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•Monomers are made inside of cells and exported to the wall by unknown mechanism(s).•Possible mechanisms include transporters such as ATP-binding cassette transporters.•Diffusion of ...monomers through membranes, driven by polymer formation, is possible.•In gymnosperms, monolignol glucosides represent a possible alternate route to lignin.•Cells can export lignin monomers into the shared walls of neighboring cells.
Lignin is a highly abundant polymer in plant cell walls that is essential for land plants’ ability to stand upright and transport water. Inside plant cells, lignin monomers, called monolignols, are made from phenylalanine via a multistep pathway. In the cell wall, monomers move freely, until they encounter stationary oxidative enzymes that determine where the lignin polymer forms. However, it remains unclear how lignin monomers are trafficked from inside the cell to the cell wall. Although multiple lines of circumstantial evidence implicate transporters, additional possible mechanisms include the diffusion of monomers across lipid bilayers and the release of monolignol glucosides stored in vacuoles. There are therefore potentially diverse and overlapping mechanisms of monolignol export.
Lignin, the second most abundant biopolymer, is a promising renewable energy source and chemical feedstock. A key element of lignin biosynthesis is unknown: how do lignin precursors (monolignols) get ...from inside the cell out to the cell wall where they are polymerized? Modeling indicates that monolignols can passively diffuse through lipid bilayers, but this has not been tested experimentally. We demonstrate significant monolignol diffusion occurs when laccases, which consume monolignols, are present on one side of the membrane. We hypothesize that lignin polymerization could deplete monomers in the wall, creating a concentration gradient driving monolignol diffusion. We developed a two-photon microscopy approach to visualize lignifying Arabidopsis thaliana root cells. Laccase mutants with reduced ability to form lignin polymer in the wall accumulated monolignols inside cells. In contrast, active transport inhibitors did not decrease lignin in the wall and scant intracellular phenolics were observed. Synthetic liposomes were engineered to encapsulate laccases, and monolignols crossed these pure lipid bilayers to form polymer within. A sink-driven diffusion mechanism explains why it has been difficult to identify genes encoding monolignol transporters and why the export of varied phenylpropanoids occurs without specificity. It also highlights an important role for cell wall oxidative enzymes in monolignol export.
Lignin is a key secondary cell wall chemical constituent, and is both a barrier to biomass utilization and a potential source of bioproducts. The Arabidopsis transcription factors MYB58 and MYB63 ...have been shown to upregulate gene expression of the general phenylpropanoid and monolignol biosynthetic pathways. The overexpression of these genes also results in dwarfism. The vascular integrity, soluble phenolic profiles, cell wall lignin, and transcriptomes associated with these MYB‐overexpressing lines were characterized. Plants with high expression of MYB58 and MYB63 had increased ectopic lignin and the xylem vessels were regular and open, suggesting that the stunted growth is not associated with loss of vascular conductivity. MYB58 and MYB63 overexpression lines had characteristic soluble phenolic profiles with large amounts of monolignol glucosides and sinapoyl esters, but decreased flavonoids. Because loss of function lac4 lac17 mutants also accumulate monolignol glucosides, we hypothesized that LACCASE overexpression might decrease monolignol glucoside levels in the MYB‐overexpressing plant lines. When laccases related to lignification (LAC4 or LAC17) were co‐overexpressed with MYB63 or MYB58, the dwarf phenotype was rescued. Moreover, the overexpression of either LAC4 or LAC17 led to wild‐type monolignol glucoside levels, as well as wild‐type lignin levels in the rescued plants. Transcriptomes of the rescued double MYB63‐OX/LAC17‐OX overexpression lines showed elevated, but attenuated, expression of the MYB63 gene itself and the direct transcriptional targets of MYB63. Contrasting the dwarfism from overabundant monolignol production with dwarfism from lignin mutants provides insight into some of the proposed mechanisms of lignin modification‐induced dwarfism.
Canola is primarily grown for its high‐quality oil, which is extracted from the seeds and has highly desirable physical properties including a high smoke point, neutral flavour and an unsaturated ...fatty acid profile beneficial to human health (Lin et al., ). ABI3 overexpression leads to improved embryo degreening following frost exposure and enhanced pod strength. (a) Progression of embryo degreening in canola under normal and frost‐exposed conditions in relation to oil quality. (b) Semi‐quantitative colour analysis of untreated and frost‐exposed seeds from Westar (wild‐type) and two independent 35S::BnABI3 lines. (c) Seed chlorophyll concentration (mg/kg) in WT and 35S::BnABI3 lines following frost and non‐frosted conditions. Error bars indicate ± SEM (n = 7) Significance determined by Student's t‐test. (* = P < 0.05, ** = P < 0.01, *** = P < 0.001) (g, h). (i) Thickness of the pod wall (valve). (j) Diameter of pedicel. (k) Phenotypic overview of replum tissue in Westar (WT) and 35S::BnABI3, Scale = 1 mm. (l) Replum‐valve joint area index. (m) Seed oil content from WT and transgenic 35S::BnABI3 seeds. Key unsaturated fatty acid profile (18:1 oleic, 18:2 linoleic and 18:3 linolenic acids). (n) Nervonic acid content in WT and transgenic 35S::BnABI3 seeds. (o) ABI3 confers frost‐tolerant degreening through hyper‐activating seed SGR2 levels (1) and through up‐regulating protective genes (2), along with promoting pod strength by enhancing valve thickness (3) and increasing replum‐valve junction index (4).
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