9.C.8 The ABC Lignin Precursor Transporters (ALPT) Family
Lignin is a complex biopolymer derived primarily from the condensation of three monomeric precursors, the monolignols. The synthesis of monolignols occurs in the cytoplasm. To reach the cell wall where they are oxidized and polymerized, they must be transported across the cell membrane. Using isolated plasma and vacuolar membrane vesicles prepared from Arabidopsis, together with applying different transporter inhibitors in the assays, Miao and Liu (2010) examined the uptake of monolignols and their derivatives by these native membrane vesicles. They demonstrated that the transport of lignin precursors across plasmalemma and their sequestration into vacuoles are ATP-dependent primary-transport processes, involving ATP-binding cassette-like transporters. Moreover, both plasma and vacuolar membrane vesicles selectively transported different forms of lignin precursors. In the presence of ATP, the inverted plasma membrane vesicles preferentially took up monolignol aglycones, whereas the vacuolar vesicles were more specific for glucoconjugates, suggesting that the different ATP-binding cassette-like transporters recognize different chemical forms in conveying them to distinct sites, and that glucosylation of monolignols is necessary for their vacuolar storage but not required for direct transport to the cell wall in Arabidopsis (Liu et al., 2011).