Maize csmd1 exhibits pre-meiotic somatic and post-meiotic microspore and somatic defects but sustains anther growth.

Abstract

Maize male reproductive development is complex and lengthy, and anther formation and pollen maturation are precisely and spatiotemporally regulated. Here, we document that callose, somatic, and microspore defect 1 (csmd1), a new male-sterile mutant, has both pre-meiotic somatic and post-meiotic gametophyte and somatic defects. Chromosome behavior and cell developmental events were monitored by nuclear staining viewed by bright field microscopy; cell dimensions were charted by Volocity analysis of confocal microscopy images. Aniline blue staining and quantitative assays were performed to record callose deposition, and expression of three callose synthase genes was measured by qRT-PCR. Despite numerous defects and unlike other maize male-sterile mutants that show growth arrest coincident with locular defects, csmd1 anther elongation is nearly normal. Pre-meiotically and during prophase I, there is excess callose surrounding the meiocytes. Post-meiotically csmd1 epidermal cells have impaired elongation but excess longitudinal divisions, and uninucleate microspores cease growth; the microspore nucleoli degrade followed by cytoplasmic vacuolization and haploid cell collapse. The single vascular bundle within csmd1 anthers senesces precociously, coordinate with microspore death. Although csmd1 anther locules contain only epidermal and endothecial cells at maturity, locules are oval rather than collapsed, indicating that these two cell types suffice to maintain an open channel within each locule. Our data indicate that csmd1 encodes a crucial factor important for normal anther development in both somatic and haploid cells, that excess callose deposition does not cause meiotic arrest, and that developing pollen is not required for continued maize anther growth.