A QTL that confers resistance to Colorado potato beetle (Leptinotarsa decemlineata [Say]) in tetraploid potato populations segregating for leptine

Genetic resistance to Colorado potato beetle (Leptinotarsa decemlineata [Say]) from Solanum chacoense has been incorporated in the tetraploid potato selection, ND4382-19, which is highly resistant and contains moderate level of foliar leptines. We recently reported using ND4382-19 progeny, population ND5873 (ND4382-19 × Chipeta), to map two genes that segregated as complementary epistatic genes that allow accumulation of leptinidine (Lep) and acetyl-leptinidine (AL) on chromosomes 2 and 8, respectively. We describe here the characterization of a second half-sib population NDG116 (ND4382-19 × N142-72). In this population, solasodine from parent N142-72, which has Solanum berthaultii in its background, was predominant over solanidine-based alkaloids. Concentrations of solanidine, leptinidine, and acetyl-leptinidine were 15-, 5-, and 14-fold lower than in the ND5873 population. Nevertheless, Lep and AL mapped to the same locations on chromosomes 2 and 8 of parent ND4382-19, respectively. The two populations were evaluated for resistance to Leptinotarsa in field assays, and by detached leaf assay for population NDG116. In both families, QTL analysis identified a major QTL from ND4382-19 on the distal end of chromosome 2, close to the Lep locus. The contribution of this QTL to resistance ranged from 11 to 34% for ND5873 at four field sites. Contribution to resistance from the linkage group that contains the gene AL for the accumulation of leptine was not detected. In family NDG116, the same chromosome 2 QTL was detected for field and detached leaf assays, explaining 26 and 12% of the variance for defoliation and larval development, respectively. These data may indicate another resistance mechanism besides leptine in the Leptinotarsa resistance observed in these populations.     

Mapping of genes associated with leptine content of tetraploid potato

High content of leptine glycoalkaloids present in Solanum chacoense has been associated with genetic resistance to Colorado potato beetle (Leptinotarsa decemlineata [Say]). From an unrecorded accession of S. chacoense, the North Dakota State University breeding program has developed a tetraploid genotype, ND4382-19, that contains foliar leptines. In this study, using a segregating population, ND5873 (ND4382-19 × Chipeta), and GC-MS to analyze foliar content of alkaloids, two loci, involved in the synthesis of leptines were identified. They segregated as two complementary epistatic genes that allowed the synthesis of leptinidine (Lep) and acetyl-leptinidine (AL), respectively. Partial AFLP maps for both parents were developed using 97 individuals from population ND5873. The total lengths mapped for ND4382-19 and Chipeta were 1,883 and 1,021 cM, respectively. The marker for Lep was located at the distal end of simplex-coupling linkage group R37. Expansion of the initial mapping population and analysis of Lep-containing individuals allowed us to identify the linkage group (R35) that enabled synthesis of AL. By the use of simple sequence repeat markers, linkage group R37 (Lep) and linkage group R35 (AL) have been identified as homologs of chromosomes II and VIII, respectively.H. Casper (deceased)     

Identification of molecular markers associated with leptine production in a population of Solanum chacoense Bitter

  Solanum chacoense Bitter, a wild relative of the cultivated potato, produces several glycoalkaloids, including solanine, chaconine, and the leptines. The foliar-specific leptine glycoalkaloids are believed to confer resistance to the Colorado Potato Beetle (CPB). Using two bulked DNA samples composed of high- and low-percent leptine individuals from a segregating F₁ population of S. chacoense, we have identified two molecular markers that are closely linked to high percent solanine+chaconine and, conversely, to nil/low percent leptine. One of these, a 1,500-bp RAPD product (UBC370-1500), had a recombination value of 3% in the F₁ progeny, indicating tight linkage. UBC370-1500 mapped to the end of the short arm of potato chromosome 1, in the region of a previously mapped major QTL for solanidine, from a S. tuberosum (solanidine)×S. berthaultii (solasodine) cross. Taken together, these results suggest that either (1) a major locus determining solanidine accumulation in Solanum spp. is on chromosome 1 in the region defined by the RFLP markers TG24, CT197, and CT233, or (2) this region of chromosome 1 may harbor two or more important genes which determine accumulation of steroidal aglycones. These findings are important for the genetics of leptine (as well as other glycoalkaloid) accumulation and for the development of CPB-resistant potato varieties. Received: 5 March 1998 / Accepted: 28 July 1998     

Partial preparative purification of leptine I from foliage of the wild potato, Solanum chacoense (Bitt.)

