THE UNIVERSITY OF ADELAIDE
Waite Campus, Plant Science, Glen Osmond, SA 506, Australia.
Daryl Mares and Anna Campbell (Current address: Animal Genomics, AgResearch, Invermay, Private Bag 50034, Mosgeil, New Zealand).
Flour and noodle color, together with components of color such as xanthophyll content and polyphenol oxidase activity, were examined in a number of DH mapping populations over two seasons. Flour yellowness (b*) was highly heritable and strongly correlated with xanthophyll content. Highly significant (P < 0.001) QTLs for xanthophyll content located on chromosomes 3B and 7A (Sunco/Tasman) also were associated with variation in flour b* and noodle b* (white-salted (WSN) and yellow-alkaline (YAN) noodles). Sunco contributed the higher value allele at the 3B locus, whereas Cranbrook contributed the higher value allele at the 7A locus. Within the Sunco/Tasman DH population, there was significant transgressive segregation for flour and noodle b* on either side of the parents. In part, this was explained by the additive nature of the alleles contributed by the two parents. In the 'Cranbrook/Halberd' and 'CD87/Katepwa' DH populations, QTLs for flour b* were identified on chromosomes 3B and 7A and 3A and 7B, respectively. A highly significant QTL associated with variation in PPO activity was located on chromosome 2D. PPO and the 2D QTL were not associated with variation in initial flour and noodle brightness but were strongly correlated with noodle darkening. A second, weaker QTL was located on chromosome 2A.
Daryl Mares and Kolumbina Mrva.
Sprouting tolerance derived from AUS1408, a white-grained genotype originating in the Transvaal region of South Africa, has been introgressed into locally adapted germ plasm. Improved cultivars should be available to wheat growers in the near future. Some of this material exhibits tolerance equal to the original donor and, at least under Australian conditions, similar to some of the better red-grained wheats and would substantially reduce the incidence and severity of sprouting in Australia. Other material exhibits only an intermediate level of tolerance and appears to have lost one of the two putative genes controlling dormancy in AUS1408. This intermediate level of tolerance, nevertheless, also represents an improvement over most current commercial varieties. Newer sources of sprouting tolerance originating from China have grain dormancy similar to AUS1408 together with other useful disease and agronomic traits. Preliminary results from a half-diallel cross suggest that genetic control of dormancy in the two sources may be very similar. Intermediate dormancy/sprouting tolerance also is characteristic of the older, Australian cultivar Halberd. Variation for grain dormancy in a DH mapping population, Cranbrook (nondormant, very susceptible to sprouting)/Halberd was associated with QTLs on chromosomes 2A, 2D, and 4A. Of particular interest was the QTL on chromosome 4A that appeared to correspond with a QTL reported in other populations, including a red-grained, dormant/nondormant population.
Kolumbina Mrva and Daryl Mares.
QTLs controlling the expression of LMA in wheat were detected in a DH population derived from wheat cultivars Cranbrook (LMA source) and Halberd (nonLMA). Cool-temperature treatment of detached tillers was used to induce the expression of LMA in lines carrying the defect. There was a highly significant (P < 0.001) QTL on the long arm of chromosome 7B (accounting for 31 % of the variation in the first experiment), with Cranbrook contributing the higher value allele. A second QTL that accounted for 13 % of the variation was found close to the centromere on chromosome 3B. These results indicate that the gene responsible for LMA in Cranbrook is located on the long arm of chromosome 7B and situated distal to the alpha-Amy-2 gene that codes for low pI alpha-amylase isozymes synthesized in developing grains or later stages of germination.
VICTORIAN INSTITUTE FOR DRYLAND AGRICULTURE
Department of Natural Resources and Environment, Private Bag 260, Horsham, VIC 3401, Australia.
F.C. Ogbonnaya, F. Dreccer, R.F. Eastwood, P.R. Hearnden, E. Martin, J. Oman, D. Rodríguez, and J.S. Brown.
Drought and heat stresses are among the most important environmental constraints to extensive wheat production in the Australian wheat belt. Both the amount of rainfall during the cropping season and its reliability are low. For example, the majority of the cropping belt of southern Australia has an average annual rainfall of 250 to 450/500 mm with a 20-30 % annual variation. In addition, large areas are affected by different forms of salinity including sodic or saline subsoils and transient salinity, which occur when salts in the subsoil concentrate as the soil dries out due to high evaporative demand. Rising water tables also can bring salt to the root zone.
