Phytohormone-mediated transformation of sugars to starch in relation to the activities of amylases, sucrose-metabolising enzymes in sorghum grain

Indole-acetic acid (IAA) and abscisic acid (ABA) were fed through complete liquid medium (containing 2, 4, 8% sucrose) to detached ear heads of sorghum. The effect of these phytohormones on interconvertion of sugars and their transformation to starch in relation to the activities of alpha-, beta-amylases, sucrose-synthase (synthesis), sucrose-phosphate synthase and soluble invertases was studied in the grain. This effect on the uptake of (U-14C) sucrose by detached ear heads and incorporation of 14C into free sugars and starch of grain and into free sugars of inflorescence parts was also studied. At concentrations of up to 4% sucrose in the culture medium, IAA increased the content of total free sugars in the grain. However, accumulation of starch and activities of alpha- and beta-amylases increased when IAA was present even beyond the 4% sucrose concentration in the culture medium. At all sucrose concentrations, the effects of ABA and IAA were opposite. With 4% sucrose, both phytohormones caused maximum accumulation of starch in the grain. ABA enhanced the relative proportion of sucrose in the sugar pool with a concomitant reduction in the activities of soluble acid (pH 4.8) and neutral (pH 7.5) invertases. In contrast, IAA decreased the sucrose proportion of grain sugars with a simultaneous elevation and reduction in the activities of invertases and sucrose-phosphate synthase, respectively. Irrespective of sucrose concentration in the culture medium, the activity of sucrose synthase (synthesis) was enhanced with IAA as well as ABA at their 10 micromolar concentration. IAA also enhanced incorporation of 14C from (U-14C) sucrose into the EtOH extract (principally constituted by free sugars) and starch of the grain, but ABA caused the reverse effect. Based on the results, it is suggested that IAA and ABA have contrasting effects on the transformation of sucrose to starch in sorghum grain where its capacity to synthesise starch is modulated positively by IAA and negatively by ABA.