Leptine I, a glycoalkaloid only known to occur in the foliage of the wild potato species Solanum chacoense (Bitt.), is a potent feeding deterrent to the economically serious insect pest, the Colorado potato beetle (Leptinotarsa decemlineata Say). In order to demonstrate, systematically, the effectiveness of leptine I, incorporation into synthetic beetle diet trials is necessary. We describe a preparative procedure for the partial purification of leptine I by a series of steps, starting with a solid-phase C18 extraction, followed by sequential silica gel chromatography, and finally reversed-phase preparative HPLC. This preparation yields a white powder, containing leptine I as the sole glycoalkaloid, with an overall purity of greater than 65%, and is entirely suitable for incorporation into synthetic diets.     

Insecticidal activity of avidin combined with genetically engineered and traditional host plant resistance against Colorado potato beetle (Coleoptera: Chrysomelidae) larvae

Colorado potato beetle, Leptinotarsa decemlineata (Say), is a destructive pest of potato, Solanum tuberosum (L.), in North America. It is renowned for adapting to insecticides. With the arsenal of effective insecticides decreasing, it is important to consider alternative forms of control. Biotin is an essential coenzyme for insect growth and development. Avidin is a protein found in chicken egg that sequesters biotin and has shown insecticidal properties against a range of insect. We assessed the effectiveness of avidin against the Colorado potato beetle neonates in a no-choice detached leaf bioassay at 0, 17, 34, 51, 102, and 204 microg avidin/ml over 12 d. The LC50 was 136 microg avidin/ml (108-188 95% CL). The combined effects of avidin (136 microg avidin/ml) with Bt-Cry3A or leptines were evaluated with neonates and third instars over 12 and 6 d, respectively. Three potato lines were used: susceptible line, a line engineered to express Cry3A from Bacillus thuringiensis, and a line expressing the natural resistance factor leptines. The addition of avidin at the LC50 concentration significantly reduced consumption by neonates, but it did not affect consumption by third instars feeding on the susceptible line and the leptine line. Survival of neonates feeding on the susceptible line with avidin was significantly reduced compared with the susceptible line. Survival of third instars on the Bt-Cry3A with avidin was significantly reduced after 3 d compared with survival on the Bt-Cry3A, suggesting the addition of avidin may increase susceptibility to Bt-Cry3A.     

Identification of molecular markers associated with leptine in reciprocal backcross families of diploid potato

Solanum phureja clone 1-3 and S. chacoense clone 80-1 have a zero and high leptine content in their foliage, respectively. An F(1) hybrid (CP2) was intermediate for the trait, but self-incompatible. Two reciprocal backcross families, PBCp ( phu 1-3 x CP2) and PBCc (CP2 x phu 1-3), and a family of monoploids derived by anther culture of CP2, were characterized for leptine as the aglycon, acetylleptinidine (ALD), content in leaves by gas chromatography. ALD was present in 43 of 87 genotypes in the PBCp backcross, implying simple genetic control by a dominant gene. However, the ALD levels were low compared to CP2. In the PBCc backcross, only 7 of 42 genotypes expressed ALD at a level generally higher than in PBCp. This ratio was significantly different from the 1:1 segregation observed in the reciprocal backcross and suggests a cytoplasmic influence. ALD levels in the CP2 monoploids ranged from 0 to 8,968 &mgr;g.g(-1) of dry weight (dw) with 18 individuals expressing ALD and five with 0 ALD content. Ten high (mean ALD = 546 &mgr;g.g(-1) of dw) and ten low (mean ALD = 0) individual plants within PBCp and seven high (mean ALD = 3,037 &mgr;g.g(-1) of dw) and eight low (mean ALD = 0) individual plants within PBCc were used for bulk segregant analysis (BSA) using 214 RAPD (randomly amplified polymorphic DNA) primers. Three RAPD primers (OPQ-2, OPT-16 and OPT-20) amplified bands exclusively in bulks containing DNA mixes of high ALD producers in both PBCp and PBCc populations. These results suggest that these markers were associated in coupling to ALD content. ANOVAs for ALD content verified association between the markers and the trait. A CAPS (cleaved amplified polymorphic sequence) marker, GP82A, was also significantly associated with ALD production in both the monoploid and the PBCp populations. None of the RAPD markers was associated to ALD in the monoploids but one was associated in repulsion. The monoploid data indicate the likelihood of a recessive gene(s) that controls leptine production, but the backcross data indicate the action of modifying loci.     

