Re-interpreting the role of endo-beta-mannanases as mannan endotransglycosylase/hydrolases in the plant cell wall

Abstract

Background: Mannans are hemicellulosic polysaccharides in the plant primary cell wall with two major physiological roles: as storage polysaccharides that provide energy for the growing seedling; and as structural components of the hemicellulose-cellulose network with a similar function to xyloglucans. Endo-beta-mannanases are hydrolytic enzymes that cleave the mannan backbone. They are active during seed germination and during processes of growth or senescence. The recent discovery that endo-beta-mannanase LeMAN4a from ripe tomato fruit also has mannan transglycosylase activity requires the role of endo-beta-mannanases to be reinterpreted.

Aims: In this review, the role of endo-beta-mannanases as mannan endotransglycosylase/hydrolases (MTHs) in remodelling the plant cell wall is considered by analogy to the role of xyloglucan endotransglucosylase/hydrolases (XTHs). The current understanding of the reaction mechanism of these enzymes, their three-dimensional protein structure, their substrates and their genes are reported.

Future outlook: There are likely to be more endohydrolases within the plant cell wall that can carry out hydrolysis and transglycosylation reactions. The challenge will be to demonstrate that the transglycosylation activities shown in vitro also exist in vivo and to validate a role for transglycosylation reactions during the growth and development of the plant cell wall.

Fig. 1.

Phylogenetic comparison of endo-β-mannanase amino acid sequences. Trees were constructed with PHYLIP and visualized in TREEVIEW (v.1·6·6). Confidence values for groupings in trees were obtained using BOOTSTRAP N-J TREE using 1000 bootstrap trials. Sequences are from Arabidopsis thaliana [AtMAN1-7, AtMANP], Oryza sativa [OsMAN1-9], Coffea arabica [CoffeeMANA, CoffeeMANB], Lactuca sativa [LettuceMAN1], Aspergillus sp. [AspergillusMAN; Yuan et al., 2007], Solanum lycopersicum [LeMAN1-4a (Gong and Bewley, 2007), LeMAN6 (SGNU316912), LeMAN7 (SGNU316863), LeMAN8 (SGNU335864), LeMAN9 (SGNU318129) (www.sgn.cornell.edu)], Actinidia arguta [AaMAN1 (accession number FJ194533)], Daucus carota [CarrotMAN1 (AAN34823)], Malus × domestica [MdMAN1 (FJ194534), MdMAN2 (FJ194535), MdMAN3 (FJ194536)], Prunus persica [PpMAN1 (ABV32547), PpMAN2 (ABV32548)], Musa acuminata [MaMAN1 (ABF69949), MaMAN2 (Zhuang et al., 2006)], Vaccinium corymbosum [VcMAN1 (FJ194537)], Vitis vinifera [VvMAN1 (CAN70632), VvMAN2 (CAN71995), VvMAN3 (CAN77937), VvMAN4 (CAO21138), VvMAN5 (CAO22814), VvMAN6 (CAO44560), VvMAN7 (CAO44561), VvMAN8 (CAO44562), VvMAN9 (CAO44563), VvMAN10 (CAO44564), VvMAN11 (CAO44565), VvMAN12 (CAO48888), VvMAN13 (CAO48890), VvMAN14 (CAO611260) and VvMAN15 (CAO69748)]. Sequences shown in bold type are expressed in fruit. The boxed sequences appear to result from recent gene duplications.