Cell-specific Na ⁺ accumulation is linked to symplastic transport in tomato leaves

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 Na⁺-excluding domesticated tomato (cv. M82), Na⁺ was largely confined to the bundle sheath, whereas Na⁺-including wild relatives accumulated Na⁺ throughout the blade mesophyll. Consistent with these cell population-specific Na⁺ patterns, M82, but not S. pennellii , exhibited reduced symplastic transport and plasmodesmal permeability under salt stress. A genetic screen combined with transcriptome profiling implicated Plasmodesmata-Located Protein 1 ( PDLP1 ), a regulator of callose-mediated plasmodesmal 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.