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

Su Y Phua,Eiji Nambara,Gonzalo M Estavillo,Dawei Yan,Kai X Chan,Barry J Pogson

doi:10.3389/fpls.2018.01171

Abstract

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.

Highlights

Plants are autotrophs that capture light energy from the sun via photosynthesis
When optimizing and finetuning resource allocation for photosynthesis, plants have to account for both the efficiency of light-harvesting through to electron transport chain under limiting conditions such as low light intensities, and the risk of photodamage by reactive oxygen species (ROS) when conditions flip to the other extremes of excessive light exposure and drought
SAL1-phosphoadenosine-5 -phosphate (PAP): Stress, Hormones and Growth chloroplast can act as an environmental sensor that subsequently triggers downstream mechanisms for adjustment of plant function and growth according to the environment

Summary

Introduction

Plants are autotrophs that capture light energy from the sun via photosynthesis. The quantity (intensity) and quality (wavelength) of light can affect the assembly and efficiency of photosynthetic machineries in the chloroplasts (Dietz, 2015), which in turn will determine the amount of carbon available for plants to convert into energy for growth and survival. Altered phenotypes related to stress (Lee et al, 1999; Xiong et al, 2001, 2004; Wilson et al, 2009), RNA metabolism (Gy et al, 2007), nutrient uptake (Hirsch et al, 2011), leaf morphology (Robles et al, 2010) or plant hormones (Rodríguez et al, 2010; Zhang et al, 2011) have been associated with mutations in SAL1, known by a diversity of gene names arising from the different alleles characterized (Supplementary Table S1) – hos2 (high expression of osmotically responsive genes), fry1 (fiery 1), alx8 (altered expression of APX2 8), ron1 (rotunda 1), fou8 (fatty acid oxygenation up-regulated), and supo1 (suppressor of PIN1 overexpression phenotype).

Results

Conclusion