Allelic changes in bread wheat cultivars were associated with long-term wheat trait improvements

Genetic impacts under selective breeding of agricultural crops have been frequently investigated with molecular tools, but inadequate attention has been paid to assess genetic changes under long-term genetic improvement of plant traits. Here we analyzed allelic changes with respect to wheat trait improvement in 78 Canadian hard red spring wheat cultivars released from 1845 to 2004 and screened with 370 mapped SSR markers. The improvements in quality, maturity, yield, disease, stem rust, leaf rust, sawfly resistance, and agronomy were considered. A total of 154 (out of 370) loci with significant allelic changes across 21 chromosomes were detected in the 78 wheat cultivars separated into improved versus non-improved groups for eight traits. The number of significant loci for improving a trait ranged from four for quality to 68 for yield and averaged 35. Many more loci with significant allelic reduction for improving a trait were detected than those with significant allelic increase. Selection for early maturity introduced more alleles, but improving the other traits purged more alleles. Significantly lower numbers of unique alleles were found in the cultivars with improved traits. The distributions of unique allele counts also varied greatly across the 21 chromosomes with respect to trait improvement. Significant SSR variation between two cultivar groups was observed for improvement in seven traits, but not in stem rust. The proportional SSR variation residing between two groups ranged from 0.014 to 0.118. The proportional SSR variations within the improved cultivar groups consistently were much lower than those within the non-improved groups. These findings clearly demonstrate the association between allelic changes and wheat trait improvements and are useful for understanding the genetic modification of the wheat genome by long-term wheat breeding.     

Rust-proofing wheat for a changing climate

This paper offers projections of potential effects of climate change on rusts of wheat and how we should factor in a changing climate when planning for the future management of these diseases. Even though the rusts of wheat have been extensively studied internationally, there is a paucity of information on the likely effects of a changing climate on the rusts and their influence on wheat production. Due to the lack of published empirical research we relied on the few published studies of other plant diseases, our own unpublished work and relevant information from the vast literature on rusts of wheat to prepare this overview. Three broad areas of potential risks from a changing climate were described: increased loss from wheat rusts, new rust pathotypes evolving faster and the reduced effectiveness of rust resistances. Increased biomass of wheat crops grown in the presence of elevated CO₂ concentrations and higher temperatures will increase the leaf area available for attack by the pathogen leading to increased inoculum production. If changed weather conditions were to accelerate the life cycle of a pathogen, the increased inoculum can lead to severe rust epidemics in many environments. Likewise should the effects of climate change result in more conducive conditions for rust development there will also be a corresponding increase in the rate of evolution of new pathotypes which could increase the rate of appearance of new virulences. The effectiveness of some rust resistance genes is influenced by temperature and crop development stage. Climate change may directly or indirectly influence the effectiveness of some resistance genes but this can not be ascertained due to a complete lack of knowledge. Since disease resistance breeding is a long term strategy it is important to determine if any of the important genes may become less effective due to climate change. Studies must be made to acquire new information on the rust disease triangle to increase the adaptive capacity of wheat under climate change. Leadership within the Borlaug Global Rust Initiative (BGRI) is needed to broker research on rust evolution and the durability of resistance under climate change.     

Selection for <Emphasis Type="Italic">Phytophthora</Emphasis> field resistance in the F<Subscript>2</Subscript> generation of organic outdoor tomatoes

Tomatoes are the most important vegetable, globally as well as in Germany. Outdoor tomato production is seriously impaired due to increasing infections with evolving late blight (Phytophthora infestans) populations. Within organic agriculture, research is being conducted to develop regionally adapted and open pollinated cultivars of outdoor tomatoes with late blight field resistance. In the present experiment, three crosses, including wild, cocktail, and beefsteak tomatoes, were selected for field resistance against late blight in F₂ at one location per cross. The comparison of positive and negative selection in F₃ revealed the selection of single F₂ plants to be efficient in all three crosses. F₂ selection has proved to be a robust and efficient tool for breeding programs. The correlated response to selection in other traits, including yield, fruit weight, days to maturity, harvest period, and plant height, depended on the cross. It was evident that selection for desired traits combined with field resistance against late blight is promising, even in wide crosses. The most undesired attribute of wild tomatoes is the formation of shoots on leaves and in inflorescences. No correlation was observed between field resistance and shoot formation, allowing the selection of genotypes with improved field resistance and yield, but without morphological disadvantages.     

Marker-assisted backcross selection in an interspecific <Emphasis Type="Italic">Cucumis</Emphasis> population broadens the genetic base of cucumber (<Emphasis Type="Italic">Cucumis sativus</Emphasis> L.)

