A modified enzymic browning assay for potato glandular trichomes

Tetralobulate (Type A) glandular trichomes are present on foliage of many wild as well as cultivated species of potatoes. These trichomes confer resistance to insect pests if their density, gland size and biochemical products are similar as those ofSolanum berthaultti Hawkes andS. polyadenium Greenm. Hybrids ofS. tuberosum L. ×S. berthaultii have been screened for resistance by measurement of phenolic oxidation activity in their Type A trichomes by use of an enzymic browning assay. However, variability in the procedure has prompted the development of an improved assay.     

Glandular trichomes: Enzymic browning assays for improved selection of resistance to the green peach aphid

Polyphenoloxidase (PPO) activity in the glandular trichomes of 32Solanum tuberosum ×S. berthaultii F₃ hybrid clones differentially resistant to the green peach aphid,Myzus persicae (Sulzer) was determined. Within this population of segregating clones, aphid infestation levels varied 8-fold, while a 5-fold difference in glandular trichome PPO activity was observed. PPO activity and aphid resistance at three sample dates were correlated as follows: r = 0.38 (P = 0.05), r = 0.31 (P = 0.10), and r = 0.55 (P = 0.01), respectively. The total browning potential test (TBP), a modification of the PPO assay, was developed for use in screening for aphid resistance. The TBP procedure is rapid, inexpensive, and highly effective in identifying aphid resistant clones.     

Phenolic constituents of glandular trichomes onSolanum berthaultii andS. polyadenium

Phenolic compounds in Type A glandular trichomes of two insectresistant potato species were separated and identified using a combination of high pressure liquid chromatography (HPLC), UV-visible spectrometry, and¹H proton nuclear magnetic resonance (NMR) procedures. Type A trichomes ofSolanum berthaultii andS. polyadenium contained a phenolic compound tentatively identified as the glucose ester of p-hydroxyphenylpropionic acid. Its presence in trichomes of both species suggests that this compound may play a major role in formation of the brownish, insect entrapping exudate characteristic of trichome-mediated insect resistance in these species. Chlorogenic acid, a major phenolic constituent of Type A trichomes ofSolanum polyadenium was not detected in trichomes of 5.berthaultii. The exclusive presence of chlorogenic acid in trichome exudates ofS. polyadenium may be responsible, at least in part, for the greater levels of insect entrapment by this species, compared with that ofS. berthaultii.     

Glandular pubescence,glycoalkaloid composition,and resistance to the green peach aphid,potato leafhopper,and potato fleabeetle inSolanum berthaultii

Fifteen accessions ofSolarium berthaultii Hawkes andS. berthaultii xS. tarijense Hawkes were assessed for resistance to field infestations of the green peach aphid,Myzus persicae (Sulzer), potato leafhopper,Empoasca fabae (Harris), and potato fleabeetle,Epitrix cucumeris (Harris). Accessions bearing both Type A and Type B glandular trichomes were much more resistant to the green peach aphid and potato leafhopper than accessions bearing Type A hairs alone. All accessions had significantly smaller populations of these 3 pests than S.tuberosum cultivars. Total glycoalkaloid (TGA) content of foliage and tubers was not correlated with insect populations. Foliar TGA levels of field-grown plants varied among accessions, ranging from < 2–240 mg/100 g fresh wt. Solasonine and solamargine were the major foliar glycoalkaloids while solamarines predominated in tubers.     

Green peach aphid resistance by glandular trichomes inSolanum tuberosum xS. berthaultii hybrids

Three interspecific F₃ hybrid clones ofSolarium tuberosum L. x 5.berthaultii Hawkes bearing both Type A and Type B glandular trichomes were assessed for resistance to the green peach aphid,Myzus persicae (Sulzer). In glasshouse trials, aphid mortality and population development on hybrid clones were similar to those on the aphid-resistant parent,S. berthaultii. Trichome exudate of the hybrid, B227-63, was as effective in aphid immobilization as that ofS. berthaultii. Field populations of the green peach aphid on hybrids were intermediate between those onS. berthaultii and susceptibleS. tuberosum cultivars.     

