ITRAQ protein profile analysis of soybean stems reveals new aspects critical for lodging in intercropping systems

Abstract

Soybean is often intercropped with maize, sugarcane, and sorghum. Because of the shade coming from the latter, the soybean stem lodging is often a very serious problem in intercropping systems. The aim of this study is to characterize the possible mechanisms in the stem of shade-induced promotion of seedling soybean lodging in intercropping systems at the proteome level. We found that the soybean stem became slender and prone to lodging when it was planted with maize in an intercropping system. The inhibition of lignin biosynthesis and lack of photosynthate (soluble sugar) for the biosynthesis of the cell wall led to the lower internode breaking strength. A total of 317 proteins were found to be affected in the soybean stem in response to shade. Under the shade stress, the down-expression of key enzymes involving the phenylpropanoid metabolic pathway inhibited lignin biosynthesis. The up-regulation of expansin and XTHs protein expression relaxed the cell wall and promoted the elongation of internodes. Although the expression of the enzymes involving sucrose synthesis increased in the soybean stem, the lack of a carbon source prevented rapid stem growth. This metabolic deficit is the principal cause of the lower cellulose content in the stem of intercropped soybean, which leads to weakened stems and a propensity for lodging.