Thioredoxin-regulated β-amylase (BAM1) triggers diurnal starch degradation in guard cells, and in mesophyll cells under osmotic stress

Abstract

BAM1 is a plastid-targeted β-amylase of Arabidopsis thaliana specifically activated by reducing conditions. Among eight different chloroplast thioredoxin isoforms, thioredoxin f1 was the most efficient redox mediator, followed by thioredoxins m1, m2, y1, y2, and m4. Plastid-localized NADPH-thioredoxin reductase (NTRC) was also able partially to restore the activity of oxidized BAM1. Promoter activity of BAM1 was studied by reporter gene expression (GUS and YFP) in Arabidopsis transgenic plants. In young (non-flowering) plants, BAM1 was expressed both in leaves and roots, but expression in leaves was mainly restricted to guard cells. Compared with wild-type plants, bam1 knockout mutants were characterized by having more starch in illuminated guard cells and reduced stomata opening, suggesting that thioredoxin-regulated BAM1 plays a role in diurnal starch degradation which sustains stomata opening. Besides guard cells, BAM1 appears in mesophyll cells of young plants as a result of a strongly induced gene expression under osmotic stress, which is paralleled by an increase in total β-amylase activity together with its redox-sensitive fraction. Osmotic stress impairs the rate of diurnal starch accumulation in leaves of wild-type plants, but has no effect on starch accumulation in bam1 mutants. It is proposed that thioredoxin-regulated BAM1 activates a starch degradation pathway in illuminated mesophyll cells upon osmotic stress, similar to the diurnal pathway of starch degradation in guard cells that is also dependent on thioredoxin-regulated BAM1.

Regulation of Leaf Starch Degradation by Abscisic Acid Is Important for Osmotic Stress Tolerance in Plants

“…Mutant studies substantiate the hypothesis that BAM1 induction is critically important in the osmotic stress response. As first reported by Valerio et al (2011), we also found that bam1 mutants had reduced starch degradation in response to was used for confirmation. BAM1 was detected using polyclonal antibody raised against recombinant BAM1.…”

“…Under standard growth conditions, BAM1 is preferentially and highly expressed in guard cells, whereas BAM3 is considerably less abundant (Horrer et al, 2016). Consistent with the gene expression pattern, bam1 mutants have elevated guard cell starch levels compared with the wild type, whereas bam3 accumulates starch in this cell type similarly to the wild type (Horrer et al, 2016;Valerio et al, 2011). Altogether, these findings indicate that BAM1 and BAM3 are active under different conditions and in a cell typespecific manner.…”

“…In response to acute temperature shock, plants mobilize starch at time when biosynthesis would be expected (e.g., in the middle of the light period), resulting in the accumulation of maltose, the major starch catabolite, and of its deriving sugars (Usadel et al, 2008a;Purdy et al, 2013;Guy, 2005, 2004;Sitnicka and Orzechowski, 2014;Yano et al, 2005;Kaplan et al, 2007). Similar rearrangements of starch metabolism are observed when plants are subject to short periods of oxidative or osmotic stress (Scarpeci and Valle, 2008;Zanella et al, 2016;Valerio et al, 2011;Geigenberger et al, 1997). It is proposed that soluble sugars and other charged metabolites, such as proline or glycine, may function as osmoprotectants during stress responses.…”

“…BAM1 is highly expressed in guard cells and, in synergy with AMY3, degrades starch in these cells for light-induced stomatal opening (Horrer et al, 2016;Blatt, 2016). Under osmotic stress, BAM1 is activated in the leaves and contributes to diurnal starch degradation for sugars and proline biosynthesis (Valerio et al, 2011;Zanella et al, 2016). Thus, it appears that BAM1 is important for stress-induced leaf starch degradation.…”

Nuclear Accumulation of Cytosolic Glyceraldehyde-3-Phosphate Dehydrogenase in Cadmium-Stressed Arabidopsis Roots

“…The amplicon was digested with EcoRI and ligated into a modified pGreen0029 binary vector upstream of the GUS coding sequence fused with the nopaline synthase terminator (Valerio et al, 2011). The obtained vector was named pGAPC1-GUS.…”