Phosphatidylserine synthase (PSS)-mediated phosphatidylserine (PS) synthesis is crucial for plant development. However, little is known about the contribution of PSS to Na⁺ homeostasis regulation and salt tolerance in plants. Here, we cloned the IbPSS1 gene, which encodes an ortholog of Arabidopsis AtPSS1, from sweet potato (Ipomoea batatas (L.) Lam.). The transient expression of IbPSS1 in Nicotiana benthamiana leaves increased PS abundance. We then established an efficient Agrobacterium rhizogenes-mediated in vivo root transgenic system for sweet potato. Overexpression of IbPSS1 through this system markedly decreased cellular Na⁺ accumulation in salinized transgenic roots (TRs) compared with adventitious roots. The overexpression of IbPSS1 enhanced salt-induced Na⁺/H⁺ antiport activity and increased plasma membrane (PM) Ca²⁺-permeable channel sensitivity to NaCl and H₂O₂ in the TRs. We confirmed the important role of IbPSS1 in improving salt tolerance in transgenic sweet potato lines obtained from an Agrobacterium tumefaciens-mediated transformation system. Similarly, compared with the wild-type (WT) plants, the transgenic lines presented decreased Na⁺ accumulation, enhanced Na⁺ exclusion, and increased PM Ca²⁺-permeable channel sensitivity to NaCl and H₂O₂ in the roots. Exogenous application of lysophosphatidylserine triggered similar shifts in Na⁺ accumulation and Na⁺ and Ca²⁺ fluxes in the salinized roots of WT. Overall, this study provides an efficient and reliable transgenic method for functional genomic studies of sweet potato. Our results revealed that IbPSS1 contributes to the salt tolerance of sweet potato by enabling Na⁺ homeostasis and Na⁺ exclusion in the roots, and the latter process is possibly controlled by PS reinforcing Ca²⁺ signaling in the roots.

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