Molecular markers associated with leptinine production are located on chromosome 1 in Solanum chacoense

Leptines of Solanum chacoense are effective natural deterrents against the Colorado potato beetle. Leptines are the acetylated forms of the glycoalkaloids solanine and chaconine and are supposed to be synthesised via hydroxylated derivatives, called leptinines. Inheritance of leptinine production was studied in crosses of closely related S. chacoense genotypes. The segregation data supported a single-gene model for the inheritance of leptinine production. In the segregating F₁ population of a S. chacoense cross, AFLP, RFLP and RAPD markers segregating with the leptinine production have been identified. The locus involved in leptinine synthesis was localised to the short arm of chromosome 1 of the potato where a major QTL for solanidine production, and markers with tight linkage to leptine production, have been mapped before. Our data further support the previous finding that the short arm of chromosome 1 is involved in steroid alkaloid synthesis in potato, and suggest that the genes involved in leptinine and leptine production are tightly linked in S. chacoense. Received: 27 June 2000 / Accepted: 4 August 2000     

Combining genetic engineering and traditional breeding to provide elevated resistance in potatoes to Colorado potato beetle

The sustainable deployment of resistant crop varieties is a critical issue for the implementation of biotechnology in crop pest management. Feeding, biomass accumulation, and mortality were evaluated for susceptible, insecticide‐resistant, and Bacillus thuringiensis (Bt) Cry 3A‐selected Colorado potato beetle (Leptinotarsa decemlineata Say) (Coleoptera, Chrysomelidae) larvae fed on: cultivated potato, a Solanum chacoense line expressing leptine glycoalkaloids, a transformed line expressing Bt toxin, or the leptine line transformed to express Bt toxin. Larvae selected for resistance to Bt‐Cry3A performed better on Bt foliage, but not as well on the leptine foliage, compared to susceptible or insecticide‐resistant larvae. Neither leptine nor Bt toxin completely inhibited the feeding and growth of 3rd and 4th instars of all three strains of Colorado potato beetle. However, for all three strains of Colorado potato beetle on leptine + Bt foliage, feeding was almost zero, growth was zero or negative, and mortality was near 100%.     

Evaluation of natural and engineered resistance mechanisms in potato against Colorado potato beetle in a no-choice field study

The Colorado potato beetle, Leptinotarsa decemlineata Say, is the major insect pest of potato, Solanum tuberosum L., in eastern North America and is renowned for resistance development, currently resistant to >40 insecticides worldwide. Host plant resistance may assist in delaying in resistance development to insecticides. We evaluated natural host plant resistance mechanisms (glandular trichomes and Solanum chacoense Bitter-derived resistance) and engineered resistance mechanisms (Bacillus thuringiensis [Bt] Berliner cry3A and cry1Ia1) in a no-choice cage study. Six different potato lines representing four host plant resistance mechanisms were evaluated over 2 yr. Egg masses were placed in each cage (one egg mass per plant). Almost no feeding was observed in the Bt-cry3A lines, and only minor feeding was observed in the Bt-cry1Ia1 lines in either year. On the S. chacoense-derived line, there was significantly less defoliation than on either the susceptible line or the glandular trichome line in 2003. In 2004, there was significantly higher defoliation on the S. chacoense-derived line than on the susceptible line or glandular trichome line. The defoliation of the Solanum chacoense-derived line was largely due to larvae clipping the petioles, rather than consumption of the leaves. Defoliation on the glandular trichome line did not differ significantly from the defoliation of the susceptible line, suggesting glandular trichomes may not be effective in controlling larvae and preventing defoliation. This study suggested that Bt can provide high levels of resistance, but the natural resistance mechanisms tested here are variable for control of Colorado potato beetle larvae in no-choice situations.     

Argentinian wild diploid Solanum species as sources of quantitative late blight resistance

Greenhouse and field evaluations of quantitative resistance to late blight in genotypes of the wild Argentine diploid species Solanum chacoense, Solanum commersonnii, Solanum microdontum and Solanum maglia were performed with a complex race containing the virulence factors 1, 3, 4, 5, 7,10, 11 of Phytophora infestans and using the percentage of leaf area affected by late blight estimated at weekly intervals. From these readings the area under the disease progress curve (AUDPC) was calculated. Highly resistant and susceptible genotypes were identified. Resistance and variability were found in S. chacoense, which together with S. commersonnii showed a significantly higher level of resistance compared to the susceptible checks. Field evaluations of an F₁ progeny of 165 individuals, obtained from non-inbred diploid parents segregating for foliage quantitative resistance in S. chacoense, were also done by means of AUDPC. The polygenic nature of the resistance and some indication of the presence of both additive and interaction effects were evident. Received: 30 August 1999 / Accepted: 30 December 1999     

Allelic variation in genes contributing to glycoalkaloid biosynthesis in a diploid interspecific population of potato

Key message

Variation for allelic state within genes of both primary and secondary metabolism influences the quantity and quality of steroidal glycoalkaloids produced in potato leaves.

