Janet Wroth

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.

Evidence suggests that Mycosphaerella pinodes infection of Pisum sativum is inherited as a quantitative trait

Before likely sources of resistance in pea to Mycosphaerella pinodes can be exploited in a breeding program, the inheritance of genetic mechanisms governing the variation in resistance need to be understood. Disease responses were first examined in a diallel cross of nine Pisum sativum genotypes. An analysis of variance of parental and F1 data detected significant additive and dominance variation but no reciprocal effects. Gene interactions were not significant in leaves with the exception of the line JI 252 and there was no association detected between the frequencies of dominant or recessive alleles and parental disease response. In stems, the slope of the regression line of the covariance if array members with their non recurrent parent on the variance of an array was significantly different from one and the additive-dominance model was therefore rejected. The non-significance of interactions among genes determining disease response in leaves was supported by the triple test cross analyses. These analyses showed that the narrow-sense heritability of disease response was higher in crosses involving SA 1160 than in the cross Wirrega × Austrian Winter. These crosses are likely to generate inbred lines more resistant to M. pinodes infection than either parent.

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.

Genetic variation in stem strength in field pea (Pisum sativum L.) and its association with compressed stem thickness

We assessed genetic variation in stem strength in field pea (Pisum sativum L.) using physical and biological measures in order to develop selection criteria for breeding programs. A diverse group of 6 pea genotypes was subjected to 2 levels of disease (ascochyta leaf and stem blight), high and low. Stem samples were tested for physical stem strength (load at breaking point and flexion) using a universal testing machine. Stem diameter and compressed stem thickness were measured as biological indicators of stem strength. The genotypes varied significantly in physical and biological measures of stem strength, and in resistance to ascochyta blight. Load at breaking point was strongly associated with compressed stem thickness but only weakly associated with stem diameter. Significant variation in compressed stem thickness was present among pea genotypes, supporting this as an inexpensive, reliable, and quantitative measure for use in the field. There was no variation in stem lignin content among genotypes. Ascochyta blight resistance and stem strength, as assessed by load, flexion, or compressed stem thickness, were independent traits (the main effects of disease level and genotype × disease level interactions for load, flexion, and compressed stem thickness were non-significant). Therefore, concurrent genetic gains in both ascochyta resistance and stem strength should be possible in the same pea breeding population.

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.

Additive genetic variance for stem strength in field pea (Pisum sativum)

Weak stem strength in field pea (Pisum sativum) is a major restriction to yield, seed quality and ease of harvest. Three aspects of stem strength: load at breaking point, flexion and compressed stem thickness, showed substantial genetic variation among a diverse range of six parents including modern cultivars, landrace accessions, and interspecific progeny. Diallel analysis of parents and F1 progeny was conducted using a simple additive-dominance model, which was adequate for load and compressed stem thickness. There were significant additive genetic effects for load and compressed stem thickness with no evidence of dominance or maternal effects, and also significant additive genetic effects for flexion which was subject to more complex genetic control. Valuable alleles for these stem strength traits were present in commercial cultivars and landrace types of field pea. Efficient and practical breeding for improved stem strength will involve several recurrent selection cycles with moderate selection pressure for compressed stem thickness in early generations, followed by verification of improvements in lodging resistance in subsequent field trials. Compressed stem thickness is relatively easy to measure on individual plants in the field and is closely associated with load.