Association between trichome characters and agronomic traits inSolanum tuberosum (L.)XS. berthaultii (hawkes) hybrids

In 1977, a program to develop potato cultivars with insect resistance based on the glandular trichomes found onSolanum berthaultii was initiated at Cornell University. Associations between agronomic and trichome characters were suspected due to difficulties in selecting hybrid plants with both insect resistance and acceptable agronomic characteristics. Plants from backcrosses and intercrosses were studied. A strong association between the presence of B trichome droplets and undesirable agronomic characteristics was established in backcrossed generations. The backcross plants bearing droplets produced significantly lower yields, fewer tubers, later maturing plants, and poorer foliage and tuber appearance than their sibs without the droplets. In intercrosses, no associations were found. Good MEBA scores were seldom found in backcrossed plants without B droplets. The associations observed are speculated to be due to linkage or structural genomic differentiation between the genomes of the species involved in these populations.     

Mechanisms of resistance to potato leafhopper,Empoasca fabae (Harris), in potato

Potato genotypes were evaluated for resistance to natural infestations of the potato leafhopper,Empoasca fabae (Harris), in the field. Characterization of Type A and B glandular trichomes, phenolic properties of Type A trichomes, glycoalkaloid content and composition,E. fabae adult and nymph infestation, leaf damage (hopperburn), yield, and yield reduction were determined on genotypes PI 473331 and PI 473334 (accessions ofSolatium berthaultii (Hawkes)), NY123 and Q174-2 (advanced hybrids), and Elba and Allegany (commercial cultivars). Regarding the Type A trichome, PI 473334 was most pubescent, followed by PI 473331, both had high levels of enzymatic browning; on PI 473334 no glycoalkaloids were detected, while on PI 473331 solamargine and solasonine were present. Those genotypes were the least infested by adults; moreover, nymphs were never found on their leaves. PI 473331 and Q174-2 possess Type B trichomes, the latter had levels of Type A trichome comparable to the first and to NY123, while displaying high levels of phenolic oxidation of Type A and very low levels of glycoalkaloids. Although Q174-2 had intermediate infestation ofE. fabae, it suffered a yield reduction comparable to the control cultivar Allegany. The density of Type A trichomes of NY123 was relatively low, even though the browning assay results were significantly elevated. Foliage of this genotype was found to contain the four types of glycoalkaloids studied at high levels.Empoasca fabae infestation on NY123 was intermediate, and it yielded well on infested plots, but suffered an intermediate yield reduction. Elba presented densities of Type A similar to NY123. It suffered the lowest yield reduction despite its high levels of leafhopper infestation and hopperburn. Its tolerance might be associated with its phenology, since it is a late-maturing cultivar. High scores of the modified enzymatic browning assay were correlated with high infestation and high leaf damage. Results indicated a significant correlation between leafhopper infestation and hopperburn.     

Feeding behavior and survival of Colorado potato beetle,Leptinotarsa decemlineata (Say), larvae onSolanum berthaultii Hawkes and an F6 S. tuberosum L. XS. berthaultii hybrid

Newly hatchedLeptinotarsa decemlineata larvae were less likely to initiate feeding onSolanum berthaultii than onS. tuberosum. Among larvae that fed there was no difference in the time to initiation of feeding; however, the duration of the initial feeding bout was less onS. berthaultii than onS. tuberosum. Larvae spent less time feeding and more time walking onS. berthaultii than onS. tuberosum. All larvae caged onS. tuberosum fed within 24 hours, but larvae were less likely to feed on a F₆ S. tuberosum XS. berthaultii hybrid and least likely to feed onS. berthaultii. Lack of feeding during the first 24 h was highly correlated with mortality at 72 h. Whether or not larvae feed within the first five minutes of being placed on a leaflet is a reliable predictor of larval mortality at 72 h and is a useful test for determining the susceptibility of plants to feeding byL. decemlineata. The amount of trichome exudate accumulated on the tarsi over a 72 h period is correlated with resistance. Resistance ofS. berthaultii to neonateL. decemlineata is characterized by a refusal to initiate feeding and a reduced feeding rate by larvae that do feed.     