Abstract

Genetic factors associated with the biosynthesis and accumulation of steroidal glycoalkaloids (SGAs) in potato were addressed by a candidate gene approach and whole genome single nucleotide polymorphism (SNP) genotyping. Allelic sequences spanning coding regions of four candidate genes [3-hydroxy-3-methylglutaryl coenzyme A reductase 2 (HMG2); 2,3-squalene epoxidase; solanidine galactosyltransferase; and solanidine glucosyltransferase (SGT2)] were obtained from two potato species differing in SGA composition: Solanum chacoense (chc 80-1) and Solanum tuberosum group Phureja (phu DH). An F₂ population was genotyped and foliar SGAs quantified. The concentrations of α-solanine, α-chaconine, leptine I, leptine II and total SGAs varied broadly among F₂ individuals. F₂ plants with chc 80-1 alleles for HMG2 or SGT2 accumulated significantly greater leptines and total SGAs compared to plants with phu DH alleles. Plants with chc 80-1 alleles at both loci expressed the greatest levels of total SGAs, α-solanine and α-chaconine. A significant positive correlation was found between α-solanine and α-chaconine accumulation as well as between leptine I and leptine II. A whole genome SNP genotyping analysis of an F₂ subsample verified the importance of chc 80-1 alleles at HMG2 and SGT2 for SGA synthesis and accumulation and suggested additional candidate genes including some previously associated with SGA production. Loci on five and seven potato pseudochromosomes were associated with synthesis and accumulation of SGAs, respectively. Two loci, on pseudochromosomes 1 and 6, explained phenotypic segregation of α-solanine and α-chaconine synthesis. Knowledge of the genetic factors influencing SGA production in potato may assist breeding for pest resistance.     

Host acceptance behaviour of Colorado potato beetle larvae influenced by potato glandular trichomes

ABSTRACT. In no-choice tests, larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae), avoided contact with, and were less frequently observed feeding on, excised foliage of the resistant wild potato Solanum berthaultii Hawkes than on leaves of the cultivated potato, S.tuberosum L. For older larvae, reduced feeding was a consequence of less frequent contact with S.berthaultii leaves. However, first instars appeared unable to penetrate the adhesive barrier presented by glandular trichomes of S. berthaultii , and those that remained on the leaflets fed less often than did first instars on S. tuberosum. Removal of the trichome barrier by wiping leaflets with tissue paper did not attenuate the apparent repellent effect of S. berthaulti foliage, but led to increased incidence of feeding by first instars. Inhibition of larval feeding may therefore rely on a mechanical barrier provided by the glandular trichomes of S.berthaultii.     

Characterization of the expression and inheritance of potato leafroll virus (PLRV) and potato virus Y (PVY) resistance in three generations of germplasm derived from Solanum etuberosum

Potato virus Y (PVY) and potato leafroll virus (PLRV) are two of the most important viral pathogens of potato. Infection of potato by these viruses results in losses of yield and quality in commercial production and in the rejection of seed in certification programs. Host plant resistance to these two viruses was identified in the backcross progeny of a Solanum etuberosum Lindl. somatic hybrid. Multiple years of field evaluations with high-virus inoculum and aphid populations have shown the PVY and PLRV resistances of S. etuberosum to be stably expressed in two generations of progeny. However, while PLRV resistance was transmitted and expressed in the third generation of backcrossing to cultivated potato (Solanum tuberosum L. subsp. tuberosum), PVY resistance was lost. PLRV resistance appears to be monogenic based on the inheritance of resistance in a BC₃ population. Data from a previous evaluation of the BC₂ progeny used in this study provides evidence that PLRV resistance was partly conferred by reduced PLRV accumulation in foliage. The field and grafting data presented in this study suggests that resistance to the systemic spread of PLRV from infected foliage to tubers also contributes to the observed resistance from S. etuberosum. The PLRV resistance contributed by S. etuberosum is stably transmitted and expressed through sexual generations and therefore would be useful to potato breeders for the development of PLRV resistant potato cultivars.     

