SUMMARYHere, we describe the characteristics of a Brassica napus male sterile mutant 7365A with loss of the BnMs3 gene, which exhibits abnormal enlargement of the tapetal cells during meiosis. Later in development, the absence of the BnMs3 gene in the mutant results in a loss of the secretory function of the tapetum, as suggested by abortive callose dissolution and retarded tapetal degradation. The BnaC.Tic40 gene (equivalent to BnMs3) was isolated by a map-based cloning approach and was confirmed by genetic complementation. Sequence analyses suggested that BnaC.Tic40 originated from BolC.Tic40 on the Brassica oleracea linkage group C9, whereas its allele Bnms3 was derived from BraA.Tic40 on the Brassica rapa linkage group A10. The BnaC.Tic40 gene is highly expressed in the tapetum and encodes a putative plastid inner envelope membrane translocon, Tic40, which is localized into the chloroplast. Transmission electron microscopy (TEM) and lipid staining analyses suggested that BnaC.Tic40 is a key factor in controlling lipid accumulation in the tapetal plastids. These data indicate that BnaC.Tic40 participates in specific protein translocation across the inner envelope membrane in the tapetal plastid, which is required for tapetal development and function.

Chimeric genes contribute to the evolution of diverse functions in plants and animals. However, new chimeric genes also increase the risk of developmental defects. Here, we show that the chimeric gene ( ) is responsible for genic male sterility in the widely used canola line 7365A ( ). originated via exon shuffling ∼4.6 million years ago. It causes defects in the normal functions of plastids and induces aborted anther formation and/or albino leaves and buds. Evidence of the age of the mutation, its tissue expression pattern, and its sublocalization indicated that it coevolved with (). In , results in complete male sterility that can be rescued by , suggesting that might restore fertility through effects on protein level. Another suppressor gene, , rescues sterility by reducing the level of transcription of Our results suggest that plants have coevolved altered transcription patterns and neofunctionalization of duplicated genes that can block developmental defects resulting from detrimental chimeric genes.

As the major determinant for nutrient uptake, root system architecture (RSA) has a massive impact on nitrogen use efficiency (NUE). However, little is known the molecular control of RSA as related to NUE in rapeseed. Here, a rapeseed recombinant inbred line population (BnaZNRIL) was used to investigate root morphology (RM, an important component for RSA) and NUE-related traits under high-nitrogen (HN) and low-nitrogen (LN) conditions by hydroponics. Data analysis suggested that RM-related traits, particularly root size had significantly phenotypic correlations with plant dry biomass and N uptake irrespective of N levels, but no or little correlation with N utilization efficiency (NUtE), providing the potential to identify QTLs with pleiotropy or specificity for RM- and NUE-related traits. A total of 129 QTLs (including 23 stable QTLs, which were repeatedly detected at least two environments or different N levels) were identified and 83 of them were integrated into 22 pleiotropic QTL clusters. Five RM-NUE, ten RM-specific and three NUE-specific QTL clusters with same directions of additive-effect implied two NUE-improving approaches (RM-based and N utilization-based directly) and provided valuable genomic regions for NUE improvement in rapeseed. Importantly, all of four major QTLs and most of stable QTLs (20 out of 23) detected here were related to RM traits under HN and/or LN levels, suggested that regulating RM to improve NUE would be more feasible than regulating N efficiency directly. These results provided the promising genomic regions for marker-assisted selection on RM-based NUE improvement in rapeseed.

Male sterility is an important contributor to heterosis in Brassica napus L. The B. napus line 7-7365ABC is a recessive epistatic genic male sterile (REGMS) three-line system. The 7-7365A line with the genotype Bnms3ms3ms4ms4RfRf is male-sterile, while the 7-7365B line with the genotype BnMs3ms3ms4ms4RfRf is male-fertile, and 7-7365C with homozygous recessive genotypes at the three loci shows male fertility because the loss function of Bnrf gene causes the inhibition of the genetic trait of the double mutant Bnms3 Bnms4. Histological studies addressing male sterility, transcriptional regulation pathways and the role of abscisic acid (ABA) in the anther development of REGMS plants are reported here. In the male-sterile line 7-7365A, tapetum cell and microspore mother cell separation were affected, and this led to failure of microspore release. The activity of polygalacturonase and the expression of the pectin methylesterase gene (AT3g06830) were significantly downregulated. Nine genes were downregulated in 7-7365A compared to 7-7365B and 7-7365C, including genes specifically expressed in tapetum (A3, A9, MS1) and the ABA-responsive gene KIN1. ABA concentration in 7-7365B was significantly higher than in 7-7365A and 7-7365C in young flower buds. Furthermore, temperature treatment made some sterile 7-7365A flowers become fertile. The stamens in these flowers produced viable pollen, and filament elongation was restored to its level in 7-7365C. We propose that ABA might control the expression of genes involved in cell separation during early anther development. The REGMS phenotype could be controlled by a primary pathway of male sterile metabolism positively regulated by the BnMs3 gene and a supplementary pathway negatively regulated by the BnRf gene.