Soybean Plant Growth and Tre6P Metabolism Under Red/Far-Red and Blue Light

Soybean plants regulate morphology and carbohydrate metabolism for growth adaptation when leaves receive low red/far-red (R/Fr) and blue light (B) under shading. In this study, we assessed the effects of shoot morphology and trehalose-6-phosphate (Tre6P) metabolism on soybean growth under different combinations of R/Fr and B light. Soybean plants were treated with low and high light intensities of 200 and 400 mu mol m-2 s-1, respectively, in combination with white light (W) and three levels of R/Fr + B. After 30 days, we conducted morphological measurements and determined Tre6P metabolism in leaves and transcriptomics in stems. Soybean seedlings grown under R76/Fr48 + B76 had higher Tre6P content, lower trehalose 6-phosphate phosphatase (TPP) activity in leaves, and significantly longer second petioles than those grown under R100/Fr10 + B90. Tre6P was stored in leaves under R76/Fr48 + B76 treatment, thereby contributing to photosynthate allocation into the second petiole elongation. Compared to stems under R100/Fr10 + B90, those under R76/Fr48 + B76 showed enhanced trehalose biosynthesis, which reduced Tre6P content and limited stem elongation to support the stems. Stems under R50/Fr100 + B250 and R25/Fr50 + B125 had significantly elongated first and second petioles compared with those under W400 and W200, respectively. However, soybean plants under R25/Fr50 + B125 showed inhibited stem apex growth and had no third leaf. Soybean plants responded to R/Fr- and B-light exposure, thus regulating Tre6P metabolism to improve petiole elongation when R/Fr decreased from 10 to 2 under low light intensity. Under shade, increasing the B-light intensity (R/Fr = 0.5) inhibited stem apex growth and increased petiole length, to prompt leaves to capture light.