Fine mapping quantitative resistances to downy mildew in lettuce revealed multiple sub-QTLs with plant stage dependent effects reducing or even promoting the infection

Key message

Three regions with quantitative resistance to downy mildew of non-host and wild lettuce species, Lactuca saligna , disintegrate into seventeen sub-QTLs with plant-stage-dependent effects, reducing or even promoting the infection.

Abstract

Previous studies on the genetic dissection of the complete resistance of wild lettuce, Lactuca saligna, to downy mildew revealed 15 introgression regions that conferred plant stage dependent quantitative resistances (QTLs). Three backcross inbred lines (BILs), carrying an individual 30–50 cM long introgression segment from L. saligna in a cultivated lettuce, L. sativa, background, reduced infection by 60–70 % at young plant stage and by 30–50 % at adult plant stage in field situations. We studied these three quantitative resistances in order to narrow down their mapping interval and determine their number of loci, either single or multiple. We performed recombinant screenings and developed near isogenic lines (NILs) with smaller overlapping L. saligna introgressions (substitution mapping). In segregating introgression line populations, recombination was suppressed up to 17-fold compared to the original L. saligna × L. sativa F ₂ population. Recombination suppression depended on the chromosome region and was stronger suppressed at the smallest introgression lengths. Disease evaluation of the NILs revealed that the resistance of all three BILs was not explained by a single locus but by multiple sub-QTLs. The 17 L. saligna-derived sub-QTLs had a smaller and plant stage dependent resistance effect, some segments reducing; others even promoting downy mildew infection. Implications for lettuce breeding are outlined.     

Bidirectional backcrosses between wild and cultivated lettuce identify loci involved in nonhost resistance to downy mildew

Key message

The nonhost resistance of wild lettuce to lettuce downy mildew seems explained by four components of a putative set of epistatic genes.

Abstract

The commonplace observation that plants are immune to most potential pathogens is known as nonhost resistance (NHR). The genetic basis of NHR is poorly understood. Inheritance studies of NHR require crosses of nonhost species with a host, but these crosses are usually unsuccessful. The plant-pathosystem of lettuce and downy mildew, Bremia lactucae, provides a rare opportunity to study the inheritance of NHR, because the nonhost wild lettuce species Lactuca saligna is sufficiently cross-compatible with the cultivated host Lactuca sativa. Our previous studies on NHR in one L. saligna accession led to the hypothesis that multi-locus epistatic interactions might explain NHR. Here, we studied NHR at the species level in nine accessions. Besides the commonly used approach of studying a target trait from a wild donor species in a cultivar genetic background, we also explored the opposite, complementary approach of cultivar introgression in a wild species background. This bidirectional approach encompassed (1) nonhost into host introgression: identification of L. saligna derived chromosome regions that were overrepresented in highly resistant BC1 plants (F1?×?L. sativa), (2) host into nonhost introgression: identification of L. sativa derived chromosome regions that were overrepresented in BC1 inbred lines (F1?×?L. saligna) with relatively high infection levels. We demonstrated that NHR is based on resistance factors from L. saligna and the genetic dose for NHR differs between accessions. NHR seemed explained by combinations of epistatic genes on three or four chromosome segments, of which one chromosome segment was validated by the host into nonhost approach.

     

Efficient QTL detection for nonhost resistance in wild lettuce: backcross inbred lines versus F2 population

