Enhancement of genetic diversity in canola-quality Brassica napus and B. juncea by interspecific hybridisation

Reciprocal crosses were made between Brassica napus cv. Mystic (canola) and B. juncea JN29 (near canola quality). The F1 hybrids were selfed and backcrossed in all possible combinations to parent plants. The greatest number of selfed fertile progeny were obtained when Mystic was the maternal parent, and its F1 was most successful in backcrosses to Mystic or JN29 as maternal or paternal parent. The predominant morphological type of fertile progeny was B. napus, but several B. juncea morphological types occurred in F2 and BC1-derived lines. F2 : 3 and BC1S0 : 1 progeny showed transgressive segregation for agronomic and seed quality traits in two contrasting field environments. Several of the B. juncea-type progeny had improved seed quality (lower total seed glucosinolates and higher % oleic acid) over the B. juncea parent. Selfing of interspecific hybrids between canola-quality B. napus and B. juncea has the potential to greatly enhance genetic diversity in canola-quality progeny of both species, without the loss of donor alleles that normally occurs with repeated backcrossing.

Tracing B-genome chromatin in Brassica napus × B. juncea interspecific progeny

We used polymerase chain reaction (PCR) and fluorescence in situ hybridization (FISH) techniques to demonstrate the presence of Brassica B-genome chromosomes and putative B-genome introgressions in B. napus × B. juncea interspecific progeny. The B-genome - specific repeat sequence pBNBH35 was used to generate PCR products and FISH probes. The highest frequencies of viable progeny were obtained when B. napus was the maternal parent of the interspecific hybrid and the first backcross. B-genome - positive PCR assays were found in 34/51 fertile F2 progeny (67%), which was more than double the proportion found in fertile BC1 progeny. Four B-genome - positive F2-derived families and 1 BC1-derived family were fixed or segregating for B. juncea morphology in the F4 and BC1S2, respectively, but in only 2 of these families did B. juncea-type plants exhibit B. juncea chromosome count (2n = 36) and typical B-genome FISH signals on 16 chromosomes. The remaining B. juncea-type plants had B. napus chromosome count (2n = 38) and no B-genome FISH signals, except for 1 exceptional F4-derived line that exhibited isolated and weak B-genome FISH signals on 11 chromosomes and typical A-genome FISH signals. B. juncea morphology was associated with B-genome - positive PCR signals but not necessarily with 16 intact B-genome chromosomes as detected by FISH. B-genome chromosomes tend to be eliminated during selfing or backcrossing after crossing B. juncea with B. napus, and selection of lines containing B-genome chromatin during early generations would be promoted by use of this B-genome repetitive marker.

A PCR based B-genome-specific marker in Brassica species

Previous hybridisation studies showed that the repetitive DNA sequence pBNBH35 from Brassica nigra (genome BB, 2n=16) bound specifically to the B-genome and not to the A- or C-genomes of Brassica species. We amplified a sub-fragment of pBNBH35 from B. nigra by PCR, cloned and sequenced this sub-fragment, and confirmed that it was a 329-bp sub-fragment of pBNBH35. PCR and hybridisation techniques were used to confirm that the pBNBH35 sub-fragment was Brassica B-genome-specific. Fluorescence in situ hybridisation (FISH) in B. nigra, B. juncea (AABB, 2n=36) and B. napus (AACC, 2n=38) showed that the pBNBH35 sub-fragment was present on all eight Brassica B-genome chromosomes and absent from the A- and C-genome chromosomes. The pBNBH35 repeat was localised to the centromeric region of each B-genome chromosome. FISH clearly distinguished the B-genome chromosomes from the A-genome chromosomes in the amphidiploid species B. juncea. This is the first known report of a B-genome repetitive marker that is present on all B-genome chromosomes. It will be a useful tool for the detection of B chromosomes in interspecific hybrids and may prove useful for phylogenetic studies in Brassica species.

Pro-embryos of Lupinus spp. produced from isolated microspore culture

Several species of lupin (Lupinus spp.) are grown in Australia as crop and pasture plants. Lupin breeding, and legume breeding in general, is constrained by the inability to produce doubled haploid (DH) plants, which would accelerate the selection and release of new varieties. This technology is still in the developmental phase for legumes, although other major grain crops such as wheat, barley, and canola successfully use DHs on a commercial scale. A new, reproducible method of microspore culture that leads to cell division and pro-embryos in lupin is reported here. Microspores at the late uninucleate stage of development are mechanically isolated from lupin buds and embryogenesis induced by a combined heat shock and sucrose starvation stress treatment. Addition of further components to the growth medium promotes division of up to 50% of microspores to ≥16 cells within 24 h. Further development of these multicellular structures or pro-embryos appears to be limited by the rigid outer exine layer, which needs to rupture for continued cell division to the globular embryo stage. Further research is required to break this barrier to development of haploid lupin embryos.