Mechanical interaction between larvae of the Colorado potato beetle and glandular trichomes ofSolanum berthaultii Hawkes

The rate of accumulation of glandular trichome exúdate ofSolanum berthaultii on the tarsi of Colorado potato beetle (Leptinotarsa decemlineata) larvae depended on larval size (instar), with large larvae bearing the hardened, darkened exudate on more of their tarsi than did small larvae. In greenhouse studies, third instar larvae accumulated large amounts of exudate regardless of which of 2S. berthaultii accessions they were exposed to, but exudate accumulation by first instars was influenced significantly by plant accession. Tarsal accumulation of exudate was lower in field plots of bothS. berthaultii accessions, but tended to increase with time of exposure. The significant differences in exudate accumulation among larvae exposed to the 2 accessions in the greenhouse were not evident in the field. The density of type A glandular trichomes on the abaxial leaflet surface differed significantly between the 2 accessions in both greenhouse and field. Despite these effects of host and environment on the accumulation of readily observed amounts of trichome exudate on larval tarsi, no larvae were trapped by the adhesive exudate.     

The germplasm release of NYL 235–4, a clone with resistance to the Colorado potato beetle

Clones selected from the wild diploid species,Solanum berthaultii, have been shown to possess valuable levels of resistance to the Colorado potato beetle (Leptinotarsa decemlineata), as well as to insects such as aphids, flea beetles, leafhoppers and the potato tuber moth. Resistance is associated with the presence of glandular trichomes on the foliage ofS. berthaultii (2). Six generations after producing a hybrid between the diploid species and a tetraploid variety, a clone with good beetle and leafhopper resistance, as well as reasonable agronomic characteristics has been produced.     

Variation in sucrose esters from the type B glandular trichomes of certain wild potato species

Accessions of four insect resistant wild potato species (i.e., S. berthaultii,S. neocardenasii, S. polyadenium andS. tarijense) were examined for the presence of glandular trichomes and any associated sucrose esters. Of the thirty accessions examined, ten proved positive for the presence of type B glandular trichomes and accompanying sucrose esters. Three distinct complexes of sucrose esters were identified (i.e., 3,4,6-tri-0, 3,3′,4,6-tetra-0-, and 2,3,3′,4-tetra-0-acylated) and the acyl substituents were found to vary significantly within each complex. Possible rationales for the observed distribution of the sucrose esters within the species are discussed and comparative data from some non-tuberousSolanums and other genera of the Solanaceae family are presented.     

Inheritance of glandular trichomes in crosses withSolanum berthaultii

The inheritance of trichome density and droplet size was investigated in diploid and tetraploid crosses. In haploidTuberosum ×Solanum berthaultii and (S.phureja ×haploid Tuberosum) × S.berthaultii crosses, droplet size on type B trichomes was small or droplets were lacking on F₁ hybrids. Parental phenotypes were recovered in the F₂ and backcross generations. Trichome density was more variable within a clone that was droplet size. Density of both type A and type B trichomes appears to be under the control of small numbers of genes, as parental phenotypes were recovered in the F₂ and backcross generations. The variation observed for each of the three traits studied could not be explained by hypotheses based on the segregation of one or two loci. The insect resistance ofSolanum berthaultii appears to be due to the interaction of several chemical and physical factors, thus potato breeders should handle it as a quantitatively inherited trait.     

