The making of suberin

Abstract

Outer protective barriers of animals use a variety of bio-polymers, based on either proteins (e.g. collagens), or modified sugars (e.g. chitin). Plants, however, have come up with a particular solution, based on the polymerisation of lipid-like precursors, giving rise to cutin and suberin. Suberin is a structural lipophilic polyester of fatty acids, glycerol and some aromatics found in cell walls of phellem, endodermis, exodermis, wound tissues, abscission zones, bundle sheath and other tissues. It deposits as a hydrophobic layer between the (ligno)cellulosic primary cell wall and plasma membrane. Suberin is highly protective against biotic and abiotic stresses, shows great developmental plasticity and its chemically recalcitrant nature might assist the sequestration of atmospheric carbon by plants. The aim of this review is to integrate the rapidly accelerating genetic and cell biological discoveries of recent years with the important chemical and structural contributions obtained from very diverse organisms and tissue layers. We critically discuss the order and localisation of the enzymatic machinery synthesising the presumed substrates for export and apoplastic polymerisation. We attempt to explain observed suberin linkages by diverse enzyme activities and discuss the spatiotemporal relationship of suberin with lignin and ferulates, necessary to produce a functional suberised cell wall.

Keywords: apoplastic barrier; cell wall; fatty acyl metabolism; ferulic acid; lignin; lipid intracellular transport; suberin; suberin lamellae.

Fig. 1

Biosynthesis of suberin monomeric precursors. Different background colours indicate compartments in which reactions are thought to take place. Reaction steps indicated in red, enzyme classes catalysing these reactions are in blue. The putative end‐products of biosynthesis, termed suberin monomeric precursors are highlighted in yellow. These are defined as the products that are subject to export into the cell wall and use as substrates for the polymerising enzymes. Although we know that all the above‐mentioned reactions are necessary to accumulate suberin, the order of the reactions that produce these suberin monomeric precursors is not however certain in all cases. Biochemical and genetic data support the idea that ω‐hydroxylation by CYP86s would precede the acyl transfer to glycerol at sn‐2 position by GPAT5 (Yang et al., 2010, 2012), but the integration of feruloylation reaction and the acyl‐CoA activation by long‐chain acyl‐CoA synthetases (LACS) is still unclear. (n) indicates a variable number of CH₂ groups. Possible mid‐chain modifications of fatty acyl‐derivatives, such as mid‐chain hydroxylation, double bonds, etc., have been omitted for simplicity. Fatty acids and primary alcohols, as well as their feruloylated and glycerylated forms, respectively, are also identified as suberin‐associated wax compounds. ω‐Hydroxyacids and α,ω‐diacids are found exclusively in suberin depolymerisation products. G3P, glycerol 3‐phospate. Specific enzymes are identified in Table 1.

Fig. 2

Location and transport of suberin precursors in the cell. Two scenarios for transport of suberin monomeric precursors: ATP‐binding cassette (ABC) transporter of the G‐clade (ABCG) transporter‐mediated (dark grey, rounded rectangles) or vesicle transport‐mediated are shown. Precursors (yellow dots) are indicated as membrane resident molecules here, but they may be part of the lumen. The precise vesicle transport pathways of monomeric precursors from the endoplasmic reticulum (ER) to the apoplast are not known, possible pathways are indicated with question marks. Blue rounded rectangles, suberin biosynthetic enzymes described in Fig. 1. Orange circles, unknown, potential carrier proteins for suberin monomeric precursors. Yellow/orange border, suberin lamellae. EV, extracellular vesicles; MVB, multivesicular bodies; PM, plasma membrane.

Fig. 3

Overview of possible transesterification reactions during suberisation. (a–f) Proposed suberin monomeric substrates and GELP‐mediated transesterification reactions to produce ester‐linked dimers/trimers detected in suberin partial depolymerisation data. Purple shade highlights the ester linkage of molecules found upon partial depolymerisation of suberin. Red shade indicates ester bonds of potential GELP enzyme substrate, whose breakage and transfer to an alcohol acceptor group (blue shaded), could explain the observed purple linkage types. GELPv‐z is meant to indicate that each of these reactions might be mediated by a dedicated GELP. (g) Speculative ‘double transesterification’ model in which the second ester linkage is shaded in green and the newly formed linkage in the product is shaded in yellow.

Fig. 4

Possible scenario for the spatiotemporal deposition and arrangement of suberin, lignin and ferulates within the suberised cell wall. Schematic, depicting the progression of cell‐wall modifications in a nonspecified, suberising cell type (e.g. phellem). (a) Cell walls of undifferentiated cells, with unmodified primary walls and middle lamella (dark grey). (b) Lignification (red) of primary cell wall precedes suberisation and starts in the middle lamella. (c) Lignification eventually impregnates the entire primary wall. (d) Ferulates, possibly as aliphatic esters, become physically integrated into the lignin network through oxidative, lignin‐like coupling. This would provide a ‘priming surface’ for suberin polyester growth by the transesterification of glycerylated aliphatic precursors to the ω‐hydroxyacid carboxyl groups (Fig. 3) (ferulate group highlighted in bold in blow‐up). (e) Formation of suberin lamellae (yellow and orange colours correspond to light and dark lamellae, respectively. Dark lamellae indicated by arrows. Enlarged image depicts current models, which propose that dark lamellae result from an increased presence of phenolics, predominantly or exclusively made of ferulates, but with a currently undetermined mode of coupling. Note that this would be a second role for ferulates in suberin formation, distinct from its priming/connectivity function to lignin, depicted in (d).