Reaction to phytotoxins in a potato population derived from mesophyll protoplasts

Functional Analyses of the Diels-Alderase Gene sol5 of Ascochyta rabiei and Alternaria solani Indicate that the Solanapyrone Phytotoxins Are Not Required for Pathogenicity

Ascochyta rabiei and Alternaria solani, the causal agents of Ascochyta blight of chickpea (Cicer arietinum) and early blight of potato (Solanum tuberosum), respectively, produce a set of phytotoxic compounds including solanapyrones A, B, and C. Although both the phytotoxicity of solanapyrones and their universal production among field isolates have been documented, the role of solanapyrones in pathogenicity is not well understood. Here, we report the functional characterization of the sol5 gene, which encodes a Diels-Alderase that catalyzes the final step of solanapyrone biosynthesis. Deletion of sol5 in both Ascochyta rabiei and Alternaria solani completely prevented production of solanapyrones and led to accumulation of the immediate precursor compound, prosolanapyrone II-diol, which is not toxic to plants. Deletion of sol5 did not negatively affect growth rate or spore production in vitro, and led to overexpression of the other solanapyrone biosynthesis genes, suggesting a possible feedback regulation mechanism. Phytotoxicity tests showed that solanapyrone A is highly toxic to several legume species and Arabidopsis thaliana. Despite the apparent phytotoxicity of solanapyrone A, pathogenicity tests showed that solanapyrone-minus mutants of Ascochyta rabiei and Alternaria solani were equally virulent as their corresponding wild-type progenitors, suggesting that solanapyrones are not required for pathogenicity.

Functional Analyses of the Diels-Alderase Gene sol5 of Ascochyta rabiei and Alternaria solani Indicate that the Solanapyrone Phytotoxins Are Not Required for Pathogenicity

Ascochyta rabiei and Alternaria solani, the causal agents of Ascochyta blight of chickpea (Cicer arietinum) and early blight of potato (Solanum tuberosum), respectively, produce a set of phytotoxic compounds including solanapyrones A, B, and C. Although both the phytotoxicity of solanapyrones and their universal production among field isolates have been documented, the role of solanapyrones in pathogenicity is not well understood. Here, we report the functional characterization of the sol5 gene, which encodes a Diels-Alderase that catalyzes the final step of solanapyrone biosynthesis. Deletion of sol5 in both Ascochyta rabiei and Alternaria solani completely prevented production of solanapyrones and led to accumulation of the immediate precursor compound, prosolanapyrone II-diol, which is not toxic to plants. Deletion of sol5 did not negatively affect growth rate or spore production in vitro, and led to overexpression of the other solanapyrone biosynthesis genes, suggesting a possible feedback regulation mechanism. Phytotoxicity tests showed that solanapyrone A is highly toxic to several legume species and Arabidopsis thaliana. Despite the apparent phytotoxicity of solanapyrone A, pathogenicity tests showed that solanapyrone-minus mutants of Ascochyta rabiei and Alternaria solani were equally virulent as their corresponding wild-type progenitors, suggesting that solanapyrones are not required for pathogenicity.

Applications of biotechnology and genomics in potato improvement

Potato is the third most important global food crop and the most widely grown noncereal crop. As a species highly amenable to cell culture, it has a long history of biotechnology applications for crop improvement. This review begins with a historical perspective on potato improvement using biotechnology encompassing pathogen elimination, wide hybridization, ploidy manipulation and applications of cell culture. We describe the past developments and new approaches for gene transfer to potato. Transformation is highly effective for adding single genes to existing elite potato clones with no, or minimal, disturbances to their genetic background and represents the only effective way to produce isogenic lines of specific genotypes/cultivars. This is virtually impossible via traditional breeding as, due to the high heterozygosity in the tetraploid potato genome, the genetic integrity of potato clones is lost upon sexual reproduction as a result of allele segregation. These genetic attributes have also provided challenges for the development of genetic maps and applications of molecular markers and genomics in potato breeding. Various molecular approaches used to characterize loci, (candidate) genes and alleles in potato, and associating phenotype with genotype are also described. The recent determination of the potato genome sequence has presented new opportunities for genomewide assays to provide tools for gene discovery and enabling the development of robustly unique marker haplotypes spanning QTL regions. The latter will be useful in introgression breeding and whole-genome approaches such as genomic selection to improve the efficiency of selecting elite clones and enhancing genetic gain over time.

