Diploid Brassica Rapa Research Articles

Genome balance and dosage effect drive allopolyploid formation in Brassica

Polyploidy is a major evolutionary force that has shaped plant diversity. However, the various pathways toward polyploid formation and interploidy gene flow remain poorly understood. Here, we demonstrated that the immediate progeny of allotriploid AAC Brassica (obtained by crossing allotetraploid Brassica napus and diploid Brassica rapa) was predominantly aneuploids with ploidal levels ranging from near-triploidy to near-hexaploidy, and their chromosome numbers deviated from the theoretical distribution toward increasing chromosome numbers, suggesting that they underwent selection. Karyotype and phenotype analyses showed that aneuploid individuals containing fewer imbalanced chromosomes had higher viability and fertility. Within three generations of self-fertilization, allotriploids mainly developed into near or complete allotetraploids similar to B. napus via gradually increasing chromosome numbers and fertility, suggesting that allotriploids could act as a bridge in polyploid formation, with aneuploids as intermediates. Self-fertilized interploidy hybrids ultimately generated new allopolyploids carrying different chromosome combinations, which may create a reproductive barrier preventing allotetraploidy back to diploidy and promote gene flow from diploids to allotetraploids. These results suggest that the maintenance of a proper genome balance and dosage drove the recurrent conversion of allotriploids to allotetraploids, which may contribute to the formation and evolution of polyploids.

Cell division and endoreduplication play important roles in stem swelling of tuber mustard (Brassica juncea Coss. var. tumida Tsen et Lee)

We investigated spatio-temporal variations in cell division and the occurrence of endoreduplication in cells of tuber mustard stems during development. Cells in the stem had 8C nuclei (C represents DNA content of a two haploid genome), since it is an allotetraploid species derived from diploid Brassica rapa (AA) and B. nigra (BB), thus indicating the occurrence of endoreduplication. Additionally, we observed a dynamic change of cell ploidy in different regions of the swollen stems, with a decrease in 4C proportion in P4-1 and a sharp increase in 8C cells that became the dominant cell type (86.33% at most) in the inner pith cells. Furthermore, cDNAs of 14 cell cycle genes and four cell expansion genes were cloned and their spatial transcripts analysed in order to understand their roles in stem development. The expression of most cell cycle genes peaked in regions of the outer pith (P2 or P3), some genes regulating S/G2 and G2/M (BjCDKB1;2, BjCYCB1;1 and BjCYCB1;2) significantly decrease in P5 and P6, while G1/S regulators (BjE2Fa, BjE2Fb and BjE2Fc) showed a relative high expression level in the inner pith (P5) where cells were undergoing endoreduplication. Coincidentally, BjXTH1and BjXTH2 were exclusively expressed in the endoreduplicated cells. Our results suggest that cells of outer pith regions (P2 and P3) mainly divide for cell proliferation, while cells of the inner pith expand through endoreduplication. Endoreduplication could trigger expression of BjXTH1 and BjXTH2 and thus function in cell expansion of the pith tissue.

Structural and functional comparative mapping between the Brassica A genomes in allotetraploid Brassica napus and diploid Brassica rapa

Brassica napus (AACC genome) is an important oilseed crop that was formed by the fusion of the diploids B. rapa (AA) and B. oleracea (CC). The complete genomic sequence of the Brassica A genome will be available soon from the B. rapa genome sequencing project, but it is not clear how informative the A genome sequence in B. rapa (A(r)) will be for predicting the structure and function of the A subgenome in the allotetraploid Brassica species B. napus (A(n)). In this paper, we report the results of structural and functional comparative mapping between the A subgenomes of B. napus and B. rapa based on genetic maps that were anchored with bacterial artificial chromosomes (BACs)-sequence of B. rapa. We identified segmental conservation that represented by syntenic blocks in over one third of the A genome; meanwhile, comparative mapping of quantitative trait loci for seed quality traits identified a dozen homologous regions with conserved function in the A genome of the two species. However, several genomic rearrangement events, such as inversions, intra- and inter-chromosomal translocations, were also observed, covering totally at least 5% of the A genome, between allotetraploid B. napus and diploid B. rapa. Based on these results, the A genomes of B. rapa and B. napus are mostly functionally conserved, but caution will be necessary in applying the full sequence data from B. rapa to the B. napus as a result of genomic rearrangements in the A genome between the two species.

