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Identification, Gene Structure, and Expression of BnMicEmUP: A Gene Upregulated in Embryogenic Brassica napus Microspores

Microspores of Brassica napus can be diverted from normal pollen development into embryogenesis by treating them with a mild heat shock. As microspore embryogenesis closely resembles zygotic embryogenesis, it is used as model for studying the molecular mechanisms controlling embryo formation. A previous study comparing the transcriptomes of three-day-old sorted embryogenic and pollen-like (non-embryogenic) microspores identified a gene homologous to AT1G74730 of unknown function that was upregulated 8-fold in the embryogenic cells. In the current study, the gene was isolated and sequenced from B. napus and named BnMicEmUP (B. napus microspore embryogenesis upregulated gene). Four forms of BnMicEmUP mRNA and three forms of genomic DNA were identified. BnMicEmUP2,3 was upregulated more than 7-fold by day 3 in embryogenic microspore cultures compared to non-induced cultures. BnMicEmUP1,4 was highly expressed in leaves. Transient expression studies of BnMicEmUP3::GFP fusion protein in Nicotiana benthamiana and in stable Arabidopsis transgenics showed that it accumulates in chloroplasts. The features of the BnMicEmUP protein, which include a chloroplast targeting region, a basic region, and a large region containing 11 complete leucine-rich repeats, suggest that it is similar to a bZIP PEND (plastid envelope DNA-binding protein) protein, a DNA binding protein found in the inner envelope membrane of developing chloroplasts. Here, we report that the BnMicEmUP3 overexpression in Arabidopsis increases the sensitivity of seedlings to exogenous abscisic acid (ABA). The BnMicEmUP proteins appear to be transcription factors that are localized in plastids and are involved in plant responses to biotic and abiotic environmental stresses; as well as the results obtained from this study can be used to improve crop yield.

Brassica napus Rop GTPases and their expression in microspore cultures

Androgenesis in plants involves a shift in development that causes cultured microspore cells to form embryos rather than continue to develop pollen. In Brassica napus microspore culture a mild heat stress is used to switch on embryo development. An early hallmark of embryogenesis in this system is a symmetrical division of the nucleus instead of the asymmetric division that occurs during pollen formation. ROP GTPases act as molecular switches in a variety of developmental processes; therefore, the current study was initiated to examine whether they might be involved in androgenesis. Five distinct Rop genes with nucleic acid similarities ranging from 82 to 93% to Arabidopsis Rop1 were isolated from B. napus cv Topas. A Southern blot hybridization with a BnRop sequence probe suggested that there are 11-15 ROP gene family members in B. napus. RT-PCR reactions with PCR primers specific to BnRop5, BnRop6, BnRop9 and BnRop10 showed that expression of the BnRop5 was restricted to pollen but the others were detected in leaf, root, stem and pollen tissue. Pollen-like cells obtained from 3-day-old cultures by flow cytometric sorting had BnRop5 transcript levels that were 2.8 times higher than in flow sorted embryogenic microspores. Conversely, the BnRop9 transcript levels were 2.5-fold higher in the embryogenic cells than in the pollen-like cells. The potential involvement of specific ROPs in early stage microspore culture responses is discussed.