PG prevention precipitates poor pollen performance

Huang et al., at review data showing that a subset of the polygalacturonase (PG) gene family is expressed during pollen formation in several species. What then is the role of PGs, normally associated with cell wall softening, in pollen formation and/or activity? The authors compared the transcriptomes of flower buds in male-sterile and wild-type Brassica campestris ssp. chinensis. Amongst the transcripts missing from the male-sterile plants were three that, based on their sequences, clearly encoded PG enzymes. One of these, BcMF9, a novel, previously uncharacterized gene, was selected for further study. Northern blotting and RT-PCR suggested that expression of BcMF9 is initiated at the time of tetrad formation and is maintained through to pollen maturity. In situ hybridization showed that gene expression mainly occurs in the tapetum and in the microspore. The gene is thus active both in the sporophyte generation (the tapetum is a sporophytically derived tissue) and in the gametophyte. When antisense RNA was used to knock out expression of BcMF9, a variety of effects ensued. In sporophytic tissues, tapetum degradation occurred prematurely, leading to incomplete exine formation and pollen grain deformities. In the gametophyte, the outer layer of the intine (the exintine) was extensively over-developed, largely at the expense of the inner layer. It is not at all obvious that all these lesions may be ascribed to PG hydrolytic activity and the authors suggest that the enzyme may also have a regulatory role in the ‘dynamic metabolism of pectin’. Whatever the specific role(s) of BcMF9 PG it is clear that its knock-out had severe effects on pollen performance. Although percentage germination of pollen was similar to that in control plants, most of the growing antisense pollen tubes burst at the tip. This is entirely consistent with the very low levels of seed set observed in self-pollinated antisense plants.