In several dicotyledonous species, NAC transcription factors act as master switches capable of turning on programmes of secondary cell-wall synthesis and cell death. This work used an oestradiol-inducible system to overexpress the NAC transcription factor BdSWN5 in the monocot model Brachypodium distachyon. This resulted in ectopic secondary cell-wall formation in both roots and shoots. Some of the genes upregulated in the process were a secondary cell-wall cellulose synthase (BdCESA4), a xylem-specific protease (BdXCP1) and an orthologue of AtMYB46 (BdMYB1). While activation of BdMYB1 may not be direct, this study showed that BdSWN5 is capable of transactivating the BdXCP1 promoter through two conserved binding sites. In the course of Brachypodium development, the BdXCP1 promoter was observed to be active in all types of differentiating tracheary elements. Together, these results suggest that Brachypodium SWNs can act as switches that turn on secondary cell-wall synthesis and programmed cell death.

Group 1 grass pollen allergens comprise a distinctive clade within the beta-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 beta-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 beta-expansins) in maize pollen development, pollen dispersal, pollen tube penetration into the style, and pollen tube growth through the transmitting tract.

Worldwide, 400 million people suffer from hay fever and seasonal asthma. The major causative agents of these allergies are pollen specific proteins called the group-1 grass pollen allergens. Although details of their antigenicity have been studied for 40 years with an eye towards immunotherapy, their function in the plant has drawn scant attention. Zea m 1 constitutes a class of abundant grass pollen allergens coded for by several genes that loosen the walls of grass cells, including the maize stigma and style. We have examined the impact of a transposon insertion into one of these genes (EXPB1, the most abundant isoform of Zea m 1) on the production of Zea m 1 protein, pollen viability, and pollen tube growth, both in vitro and in vivo. We also examined the effect of the insertional mutation on the competitive ability of the pollen by experimentally varying the sizes of the pollen load deposited onto stigmas using pollen from heterozygous plants and then screening the progeny for the presence of the transposon using PCR. We found that the insertional mutation reduced the levels of Zea m 1 in maize pollen, but had no effect on pollen viability, in vitro pollen tube growth or the proportion of progeny sired when small pollen loads are deposited onto stigmas. However, when large pollen loads are deposited onto the stigmas, the transposon mutation is vastly underrepresented in the progeny, indicating that this major pollen allergen has a large effect on pollen tube growth rates in vivo, and plays an important role in determining the outcome of the pollen-pollen competition for access to the ovules. We propose that the extraordinary abundance (4% of the extractable protein in maize pollen) of this major pollen allergen is the result of selection for a trait that functions primarily in providing differential access to ovules.

The dominant allergenic components of grass pollen are known by immunologists as group 1 allergens. These constitute a set of closely related proteins from the β-expansin family and have been shown to have cell wall-loosening activity. Group 1 allergens may facilitate the penetration of pollen tubes through the grass stigma and style. In maize (Zea mays), group 1 allergens are divided into two classes, A and B. We have identified 15 genes encoding group 1 allergens in maize, 11 genes in class A and four genes in class B, as well as seven pseudogenes. The genes in class A can be divided by sequence relatedness into two complexes, whereas the genes in class B constitute a single complex. Most of the genes identified are represented in pollen-specific expressed sequence tag libraries and are under purifying selection, despite the presence of multiple copies that are nearly identical. Group 1 allergen genes are clustered in at least six different genomic locations. The single class B location and one of the class A locations show synteny with the rice (Oryza sativa) regions where orthologous genes are found. Both classes are expressed at high levels in mature pollen but at low levels in immature flowers. The set of genes encoding maize group 1 allergens is more complex than originally anticipated. If this situation is common in grasses, it may account for the large number of protein variants, or group 1 isoallergens, identified previously in turf grass pollen by immunologists.