In plants, several population types [F₂, recombinant inbred lines, backcross inbred lines (BILs), etc.] are used for quantitative trait locus (QTL) analyses. However, dissection of the trait of interest and subsequent confirmation by introgression of QTLs for breeding purposes has not been as successful as that predicted from theoretical calculations. More practical knowledge of different QTL mapping approaches is needed. In this recent study, we describe the detection and mapping of quantitative resistances to downy mildew in a set of 29 BILs of cultivated lettuce (L. sativa) containing genome segments introgressed from wild lettuce (L. saligna). Introgression regions that are associated with quantitative resistance are considered to harbor a QTL. Furthermore, we compare this with results from an already existing F₂ population derived from the same parents. We identified six QTLs in our BIL approach compared to only three in the F₂ approach, while there were two QTLs in common. We performed a simulation study based on our actual data to help us interpret them. This revealed that two newly detected QTLs in the BILs had gone unnoticed in the F₂, due to a combination of recessiveness of the trait and skewed segregation, causing a deficit of the wild species alleles. This study clearly illustrates the added value of extended genetic studies on two different population types (BILs and F₂) to dissect complex genetic traits.     

The development of lettuce backcross inbred lines (BILs) for exploitation of the<Emphasis Type="Italic"> Lactuca saligna</Emphasis> (wild lettuce) germplasm

Backcross inbred lines (BILs) were developed in which chromosome segments of Lactuca saligna (wild lettuce) were introgressed into L. sativa (lettuce). These lines were developed by four to five backcrosses and one generation of selfing. The first three generations of backcrossing were random. Marker-assisted selection began in the BC₄ generation and continued until the final set of BILs was reached. A set of 28 lines was selected that together contained 96% of the L. saligna genome. Of these lines, 20 had a single homozygous introgression (BILs), four had two homozygous introgressions (doubleBILs) and four lines had a heterozygous single introgression (preBILs). Segregation ratios in backcross generations were compared to distorted segregation ratios in an F₂ population, and the results indicated that most of the distorted segregations can be explained by genetic effects on pollen- or egg-cell fitness. By means of BIL association mapping we were able to map 12 morphological traits and hundreds of additional amplified fragment length polymorphic (AFLP) markers. The total AFLP map now comprises 757 markers. This set of BILs is very useful for future genetic studies.Communicated by F. Salamini     

High-frequency generation and characterization of intergeneric hybrids and haploids from new wheat–barley crosses

Key message

Hybrid plants and a high frequency of maternal haploids were obtained using an efficient wheat–barley hybridization system (with new genotype combinations) and confirmed by several cytological and molecular tools.

Abstract

An efficient hybridization system between wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.) is presented on the basis of three new genotype combinations. A particularly high, 14 % frequency of plant regeneration per florets was achieved in the wheat–barley genotype combination of ‘Sichuan’ × ‘Morex’. The genome composition in 42 of the 95 plants regenerated by embryo rescue was determined using ploidy analysis, genomic in situ hybridization and the application of chromosome arm-specific molecular markers (SSR and STS). A high overall frequency (76 %) of maternal (wheat) haploids was observed in all the tests for all three cross combinations. A major implication of this observation is that this new hybridization system represents a useful tool to study the mechanism of uniparental chromosome elimination in cereals.     

Characterization of backcross introgression lines derived from <Emphasis Type="Italic">Oryza nivara</Emphasis> accessions for photosynthesis and yield

Improvement of photosynthetic traits is a promising strategy to break the yield potential barrier of major food crops. Leaf photosynthetic traits were evaluated in a set of high yielding Oryza sativa, cv. Swarna?×?Oryza nivara backcross introgression lines (BILs) along with recurrent parent Swarna, both in wet (Kharif) and dry (Rabi) seasons in normal irrigated field conditions. Net photosynthesis (P_N) ranged from 15.37 to 23.25 µmol (CO₂) m^?2 s^?1 in the BILs. Significant difference in P_N was observed across the seasons and genotypes. Six BILs showed high photosynthesis compared with recurrent parent in both seasons. Chlorophyll content showed minimum variation across the seasons for any specific BIL but significant variation was observed among BILs. Significant positive association between photosynthetic traits and yield traits was observed, but this association was not consistent across seasons mainly due to contrasting weather parameters in both seasons. BILs 166s and 248s with high and consistent photosynthetic rate exhibited stable high yield levels in both the seasons compared to the recurrent parent Swarna. There is scope to exploit photosynthetic efficiency of wild and weedy rice to identify genes for improvement of photosynthetic rate in cultivars.     