Turnip mosaic virus (TuMV) Is Able to Use Alleles of Both eIF4E and eIF(iso)4E from Multiple Loci of the Diploid Brassica rapa

Three copies of eIF4E and three copies of eIF(iso)4E have been identified and sequenced from a Turnip mosaic virus (TuMV)-susceptible, inbred, diploid Brassica rapa line, R-o-18. One of the copies of eIF4E lacked exons 2 and 3 and appeared to be a pseudogene. The two other copies of eIF4E and two of the three copies of eIF(iso)4E were isolated from a bacterial artificial chromosome library of R-o-18. Using an Arabidopsis line (Col-0::dSpm) with a transposon knock-out of the eIF(iso)4E gene which resulted in a change from complete susceptibility to complete resistance to TuMV, complementation experiments were carried out with the two versions of eIF4E and the two versions of eIF(iso)4E. When transformed into Col-0::dSpm, all four Brassica transgenes complemented the Arabidopsis eIF(iso)4E knock-out, conferring susceptibility to both mechanical and aphid challenge with TuMV. One of the copies of eIF4E did not appear to support viral replication as successfully as the other copy of eIF4E or the two copies of eIF(iso)4E. The results show that TuMV can use both eIF4E and eIF(iso)4E from B. rapa for replication and, for the first time, that a virus can use eIF4E and eIF(iso)4E from multiple loci of a single host plant.

Hybridisation between oilseed rape ( Brassica napus) and tetraploid Brassica rapa under field conditions

Cross-compatible relatives of crop species contribute to the uncertainty regarding the potential risk of transgene escape from genetically modified varieties. The most successful crossing partner of oilseed rape ( Brassica napus L.) is diploid Brassica rapa L. Variation of ploidy level among B. rapa cultivars has, until recently, been neglected in the context of gene flow and hybridisation with oilseed rape. We estimated the extent of hybridisation between autotetraploid B. rapa varieties (female) and B. napus (pollen donor) under experimental field conditions. Morphology, variation of relative DNA amount, and microsatellite markers were used to distinguish between intraspecific offspring of tetraploid B. rapa and interspecific hybrids with B. napus. Of 517 seed progenies of tetraploid B. rapa, 45 juvenile plants showed species specific morphological traits of oilseed rape. The detection of putative hybrids based on variation in relative DNA amounts was problematic due to the occurrence of aneuploidy. In total, 84 offspring showed relative DNA amounts deviating from tetraploid B. rapa, four of which were hexaploids. Of the 205 offspring analysed at three microsatellite loci, 67 had oilseed rape alleles. Based on molecular evidence a minimum hybridisation rate of 13.0% was estimated. A few mother plants accounted for the majority of hybrids. The mean pollen viability of hybrids between B. napus and tetraploid B. rapa (80.6%) was high in comparison with mean pollen viability of triploid hybrids between B. napus and diploid B. rapa. Therefore, the occurrence of tetraploid B. rapa should be taken into consideration when estimating the likelihood of gene flow from oilseed rape to close relatives at the landscape level. Tetraploid B. rapa is a common component of several seed mixtures and establishes feral populations in northwest Germany. Assuming a similar abundance of diploid and tetraploid B. rapa, gene flow from B. napus to tetraploid may be more likely than gene flow to diploid B. rapa.

Brassica rapa has three genes that encode proteins associated with different neutral lipids in plastids of specific tissues.

Plastid lipid-associated protein (PAP), a predominant structural protein associated with carotenoids and other non-green neutral lipids in plastids, was shown to be encoded by a single nuclear gene in several species. Here we report three PAP genes in the diploid Brassica rapa; the three PAPs are associated with different lipids in specific tissues. Pap1 and Pap2 are more similar to each other (84% amino acid sequence identity) than to Pap3 (46% and 44%, respectively) in the encoded mature proteins. Pap1 transcript was most abundant in the maturing anthers (tapetum) and in lesser amounts in leaves, fruit coats, seeds, and sepals; Pap2 transcript was abundant only in the petals; and Pap3 transcript had a wide distribution, but at minimal levels in numerous organs. Immunoblotting after sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated that most organs had several nanograms of PAP1 or PAP2 per milligram of total protein, the highest amounts being in the anthers (10.9 microg x mg(-1) PAP1) and petals (6.6 microg x mg(-1) PAP2), and that they had much less PAP3 (<0.02 microg x mg(-1)). In these organs PAP was localized in isolated plastid fractions. Plants were subjected to abiotic stresses; drought and ozone reduced the levels of the three Pap transcripts, whereas mechanical wounding and altering the light intensity enhanced their levels. We conclude that the PAP gene family consists of several members whose proteins are associated with different lipids and whose expressions are controlled by distinct mechanisms. Earlier reports of the expression of one Pap gene in various organs in a species need to be re-examined.