Potato protoplasts in crop improvement

Clonal populations regenerated from single-leaf cell protoplasts of the potato cultivar ;Russet Burbank' display a high frequency of variation for several horticultural and disease resistance characters. Observations over a period of three tuber generations suggest stable changes in tuber shape, yield, and maturity date, in photo-period requirements for flowering, and in plant morphology. Enhanced resistance to early blight (Alternaria solani) and late blight (Phytophthora infestans) diseases also regularly occurs within regenerated populations. These findings are discussed in the context of possible application to varietal improvement, particularly as they pertain to asexually propagated plants.

Stable and extreme resistance to common scab of potato obtained through somatic cell selection

Somatic cell selection with thaxtomin A as a positive selection agent was used to isolate variants of potato cv. Russet Burbank with strong to extreme resistance to common scab. Glasshouse and field trials identified 51 variants with significantly reduced disease incidence (frequency of infected tubers) and severity (tuber lesion coverage) compared with the parent cultivar. The most promising variants exhibited extreme disease resistance, rarely showing lesions, which were invariably superficial and shallower than those on the parent. Resistance traits were consistently expressed both in 10 glasshouse and two field trials at different locations, with varied inoculum and disease pressure. Disease-resistant variants differed in their response to thaxtomin A in tuber slice bioassays. Of 23 variants tested, 10 showed reduced thaxtomin A susceptibility, with the remaining 13 responding similar to that of the parent. Thus, toxin tolerance was not the only factor responsible for observed disease resistance; however, four of the five most disease-resistant variants had enhanced thaxtomin A tolerance, suggesting that this factor is important in the expression of strong disease resistance. Pathogenicity and toxin tolerance remained stable over a 6-year period, demonstrating that selected phenotypes were robust and genetic changes stable. The majority of disease-resistant variants had tuber yields equivalent to the parent cultivar in glasshouse trials. This suggests that selection for disease resistance was not associated with negative tuber attributes and that certain variants may have commercial merit, worthy of further agronomic testing.

Toxic metabolites from culture filtrate of Fusarium oxysporum and its effects on cucumber cells and plantlets

Resistance of cucumber plantlets to culture filtrate of Fusarium oxysporum is correlated with resistance of single cells from callus. Single cells and plantlets of two cultivars of cucumber were incubated with culture filtrates. Rapid cell death occurred, as assessed by the stain fluorescein diacetate. More cell death ocurred in the cells of the cultivar Aodai than in to cells of the cultivar Caipira, which presented high level of resistance. Maximum toxic activity of culture filtrates was attained after 21-25 days of growth of the fungus.

Anthranilate N-hydroxycinnamoyl/benzoyltransferase gene from carnation: rapid elicitation of transcription and promoter analysis

Hydroxycinnamoyl/benzoyl-CoA:anthranilate N-hydroxycinnamoyl/benzoyltransferase (HCBT) catalyses the committed reaction of phytoalexin biosynthesis in carnation (Dianthus caryophyllus L.). Three HCBT cDNAs were cloned previously from suspension-cultured carnation cells that had been induced with fungal elicitor. A rapid, transient induction of hcbt transcripts upon elicitation, reaching maximal abundances within about 0.5 h and returning to basal levels within 4 h, suggested the involvement of unusual cis elements. A DNA fragment of 3.8 kb, spanning the hcbt2 gene with the 5'-flanking region of roughly 1.8 kb, was cloned from carnation plants. The gene encodes one long open reading frame lacking introns. The DNA sequence revealed a conserved TATA box, three elicitor response elements (EREs) and a 9 bp direct repeat as well as an interrupted direct repeat of 11 bp in the TATA distal region. EMSA revealed the binding of elicitor-inducible nuclear factors to the promoter region from -377 to -326 spanning two of the EREs, and their functional relevance was confirmed by transient expression assays of hcbt2 promoter-GUS reporter gene constructs in parsley protoplasts. Furthermore, an oligo(A) segment was present immediately preceding the start of translation (+140 to +150). Transient expression analysis demonstrated that the sequence upstream to -1157 at least is required in context with the 5'-UTR, particularly including the poly(A) segment, for strong expression and full elicitor induction of the hcbt2 gene. The results suggested that several sequence motifs scattered over a wide range of the 5'-flanking region and into the exonic sequence are responsible for the full elicitor regulation of the hcbt2 gene.