The potential to overcome these constraints is through the utilization of novel sources of genetic variation. Considerable variation has been found for resistance and/or tolerance to biotic stresses in wild relatives of wheat (Ogbonnaya et al. 2001a). There is a strong likelihood that Ae. tauschii and T. turgidum will have features of adaptation to marginal conditions that can be recaptured in synthetic wheats. Superior performance under drought and high temperatures has been observed in synthetic-derived wheat lines (Trethowan 2001). We also know that Ae. tauschii contributes substantially to salt tolerance by regulating the exclusion of sodium absorption (Ducobsky et al. 1996). In addition, in the process of making synthetic wheat, transgressive segregation for valuable traits may also be obtained.
The Australian Grains Research and Development Corporation (GRDC) is funding (July 2000-June 2005) a project to evaluate the potential of synthetic wheats to increase potential productivity under the drought, high temperature and salinity stresses experienced in Australia. The project involves collaboration between CIMMYT, the Victorian Institute for Dryland Agriculture (VIDA), and the Farming System Institute, Leslie Research Station, Toowoomba, Queensland. This project will evaluate primary synthetic wheats and derived synthetic lines supplied by CIMMYT under Australian conditions. The primary synthetics (~ 50 lines/year) were initially selected at CIMMYT after the imposition of both heat and drought stresses. However, because the physiological mechanism associated with the enhanced performance under such stresses are yet to be elucidated, a physiological approach (Reynolds et al. 2001) will be used to characterize the materials for tolerance to these stresses. This approach will ensure that the adaptive traits detected can be used, independently or in combination, to improve the efficiency of breeding for abiotic stress tolerance.
Physiological characterization will be complemented by an exploratory study with a simulation model (Rodríguez et al. 1999) where the chances of success of different combinations of traits will be evaluated in contrasting environments. This step will help in the definition of crop ideotypes suitable to regions with different stress patterns.
Selected primary synthetics with putative traits contributing to tolerance to the different stresses will be backcrossed to elite Australian lines to develop a pool of wheat germ plasm that can be readily assessed by wheat breeders. In addition to improving tolerance to abiotic stresses, the primary synthetics will be evaluated at VIDA for resistance to a range of biotic and other abiotic stresses limiting wheat productivity in Australia. In collaboration with Dr. Habans Bariana (Plant Breeding Institute, Cobbity, NSW, Australia), they also will be evaluated for seedling and adult-plant rust resistance. Those identified as possessing desirable traits will be used for the development of wheat germ plasm for the Australian wheat breeding entities.
Many Australian white-grained wheats lack adequate PHS resistance, causing sporadic and heavy losses in the high rainfall areas of the Northern Australian wheat belt. Thus, the development of white-grained, spouting-resistant lines is a high priority for the Australian wheat industry. The D-genome donor of bread wheat, Ae. tauschii, contains strong levels of PHS resistance, in which many mechanisms have been implicated including embryo- and glume-based dormancy.
In studies at VIDA, inheritance of embryo-related dormancy was assessed using a cross between accessions of Ae. tauschii that differed in levels of seed dormancy. Synthetic hexaploids also were produced using Ae. tauschii accessions differing in dormancy and two tetraploid durum parents. The synthetic-derived, hexaploid wheat lines along with their hexaploid and diploid parents were assessed for the expression of embryo-related and seed-related dormancy.
The results indicate that Ae. tauschii-derived PHS resistance can be expressed, at least in part, in a white grained background. Significant increases in dormancy were observed in both the naked embryo and mature seed with greater dormancy observed in the mature seed. Despite the increase in dormancy conferred by the presence of the seed coat, all hybrids assessed were shown to be white grained when subjected to an NaOH test (De Pauw and McCaig 1988). The Ae. tauschii parent, which is red-grained, expressed complete dormancy for 28 days as mature seed. When assessed as naked embryos, however, germination began by day 5. The increased dormancy expressed by the white-grained, synthetic-derived lines could, therefore, be caused by the presence of pigment precursors in the seedcoat. These precursors have been suggested to be inhibitory to germination and can be present without the full expression of the red pigment.
