Multi-layered apoplastic barrier underlying the ability of Na+ exclusion in Vigna marina

Soil salinization and ground water depletion are increasingly constraining crop production. Identifying useful mechanisms of salt tolerance is an important step toward the development of salt-tolerant crops. Of particular interest are mechanisms that are present in crop wild relatives, as they may have greater stress tolerance than crop species. The coastal species Vigna marina is one of the promising plant resources for salt tolerance. Vigna luteola is another wild species with diverse habitats, including seaside and riverbank, hereafter V. luteola-beach and V. luteola-river, respectively. By comparative transcriptome and histological analyses, this study elucidated one important aspect of how V. marina achieves an extraordinary ability to suppress Na+ uptake. Under salt stress, V. marina specifically upregulated genes involved in Casparian strip formation and developed a multi-layered lignified apoplastic barrier around endodermis, whereas V. luteola-beach formed typical, band-like Casparian strips and V. luteola-river formed only spot-like Casparian strips. As such, the ability of developing apoplastic barrier strongly correlated with those of suppressing Na+ uptake. The disruption of lignified barrier led to a dramatic increase of Na+ allocation to the shoot in V. marina, which was manifested in leaf etiolation and burning. Interestingly, despite the presence of reinforced apoplastic barrier, V. marina maintained the transport of essential ions including K+, Mg2+, and Ca2+. This study shows that the multi-layered Casparian strip-like structure plays an important role in salt tolerance.