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 of starch biosynthesis, presumably from sucrose stored in the vacuoles. Very little is known about the genes and the intracellular controls that regulate the sucrose → starch pathway in such microspores. We show here RNA profiles in sorghum microspores during the transition from vacuolated microspore 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 sterile lines that retain fully turgid immature pollen until the starch-filling stage, but remain starch-deficient, and ultimately form inviable pollen. Using maize cDNA clones, both temporal and genotypic differences were observed in the expression profiles of four metabolic genes (soluble or vacuolar invertase, Ivr2; sucrose synthase, Sus1; phosphoglucomutase, Pgm; and a subunit of ADPG pyrophosphorylase, Bt2), two similar metabolic regulatory genes (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 both a qualitative and quantitative nature were observed in all three proteins in both male-fertile and male-sterile lines. We suggest that these changes in gene expression may result from increased and decreased sink strengths and associated changes in carbohydrate metabolism in starch-filled (fertile) and starch-deficient (sterile) pollen, respectively.