Genetic loci linking improved heat tolerance in wheat (<Emphasis Type="Italic">Triticum aestivum</Emphasis> L.) to lower leaf and spike temperatures under controlled conditions

Heat stress adversely affects wheat production in many regions of the world and is particularly detrimental during reproductive development. The objective of this study was to identify novel quantitative trait loci (QTL) associated with improved heat tolerance in wheat (Triticum aestivum L.) and to confirm previous QTL results. To accomplish this, a recombinant inbred line (RIL) population was subjected to a three-day 38°C daytime heat stress treatment during early grain-filling. At maturity, a heat susceptibility index (HSI) was calculated from the reduction of three main spike yield components; kernel number, total kernel weight, and single kernel weight. The HSI, as well as temperature depression (TD) of the main spike and main flag leaf during heat stress were used as phenotypic measures of heat tolerance. QTL analysis identified 14 QTL for HSI, with individual QTL explaining from 4.5 to 19.3% of the phenotypic variance. Seven of these QTL co-localized for both TD and HSI. At all seven loci, the allele for a cooler flag leaf or spike temperature (up to 0.81°C) was associated with greater heat tolerance, indicated by a lower HSI. In a comparison to previous QTL results in a RIL population utilizing the same source of heat tolerance, seven genome regions for heat tolerance were consistently detected across populations. The genetic effect of combining three of these QTL, located on chromosomes 1B, 5A, and 6D, demonstrate the potential benefit of selecting for multiple heat tolerance alleles simultaneously. The genome regions identified in this study serve as potential target regions for fine-mapping and development of molecular markers for more rapid development of heat tolerant germplasm.     

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.     

Parental effects on the performance of cultivated × wild species hybrids in potato

Valuable genetic diversity in diploid wild Solanum species can be accessed through crosses to haploids (2n = 2×) of the tetraploid cultivated potato, Solanum tuberosum. Haploid-wild species hybrids segregate for the ability to tuberize in the field. In addition, they vary in male fertility, vine size, stolon length, and tuber size. In this study, three haploids were crossed with nine diploid wild Solanum species and 27 hybrid families were evaluated in the field for two years. The proportion of male fertile hybrid clones varied depending on the wild species parent. A large effect of the female parent was detected for vine size, stolon length, tuber size, percent tuberization, and percent plants selected for agronomic quality. An exceptional haploid (US-W4) was identified for the production of agronomically desirable haploid-wild species hybrids. In hybrids derived from US-W4, differences among wild species parents were observed for agronomic quality. Superior hybrids were produced by S. berthaultii and S. microdontum. Reciprocal crosses were evaluated for a subset of families. When the wild species was used as the female parent, male fertility was restored, but tuberization and tuber size were reduced. Careful selection of both haploid and wild species parents can result in a large proportion of fertile, agronomically desirable hybrid offspring.     

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.     

Effect of source germplasm and season on the in vivo haploid induction rate in tropical maize

For in vivo production of doubled haploid (DH) lines in maize, the rate of haploid induction is of crucial importance. Maternal haploid induction depends primarily on the inducer used as a pollinator. However, the source germplasm used as a maternal parent and the environmental conditions for induction may also influence haploid induction and these aspects have not been examined in tropical maize so far. The objectives of our study were to (i) monitor the variation for haploid induction rate (HIR) among diverse source germplasm in tropical maize, (ii) determine the relative importance of general (GCA) and specific (SCA) combining abilities for HIR, and (iii) investigate the influence of summer and winter seasons and genotype × season interactions on this trait. Ten inbreds were mated in a half diallel design. The resulting 45 F₁ single crosses were pollinated with the haploid inducer hybrid RWS × UH400 during the summer 2008 and winter 2009 seasons in a lowland tropical environment in Mexico. HIR of the single crosses averaged over seasons ranged from 2.90 to 9.66% with an overall mean of 6.74%. Mean HIR was significantly (P < 0.01) higher during the winter (7.37%) than summer season (6.11%). Significant (P < 0.01) variation was observed due to GCA effects of parental inbreds of single crosses but not for SCA, GCA × season and SCA × season interactions. Our study underpins that a higher HIR in tropical maize can be obtained by selecting appropriate source germplasm and undertaking pollination under favorable environmental conditions.     

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.     

Foliar application of the exogenous plant hormones at pre-blooming stage improves flowering and fruiting in cashew (<Emphasis Type="Italic">Anacardium occidentale</Emphasis> L.)

The problem of declining tree yield has led to an investigation into the effectiveness of foliar application of exogenous hormones to improve flowering, fruit set, and fruit retention in cashew. Five exogenous hormones, one Gibberellic Acid (GA₃) and four Auxins (IAA, IBA, NAA, and 2,4-D) at seven different rates of application (0 mg L⁻¹, 10 mg L⁻¹, 25 mg L⁻¹, 50 mg L⁻¹, 100 mg L⁻¹, 250 mg L⁻¹, and 500 mg L⁻¹) were tested on six yield-related components of the two Brazilian cashew genotypes. This trial was a factorial split-split-plot design with each treatment replicated five times within a tree and three replications (three trees) per genotype. Responses varied significantly between exogenous hormones, concentrations and genotypes. The cashew plants used showed hormone-specific and optimum concentration response patterns. Of the five exogenous hormones tested, GA₃ was most effective as its application at 50–100 mg L⁻¹ gave five-fold improvements in flowering (precocity and number of hermaphrodite flowers) and fruiting, and about 69% increase in fruit retention ability and 25% in nut size. Panicles treated with GA₃ also produced relatively bigger nuts compared to the untreated. Days to flowering was found to be hormone sensitive, while production of hermaphrodite flowers, fruit set, and nut development tended to be concentration specific. The GA₃ exhibited a broad concentration tolerance among the five exogenous hormones investigated. Our data showed that using GA₃ at 50 mg L⁻¹ will enhance flowering precocity, shorten flowering duration, increase production of hermaphrodite flowers and fruit set significantly, and resultant nuts develop optimally with high percentage retention. Thus, it suggests cashew yield could be increased by exogenous foliar application of GA₃ at 50–100 mg L⁻¹ at pre-blooming stage.