Marker-assisted introgression of a QTL region to improve rust resistance in three elite and popular varieties of peanut (Arachis hypogaea L.)

Key message

Successful introgression of a major QTL for rust resistance, through marker-assisted backcrossing, in three popular Indian peanut cultivars generated several promising introgression lines with enhanced rust resistance and higher yield.

Abstract

Leaf rust, caused by Puccinia arachidis Speg, is one of the major devastating diseases in peanut (Arachis hypogaea L.). One QTL region on linkage group AhXV explaining upto 82.62 % phenotypic variation for rust resistance was validated and introgressed from cultivar ‘GPBD 4’ into three rust susceptible varieties (‘ICGV 91114’, ‘JL 24’ and ‘TAG 24’) through marker-assisted backcrossing (MABC). The MABC approach employed a total of four markers including one dominant (IPAHM103) and three co-dominant (GM2079, GM1536, GM2301) markers present in the QTL region. After 2–3 backcrosses and selfing, 200 introgression lines (ILs) were developed from all the three crosses. Field evaluation identified 81 ILs with improved rust resistance. Those ILs had significantly increased pod yields (56–96 %) in infested environments compared to the susceptible parents. Screening of selected 43 promising ILs with 13 markers present on linkage group AhXV showed introgression of the target QTL region from the resistant parent in 11 ILs. Multi-location field evaluation of these ILs should lead to the release of improved varieties. The linked markers may be used in improving rust resistance in peanut breeding programmes.     

Stripe rust and leaf rust resistance QTL mapping,epistatic interactions,and co-localization with stem rust resistance loci in spring wheat evaluated over three continents

Key message

In wheat, advantageous gene-rich or pleiotropic regions for stripe, leaf, and stem rust and epistatic interactions between rust resistance loci should be accounted for in plant breeding strategies.

Abstract

Leaf rust (Puccinia triticina Eriks.) and stripe rust (Puccinia striiformis f. tritici Eriks) contribute to major production losses in many regions worldwide. The objectives of this research were to identify and study epistatic interactions of quantitative trait loci (QTL) for stripe and leaf rust resistance in a doubled haploid (DH) population derived from the cross of Canadian wheat cultivars, AC Cadillac and Carberry. The relationship of leaf and stripe rust resistance QTL that co-located with stem rust resistance QTL previously mapped in this population was also investigated. The Carberry/AC Cadillac population was genotyped with DArT^® and simple sequence repeat markers. The parents and population were phenotyped for stripe rust severity and infection response in field rust nurseries in Kenya (Njoro), Canada (Swift Current), and New Zealand (Lincoln); and for leaf rust severity and infection response in field nurseries in Canada (Swift Current) and New Zealand (Lincoln). AC Cadillac was a source of stripe rust resistance QTL on chromosomes 2A, 2B, 3A, 3B, 5B, and 7B; and Carberry was a source of resistance on chromosomes 2B, 4B, and 7A. AC Cadillac contributed QTL for resistance to leaf rust on chromosome 2A and Carberry contributed QTL on chromosomes 2B and 4B. Stripe rust resistance QTL co-localized with previously reported stem rust resistance QTL on 2B, 3B, and 7B, while leaf rust resistance QTL co-localized with 4B stem rust resistance QTL. Several epistatic interactions were identified both for stripe and leaf rust resistance QTL. We have identified useful combinations of genetic loci with main and epistatic effects. Multiple disease resistance regions identified on chromosomes 2A, 2B, 3B, 4B, 5B, and 7B are prime candidates for further investigation and validation of their broad resistance.     

QTL mapping of black rot (Guignardia bidwellii) resistance in the grapevine rootstock ‘Börner’ (V. riparia Gm183 × V. cinerea Arnold)

Key message

In the grapevine cultivar ‘Börner’ QTLs for black rot resistance were detected consistently in several independent experiments. For one QTL on chromosome 14 closely linked markers were developed and a detailed map provided.