Cucumber (Cucumis sativus L.) is a major cucurbit vegetable species whose genetic base has been drastically reduced during its domestication. The crop’s narrow genetic base (3–12% DNA polymorphism) has resulted from the use of limited genetic material and intense selection during plant improvement. Recently, however, interspecific hybridization has been successful in Cucumis via mating of C. hystrix Chakr. and C. sativus, which resulted in the amphidiploid C. hytivus. We report herein a marker-assisted strategy for increasing genetic diversity in cucumber through introgression backcrossing employing C. hytivus. The comparatively late-flowering but high-yielding, indeterminate, monoecious line WI 7012A (P₁; donor parent) derived from a C. hytivus × C. sativus-derived line (long-fruited Chinese C. sativus cv. Beijingjietou) was initially crossed to the determinate, gynoecious C. sativus line WI 7023A (P₂; recurrent parent 1), and then advanced backcross generation progeny (BC₂) were crossed with the gynoecious indeterminate line WI 9-6A (P₃; recurrent parent 2). More specifically, a single F₁ individual (P₁ × P₂) was backcrossed to P₂, and then BC progeny were crossed to P₂ and P₃, where marker-assisted selection (MAS) for genetic diversity (8 mapped and 16 unmapped markers; designated Sel) or no selection (designated NSel) was applied to produce BC₃P₂ (Sel) and BC₃P₃ (Sel), and BC₂P₂ (NSel) and BC₂P₂S₁ (NSel) progeny. Relative vegetative growth, number of lateral branches (LB), days to flowering (DF), yield (fruit number), and fruit quality [as measured by length:diameter (L:D) and endocarp:total diameter (E:T) ratios] were assessed in parents and cross-progeny. DF varied from ~20 (BC₃P₂Sel) to ~25 days (BC₂P₃Sel) among the populations examined, where progeny derived from P₂ possessed the shortest DF. Differences in cumulative yield among the populations over six harvests were detected, varying from ~8 fruits per plant in BC₃P₂ (Sel) to ~39 fruits per plant in BC₂P₃ (Sel). Although the vigorous vegetative growth of line P₁ was observed in its backcross progeny, highly heterozygous and polymorphic backcross progeny derived from P₃ were comparatively more vigorous and bore many high-quality fruit. Response to selection was detected for LB, DF, L:D, and E:T, but the effectiveness of MAS depended upon the parental lines used. Data indicate that the genetic diversity of commercial cucumber can be increased by introgression of the C. hystrix genome through backcrossing.     

Progress on characterization of self-incompatibility in <Emphasis Type="Italic">Brassica napus</Emphasis> L.

Self-incompatibility (SI) is a widespread mechanism in flowering plants that promotes outbreeding and thereby increases genetic diversity. Recognition specificity in Brassica is achieved by the interaction of the female determinant S-receptor kinase (SRK) and its ligand, the male determinant S-locus protein 11 (SP11). The interaction between SP11 and SRK triggers the signaling cascade in an S-haplotype-specific manner and results in the rejection of self-pollen, but the signal components involved are still not well characterized. S haplotypes are widespread in self-compatible amphidiploid B. napus, and the interaction of heterozygous S haplotypes causes the loss of SI. This review highlights the recent advances made towards understanding the genetic analysis, distribution, and evolution of S haplotypes, the signal factors, and the potential of SI in B. napus hybrid breeding program.     

Transgenic fertile soybean plants derived from somatic embryos transformed via the combined DNA-free particle bombardment and <Emphasis Type="Italic">Agrobacterium</Emphasis> system

An Agrobacterium-mediated transformation procedure for soybean [Glycine max L. Merrill] proliferating somatic embryos is here described. The Agrobacterium tumefaciens LBA4404 strain harboring pTOK233, pCAMBIA1390-olp or pH7WG2Dwrky plasmids was used to mediate gene transfer into the plant genome. Prior to Agrobacterium inoculation, proliferative soybean embryogenic clusters were microwounded by DNA-free tungsten particle bombardment. Three independent transformation experiments were performed. In Experiment I, 26 transgenic plants were obtained from a unique clone of cv Bragg, while 580 plants were recovered from 105 clones of cv IAS5. In Experiment II, a single hygromycin-resistant clone of cv BRSMG68 Vencedora was recovered and gave rise to five plants. In Experiment III, 19 plants of cv Bragg and 48 plants of IAS5 were recovered, representing five and 14 independent transformation events, respectively. PCR and Southern analyses confirmed the transgenes’ integration into plant genomes. Transgenic plants were fertile. They flowered, set pods and seeds. Transgene segregation in two T₁ progenies fits the Mendelian pattern (3:1 transgenic:non-transgenic plants). This is the first report of transgenic fertile soybean plants obtained from somatic embryogenic tissues transformed by the system that combines DNA-free particle bombardment and Agrobacterium.     

