Pex1, a pollen-specific gene with an extensin-like domain.

We report here the identification of a pollen-specific gene from Zea mays that contains multiple Ser-(Pro)n repeats, the motif found in the cell wall-associated extensins. Sequence analysis reveals that the encoded protein has a putative globular domain at the N terminus and an extensin-like domain at the C terminus. The Pex1 (pollen extensin-like) gene is expressed exclusively in pollen, not in vegetative or female tissues, and is not induced in leaves upon wounding. We propose that the encoded protein may have a role in reproduction, either as a structural element deposited in the pollen tube wall during its rapid growth or as a sexual recognition molecule that interacts with partner molecules in the pistil.

HydroxyprolineO-arabinosyltransferase mutants oppositely alter tip growth inArabidopsis thalianaandPhyscomitrella patens

Hydroxyproline O-arabinosyltransferases (HPATs) are members of a small, deeply conserved family of plant-specific glycosyltransferases that add arabinose sugars to diverse proteins including cell wall-associated extensins and small signaling peptides. Recent genetic studies in flowering plants suggest that different HPAT homologs have been co-opted to function in diverse species-specific developmental contexts. However, nothing is known about the roles of HPATs in basal plants. We show that complete loss of HPAT function in Arabidopsis thaliana and the moss Physcomitrella patens results in a shared defect in gametophytic tip cell growth. Arabidopsis hpat1/2/3 triple knockout mutants suffer from a strong male sterility defect as a consequence of pollen tubes that fail to fully elongate following pollination. Knocking out the two HPAT genes of Physcomitrella results in larger multicellular filamentous networks due to increased elongation of protonemal tip cells. Physcomitrella hpat mutants lack cell-wall associated hydroxyproline arabinosides and can be rescued with exogenous cellulose, while global expression profiling shows that cell wall-associated genes are severely misexpressed, implicating a defect in cell wall formation during tip growth. Our findings point to a major role for HPATs in influencing cell elongation during tip growth in plants.