Abstract

Black rot is a serious grapevine disease that causes substantial yield loss under unfavourable conditions. All traditional European grapevine cultivars are susceptible to the causative fungus Guignardia bidwellii which is native to North America. The cultivar ‘Börner’, an interspecific hybrid of V. riparia and V. cinerea, shows a high resistance to black rot. Therefore, a mapping population derived from the cross of the susceptible breeding line V3125 (‘Schiava grossa’ × ‘Riesling’) with ‘Börner’ was used to carry out QTL analysis. A resistance test was established based on potted plants which were artificially inoculated in a climate chamber with in vitro produced G. bidwellii spores. Several rating systems were developed and tested. Finally, a five class scheme was applied for scoring the level of resistance. A major QTL was detected based on a previously constructed genetic map and data from six independent resistance tests in the climate chamber and one rating of natural infections in the field. The QTL is located on linkage group 14 (Rgb1) and explained up to 21.8 % of the phenotypic variation (LOD 10.5). A second stable QTL mapped on linkage group 16 (Rgb2; LOD 4.2) and explained 8.5 % of the phenotypic variation. These two QTLs together with several minor QTLs observed on the integrated map indicate a polygenic nature of the black rot resistance in ‘Börner’. A detailed genetic map is presented for the locus Rgb1 with tightly linked markers valuable for the development for marker-assisted selection for black rot resistance in grapevine breeding.     

Genetic analyses of BaMMV/BaYMV resistance in barley accession HOR4224 result in the identification of an allele of the translation initiation factor 4e (Hv-eIF4E) exclusively effective against Barley mild mosaic virus (BaMMV)

Key message

Based on a strategy combining extensive segregation analyses and tests for allelism with allele-specific re-sequencing an Hv-eIF4E allele exclusively effective against BaMMV was identified and closely linked markers for BaYMV resistance were developed.

Abstract

Soil-borne barley yellow mosaic disease is one of the most important diseases of winter barley. In extensive screenings for resistance, accession ‘HOR4224’ being resistant to three strains of Barley mild mosaic virus (BaMMV-ASL1, BaMMV-Sil, and BaMMV-Teik) and two strains of Barley yellow mosaic virus (BaYMV-1 and BaYMV-2) was identified. Analyses using Bmac29, being to some extent diagnostic for the rym4/5 locus, gave hint to the presence of the susceptibility-encoding allele at this locus. Therefore, 107 DH lines derived from the cross ‘HOR4224’ × ‘HOR10714’ (susceptible) were screened for resistance. Genetic analyses revealed an independent inheritance of resistance to BaMMV and BaYMV ( $\chi_{1:1:1:1}^{2}$  = 5.58) both encoded by a single gene (BaMMV $\chi_{1:1}^{2}$  = 0.477; BaYMV $\chi_{1:1}^{2}$  = 0.770). Although Bmac29 indicated the susceptibility-encoding allele, BaMMV resistance of ‘HOR4224’ co-localized with rym4/rym5. The BaYMV resistance was mapped to chromosome 5H in the region of rym3. Sequencing of full length cDNA of the Hv-eIF4E gene displayed an already sequenced allele described to be efficient against BaMMV and BaYMV. However, the F₁ progenies of crosses involving ‘HOR4224’ and rym4/rym5 donors were all resistant to BaMMV but susceptible to BaYMV. Therefore, this is the first report of an allele at the rym4/rym5 locus exclusively efficient against BaMMV. Changes in the specificity are due to one non-synonymous amino acid substitution (I118K). Results obtained elucidate that combining extensive segregation analyses and tests for allelism involving different strains of BaMMV/BaYMV in combination with allele-specific re-sequencing is an efficient strategy for gene and allele detection in complex pathosystems.     

Somatic embryogenesis and plant regeneration from immature cotyledons of European chestnut (Castanea sativa Mill.)

