Transgenically enhanced sorbitol synthesis facilitates phloem‐boron mobility in rice

Abstract

The tolerance of crops to a shortage of boron (B) in the soil varies markedly among species. This variation in tolerance is due, in part, to a species ability to form phloem mobile B‐sugar‐alcohol complexes (such as B‐mannitol or B‐sorbitol) which enhance the remobilization of B within the plant. Species lacking the capacity to form B‐sugar alcohol complexes are intolerant of even short‐term deficits in soil B supply. Here we have genetically engineered rice (Oryza sativa L.) cultivar Taipei 309 (TP309) with the sorbitol‐6‐phosphate dehydrogenase (S6PDH) gene, a key enzyme for sorbitol production, and determined the effect of this transformation on the physiology of B remobilization. Sorbitol was detected in the S6PDH transgenic plants as well as in vector‐transformed plants and wild‐type (TP 309) plants, although the concentration of sorbitol in the S6PDH transgenic plants was significantly enhanced. Remobilization of B from mature leaves to flag leaves correlated with increased levels of sorbitol. The presence of sorbitol and detection of B remobilization in the wild‐type and vector‐transformed plants suggests that rice utilizes an unknown pathway for sorbitol synthesis and may partly explain the relative insensitivity of rice to B deficits when compared to other graminaceous crops.