), slowing metabolism and consequently improving survival during periods of stress. In agreement with this concept we have demonstrated that application of osmotic stress slows the circadian system. This results in repression of genes normally expressed during the evening. We have demonstrated that a signaling metabolite that accumulates in response to osmotic stress is sufficient to induce a comparable delay in the circadian system. Such data demonstrates how changes in metabolism arising from the application of stress can induce changes in gene expression, ultimately altering plant behavior.The circadian system induces rhythmic expression of approximately one third of a plants genome but we do not have a precise understanding of how changes in metabolism alter the pace of the endogenous biological timer. Importantly, circadian timing components originally identified in the experimental workhorse Arabidopsis thaliana have been found to be conserved throughout the plant kingdom, with naturally-occurring alleles of known clock components being historically introduced into commercially grown varieties of barley and tomato. This study will take advantage of the genetic resources available in Arabidopsis. This will allow for rapid progress before our understanding is transferred into crops such as barley and wheat. Such work will advance our understanding of plants responses to osmotic stress and directly inform BBSRC's priorities to design crops with greater drought resilience that make more efficient use of available water resources.