Here, we established a protocol for induction of somatic embryogenesis and plant regeneration from immature cotyledons of open-pollinated seeds of European chestnut (Castanea sativa Mill.) cultivars ‘Osmano?lu’ and ‘Sar?a?lama’. Basal media, Murashige and Skoog medium (MS), Driver and Kuniyuki Walnut medium (DKW), and Woody Plant Medium (WPM) supplemented with l-glutamine or casein hydrolysate, with or without silver nitrate, agar or gelrite, and various plant growth regulator (PGR) combinations were tested in initial cultures for induction of somatic embryos. The effects of initial cultures on the percentage of somatic embryos and average number of embryos per cotyledon explant, subcultured monthly, were determined at the end of 4 mo. Interactions were observed among the different treatments for ‘Osmano?lu’ cultivar, with the highest rates of somatic embryogenesis (4.7–9.7%) being obtained in MS, DKW, or WPM basal media supplemented with (1) 6-benzyladenine (BA; 1 mg/L)?+?kinetin (KIN; 2 mg/L)?+?indole-3-butyric acid (IBA; 0.01 mg/L); (2) BA (1 mg/L)?+?1-phenyl-3-(1,2,3-thiadiazol-5-yl; TDZ 0.1 mg/L)?+?IBA (0.01 mg/L), and (3) KIN (2 mg/L)?+?TDZ (0.1 mg/L)?+?IBA (0.01 mg/L) PGR combinations plus l-glutamine or casein hydrolysate, with or without silver nitrate, and with either gelrite or agar. The highest percentages (12.0% and 11.2%) of somatic embryogenesis for ‘Sar?a?lama’ were obtained in DKW supplemented with PGR combinations of (1) BA (1 mg/L)?+?KIN (2 mg/L)?+?IBA (0.01 mg/L), (2) BA (1 mg/L)?+?TDZ (0.1 mg/L)?+?IBA (0.01 mg/L), respectively. The average number of somatic embryos ranged between 0 and 0.65 per explant for ‘Osmano?lu’ and between 0 and 0.49 per ‘Sar?a?lama’ explant. For germination of somatic embryos, root, shoot, and plantlet regeneration, different treatments included desiccation, cold and gibberellic acid (GA₃), and BA alone or in combination with auxins (IBA or α-naphthaleneacetic acid, NAA; 0.1 mg/L). The highest rate of somatic embryos regeneration (27.5%) occurred using MS basal media with half-strength microelements containing 0.1 mg/L BA?+?0.1 mg/L NAA, after treatments of desiccation, or desiccation plus cold or GA₃ (3 mg/L).     

Golden SusPtrit: a genetically well transformable barley line for studies on the resistance to rust fungi

Key message

We developed ‘Golden SusPtrit’, i.e., a barley line combining SusPtrit’s high susceptibility to non-adapted rust fungi with the high amenability of Golden Promise for transformation.

Abstract

Nonhost and partial resistance to Puccinia rust fungi in barley are polygenically inherited. These types of resistance are principally prehaustorial, show high diversity between accessions of the plant species and are genetically associated. To study nonhost and partial resistance, as well as their association, candidate gene(s) for resistance must be cloned and tested in susceptible material where SusPtrit would be the line of choice. Unfortunately, SusPtrit is not amenable to Agrobacterium-mediated transformation. Therefore, a doubled haploid (DH) mapping population (n = 122) was created by crossing SusPtrit with Golden Promise to develop a ‘Golden SusPtrit’, i.e., a barley line combining SusPtrit’s high susceptibility to non-adapted rust fungi with the high amenability of Golden Promise for transformation. We identified nine genomic regions occupied by resistance quantitative trait loci (QTLs) against four non-adapted rust fungi and P. hordei isolate 1.2.1 (Ph.1.2.1). Four DHs were selected for an Agrobacterium-mediated transformation efficiency test. They were among the 12 DH lines most susceptible to the tested non-adapted rust fungi. The most efficiently transformed DH line was SG062N (11–17 transformants per 100 immature embryos). The level of non-adapted rust infection on SG062N is either similar to or higher than the level of infection on SusPtrit. Against Ph.1.2.1, the latency period conferred by SG062N is as short as that conferred by SusPtrit. SG062N, designated ‘Golden SusPtrit’, will be a valuable experimental line that could replace SusPtrit in nonhost and partial resistance studies, especially for stable transformation using candidate genes that may be involved in rust-resistance mechanisms.     

