Role of ABA Signaling in Regulation of Stem Sugar Metabolism and Transport under Post- Flowering Drought Stress in Sweet Sorghum

Sugar remobilization from vegetative to reproductive tissues is an important process that determines grain yield in crops. Sweet sorghum stems store sugar and introgression of Stay green1 (Stg 1) locus from the grain sorghum genotype B35 into the sweet sorghum genotype S35 was previously shown to bring about a 2-fold higher stem sugar accumulation in the near-isogenic line (NIL) S35SG06040. We hypothesized that remobilization of stem sugar augments grain yield on exposure to drought stress and that the phytohormone abscisic acid (ABA) has a role to play in this process. ABA levels were three times higher in the NIL as compared to those in S35 on drought stress exposure. Remobilization of stem sugars in the NIL was evident by the observed decrease in reducing sugar content in the stem but not in the peduncle in response to prolonged drought stress exposure. Drought-induced expression of some ABA response factors (ABFs) as well as invertase and sucrose transporter genes was seen to be higher in the NIL as compared to S35. An over-representation of ABA-responsive elements (ABREs) and sugar signaling motifs in the differentially expressed genes indicated the involvement of ABA and sugar signaling in regulation of their expression. Two ABF genes located on the Stg1 locus showed single nucleotide polymorphism, which possibly accounted for their differential regulation in S35 and the NIL. The results suggest that ABA signaling plays an important role in post-flowering drought-induced remobilization of sugars to the reproductive sinks.