ABC transporters coordinately expressed during lignification of Arabidopsis stems include a set of ABCBs associated with auxin transport

Abstract

The primary inflorescence stem of Arabidopsis thaliana is rich in lignified cell walls, in both vascular bundles and interfascicular fibres. Previous gene expression studies demonstrated a correlation between expression of phenylpropanoid biosynthetic genes and a subset of genes encoding ATP-binding cassette (ABC) transporters, especially in the ABCB/multi-drug resistance/P-glycoprotein (ABCB/MDR/PGP) and ABCG/pleiotropic drug resistance (ABCG/PDR) subfamilies. The objective of this study was to characterize these ABC transporters in terms of their gene expression and their function in development of lignified cells. Based on in silico analyses, four ABC transporters were selected for detailed investigation: ABCB11/MDR8, ABCB14/MDR12, ABCB15/MDR13, and ABCG33/PDR5. Promoter::glucuronidase reporter assays for each gene indicated that promoters of ABCB11, ABCB14, ABCB15, and ABCG33 transporters are active in the vascular tissues of primary stem, and in some cases in interfascicular tissues as well. Homozygous T-DNA insertion mutant lines showed no apparent irregular xylem phenotype or alterations in interfascicular fibre lignification or morphology in comparison with wild type. However, in abcb14-1 mutants, stem vascular morphology was slightly disorganized, with decreased phloem area in the vascular bundle and decreased xylem vessel lumen diameter. In addition, abcb14-1 mutants showed both decreased polar auxin transport through whole stems and altered auxin distribution in the procambium. It is proposed that both ABCB14 and ABCB15 promote auxin transport since inflorescence stems in both mutants showed a reduction in polar auxin transport, which was not observed for any of the ABCG subfamily mutants tested. In the case of ABCB14, the reduction in auxin transport is correlated with a mild disruption of vascular development in the inflorescence stem.

Impact of the Absence of Stem-Specific β-Glucosidases on Lignin and Monolignols

“…The hypothesis was that monolignol glucosides would be stored in the vacuole or transported from the cytosol to the cell wall and then deglucosylated by b-glucosidases before their polymerization in muro (Freundenberg et al, 1952;Sarkanen, 1971). However, recent studies suggest that the monolignol glucosides could be, at least in angiosperms, a storage form of the monolignols and not their direct transportation form (Miao and Liu, 2010;Kaneda et al, 2011;Liu et al, 2011;Liu, 2012).…”

A Critical View on ABC Transporters and Their Interacting Partners in Auxin Transport

“…As substrate specificities cannot easily be deduced from sequence Murphy 2006, Bailly et al 2011), it is currently unclear how many of the 17 remaining ABCBs function as auxin transporters. However, a recent publication suggested that there are eventually more to come: ABCB14 and ABCB15, both apparently involved in stem lignification, promote auxin transport since inflorescence stems in both mutants showed a reduction in PAT (Kaneda et al 2011) (see Fig. 1).…”

Laccases Direct Lignification in the Discrete Secondary Cell Wall Domains of Protoxylem

“…ABCB11, ABCG29, and ABCG33 transporters were previously identified as candidate monolignol exporters (Ehlting et al, 2005;Kaneda et al, 2011;Alejandro et al, 2012). To test the hypothesis that these putative monolignol transporters could have a restricted plasma membrane localization adjacent to secondary cell wall domains, GFP translational fusions of ABCB11, ABCG29, and ABCG33 were transformed into VND7-VP16-GR plants.…”