Identification and fine mapping of qRBSDV-6 MH , a major QTL for resistance to rice black-streaked dwarf virus disease

Rice black streaked-dwarf virus (RBSDV) disease is recently expanding in southern China and poses a serious threat to rice crops. Few studies related to the genetics and breeding of RBSDV resistance have been reported. We have previously mapped a number of quantitative trait loci (QTLs) for RBSDV resistance by using a recombinant inbred line population of ‘Zhenshan 97’ (ZS97, susceptible)/‘Minghui 63’ (MH63, resistant) with natural infection data in two locations. In the present study, we confirmed the presence of a number of resistant QTLs on chromosomes 6, 7, and 9 from MH63 by using the same population in four different locations. We then focused on a major QTL, qRBSDV-6 ^MH , on chromosome 6 and introduced it into a highly susceptible japonica rice variety, ‘Huaidao 5’, using MH63 as the donor via marker-assisted selection, to generate seven backcross inbred lines (BILs). Natural infection and artificial inoculation-based tests revealed that all of the BILs had a significantly higher resistance to RBSDV than the recurrent parent. These results demonstrate that qRBSDV-6 ^MH is a stable major resistance QTL of high breeding value. We also constructed a set of chromosome segment substitution lines (CSSLs) specific to the qRBSDV-6 ^MH region and these used as fine mapping population. Combining the genotypes of CSSLs with the phenotypes from natural infection data in a highly RBSDV epidemic area during two different sowing seasons, we were able to precisely map qRBSDV-6 ^MH to the markers S18 and S23 at a physical distance of 627.6 kb on the Nipponbare reference genome.     

Inheritance of resistance to lettuce root aphid in the lettuce cultivars 'Avoncrisp' and 'Lakeland"

The inheritance of resistance in two lettuce cultivars to lettuce root aphid, Pemphigus bursarius, was studied in a series of laboratory and field experiments at Wellesbourne between 1989 and 1992. A source of total resistance in the cv. ‘Avoncrisp’ which is linked to the downy mildew resistance gene Dm6, was shown to be governed by a single dominant gene. There were no maternal effects evident in the inheritance of this resistance. The basis of the high level of resistance which exists in the cv. ‘Lakeland’ (formerly known as ‘Jubilee’) was also shown to be controlled by the same dominant gene. The linkage between Dm6 and root aphid resistance was broken in ‘Lakeland’ as this cultivar does not possess the Dm6 gene. The linkage was presumably broken when the original cross between the parents of cv. ‘Lakeland’, ‘Calmar’ and ‘Avoncrisp’ was made. Under laboratory conditions small numbers of aphids commence development on cv. ‘Lakeland’ but colonies fail to develop and under field conditions the resistance provides adequate Protectión against the pest. The resistance in both ‘Avoncrisp’ and ‘Lakeland’ was effective against a population of lettuce root aphid collected from an endive crop in southern France as well as being effective against the Wellesbourne population of this aphid.     

Genome-wide association analysis identified SNPs closely linked to a gene resistant to Soil-borne wheat mosaic virus

Key message

Using association and linkage mapping, two SNP markers closely linked to the SBWMV resistance gene on chromosome 5D were identified and can be used to select the gene in breeding.

