Class B β-expansins are needed for pollen separation and stigma penetration

Group 1 grass pollen allergens comprise a distinctive clade within the β-expansin family of cell wall-loosening proteins and are divided by sequence divergence into two phylogenetically separable classes (A and B). They have been proposed to loosen the walls of the stigma and style. Supporting this idea, we recently showed that a transposon insertion in one of the maize group-1 allergen genes reduces the ability of pollen to effect fertilization under conditions of pollen competition. In this work, we provide additional information on the phenotype of this mutant, showing that pollen deficient in β-expansin gene expression tended to form large aggregates, leading to poor pollen dispersal on anther dehiscence, and that emerging pollen tubes had difficulties entering the silk. In addition, a silencing construct was created to reduce expression of all the class B genes with results that are consistent with those seen with the transposon insertional line, including reduced transgene transmission through the pollen. Our results provide a more detailed understanding of the role of group 1 allergens (pollen β-expansins) in maize pollen development, pollen dispersal, pollen tube penetration into the style, and pollen tube growth through the transmitting tract.