Development of strategies for genetic manipulation and fine-tuning of a chloroplast retrograde signal 3'-phosphoadenosine 5'-phosphate

Potential abiotic stress targets for modern genetic manipulation

Research into crop yield and resilience has underpinned global food security, evident in yields tripling in the past 5 decades. The challenges that global agriculture now faces are not just to feed 10+ billion people within a generation, but to do so under a harsher, more variable, and less predictable climate, and in many cases with less water, more expensive inputs, and declining soil quality. The challenges of climate change are not simply to breed for a "hotter drier climate," but to enable resilience to floods and droughts and frosts and heat waves, possibly even within a single growing season. How well we prepare for the coming decades of climate variability will depend on our ability to modify current practices, innovate with novel breeding methods, and communicate and work with farming communities to ensure viability and profitability. Here we define how future climates will impact farming systems and growing seasons, thereby identifying the traits and practices needed and including exemplars being implemented and developed. Critically, this review will also consider societal perspectives and public engagement about emerging technologies for climate resilience, with participatory approaches presented as the best approach.

Secondary sulfur metabolism in cellular signalling and oxidative stress responses

The sulfur metabolism pathway in plants produces a variety of compounds that are central to the acclimation response to oxidative stresses such as drought and high light. Primary sulfur assimilation provides the amino acid cysteine, which is utilized in protein synthesis and as a precursor for the cellular redox buffer glutathione. In contrast, the secondary sulfur metabolism pathway produces sulfated compounds such as glucosinolates and sulfated peptides, as well as a corresponding by-product 3'-phosphoadenosine 5'-phosphate (PAP). Emerging evidence over the past decade has shown that secondary sulfur metabolism also has a crucial engagement during oxidative stress. This occurs across various cellular, tissue, and organismal levels including chloroplast-to-nucleus retrograde signalling events mediated by PAP, modulation of hormonal signalling by sulfated compounds and PAP, control of physiological responses such as stomatal closure, and potential regulation of plant growth. In this review, we examine the contribution of the different components of plant secondary metabolism to oxidative stress homeostasis, and how this pathway is metabolically regulated. We further outline the key outstanding questions in the field that are necessary to understand how and why this 'specialized' metabolic pathway plays significant roles in plant oxidative stress tolerance.

The Arabidopsis SAL1-PAP Pathway: A Case Study for Integrating Chloroplast Retrograde, Light and Hormonal Signaling in Modulating Plant Growth and Development?

Plant growth and development are dependent on chloroplast development and function. Constitutive high level accumulation of a chloroplast stress signal, 3'-phosphoadenosine-5'-phosphate (PAP), confers drought tolerance to plants, but slow downs and alters plant growth and development. PAP, a by-product of sulfur metabolism, is maintained at very low levels by the SAL1 phosphatase during vegetative growth of Arabidopsis and accumulates in rosettes during drought and excess light. Eight independent forward genetic screens in Arabidopsis identified SAL1 as the regulator of multiple phenotypes related to stress responses, hormonal signaling and/or perception. In this perspective article, we collate all the sal1 phenotypes published in the past two decades, and distill the different pathways affected. Our meta-analysis of publicly available sal1 microarray data coupled to preliminary hormonal treatment and profiling results on sal1 indicate that homeostasis and responses to multiple hormones in sal1 are altered during rosette growth, suggesting a potential connection between SAL1-PAP stress retrograde pathway and hormonal signaling. We propose the SAL1-PAP pathway as a case study for integrating chloroplast retrograde signaling, light signaling and hormonal signaling in plant growth and morphogenesis.