Mitochondrial gene expression in developing male gametophytes of male-fertile and S male-sterile maize

Mitochondria play a critical role in the normal development of the plant ma le gametophyte and in the disruption of normal gametophyte development asso ciated with cytoplasmically inherited male sterility (CMS). To investigate the role of mitochondria in these processes, the accumulation of mitochondr ial gene transcripts and the accumulation of nuclear gene transcripts encod ing mitochondrial proteins were investigated through male gametophyte devel opment in normal maize and through the course of pollen abortion in CMS-S m aize. Male gametophytes differing in developmental stage were isolated from male-fertile or male-sterile plants by sucrose density gradient centrifuga tion. Mature pollen was collected from dehiscent anthers of male-fertile pl ants. Aborted pollen, which collapsed during starch accumulation, was isola ted from emergent tassels of CMS-S male-sterile plants. Microspores, develo ping pollen and mature pollen exhibited striking differences in mitochondri al transcript accumulation. Mature pollen lacked detectable mitochondrial t ranscripts. Aborted pollen of CMS-S plants contained abundant, intact trans cripts of all mitochondrial genes studied, but prematurely degraded transcr ipts of several nuclear genes. Transcripts of the CMS-S associated mitochon drial open reading frames (orf355 and orf77) were detected from the early s tages of microspore development through the aborted pollen stage. The impli cations of these findings are discussed in terms of the mitochondrial requi rements for pollen function and the mechanism of pollen abortion in CMS-S m aize.