The sessile nature of plants introduces an evolutionary pressure to develop a highly sophisticated network of signals that encode perception, response and adaptation to a dynamic range of stressors. When a plant experiences drought stress, several molecular and physiological processes occur in parallel. Production of the retrograde chloroplast-to-nucleus stress signal 3’phosphoadenosine 5’phosphate (PAP) which up-regulates the transcription of stress-responsive genes, the initiation of reactive oxygen species (ROS) and stomatal closure signalling pathways, are all co-occurring in response to stress. However, the functional role of chloroplasts and PAP-mediated signalling has remained largely enigmatic when considering any intersection between the pathways. Thus, in my PhD, I tackled these intertwining pathways that occur in response to stress: retrograde signalling, ROS signalling and stomatal closure signalling.
I investigate how PAP affects ROS homeostasis, signalling thresholds, the ‘biphasic ROS wave’ and the resulting physiological consequences. Here I present evidence suggesting ROS can induce signalling cascades, as well as being part of the pathway that propagates signals. As an exemplar proof of concept, I use the guard cell model to show how PAP influences this network of information. Gatekeepers of gas exchange, guard cells mediate stomatal closure as a short-term physiological response that plants utilise to prevent water loss. I show that exogenous application of PAP leads to rapid stomatal closure to a similar extent as the guard cell regulatory hormone abscisic acid (ABA). Significantly, genetic analyses and biochemical manipulation experiments indicate that PAP signalling can supplement the canonical ABA-mediated signalling pathway in guard cells. PAP signalling in stomatal closure includes a dynamic interaction with secondary messengers (including ROS), ion flux induction, and the utilisation of chloroplastic mechanisms separate to ABA signalling. PAP signalling can act, at least in part, by up-regulating a number of novel calcium protein kinases.
My thesis presents evidence of the chloroplast signal PAP having a functional role as a molecular signal in response to stress. While we can measure outcomes of drought stress, the complexity of the preceding stress pathways serve as an example of how the journey is just as important as the ending.