Role of Glycosyltransferases in Pollen Wall Primexine Formation and Exine Patterning.

The pollen cell wall is important for protection of male sperm from physical stresses and consists of an inner gametophyte-derived intine layer and a sporophyte-derived exine layer. The polymeric constituents of the robust exine are termed sporopollenin. The mechanisms by which sporopollenin is anchored onto microspores and polymerized in specific patterns are unknown, but the primexine, a transient cell wall matrix formed on the surface of microspores at the late tetrad stage, is hypothesized to play a key role. Arabidopsis (Arabidopsis thaliana) spongy (spg) and uneven pattern of exine (upex) mutants exhibit defective and irregular exine patterns. SPG2 (synonymous with IRREGULAR XYLEM9-LIKE [IRX9L]) encodes a family GT43 glycosyltransferase involved in xylan backbone biosynthesis, while UPEX1 encodes a family GT31 glycosyltransferase likely involved in galactosylation of arabinogalactan proteins. Imaging of developing irx9l microspores showed that the earliest detectable defect was in primexine formation. Furthermore, wild-type microspores contained primexine-localized epitopes indicative of the presence of xylan, but these were absent in irx9l These data, together with the spg phenotype of a mutant in IRX14L, which also plays a role in xylan backbone elongation, indicate the presence of xylan in pollen wall primexine, which plays a role in exine patterning on the microspore surface. We observed an aberrant primexine and irregular patterns of incipient sporopollenin deposition in upex1, suggesting that primexine-localized arabinogalactan proteins could play roles in sporopollenin adhesion and patterning early in microspore wall development. Our data provide new insights into the biochemical and functional properties of the primexine component of the microspore cell wall.

BEL1-LIKE HOMEODOMAIN6 and KNOTTED ARABIDOPSIS THALIANA7 interact and regulate secondary cell wall formation via repression of REVOLUTA.

The TALE homeodomain transcription factor KNOTTED ARABIDOPSIS THALIANA7 (KNAT7) is part of a regulatory network governing the commitment to secondary cell wall biosynthesis of Arabidopsis thaliana, where it contributes to negative regulation of this process. Here, we report that BLH6, a BELL1-LIKE HOMEODOMAIN protein, specifically interacts with KNAT7, and this interaction influences secondary cell wall development. BLH6 is a transcriptional repressor, and BLH6-KNAT7 physical interaction enhances KNAT7 and BLH6 repression activities. The overlapping expression patterns of BLH6 and KNAT7 and phenotypes of blh6, knat7, and blh6 knat7 loss-of-function mutants are consistent with the existence of a BLH6-KNAT7 heterodimer that represses commitment to secondary cell wall biosynthesis in interfascicular fibers. BLH6 and KNAT7 overexpression results in thinner interfascicular fiber secondary cell walls, phenotypes that are dependent on the interacting partner. A major impact of the loss of BLH6 and KNAT7 function is enhanced expression of the homeodomain-leucine zipper transcription factor REVOLUTA/INTERFASCICULAR FIBERLESS1 (REV/IFL1). BLH6 and KNAT7 bind to the REV promoter and repress REV expression, while blh6 and knat7 interfascicular fiber secondary cell wall phenotypes are suppressed in blh6 rev and knat7 rev double mutants, suggesting that BLH6/KNAT7 signaling acts through REV as a direct target.