Effect of Alternaria solani culture filtrate on adventitious shoot regeneration in potato

The effect of Alternaria solani culture filtrate on adventitious shoot regeneration from tuber discs was evaluated using five potato cultivars, which were selected based on their field reaction to Alternaria solani and which represented a range of disease reactions. The culture filtrate stimulated regeneration, a response that could prove to be very useful in the wider utilization of transformation and in vitro selection technology.

Field assessment of dihaploid Solatium tuberosum and S. brevidens somatic hybrids

Following both chemical and electrical fusion of protoplasts of a dihaploid line of potato (Solanum tuberosum), (PDH40), with those of the wild species, Solanum brevidens, 11 and 40 somatic hybrid plants, respectively were obtained. Fifteen of these somatic hybrid genotypes and the two parents were studied further in a small field trial to assess field performance and phenotypic variability. In the UK, somatic hybrid plants are classified as genetically engineered organisms, and the UK Advisory Committee on Genetic Manipulation have imposed various restrictions on field experiments. Examination of the somatic hybrids in the field showed extensive phenotypic variability, and no two genotypes were identical. Some of the variation reflected changes in chromosome numbers, but other factors were also involved. Half the somatic hybrid genotypes produced tubers in the field, although the tubers were smaller and differed morphologically from those of PDH40. The results of the study suggest that the extent of somaclonal variation manifested in somatic hybrids is greater than that found in protoplast-derived plants of potato. The implications of this observation and the current regulations concerning field experiments of somatic hybrid plants in the UK are discussed.

In vitro selection of alfalfa plants resistant to Fusarium oxysporum f. sp. medicaginis

From two lines of Medicago sativa characterized by a high regeneration capability, calli resistant to culture filtrate of Fusarium oxysporum f. sp. medicaginis have been selected. In these calli regeneration capability was greatly reduced and only one plant per callus was recovered. Regenerated plants have been evaluated for resistance to culture filtrate and for in vivo resistance to the pathogen. Three plants out of eight were resistant to the fungus and a high correlation between resistance to culture filtrate and in vivo resistance was observed.

Uniformity of plants regenerated from orange (Citrus sinensis Osb.) protoplasts

Using 25 plants (protoclones) regenerated from orange (Citrus sinensis Osb.) protoplasts, several characters, including leaf and flower morphology, leaf oil, isozyme patterns and chromosome number, were examined. No significant variations in each character were recorded among the protoclones. Uniformity observed among protoclones was identical to that of nucellar seedlings.

Is the Phytophthora citrophthora culture filtrate a reliable tool for the in vitro selection of resistant Citrus variants?

Nucellar calli from four Citrus cultivars with known resistance to the Phytophthora citrophthora pathogen were chosen as experimental material to test the pathogen's response to culture filtrate (CF). Sensitivity of the four calli to CF of the fungus was in reverse order to what is known on the susceptibility of the cultivars in vivo. Sensitivity of protoplasts derived from the same four calli to 2,4-dichlorophenoxyacetic acid (2,4-D) was in the same order as that of calli to CF. Protoplasts derived from calli selected for tolerance to CF showed a higher plating efficiency with increasing concentration of CF in the medium. TLC and GLC determinations showed the presence of indole acetic acid in the culture filtrate. Results indicate that CF of P. citrophthora cannot be used as a selection tool in vitro.

