Analysis of the sucrose synthase gene family in Arabidopsis

Abstract

). † These authors contributed equally to the work. SummaryThe properties and expression patterns of the six isoforms of sucrose synthase in Arabidopsis are described, and their functions are explored through analysis of T-DNA insertion mutants. The isoforms have generally similar kinetic properties. Although there is variation in sensitivity to substrate inhibition by fructose this is unlikely to be of major physiological significance. No two isoforms have the same spatial and temporal expression patterns. Some are highly expressed in specific locations, whereas others are more generally expressed. More than one isoform is expressed in all organs examined. Mutant plants lacking individual isoforms have no obvious growth phenotypes, and are not significantly different from wild-type plants in starch, sugar and cellulose content, seed weight or seed composition under the growth conditions employed. Double mutants lacking the pairs of similar isoforms sus2 and sus3, and sus5 and sus6, are also not significantly different in these respects from wild-type plants. These results are surprising in the light of the marked phenotypes observed when individual isoforms are eliminated in crop plants including pea, maize, potato and cotton. A sus1/sus4 double mutant grows normally in well-aerated conditions, but shows marked growth retardation and accumulation of sugars when roots are subjected to hypoxia. The sucrose synthase activity in roots of this mutant is 3% or less of wild-type activity. Thus under well-aerated conditions sucrose mobilization in the root can proceed almost entirely via invertases without obvious detriment to the plant, but under hypoxia there is a specific requirement for sucrose synthase activity.

Linking the Salt Transcriptome with Physiological Responses of a Salt-Resistant Populus Species as a Strategy to Identify Genes Important for Stress Acclimation

“…In contrast, its homolog AtSUS1 increases under stress (Ma et al, 2006). AtSUS1 also responds to elevated carbohydrate concentrations (Bieniawska et al, 2007). However, our metabolite profiling did not provide evidence for strong redirecting of carbohydrate metabolism during salt stress.…”

Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

“…However, it seems highly unlikely that SUS5 and SUS6 are outside the cell. Both have kinetic properties, including a neutral pH optimum, similar to other, intracellular SUS isoforms (Bieniawska et al, 2007). We suggest that GSL7, SUS5, and SUS6 may form part of a tight complex that spans the plasma membrane of the sieve element and links into the wall.…”

Sucrose synthase activity in the sus1/sus2/sus3/sus4 Arabidopsis mutant is sufficient to support normal cellulose and starch production

“…Studies of the predicted amino acid sequences and gene structure have shown that the Arabidopsis SUS family consists of six SUS genes displaying different developmental expression patterns (40,41). The involvement of SUS in starch and cellulose biosynthesis in Arabidopsis has been recently questioned by Bieniawska et al (42) and Barratt et al (43), who showed that (i) leaves, siliques, stems, and roots of the sus1/sus2/sus3/sus4 quadruple SUS mutant impaired in SUS activity accumulate WT content of ADPG, UDPG, cellulose and starch, and (ii) SUS activity in WT leaves is too low to account for the rate of starch accumulation in illuminated leaves. However, these authors assayed SUS activity in the sucrose synthetic direction under conditions that are far from optimal, which inadvertently lead to their conclusion.…”

“…Several assay conditions that departed from optimal were: First, the SUS reaction assay mixture used by the authors contained 6 mM fructose, a concentration comparable to or lower than the reported K m values for fructose in SUS from many species (10,11,44). Second, the pH of the SUS reaction assay mixture used by Bieniawska et al (42) and Barratt et al (43) was 9.4, which is far too basic with respect to both cytosolic pH and to previously reported optimum pH values for SUS activity in the synthetic direction (10,(45)(46)(47)(48). Third, SUS assay reactions were stopped after 20 min of incubation, a condition that does not allow the attainment of reliable data under SUS initial velocity conditions.…”