BETA-AMYLASE9 is a plastidial nonenzymatic regulator of leaf starch degradation.

Laure C David,Melanie R Abt,Samuel C Zeeman,Marie-Aude Tschopp,Katarzyna Sanchez,Michaela Fischer-Stettler,Eduard Bruderer,Erik M Solhaug,Sang-Kyu Lee

doi:10.1093/plphys/kiab468

Abstract

β-Amylases (BAMs) are key enzymes of transitory starch degradation in chloroplasts, a process that buffers the availability of photosynthetically fixed carbon over the diel cycle to maintain energy levels and plant growth at night. However, during vascular plant evolution, the BAM gene family diversified, giving rise to isoforms with different compartmentation and biological activities. Here, we characterized BETA-AMYLASE 9 (BAM9) of Arabidopsis (Arabidopsis thaliana). Among the BAMs, BAM9 is most closely related to BAM4 but is more widely conserved in plants. BAM9 and BAM4 share features including their plastidial localization and lack of measurable α-1,4-glucan hydrolyzing capacity. BAM4 is a regulator of starch degradation, and bam4 mutants display a starch-excess phenotype. Although bam9 single mutants resemble the wild-type (WT), genetic experiments reveal that the loss of BAM9 markedly enhances the starch-excess phenotypes of mutants already impaired in starch degradation. Thus, BAM9 also regulates starch breakdown, but in a different way. Interestingly, BAM9 gene expression is responsive to several environmental changes, while that of BAM4 is not. Furthermore, overexpression of BAM9 in the WT reduced leaf starch content, but overexpression in bam4 failed to complement fully that mutant’s starch-excess phenotype, suggesting that BAM9 and BAM4 are not redundant. We propose that BAM9 activates starch degradation, helping to manage carbohydrate availability in response to fluctuations in environmental conditions. As such, BAM9 represents an interesting gene target to explore in crop species.

Highlights

Starch is the major storage carbohydrate in plants and is composed of a-1,4- and a-1,6-linked glucose polymers
Bam9 single mutants resemble the wild-type (WT), genetic experiments reveal that the loss of BETA-AMYLASE 9 (BAM9) markedly enhances the starch-excess phenotypes of mutants already impaired in starch degradation
BAM9 is a plastidial protein expressed in green tissues We studied the expression pattern and subcellular localization of BAM9 using a transcriptional fusion construct

Summary

Introduction

Starch is the major storage carbohydrate in plants and is composed of a-1,4- and a-1,6-linked glucose polymers. Starch is synthetized in chloroplasts during the day and degraded at night, when energy from photosynthesis is unavailable. This pattern of diel turnover is important for optimizing plant growth (Stitt and Zeeman, 2012) and is closely linked with the endogenous circadian clock (Graf et al, 2010; Scialdone et al, 2013; Seki et al, 2017). There are appreciable changes in allocation to, or retrieval from the starch pool during the day (Kolling et al, 2015; Fernandez et al, 2017), in response to environmental fluctuations (Gibon et al, 2004), and in response to various stresses (Kaplan and Guy, 2004; Thalmann et al, 2016; Dong et al, 2018). The regulation of starch turnover to ensure availability of carbohydrates seems to be mediated by changes in both the starch synthesis and degradation rates (Mugford et al, 2014; Thalmann et al, 2016; Fernandez et al, 2017)

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