A Glycosyltransferase from Nicotiana alata Pollen Mediates Synthesis of a Linear (1,5)-α-L-Arabinan When Expressed in Arabidopsis.

The walls of Nicotiana alata pollen tubes contain a linear arabinan composed of (1,5)-α linked arabinofuranose (Araf) residues. Although generally found as a side-chain on the backbone of the pectic polysaccharide rhamnogalacturonan I (RG I), the arabinan in N. alata pollen tubes is considered 'free' as there is no detectable RG I in these walls. Carbohydrate-specific antibodies detected arabinan epitopes at the tip and along the shank of N. alata pollen tubes that are predominantly part of the primary layer of the bilayered wall. A sequence related to ARABINAN DEFICIENT 1 (AtARAD1), a presumed arabinan arabinosyltransferase from Arabidopsis thaliana, was identified by searching a N. alata pollen transcriptome. Transcripts for this ARAD1-like sequence, which we have named N. alata ARABINAN DEFICIENT-LIKE 1 (NaARADL1), accumulate in various tissues, most abundantly in the pollen grain and tube, and encode a protein that is a type II membrane protein with its catalytic C-terminus located in the Golgi lumen. NaARADL1 protein can form homodimers when transiently expressed in N. benthamiana leaves and heterodimers when co-expressed with ARAD1. Expression of NaARADL1 in Arabidopsis led to plants with more arabinan in their walls and that also exuded a guttation fluid rich in arabinan. Chemical and enzymatic characterisation of the guttation fluid showed that a soluble, linear α-(1,5)-arabinan was the most abundant polymer present. These results are consistent with NaARADL1 having an arabinan (1,5)-α-arabinosyltransferase activity.