Salinity is a severe abiotic stress limiting agricultural yield and productivity. Plants have evolved various strategies to cope with salt stress. Chloroplasts are important photosynthesis organelles, which are sensitive to salinity. An understanding of molecular mechanisms in chloroplast tolerance to salinity is of great importance for genetic modification and plant breeding. Previous studies have characterized more than 53 salt-responsive genes encoding important chloroplast-localized proteins, which imply multiple vital pathways in chloroplasts in response to salt stress, such as thylakoid membrane organization, the modulation of photosystem II (PS II) activity, carbon dioxide (CO₂) assimilation, photorespiration, reactive oxygen species (ROS) scavenging, osmotic and ion homeostasis, abscisic acid (ABA) biosynthesis and signaling, and gene expression regulation, as well as protein synthesis and turnover. This review presents an overview of salt response in chloroplasts revealed by gene characterization efforts.

When plants are infected by pathogens two distinct responses can occur, the early being a local response in the infected area, and later a systemic response in non-infected tissues. Closure of stomata has recently been found to be a local response to bacterial pathogens. Stomata closure is linked to both salicylic acid (SA), an essential hormone in local responses and systemic acquired resistance (SAR), and absisic acid (ABA) a key regulator of drought and other abiotic stresses. SAR reduces the effects of later infections. In this review we discuss recent research elucidating the role of guard cells in local and systemic immune responses, guard cell interactions with abiotic and hormone signals, as well as putative functions and interactions between long-distance SAR signals.

Systemic Acquired Resistance (SAR) improves immunity of plant systemic tissue after local exposure to a pathogen. Guard cells that form stomatal pores on leaf surfaces recognize bacterial pathogens via pattern recognition receptors, such as Flagellin Sensitive 2 (FLS2). However, how SAR affects stomatal immunity is not known. In this study, we aim to reveal molecular mechanisms underlying the guard cell response to SAR using multi-omics of proteins, metabolites and lipids. Arabidopsis plants previously exposed to pathogenic bacteria Pseudomonas syringae pv. tomato DC3000 (Pst) exhibit an altered stomatal response compared to control plants when they are later exposed to the bacteria. Reduced stomatal apertures of SAR primed plants lead to decreased number of bacteria in leaves. Multi-omics has revealed molecular components of SAR response specific to guard cells functions, including potential roles of reactive oxygen species (ROS) and fatty acid signaling. Our results show an increase in palmitic acid and its derivative in the primed guard cells. Palmitic acid may play a role as an activator of FLS2, which initiates stomatal immune response. Improved understanding of how SAR signals affect stomatal immunity can aid biotechnology and marker-based breeding of crops for enhanced disease resistance.

After localized invasion by bacterial pathogens, systemic acquired resistance (SAR) is induced in uninfected plant tissues, resulting in enhanced defense against a broad range of pathogens. Although SAR requires mobilization of signaling molecules via the plant vasculature, the specific molecular mechanisms remain elusive. The lipid transfer protein-defective in induced resistance 1-1 (DIR1-1) was identified in Arabidopsis thaliana by screening for mutants that were defective in SAR. Here we demonstrate that stomatal response to pathogens is altered in systemic leaves by SAR, and this guard cell SAR defense requires DIR1. Using a multi-omics approach, we have determined potential SAR signaling mechanisms specific for guard cells in systemic leaves by profiling metabolite, lipid, and protein differences between guard cells in wild type and dir1-1 mutant during SAR. We identified two 18C fatty acids and two 16C wax esters as putative SAR-related molecules dependent on DIR1. Proteins and metabolites related to amino acid biosynthesis and response to stimulus were also changed in guard cells of dir1-1 compared to wild type. Identification of guard cell-specific SAR-related molecules may lead to new avenues of genetic modification/molecular breeding for disease resistant plants. One-sentence summary: DIR1 affects many biological processes in stomatal guard cells during systemic acquired resistance (SAR), as revealed by multi-omics, and it may function through transporting two 18C fatty acids during SAR.