Improvements in regeneration from protoplasts of potato and studies on chromosome stability : 1. The effect of initial culture media

Regeneration of plants from protoplasts of potato (Solanum tuberosum) cv. 'Maris Bard' has been achieved from four different initial culture media (ET2, ET3, CLG, VkCLG). These media differed in their hormone, salt and sugar content. Plating efficiencies were highest in the VkCLG medium, but no correlation was found between plating efficiency and regeneration frequency (i.e. the percentage of calli producing shoots). Regeneration frequencies were high on all four media; up to 95% on ET3. Chromosome counts of up to 50 regenerants selected at random from the four treatments showed no significant differences between any of the treatments, in the proportions of plants which were euploid (48), aneuploid at the tetraploid level (48±), and aneuploid with high chromosome numbers (48+ +). Highly significant differences were present, however, between shoots which rooted quickly (predominantly euploid) and those which rooted only after transfer to a rooting medium (predominantly 48+ +). Overall more than 60% of the regenerants were normal (2n=4x=48) and this is a considerable improvement on our earlier work in this cultivar (4% normal). These findings are discussed in relation to factors affecting chromosome stability. Chromosome structural rearrangements are also described.

Expression of nuclear-cytoplasmic genomic incompatibility in interspecific Petunia somatic hybrid plants

Somatic hybrid plants were regenerated following calcium-high pH fusion of the unidirectional, sexually incompatible cross of Petunia parodii wild-type leaf mesophyll protoplasts with protoplasts from a cytoplasmic determined chlorophyll-deficient mutant of P. inflata. Genic complementation to chlorophyll synthesis and sustained growth in the selective medium was used to visually identify hybrid calluses. Hybrid calluses were subsequently regenerated to shoots, rooted, and confirmed as somatic hybrids by their intermediate floral and leaf morphology based on comparison to the 2 n = 4 x = 28 sexual counterpart, dominant anthocyanin expression in the corolla, chromosome number, and peroxidase and maleic dehydrogenase isozyme patterns. Certain cytologically stable somatic hybrids displayed aberrant reproductive and floral morphologies including subtle to moderate corolla and leaf pigment variegation, floral dimension changes and reduced pollen viability. In contrast, cytologically unstable somatic hybrids showed various degrees of aneuploidy coupled with corolla splitting, and irregularities in reproductive organs such as double stigmas and styles in addition to reduced pollen viability. Postulated mechanisms to account for these phenotypic changes in stable and unstable somatic hybrids include nuclear-cytoplasmic genomic incompatibility, chromosome loss in a biparental cytoplasm, or a phenomenon similar to hybrid dysgenesis occurring as a result of somatic fusion.

Comparison between a Stable NaCl-Selected Nicotiana Cell Line and the Wild Type : K, Na, and Proline Pools as a Function of Salinity

An NaCl-resistant line has been developed from suspension-cultured tobacco cells (Nicotiana tabacum/gossii) by stepwise increases in the NaCl concentration in the medium. Resistance showed stability through at least 24 generations in the absence of added NaCl.Above an external NaCl concentration of 35 millimolar, proline concentration in the selected cells rose steeply with external NaCl, particularly so above 100 millimolar NaCl. Proline accumulation in the wild type was far slighter. Selected cells which had been grown for 24 generations in the absence of added NaCl accumulated proline strongly on re-exposure to NaCl medium, indicating stability of this character. Proline accumulation was fully reversible with a half-time of about 6 hours. When selected cells were transferred sequentially to lower and lower NaCl concentrations, their proline content fell to the level corresponding to the new NaCl concentration. The NaCl-selected cells responded to water stress (i.e. added mannitol) by accumulating markedly more proline than did the wild type.The addition of Ca(2+) to the growing and rinsing media minimized Na(+) and K(+) binding in the Donnan free space of cell walls and thus allowed assessment of intracellular Na(+) and K(+). In both cell types, internal Na(+) content rose steadily as a function of external NaCl concentration. In the course of 7 days in NaCl media, the wild type cells lost a considerable part of their K(+) content, the extent of the loss increasing with rise in external NaCl concentration. The selected cells, by contrast, lost no K(+) at external NaCl concentrations below 50 millimolar external NaCl, and at higher concentrations lost less than the wild type.

Resistant tobacco plants from protoplast-derived calluses selected for their resistance to Pseudomonas and Alternaria toxins

Protoplast-derived calluses of tobacco (Nicotiana tabacum cv. 'Samsun') were selected for their resistance to toxins from Pseudomonas syringae pv. tabaci, which causes wildfire disease, and from Alternaria alternata pathotype tobacco, which causes brown spot. A number of plants were regenerated from each of the toxin-selected protoplast-derived calluses. A large percentage of the plants obtained from the second selection cycle calluses were resistant to infection by these pathogens. Resistance to wildfire disease, however, seems to be unrelated to resistance to brown spot disease. Variations in the morphological characteristics of the regenerated plants were found. Results of an assay of the R1 generation indicate that the resistance shown by R0 plants against both disease is heritable.