Abstract

Soil-borne wheat mosaic virus (SBWMV) disease is a serious viral disease of winter wheat growing areas worldwide. SBWMV infection can significantly reduce grain yield up to 80 %. Developing resistant wheat cultivars is the only feasible strategy to reduce the losses. In this study, wheat Infinium iSelect Beadchips with 9 K wheat SNPs were used to genotype an association mapping population of 205 wheat accessions. Six new SNPs from two genes were identified to be significantly associated with the gene for SBWMV resistance on chromosome 5D. The SNPs and Xgwm469, an SSR marker that has been reported to be associated with the gene, were mapped close to the gene using F₆-derived recombinant inbred lines from the cross between a resistant parent ‘Heyne’ and a susceptible parent ‘Trego’. Two representative SNPs, wsnp_CAP11_c209_198467 and wsnp_JD_c4438_5568170, from the two linked genes in wheat were converted into KBioscience Competitive Allele-Specific Polymerase assays and can be easily used in marker-assisted selection to improve wheat resistance to SBWMV in breeding.     

Physiological adaptations of five poplar genotypes grown under SRC in the semi-arid Mediterranean environment

Key message

The genotype ‘Neva’ under high plant density showed the highest biomass yield and optimal physiological strategies and could be the most suitable choice under semi-arid environment

Abstract

The poplars (Populus spp.) are the most sensitive plants to water deficit conditions among the woody species utilized for biomass production for energetic purposes; their productivity is associated with water availability in the soil. In the Mediterranean environment, crops are mainly limited by evapotranspirative demand that is not balanced by rainfall supply. As new hybrids with high growth rates and resistance to water stress are selected, the use of poplar as an energy crop may increase in Southern regions of Mediterranean Europe. The growth dynamics and physiological characteristics of poplar hybrid genotypes have been monitored for 2 years at a site with a Mediterranean climate, Apulia region, that could be used for energy crops. Unrooted cuttings of three recently selected genotypes of poplar (‘Neva’, ‘Dvina’ and ‘Lena’) and two “traditional” genotypes (‘Luisa Avanzo’ and ‘Bellini’) were planted in the spring of 2010 at two different densities: (a) low plant density = 1,667 cuttings ha^?1 (LPD); (b) high plant density = 6,667 cuttings ha^?1 (HPD). The genotypes ‘Lena’ and ‘Dvina’ showed the lowest survival rates and the poorest growth among the hybrid poplar tested. The genotype ‘Bellini’ had low stomatal sensitivity to soil water content and a moderate productive performance. The genotypes ‘Luisa Avanzo’ and ‘Neva’ had a good degree of rooting and sprouting, high values of leaf relative water content (RWC_l) and low values of stomatal conductance (g _s) during the summer months. In “Neva”, these characteristics were associated with the best yields (4 t ha^?1) in HPD.     

Differences in boron distribution and forms in four citrus scion–rootstock combinations with contrasting boron efficiency under boron-deficient conditions

Key message

Boron efficiency of scion ‘Fengjie-72' is related to its less reduced boron concentration and distribution in leaves, achieved by decreasing the ratio of available boron in roots under boron-deficient conditions.

Abstract

Boron (B) deficiency is widespread in citrus orchards. Previous studies have demonstrated that the B-efficient navel orange scion ‘Fengjie-72' (Fs) and rootstock Carrizo citrange (Cr) are more tolerant to B deficiency than the closely related B-inefficient scion ‘Newhall’ (Ns) and rootstock trifoliate orange (Tr), respectively. However, the mechanisms underlying such differences remain unclear. Here, we investigated the differences in B distribution and forms among four combinations (Fs/Cr, Fs/Tr, Ns/Cr, and Ns/Tr) under adequate (0.25 mg/L) or deficient (0.001 mg/L) B supply for 300 days in sand culture. The results showed that B concentrations in buds and leaves of Fs-grafted plants were significantly higher than the respective concentrations of Ns-grafted plants under B-deficient conditions. Moreover, B distribution of Fs-grafted plants due to B deficiency was reduced less in leaves, but more in roots as compared to that of Ns-grafted plants. However, Ns/Cr accumulated more B in the scion stem (24 %) than the other combinations (17–19 %) when B was limited. A correlation was established between B efficiency and the ratio of B concentration in the rootstock stem or buds to the scion stem. Under B-deficient conditions, the ratio of available B (free B and semi-bound B) was significantly higher in leaves in Cr-grafted (36 %) than Tr-grafted plants (29 %), but lower in roots of Fs-grafted (22 %) than Ns-grafted plants (28 %). These results suggest that, under B-deficient conditions, differential B efficiency arises probably because Cr transports more B into scion, Fs redistributes B more efficiently within the plant, or both.     