Differential responses of field pea (Pisum sativum L.) to ascochyta blight (Mycosphaerella pinodes): rating disease in the field

Lotus glaber Mill. is a herbaceous legume, tolerant to flooding, that in a few decades has colonised the Flooding Pampa grasslands, an extensive wetland of 90 000 km in central-east Argentina. In this study, the growth and nodulation of flooded plants in 5 local populations were evaluated. Thirty-day-old seedlings, grown in pots, were flooded outdoors during 4 months. Stem, leaf, and root biomass, as well as the number of nodules per plant, were lower in flooded plants than in non-flooded ones. In flooded plants, the submerged portions of stems were hypertrophied and populations differed in the proportion of plants with adventitious roots. Flood tolerance was significantly related to the proportion of plants with adventitious roots in stems of each population. A positive relationship was shown between the proportion of plants with adventitious root and number of nodules per plant. The plants were also tolerant of low temperatures and ice-sheeting of water. These results underline the importance of morphological adaptations related to the maintenance of plant functionality under flooding conditions. The results provide evidence of the existence of characters related to flooding tolerance, which should be taken into account in ecological studies and agronomic selection programs.

Possible role for wild genotypes of Pisum spp. to enhance ascochyta blight resistance in pea

Summary. There are no cultivars with effective field resistance to ascochyta blight currently available in Australia but a number of wild genotypes of Pisum have been identified as possible sources of resistance and these were evaluated in crosses with a commercial cultivar. Pisum fulvum JI 1006, used as the pollen parent, was crossed with P. sativum cv. Wirrega using wild type P. sativum JI 252 as a bridging cross. JI 1006 and JI 252 both respond to Mycosphaerella pinodes infection by inducing a rapid hypersensitive response. All F2 seedlings (17–20-day-old) from the cross Wirrega × (JI 252 × JI 1006) were screened for their responses to M. pinodes infection in a controlled environment and plants with the highest levels of resistance were then screened as F3 progeny families in the field to determine their responses to natural M. pinodes infection. Nine percent of these families were significantly more resistant for both leaf and stem disease compared with Wirrega and among them were 9 lines which flowered at the same time or earlier than Wirrega. However, even the most resistant line had 30% of the foliage destroyed by disease, indicating disease control was insufficient. A second resistance mechanism which impeded M. pinodes hyphal penetration in leaves (P. sativum SA 1160) was combined with the hypersensitive response in the cross SA 1160 × (JI 252 × JI 1006). The level of resistance to disease was now significantly higher than any plant in the original F3 population, despite the wild-type growth habit of these plants. It is suggested that breeding programs should focus first on maximising field resistance through isolation of some optimal combinations of resistance mechanisms in wild genotypes before turning to improving the agronomic performance through backcrossing to advanced breeding lines.

Impaired nodule function in Medicago polymorpha L. infected with alfalfa mosaic virus

The effects of alfalfa mosaic virus (AMV) on growth, nodule formation and nodule function in the annual burr medic, Medicago polymorpha cv. Circle Valley, were investigated in glasshouse pot experiments. Systemically-infected plants from AMV-infected seed produced 21% less shoot dry weight and accumulated 24% less fixed nitrogen in shoots than healthy plants when harvested 53 d after germination. At day 75, infected plants showed similar shoot dry weight losses (22%), but the quantity of nitrogen fixed fell by only 15%. At day 53, soluble sugar, starch and bacteroid concentrations in nodules were unaffected by AMV infection, but nitrogenase specific activity was decreased by 25% and soluble amino acids by 20%. Although AMV infection resulted in no differences in the number of nodules formed in the first 11 d after germination or at any harvest thereafter, nodule mass was decreased by 23% for virus-infected plants at day 53. However this difference disappeared by day 75. Growth of AMV-infected plants was decreased probably because of impaired N₂ fixation with nodule function affected rather than nodulation. Increased nodule mass relative to plant weight in virus-infected plants, seen at day 75, implied some degree of compensation for the limitation in N₂ -fixing capacity. ELISAs for AMV antigen indicated that nodules were active sites of virus multiplication.

Subterranean clover mottle virus infection of subterranean clover: widespread occurrence in pastures and effects on productivity

In 1989 and 1990, infection with subterranean clover mottle sobemovirus (SCMV) was widespread in subterranean clover ( Trifolium subterraneum L.) pastures in the south-west of Western Australia. The virus was detected in 61% of the pastures sampled and incidences of infection ranged from 1 to 50%. The virus was more common in old pastures than in pastures resown with newer cultivars during the preceeding 5 year period. When 12 isolates of SCMV were inoculated to subterranean clover plants grown in the glasshouse, symptoms varied from mild to severe. SCMV isolates P23 and F4 decreased the herbage dry weight of cw. Daliak and Woogenellup grown in plots as spaced plants by 81-88% while the Type isolate caused losses of 92%. By contrast, losses were 37-49% with cv. Karridale, a cultivar in which systemic infection was either delayed or prevented during winter. Infection decreased seed yield by c. 90% in cvv. Karridale and Woogenellup with all three isolates; seed weight was decreased 21-55%. A small proportion of cv. Woogenellup transplants outgrew the infection in new shoots during late spring to produce abundant healthy foliage. SCMV seed transmission rates in seed collected from infected transplants of cv. Woogenellup were 0.06, 0.07 and 0.43% for the Type, P23 and F4 isolates respectively. It was concluded that SCMV was present in most pastures, but at low incidences, and that it persists in them from year to year. Extended growing seasons and hard grazing are likely to increase its incidence.