Phenotypic variation and ploidy level of plants regenerated from protoplasts of tetraploid potato (Solanum tuberosum L. cv. 'Bintje')

A wide range of phenotypic variation occurred among protoplast - derived plants of tetraploid potato cultivar 'Bintje'. The variant plants had alterations in growth and vigour, and in leaf and stem characteristics. The results suggest that the altered morphologies are caused predominantly by changes in ploidy levels. Some alterations could be attributed typically to octoploidy and aneuploidy. The occurrence of mixoploidy indicates that at least part of the observed variation arose during culture stage. The exogeneous cytokinin or auxin level and their combination during in vitro phase influenced the frequency of the variants observed. The origin of variation is discussed.

Chromosome variation in protoplast-derived potato plants

Chromosomes have been studied in protoplast-derived potato plants of the tetraploid cultivars Maris Bard and Fortyfold. A high degree of aneuploidy was found amongst the regenerants of both cultivars but the nature of the chromosome variation differed. The Maris Bard regenerants were characterised by high chromosome numbers, a wide range of aneuploidy (46-92) and a low percentage of plants with the normal chromosome number (2n = 48), whereas a much higher proportion of the Fortyfold regenerants had 48 chromosomes and the variants were within a more limited aneuploid range. In both cultivars chromosome variation was found between calluses, within calluses and even within shoot cultures. The origin of the chromosome variation and the differences found between the two cultivars are discussed.

A selective tissue culture medium for growth of compact (dwarf) mutants of apple

Three strains of 'McIntosh' apple (Malus domestica Borkh.) with growth habits ranging from the standard parent type to extremely compact (dwarf) were grown in vitro as meristem-tip cultures on Murashige and Skoog medium containing a range of concentrations of benzyladenine (BA). All strains exhibited a similar optima (3 to 6 μM BA) for maximum shoot proliferation and culture weight increase. However, tolerance to supra-optimal concentration of this cytokinin was related to growth habit. For example, at 10 μM BA shoot production rates as a percent of the maximum rates were 90%, 20% and zero for the extreme compact, moderate compact and standard strains, respectively. Comparisons among field trees and meristem-tip cultures of all three strains revealed similarities in growth and development.

Resistance responses to Phoma lingam of plants regenerated from selected cell and embryogenic cultures of haploid Brassica napus

Resistant plants and plants with reduced susceptibility against the pathogen Phoma lingam could be regenerated from selected callus and embryogenic cultures of haploid rape (Brassica napus) previously treated with mutagens. In the two in vitro selection systems used - absence of fungus growth on the cultures after incubation with parasite spores and resistance to the toxic filtrate - the resistance to the toxin was effective. In addition, some regenerants with increased tolerance were obtained from unselected cultures. Resistance tests on regenerated plants were carried out by inoculation of whole plants in the greenhouse, reproducing as much as possible the infection mechanisms which take place under natural conditions. Preliminary results on resistance of the progeny of single susceptible and tolerant regenerants seem to indicate that the acquired resistances are of a genetic nature.

In vitro selection of Nicotiana sylvestris variants with limited resistance to TMV

Haploid tobacco (Nicotiana sylvestris) plants were inoculated with a yellow strain of tobacco mosaic virus (TMV-Flavum) and then exposed to 500 rads of acute gamma radiation. Leaf strips cultured on callus-inducing medium yielded two types of colonies: 1) yellow, virus-infected and 2) green, apparently healthy. Of the 3210 calli scored, approximately 5% were virus-free, and after regeneration, 0.2% were resistant at the plant stage. Later, adult plants, both TMV-resistant and TMV-susceptible, produced self-fertile, diploid flowers. Both seedling progeny and rooted cuttings from resistant stock plants showed resistance to TMV infection. This resistance was characterized by restricted virus multiplication and movement within the infected plant resulting in a 3-8 week delay in symptom expression.

Somaclonal variation - a novel source of variability from cell cultures for plant improvement

It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.