Inheritance and synergism of resistance to imidacloprid in the Colorado potato beetle (Coleoptera: Chrysomelidae)

Colorado potato beetle, Leptinotarsa decemlineata (Say), adults and larvae collected from Long Island, NY, were 100.8 and 13.2 times more resistant to imidacloprid, respectively, compared with a susceptible strain. This high level of resistance appeared in only the third field season of imidacloprid use. Analysis of probit lines from F1 reciprocal crosses indicated that resistance to imidacloprid in adults was inherited autosomaly as an incompletely recessive factor. The degree of dominance of the resistance was -0.23 and -0.10, respectively, 3 and 7 d after treatment (incompletely recessive). The chi2 analysis of response ratio statistics from F1 x susceptible back crosses compared with a monogenic model suggested that more than one locus is responsible for resistance to imidacloprid. Synergism studies with piperonyl butoxide suggested that mixed-function oxidase mediated detoxification is responsible for the resistance to imidacloprid in adults. Synergism studies with S,S,S-tributyl phosphorotrithioate (DEF) indicated that esterase mediated detoxification may be an additional resistance factor. Mixed-function oxidase mediated detoxification is probably also one of the mechanisms of resistance to imidacloprid in larvae. Because the synergists used did not completely eliminate resistance in the resistant strain, there may be additional mechanisms involved. Refugia and crop rotation decrease the frequency of homozygous resistant genotypes and may be effective resistance management strategies, because of the recessive nature of the resistance.     

Inheritance of resistance to Bacillus thuringiensis subsp. kurstaki in Trichoplusia ni

The genetic inheritance of resistance to a commercial formulation of Bacillus thuringiensis subsp. kurstaki was examined in a Trichoplusia ni colony initiated from a resistant population present in a commercial vegetable greenhouse in British Columbia, Canada. Progeny of F(1) reciprocal crosses and backcrosses between F(1) larvae and resistant (P(R)) and susceptible (P(S)) populations were assayed at different B. thuringiensis subsp. kurstaki concentrations. The responses of progeny of reciprocal F(1) crosses were identical, indicating that the resistant trait was autosomal. The 50% lethal concentration for the F(1) larvae was slightly higher than that for P(S), suggesting that resistance is partially recessive. The responses of both backcross progeny (F(1) x P(R), F(1) x P(S)) did not correspond to predictions from a single-locus model. The inclusion of a nonhomozygous resistant parental line in the monogenic model significantly increased the correspondence between the expected and observed results for the F(1) x P(R) backcross but decreased the correspondence with the F(1) x P(S) backcross results. This finding suggests that resistance to B. thuringiensis subsp. kurstaki in this T. ni population is due to more than one gene.     

Successful first and second backcrosses of S. nigrum (+) S. tuberosum somatic hybrids with both Solanum parents

Somatic hybrids of Solanum nigrum (+) 2× potato were successfully crossed with S. nigrum and with potato. First and second backcross progeny with S. nigrum could easily be obtained. One of the BC1 genotypes was already self-fertile. Backcrosses with potato had a much lower success rate. Only pollinations with tetraploid potato resulted in seed-containing berries. Two BC1 genotypes were obtained after 4362 pollinations from which 505 ovules were cultured. The first BC1 genotype grew vigorously in vitro and in the greenhouse and flowered abundantly. The second BC1 showed many abnormalities and dropped its flowers before anthesis. The first BC1 was again crossed with tetraploid potato and in this generation also the success rate was low. Over 5000 pollinations resulted in 1750 berries from which over 3000 ovules were obtained. Twelve plants germinated from these ovules and they were not as vigorous in vitro and in vivo as the BC1 parent. Some of the BC2 genotypes were used for further backcrosses but no BC3 plants were obtained. BC1 and BC2 genotypes that resulted from the backcross program with potato were tested for resistance to Phytophthora infestans. The BC1 genotype was as resistant as the S. nigrum fusion parent, but among the eight BC2 genotypes scored six were resistant, whereas two genotypes showing lesions were susceptible. Received: 19 October 1998 / Revision accepted: 20 April 1999     

Inheritance of Cry1Ac resistance and associated biological traits in the cotton bollworm, Helicoverpa armigera (Lepidoptera: Noctuidae)