Analysis of the F2 data suggests that PHS in the Ae. tauschii accession used for this study is controlled by two recessive genes that are complementary. F3 data is presently being assessed to verify this information. PHS resistance is often expressed as a polygenic trait (Lawson et al. 1997), and, as such, it is likely that more than two genes exist.
A number of mechanisms have been implicated in PHS resistance, with inheritance reported as both simple and complex. Results from the present study concur with previous reports that dormancy is under control of a recessive gene (Han et al. 1999). More specifically, it has previously been shown that Ae. tauschii-derived sprouting resistance expressed in an artificial amphiploid is inherited as a recessive trait controlled by one gene (Lan et al. 1997).
The BSA of the F2 population using RAPD primers has not revealed any variation to date, and a candidate gene approach is being investigated as an alternative approach. The Vp1 gene, which is necessary for the induction and maintenance of dormancy in maize, has homologues in rice (Hattori et al. 1994), wild oats (Jones, Peters et al. 1997), and wheat (Bailey et al. 1999; Nakamura and Toyama 2001). Homologues of the Vp1 gene also have been discovered in the noncereal species, tobacco (Phillips and Conrad 1994) and bean (Bobb et al. 1995). A related gene conferring seed dormancy, Abi3, also has been identified in A. thaliana (Bailey 1999). Using conserved regions among these gene homologues, specific primers will be developed and used to screen for D-genome homologues in the accessions differing for dormancy and possible cosegregation with embryo dormancy.
Cereal cyst nematode causes significant losses to wheat production in southern Australia. Although a number of resistance genes have been found for CCN resistance (Ogbonnaya et al. 2001b), inadequate resistance levels in current wheat cultivars as well as a wide host range, compound the magnitude of yield losses associated with the incidence of P. neglectus.
Primary synthetic hexaploids obtained from CIMMYT were evaluated for resistance to P. neglectus and H. avenae. Included in the material evaluated were bread wheat lines introgressed with Ae. ventricosa chromosomes. Five of the 50 primary synthetic wheats evaluated for resistance to CCN displayed a near complete immune response. Whether this is a different gene than Cre3 earlier found in Ae. tauschii is yet to be determined. Resistance to P. neglectus varied among the 100 primary synthetic wheats evaluated (a collaboration with Dr. S.P. Taylor, SARDI, South Australia). Some synthetic wheats displayed higher levels of resistance than those currently available in bread wheat (resistance levels were equivalent to those found in triticale, the resistant control), whereas a limited number had moderate resistance. With the exception of one line with a moderate level of resistance, the bread wheat-Ae. ventricosa introgression lines were susceptible.
The wheat, molecular-marker implementation project at VIDA is part of the Australian Wheat Molecular Marker Program funded by GRDC. The major objective of this project is the utilization of molecular markers to screen and select wheat plants in the wheat breeding populations for a number of traits of interest.
The traits and the loci that are targeted include CCN resistance (Cre1 and Cre3), BYDV resistance (TC14), and the VPM segment conferring resistance to leaf (Lr37), stem (Sr38), and stripe (Yr17) rusts. Other traits that are being investigated are boron tolerance (Bo1) and dwarfing (Rht8).
All the markers being used are PCR-based markers, requiring smaller amounts of DNA than RFLPs. These markers are mostly dominant SCAR-based markers, which have been developed by CSIROAustralia. CSIRO and VIDA (Eastwood et al. 1994; Ogbonnaya et al. 1996, 2001b) developed the CCN markers through a collaborative effort). In our program, markers are used to screen early generation BC1F1s, to select plants for DH production and to screen fixed lines for the desirable loci. Approximately 3,000-4,000 genotypes are analyzed with markers each year for both marker implementation and germ plasm enhancement in wheat.
The major impact of using markers as a selection tool for wheat breeding at VIDA is an increased rate of genetic gain and an increased efficiency in selecting plants with desirable alleles. The application of molecular markers has allowed genotypes fixed for desirable alleles to be identified in earlier generations and reduced the resource expended on material lacking critical alleles. Larger populations with critical alleles can be retained, thus increasing the probability of identifying genotypes that are superior to the recurrent parents.
A set of 75 wheat land races, collected by Dr. Gerald Halloran (retired professor of plant breeding, University of Melbourne), previously characterized for quality traits are being genotyped with 25 SSR markers. We hope to examine the utility of the SSRs reported in the literature for detecting DNA polymorphism and for estimating genetic diversity among these accessions.
References.