Abstract
Both the mechanisms of monolignol transport and the transported form of monolignols in developing xylem of trees are unknown. We tested the hypothesis of an active, plasma membrane-localized transport of monolignol monomers, dimers, and/or glucosidic forms with membrane vesicles prepared from developing xylem and lignin-forming tissue-cultured cells of Norway spruce (Picea abies L. Karst.), as well as from control materials, comprising non-lignifying Norway spruce phloem and tobacco (Nicotiana tabacum L.) BY-2 cells. Xylem and BY-2 vesicles transported both coniferin and p-coumaryl alcohol glucoside, but inhibitor assays suggested that this transport was through the tonoplast. Membrane vesicles prepared from lignin-forming spruce cells showed coniferin transport, but the Km value for coniferin was much higher than those of xylem and BY-2 cells. Liquid chromatography-mass spectrometry analysis of membrane proteins isolated from spruce developing xylem, phloem, and lignin-forming cultured cells revealed multiple transporters. These were compared with a transporter gene set obtained by a correlation analysis with a selected set of spruce monolignol biosynthesis genes. Biochemical membrane vesicle assays showed no support for ABC-transporter-mediated monolignol transport but point to a role for secondary active transporters (such as MFS or MATE transporters). In contrast, proteomic and co-expression analyses suggested a role for ABC transporters and MFS transporters.
Highlights
Lignification of plant cell walls is a developmental process in water-transporting xylem cells and support-giving sclerenchyma cells
MF were prepared from developing xylem, phloem, and ligninforming cultured cells of Norway spruce, and from tobacco BY-2 cells, to test the hypothesis that there are plasma membrane (PM)-localized, active transporters for monolignol transport
For the following reasons. (i) The preparation of MF is much faster than PM enrichment by aqueous polymer two-phase partitioning; the enzyme activity is better maintained during isolation of the membranes
Summary
Lignification of plant cell walls is a developmental process in water-transporting xylem cells and support-giving sclerenchyma cells. It can be an induced as a protective response against stresses. The plasma membrane (PM) serves the plant cell as a controlling boundary between the cell wall and the cytoplasm. This boundary has many roles, including maintaining cellular ionic homeostasis, creating concentration gradients, and regulating biochemical reactions, and detecting and mediating signals between the apoplast and the symplast. The biosynthesis of lignin is separated in space: monolignol biosynthesis takes place in the cytoplasm and lignin polymerization occurs in the cell wall. Passive diffusion has a likely role in aglycone transport (Boija and Johansson, 2006; Vermaas et al, 2019), whereas Golgi-mediated transport does not seem probable (Kaneda et al, 2008)
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