Tissue-specific accumulation of Na+ in leaves of two tomato species is linked to differences in symplastic transport

Abstract Soil salinization is a growing global threat that limits crop productivity. To cope with sodium (Na+) stress, plants have evolved tolerance mechanisms, including excluding Na+ from shoot tissues and tolerating elevated Na+ within shoots through tissue- and cellular-level mechanisms. Most current knowledge of Na+ accumulation comes from organ- or whole-plant measurements that lack the spatial resolution needed to resolve cellular tolerance mechanisms. Here, we used histological approaches to map leaf Na+ distribution in tomato (Solanum) species with contrasting salt-tolerance strategies. In the moderately salt-tolerant, Na+-excluding domesticated tomato (cv. M82), Na+ was largely confined to the bundle sheath, whereas in salt-tolerant, Na+-including wild relatives, Na+ accumulated throughout the blade mesophyll. Consistent with these cell population-specific Na+ patterns, M82, but not S. pennellii, exhibited reduced symplastic transport and plasmodesma permeability under salt stress. A genetic screen combined with transcriptome profiling implicated Plasmodesmata-Located Protein 1 (PDLP1), a regulator of callose-mediated plasmodesma closure, in establishing symplastic domains in M82 that restrict Na+ movement into the mesophyll. Moreover, PDLP1 expression negatively correlated with mesophyll Na+ levels across wild and domesticated tomatoes. Collectively, these results link cellular Na+ enrichment patterns to symplastic connectivity and suggest that PDLP1-mediated regulation of plasmodesmata contributes to leaf-level salt-tolerance strategies.