Light, temperature and circadian clock signal integration during leaf senescence

Leaf senescence is an active process involving the remobilisation of nutrients from older, photosynthetically limited tissues to developing plant structures. It is characterised by striking leaf yellowing resulting from the dismantling of chloroplasts and degradation of chlorophyll. Leaf senescence can be induced by environmental stress, such as prolonged darkness and is accelerated at high temperature. In commercial horticulture, leaf senescence in cut produce reduces crop quality and shelf-life, resulting in significant economic losses to growers. This problem is exacerbated by the transport and storage of produce in warm and dark conditions, requiring producers and retailers to invest in costly and environmentally unfriendly refrigeration technologies. Sustainably delaying leaf senescence is a key objective for the fresh food industry, yet cost-effective, practical solutions remain limited.

In the model plant Arabidopsis thaliana, dark- induced leaf senescence is promoted by the light, temperature and circadian clock-regulated transcription factors, PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5. The Franklin lab has shown that low dose (non-stressful) UV-B light treatment can promote rapid degradation of PIF4 and PIF5 proteins and suppress the expression of PIF4 and PIF5 genes in Arabidopsis seedlings. UV-B is filtered by glass in commercial greenhouses and is not a component of horticultural lighting in vertical farms. As such, UV-B is absent in the majority of protected growing environments used in commercial horticulture. We have preliminary evidence that a short (4 hour) preharvest treatment of low dose UV-B can significantly reduce dark-induced senescence in detached Arabidopsis leaves and cotyledons stored at warmer temperatures. This treatment supresses the expression of the key senescence regulator, ORE1 and delays chlorophyll degradation. The proposed work will dissect the molecular signalling pathways through which light, temperature and the circadian clock control dark-induced senescence in detached Arabidopsis leaves. In addition to furthering mechanistic insight into the regulation of a key developmental process, the work will inform strategies for using pre and postharvest light treatments to delay dark-induced leaf senescence in crops. Our preliminary data suggest that preharvest UV-B treatment can delay dark-induced senescence in young rocket (Eruca sativa) leaves, supporting our approach. Overall, the proposed work will provide a molecular framework to address a key problem in sustainable food storage.

Leaf senescence is a highly regulated process required for nutrient remobilisation in photosynthetically limited leaves. It occurs with age and in response to environmental stress. Dark-induced leaf senescence is regulated by the bHLH transcription factors, PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 and is accelerated by high temperature. PIF4/5 abundance and activity are regulated by phytochromes, cryptochromes, temperature and the circadian clock. We have shown that low dose UV-B perceived by the photoreceptor UV RESISTANCE LOCUS 8 (UVR8) promotes rapid PIF4/5 degradation through sequestration of the PIF-stabilising protein CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1). UVR8 signalling additionally reduces PIF4/5 transcript abundance through an unknown mechanism, providing a highly effective dual target approach to supress PIF4/5 activity. Our unpublished work shows that pre- and postharvest UV-B treatment can supress the dark induction of senescence marker genes and reduce chlorophyll degradation in detached Arabidopsis leaves. Here, we propose to dissect the molecular mechanisms through which light, temperature and the circadian clock control leaf senescence and use this information to optimise preharvest treatment and postharvest storage of cut leaves. Characterisation of PIF4/5 circadian rhythms in detached leaves stored in the dark will guide the timings of postharvest light treatments for maximum senescence delay. RNA sequencing will additionally be used to identify novel PIF4/5-dependent and -independent targets involved in the suppression of leaf senescence by low temperature.