Development of SSR markers linked to QTL reducing leaf hair density and grapevine downy mildew resistance in <Emphasis Type="Italic">Vitis vinifera</Emphasis>

Dense leaf hairs of grapevines have been known to act as a preexisting defense structure for preventing the incidence of grapevine downy mildew, because the pathogen, Plasmopara viticola, needs water to invade grapevines, and water is repelled by a hydrophobic surface due to dense leaf hairs. In the present study, we performed regression analyses of downy mildew resistance with leaf hair density using hybrids of Vitis labrusca origin and confirmed the effect of leaf hairs. Reducing the repelling effect of leaf hairs by detergent application canceled the effect of leaf hair, which confirmed the hypothesis. Thereafter, based on QTL analyses in the population of V. vinifera ‘Muscat of Alexandria’ × the interspecific hybrid ‘Campbell Early,’ we identified a major locus in linkage group (LG) 5 of ‘Muscat of Alexandria’ controlling leaf hair density. This locus was previously reported as a small effect QTL for downy mildew resistance, however we found that the locus had high LOD scores explaining 71.9%–78.5% of the phenotypic variance of leaf hairs. Moreover, this locus was detected as a QTL for downy mildew resistance. The effect of this locus was confirmed in two other hybrid populations. Finally, we could successfully identify three traditional V. vinifera table grapes ‘Muscat of Alexandria,’ ‘Katta Kurgan,’ and ‘Parkent’ as the origin of the allele linked to hairlessness by defining the SSR haplotypes. The use of this locus for marker-assisted selections would be a promising strategy for producing grapevines with resistance by preexisting defense structure.     

Usefulness of 10 genomic regions in soybean associated with sudden death syndrome resistance

Sudden death syndrome (SDS) is an important soybean [Glycine max (L) Merrill] disease caused by the soilborne fungus Fusarium virguliforme. Currently, 14 quantitative trait loci (QTL) had been confirmed associated with resistance or tolerance to SDS. The objective of the study was to evaluate usefulness of 10 of these QTL in controlling disease expression. Six populations were developed providing a total of 321 F2-derived lines for the study. Recombinant inbred lines (RIL) used as parents were obtained from populations of ‘Essex’ × ‘Forrest’ (EF), ‘Flyer’ × ‘Hartwig’ (FH), and ‘Pyramid’ × ‘Douglas’ (PD). Disease resistance was evaluated in the greenhouse at three different planting times, each with four replications, using sorghum infested with F. virguliforme homogeneously mixed in the soil (Luckew et al., Crop Sci 52:2215–2223, 2012). Four disease assessment criteria—foliar disease incidence (DI), foliar leaf scorch disease severity (DS), area under the disease progress curve (AUDPC), and root rot severity—were used. QTL were identified in more than one of the disease assessment criteria, mainly associated with lines in the most resistant categories. Five QTL (qRfs4, qRfs5, qRfs7, qRfs12, and Rfs16) were associated with at least one of the disease assessments across multiple populations. Of the five, qRfs4 was associated with DI, AUDPC, and root rot severity, and Rfs16 with AUDPC and root rot severity. The findings suggest it may be possible for plant breeders to focus on stacking a subset of the previously identified QTL to improve resistance to SDS in soybean.