In vitro screening and QTL analysis for drought tolerance in diploid potato

Drought stress is a major abiotic constraint limiting crop production worldwide. Screening for drought tolerance and the traits that enhance drought tolerance is not straightforward in large mapping populations. In this study, we investigated the possibility of screening a mapping population in vitro for PEG-induced water deficit stress and recovery potential. We have measured several shoot and root growth parameters or traits in the C × E diploid potato mapping population. Significant variation was observed for genotype-specific responses to water deficit and recovery potential. Genetic variation and heritability estimates were high to very high for the measured traits depending on growth conditions. In order to identify potato QTLs for drought tolerance and recovery potential an SNP marker-rich integrated linkage map was used. A total of 23 QTLs were detected under control, stress and recovery treatments explaining 10.3–22.4% of the variance for each phenotypic trait. Among these, 10 QTLs were located on chromosome 2. Three QTLs involved in the important trait root to shoot ratio were identified on linkage groups 2, 3 and 8. These loci explained together 41.1% of the variance for this trait, and may be breeding targets for stress tolerance and yield in the field as well. The SNP markers derived from EST sequences underlying these QTLs led to the identification of putative candidate genes for further study in potato. This study constitutes the first knowledge of in vitro screening of a mapping population for drought tolerance in potato.     

Estimation of genetic components of variation for salt tolerance in chickpea using the generation mean analysis

Chickpea (Cicer arietinum L.) is known to be salt-sensitive and in many regions of the world its yields are restricted by salinity. Recent identification of large variation in chickpea yield under salinity, if genetically controlled, offers an opportunity to develop cultivars with improved salt tolerance. Two chickpea land races, ICC 6263 (salt sensitive) and ICC 1431 (salt tolerant), were inter-crossed to study gene action involved in different agronomic traits under saline and control conditions. The generation mean analysis in six populations, viz. P₁, P₂, F₁, F₂, BC₁P₁ and BC₁P₂, revealed significant gene interactions for days to flowering, days to maturity, and stem Na and K concentrations in control and saline treatments, as well as for 100-seed weight under salinity. Seed yield, pods per plant, seeds per plant, and stem Cl concentration were controlled by additive effects under saline conditions. Broad-sense heritability values (>0.5) for most traits were generally higher in saline than in control conditions, whereas the narrow-sense heritability values for yield traits, and stem Na and K concentrations, were lower in saline than control conditions. The influence of the sensitive parent was higher on the expression of different traits; the additive and dominant genes acted in opposite directions which led to lower heritability estimates in early generations. These results indicate that selection for yield under salinity would be more effective in later filial generations after gene fixation.     

Reciprocal differences in DNA sequence and methylation status of the pollen DNA between F<Subscript>1</Subscript> hybrids of <Emphasis Type="Italic">Solanum tuberosum</Emphasis> × <Emphasis Type="Italic">S. demissum</Emphasis>

Reciprocal differences, mostly caused by cytoplasmic effects, are frequently observed in interspecific hybrids. Previously, we found that crosses onto Solanum demissum were much successful with the pollen of interspecific hybrids between S. tuberosum as female and S. demissum as male (TD hybrids) than the reciprocal ones (DT hybrids). To elucidate this reciprocally different crossability, we analyzed the pollen DNA of TD and DT using methylation-sensitive amplified polymorphism (MSAP) analysis. Using 126 primer combinations, MSAP analysis revealed 57 different bands between bulked pollen DNA samples of TD and DT. Individual examination of 16 TD and 9 DT plants disclosed eight bands uniformly different between TD and DT. Their sequencing results revealed two pairs of bands to be identical to each other, resulting in six distinct sequences. As expected, one band shared high homology with chloroplast DNA, and another one with mitochondrial DNA. However, one band that was apparently different at DNA sequence level and maternally transmitted from S. demissum, showed no homology with any known sequence. The remaining three bands were of DNA methylation level differences with no or uncertain homology to known sequences. To our knowledge, this is the first report detecting reciprocal differences in DNA sequence or DNA methylation other than those in cytoplasmic DNA.     

Clonal propagation of triploid <Emphasis Type="Italic">Acorus calamus</Emphasis> Linn. Using dual-phase culture system

Acorus calamus is an important medicinal plant which has been used in Indian traditional medicine since time immemorial. Various bioactive molecules, viz., acorin, α- and β-asarone, asaryldehyde, caryophylene, isoasarone, methylisoeugenol, and safrol have been isolated from this plant. However, the use of this plant for medicinal purpose has been recently banned due to the high toxic property of β-asarone. The triploid Acorus calamus is reported to be low in β-asarone content and thus found to be the ideal raw material for medicinal use. The present investigation represents our finding for successful in vitro clonal propagation of the elite triploid accessions of Acorus calamus for mass propagation. In the dual-phase culture system consisting of agar-solidified Murashige and Skoog medium overlaid by liquid fraction of the same medium, maximum multiple shoot induction was favored by supplementation of α-naphthaleneacetic acid (0.5 mg L⁻¹) and 6-benzylaminopurine (2.0 mg L⁻¹). In vitro rooting of the microshoots was maximum in the medium supplemented with indolebutyric acid at 2.0 mg L⁻¹. The well-rooted microshoots could be successfully hardened and transplanted in the field. This result can be reproduced and is a viable protocol for successful clonal propagation of the seedless triploid Acorus calamus for conservation and sustainable development.