Reply to Smith et al.: No evidence to challenge the current paradigm on starch and cellulose biosynthesis involving sucrose synthase activity

Abstract

The generally accepted view that sucrose synthase (SUS) activity is involved in the production of cellulose and starch biosynthesis (1 and references contained therein) has been challenged (2). Using a method for assaying SUS activity in the synthetic direction (conversion of UDP-glucose into sucrose), Barratt et al. (2) reported an almost total absence of SUS activity in sus1/sus2/sus3/sus4 Arabidopsis mutant stems and the presence of WT levels of starch and cellulose content in this organ. Bieniawska et al. (3) reported that SUS activities in WT Arabidopsis leaves were too low to account for the starch accumulation rate occurring during illumination. Barratt et al. (2) thus concluded that SUS is not required for starch and cellulose biosynthesis in Arabidopsis. However, in our study (1), we used a method for assaying SUS activity in the sucrose cleavage direction (UDP-glucose and ADP-glucose synthesis) and found that SUS activity in WT Arabidopsis leaves was approximately 10-fold higher than that reported by Bieniawska et al. (3), greatly exceeding the minimum needed to support the normal rate of starch accumulation during illumination.