Emerging Roles of β-Glucanases in Plant Development and Adaptative Responses

Abstract

Plant β-glucanases are enzymes involved in the synthesis, remodelling and turnover of cell wall components during multiple physiological processes. Based on the type of the glycoside bond they cleave, plant β-glucanases have been grouped into three categories: (i) β-1,4-glucanases degrade cellulose and other polysaccharides containing 1,4-glycosidic bonds to remodel and disassemble the wall during cell growth. (ii) β-1,3-glucanases are responsible for the mobilization of callose, governing the symplastic trafficking through plasmodesmata. (iii) β-1,3-1,4-glucanases degrade mixed linkage glucan, a transient wall polysaccharide found in cereals, which is broken down to obtain energy during rapid seedling growth. In addition to their roles in the turnover of self-glucan structures, plant β-glucanases are crucial in regulating the outcome in symbiotic and hostile plant-microbe interactions by degrading non-self glucan structures. Plants use these enzymes to hydrolyse β-glucans found in the walls of microbes, not only by contributing to a local antimicrobial defence barrier, but also by generating signalling glucans triggering the activation of global responses. As a counterpart, microbes developed strategies to hijack plant β-glucanases to their advantage to successfully colonize plant tissues. This review outlines our current understanding on plant β-glucanases, with a particular focus on the latest advances on their roles in adaptative responses.

Keywords: cell wall polysaccharides; environmental stress; plant development; β-glucanases; β-glucans.

Figure 1

Activity of the three types of β-glucanases found in plants and their main physiological substrates. (1,3)- and (1,4)-β-glycosidic linkages are depicted as 3 and 4, respectively. Arrows indicate the glycosidic linkage hydrolysed in each case.

Figure 2

Topology models of plant β-glucanases. Protein domains and regions are abbreviated as follows: NTS—N-terminal sequence; GH—core glycosyl hydrolase family domain; CBM—carbohydrate binding module; GPI—glycosylphosphatidylinisotol-anchor attachment; CTS—hydrophobic C-terminal sequence; CNT—cytosolic N-terminal extension; SP—secretory signal peptide; TMD—transmembrane domain. Structure models represented in this figure were built by using the AlphaFold Protein Structure Database [40,41] with the following Uniprot accessions: GH9A 1,4-β-glucanase (AtGH9A1; Q38890), GH9B 1,4-β-glucanase (AtGH9B16; Q9SVJ4) and GH9C 1,4-β-glucanase (AtGH9C1; Q9M995), group I 1,3-β-glucanase (At5g64790; Q9LV98), group II 1,3-β-glucanase (At2g05790; F4IHD3), group III 1,3-β-glucanase (At2g39640; O48812), group IV 1,3-β-glucanase (At1g77780; Q9CA16), group V 1,3-β-glucanase (At5g20340; O49353), 1,3;1,4-β-glucanase (ZmMLGH1; B6T391).