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

Abstract

Sucrose synthase (SUS) catalyzes the reversible conversion of sucrose and a nucleoside diphosphate into the corresponding nucleoside diphosphate-glucose and fructose. In Arabidopsis , a multigene family encodes six SUS (SUS1-6) isoforms. The involvement of SUS in the synthesis of UDP-glucose and ADP-glucose linked to Arabidopsis cellulose and starch biosynthesis, respectively, has been questioned by Barratt et al. [(2009) Proc Natl Acad Sci USA 106:13124–13129], who showed that ( i ) SUS activity in wild type (WT) leaves is too low to account for normal rate of starch accumulation in Arabidopsis , and ( ii ) different organs of the sus1/sus2/sus3/sus4 SUS mutant impaired in SUS activity accumulate WT levels of ADP-glucose, UDP-glucose, cellulose and starch. However, these authors assayed SUS activity under unfavorable pH conditions for the reaction. By using favorable pH conditions for assaying SUS activity, in this work we show that SUS activity in the cleavage direction is sufficient to support normal rate of starch accumulation in WT leaves. We also demonstrate that sus1/sus2/sus3/sus4 leaves display WT SUS5 and SUS6 expression levels, whereas leaves of the sus5/sus6 mutant display WT SUS1 – 4 expression levels. Furthermore, we show that SUS activity in leaves and stems of the sus1/sus2/sus3/sus4 and sus5/sus6 plants is ∼85% of that of WT leaves, which can support normal cellulose and starch biosynthesis. The overall data disprove Barratt et al. (2009) claims, and are consistent with the possible involvement of SUS in cellulose and starch biosynthesis in Arabidopsis .

Crosstalk between Two bZIP Signaling Pathways Orchestrates Salt-Induced Metabolic Reprogramming in Arabidopsis Roots

“…One of the most significantly DEGs in primary carbohydrate metabolism in the array data set is SUS4, encoding a SUCROSE SYNTHASE that participates in Suc breakdown, in particular under low oxygen stress (Baroja-Fernández et al, 2012;Pucciariello et al, 2012). Validation of these results by RT-qPCR studies revealed that SUS4 expression is partially impaired in the bzip1 bzip53 mutant ( Figure 3C).…”

“…In particular, regulation of the hypoxia marker gene SUS4, encoding a Sucdegrading enzyme (Baroja-Fernández et al, 2012), depends on bZIP1/ bZIP53, which might explain the increase in monosaccharides 3 to 6 h after the onset of stress. In the same time span, Suc levels highly increase in the bzip1 bzip53 mutant, presumably because Suc is not degraded to hexoses, as occurs in the wild type.…”

A sensitive method for confocal fluorescence microscopic visualization of starch granules in iodine stained samples

“…SuSy has also been suggested to be involved, at least in part, in the sucrosestarch conversion process in autotrophic organs. 15,16 Earlier studies 17,18 have shown that different organs of the sus1/sus2/sus3/sus4 quadruple Arabidopsis mutant accumulate WT starch content. We employed the method for confocal fluorescence microscopic observation described in this work to visualize the starch granules in the chloroplasts of sus1/sus2/sus3/sus4 mutant.…”

Recent progress toward understanding the role of starch biosynthetic enzymes in the cereal endosperm

“…The UDPGlc is converted to adenosine 5'-diphosphate glucose (ADPGlc) in the cytosol by the stepwise reactions of UDPGlc pyrophosphorylase and ADPGlc pyrophosphorylase (AGPase), while the conversion of fructose to ADPGlc involves hexokinase, phosphoglucoisomerase and phosphoglucomutase [11]. Although UDP is the preferred nucleoside diphosphate substrate for SuSy, ADP could also be an acceptor molecule of this sucrolytic enzyme to produce ADPGlc [12][13][14]. Cytosolic ADPGlc is then thought to be transported into the amyloplast by means of Brittle-1, a membrane protein located in the envelope membranes of amyloplasts [15], whose absence results in reduced starch content [16,17].…”

“…It starts with individual chains as the first level, which are formed by ADP-glucose through (1→4)-α-glycosidic linkages. Those chains in amylopectin have been further denoted as C chains having the reducing end, A chains (degree of polymerization, DP, [6][7][8][9][10][11][12] carrying no branches, B1 chains (DP [13][14][15][16][17][18][19][20][21][22][23][24] carrying A chains, B2 chains (DP DOI 10.1515/amylase-2017-0006 [25][26][27][28][29][30][31][32][33][34][35][36] carrying B1 chains, B3 chains (DP > 36) carrying B2 chains, and so on (for a recent review, see [2]). Although still occasionally one finds statements to the contrary, amylose is not solely linear, but contains a small but significant number of long-chain branches [3].…”