Gene-expression analysis of sucrose-starch metabolism during pollen maturation in cytoplasmic male-sterile and fertile lines of sorghum

During pollen maturation, single-celled microspores undergo a rapid phase o f starch biosynthesis, presumably from sucrose stored in the vacuoles. Very little is known about the genes and the intracellular controls that regula te the sucrose --> starch pathway in such microspores. We show here RNA pro files in sorghum microspores during the transition from vacuolated microspo re to immature pollen at two specific stages of pollen development: early, showing little or no detectable starch, and late, when pollen are active in starch-filling. We have also examined two near-isogenic cytoplasmic male s terile lines that retain fully turgid immature pollen until the starch-fill ing stage, but remain starch-deficient, and ultimately form inviable pollen . Using maize cDNA clones, both temporal and genotypic differences were obs erved in the expression profiles of four metabolic genes (soluble or vacuol ar invertase, Ivr2; sucrose synthase, Sus1; phosphoglucomutase, Pgm; and a subunit of ADPG pyrophosphorylase, Bt2), two similar metabolic regulatory g enes (Grf1 and Grf2, encoding 14-3-3 proteins) and a transcription factor, ZmMADS1. Western blot analyses were conducted to visualize sucrose synthase , SS2; AGPase, BT2 and the GRF (14-3-3) proteins. Temporal differences of b oth a qualitative and quantitative nature were observed in all three protei ns in both male-fertile and male-sterile lines. We suggest that these chang es in gene expression may result from increased and decreased sink strength s and associated changes in carbohydrate metabolism in starch-filled (ferti le) and starch-deficient (sterile) pollen, respectively.