The analysis of reciprocal genetic crosses between resistant Helicoverpa armigera strain (BH-R) (227.9-fold) with susceptible Vadodara (VA-S) strain showed dominance (h) of 0.65-0.89 and degree of dominance (D) of 0.299-0.782 suggesting Cry1Ac resistance as a semi-dominant trait. The D and h values of F₁ hybrids of female resistant parent were higher than female susceptible parent, showing maternally enhanced dominance of Cry1Ac resistance. The progeny of F₂ crosses, backcrosses of F₁ hybrid with resistant BH-R parent did not differ significantly in respect of mortality response with resistant parent except for backcross with female BH-R and male of F₁ (BH-R × VA-S) cross, suggesting dominant inheritance of Cry1Ac resistance. Evaluation of some biological attributes showed that larval and pupal periods of progenies of reciprocal F₁ crosses, backcrosses and F₂ crosses were either at par with resistant parent or lower than susceptible parent on treated diet (0.01 μg/g). The susceptible strain performed better in terms of pupation and adult formation than the resistant strain on untreated diet. In many backcrosses and F₂ crosses, Cry1Ac resistance favored emergence of more females than males on untreated diet. The normal larval period and the body weight (normal larval growth) were the dominant traits associated with susceptible strain as contrast to longer larval period and the lower body weight (slow growth) associated with resistance trait. Further, inheritance of larval period in F₂ and backcross progeny suggested existence of a major resistant gene or a set of tightly linked loci associated with Cry1Ac sensitivity.     

Multiple components of the resistance of potatoes to potato leafroll virus

In glasshouse experiments the ranking of potato genotypes for resistance to infection with potato leafroll virus (PLRV) using three concentrations of aphid-borne inoculum was the same as their field resistance ratings. In field-grown plants this resistance to infection increased in all genotypes as the plants aged but its rate of increase differed between genotypes. In tests on field-grown plants infected by aphid- or graft-inoculation, the proportion of virus-free progeny tubers increased the later the date of inoculation but was greater in resistant than in susceptible genotypes. This trend was most pronounced in the resistant clone G7445(1), in which the virus failed to move from the foliage to the tubers of some plants infected in glasshouse tests. The spread of PLRV will thus be minimised in crops of resistant compared with susceptible genotypes for three reasons: plants have greater resistance to infection, systemic spread of virus from their foliage to tubers is less likely and, as shown previously, the low concentration of virus particles in leaf tissue makes infected plants less potent sources of inoculum for aphids.     

Is there meaningful plant resistance to diaprepes abbreviatus (Coleoptera: Curculionidae) in citrus rootstock germplasm?

Host plant resistance to the Diaprepes root weevil, Diaprepes abbreviatus (L.) was assessed for seedlings of 54 Poncirus trifoliata (L.) Raf. selections and two families of progeny from crosses between Citrus and P. trifoliata. Weight gain was consistently lower when larvae were reared in pots containing the progeny of Citrus reticulata Blanco 'Sunki' x P. trifoliata 'Flying Dragon' compared with larvae reared on progeny of 'Pearl' (C. reticulata x C. paradisi Macf.) x 'Flying Dragon'. This is the first evidence of genetic control of resistance to the Diaprepes root weevil within sexually compatible citrus rootstock germplasm. There was a significant positive correlation between percentage root loss and larval weight gain within the resistant progeny, indicating a possible antixenotic effect. Two varieties of P. trifoliata were identified as more resistant than 'Flying Dragon' based on larval weight gain.     

Paranoid potato: Phytophthora-resistant genotype shows constitutively activated defense

Phytophthora is the most devastating pathogen of dicot plants. There is a need for resistance sources with different modes of action to counteract the fast evolution of this pathogen. In order to better understand mechanisms of defense against P. infestans, we analyzed several clones of potato. Two of the genotypes tested, Sarpo Mira and SW93-1015, exhibited strong resistance against P. infestans in field trials, whole plant assays and detached leaf assays. The resistant genotypes developed different sizes of hypersensitive response (HR)-related lesions. HR lesions in SW93-1015 were restricted to very small areas, whereas those in Sarpo Mira were similar to those in Solanum demissum, the main source of classical resistance genes. SW93-1015 can be characterized as a cpr (constitutive expressor of PR genes) genotype without spontaneous microscopic or macroscopic HR lesions. This is indicated by constitutive hydrogen peroxide (H₂O₂) production and PR1 (pathogenesis-related protein 1) secretion. SW93-1015 is one of the first plants identified as having classical protein-based induced defense expressed constitutively without any obvious metabolic costs or spontaneous cell death lesions.