Exogenous tryptophan increases soybean yield by enhancing sucrose-starch metabolism in leaves and seeds at the R6 stage under salt-alkali stress.

Background: Saline-alkali stress (SA) can significantly limit the growth and yield of soybean. The grain filling stage (R6) is a crucial growth period that determines the yield of soybeans and is also the most complex stage of sucrose-starch metabolism. Tryptophan (Trp) is an essential amino acid for protein synthesis and also an important signaling molecule in plants, plays an important role in maintaining osmotic regulation in plants and resisting adverse external environments. However, the mechanism of Trp regulation on sucrose-starch metabolism in R6-stage soybean leaves and seeds under SA is still unclear. This study investigated the effects of different Trp concentrations (100mg·L^-1、200mg·L^-1 and 300mg·L^-1) on sucrose-starch metabolism in soybean under SA (NaCl: Na₂SO₄: Na₂CO₃: NaHCO₃ = 1:9:1:9).

Results: The results showed that exogenous tryptophan alleviated the growth inhibition of R6 soybean under SA treatment. Exogenous Trp could enhance the photosynthetic capacity of soybean by increasing photosynthetic pigment content, net photosynthetic rate (Pn), intercellular carbon dioxide concentration (Ci), stomatal conductance (Gs), and the transpiration rate (Tr) of soybean leaves under SA. Exogenous Trp affected the balance of sucrose-starch metabolism in soybean leaves and seeds under SA by changing the activities of key enzymes in sucrose metabolism(SPS, SuSy, A-INV and N-INV) and expression levels of related genes. Meanwhile, exogenous Trp promoted the transport of sucrose equivalents from the source to the sink by increasing sucrose transport-related genes (GmSUC2, GmSWEET6, and GmSWEET15) under SA.

Conclusions: These results showed that exogenous Trp could improve the photosynthesis of leaves, regulate the metabolic balance of starch-sucrose, and increase the node number, pod number, and seed number, ultimately affecting high soybean yield at maturity and enhancing the saline-alkali tolerance of plants. These results can provide a new direction and theoretical basis for improving saline-alkali soil and tolerant crop breeding.