Differential expression of soluble acid invertase genes in the shoots of high-sucrose and low-sucrose species of Saccharum and their hybrids

Abstract

The hydrolytic activity of soluble acid invertase (SAI) is strongly correlated to sucrose accumulation in sugarcane (Saccharum spp.). Plants exhibiting SAI activity above a low threshold level do not accumulate high concentrations of sucrose. The present work investigates the basis for the difference in SAI activity observed between high- and low-sucrose-accumulating sugarcane lines. SAI-encoding cDNAs were isolated from two high- and one low-sucrose lines. All of these cDNAs were highly similar, with deduced proteins at least 98% identical. Expression of SAI in the stem of sugarcane was developmentally regulated, with relatively larger pools of SAI protein and mRNA in the apex and young internodes, which declined rapidly in the maturing internodes where sucroseaccumulation occurs. This developmental pattern, while qualitatively similar, was quantitatively quite different between low- and high-sucrose lines. SAI protein and mRNA pools started substantially higher, declined later, and stabilized at a significantly higher level in a low-sucrose line than in a high-sucrose line. These data indicate that differences in SAI activity between high- and low-sucrose sugarcane lines are due, at least in part, to differences inthe level of expression of essentially identical SAI genes.