Leaf effects of wild species ofSolanum and interspecific hybrids on growth and behaviour of the potato tuber moth,Phthorimaea operculella Zeller

Summary Four introductions ofSolanum berthaultii wild species and sevenS. tuberosumxS. berthaultii hybrids were evaluated by antibiosis and antixenosis laboratory tests with potato tuber moth. Some morphological and chemical traits related to the trichomes were analysed. Two clones of the wild speciesS. sparsipilum and five ofS. pinnatisectum, characterised by high tuber resistance, were tested for leaf resistance, both by antibiosis and antixenosis assays. The genotypes with high density of trichomes A and with high exudate PPO activity, showed a moderate negative effect on pupal weight and fecundity in the antibiosis tests. Trichomes B exudate showed a strong repulsive effect on oviposition of adult in the antixenosis tests. TheS. pinnatisectum clones revealed a good leaf resistance level, whereas theS. sparsipilum clones showed lower levels of antibiosis and antixenosis.     

Aphid-trapping glandular hairs on hybrids ofSolanum tuberosum andS. berthaultii

Summary The foliage of hybrids ofS. tuberosum cv. Pentland Crown andS. berthaultii bore four-lobed, glandular hairs which, when ruptured by aphid movements, released a sticky substance which glued the aphids to the plant. The transference of this property makes it more worthwhile testing as a means of decreasing the spread of aphid-borne viruses.     

Selection and utilization of solanum phureja clones with high resistance to different strains of pseudomonas solanacearum

Over 250 accessions ofSolanum phureja from Colombia and 300S. phureja—S. tuberosum hybrids were screened in the greenhouse for resistance toPseudomonas solanacearum (isolate K-60). Each plant was inoculated by stem puncture at the pre-bud stage and then held at a constant 28 C for 15 days. Seventeen clones with different levels of resistance to K-60 were inoculated with nine additional isolates representing all three races of the bacterium. The reactions of individual clones varied from resistance to almost all isolates, resistance to only one isolate, or complete susceptibility. Crosses amongS. phureja clones and betweenS. phureja and 24-chromosomeS. tuberosum stocks gave progeny with various percentages of resistant plants. Clones with resistance equal to that inS. phureja were found in theS. phureja × S. tuberosum crosses. Tests conducted in growth chambers showed that resistance was expressed more frequently at high light intensity (2000 ft-c) than at low intensities (1300 ft-c) at 24 C and 28 C. Resistance appears to be relatively simply inherited and the prospects for transferring a high level of resistance into cultivated forms are promising.     

Resistance in tuber-bearing solanum species to root knot nematode,Meloidogyne incognita

Forty-one clones of 34 accessions belonging to 25 tuber-bearingSolanum species were tested for their reaction to root knot nematode,Meloidogyne incognita. A high degree of resistance was found inS. bulbocastanum, S. gandarillasii, S. lignicaule, S. ajanhuiri andS. tuberosum groupandigena. S. vernei andS. spegazzinii were the most resistant species.     

Research inSolanum spp. of sources of resistance to the potato tuber mothPhthorimaea operculella (Zeller)

Summary The potato tuber moth (PTM) represents a dangerous pest on potato crop in Italy, spreading out from southern areas to central and northern areas. Introductions ofSolanum berthaultii, S. commersonii, S. pinnatisectum, S. sparsipilum, S. spegazzini, S. sucurense, S. tarijense wild species andS. tuberosum × S. berthaultii hybrids were evaluated for resistance to a PTM population isolated from a field in central Italy, by means of an antibiosis test on tubers in the laboratory and of an antixenosis trial in the field. SeveralS. sparsipilum andS. pinnatisectum clones showed an antibiosis effect on larval survival and antixenosis of the aerial and the hypogeal parts of the plant. In the field, the genotypes with leaf glandular trichomes did not revcal the expected antixenosis effect of the acrial part of the plant.     

Germplasm enhancement with wild tuber-bearing species: introgression of PVY resistance and high dry matter content fromSolanum berthaultii, S. gourlayi, S. tarijense andS. vernei

Summary The narrow genetic base of the cultivated potato is a severe constraint for potato breeding. Wild tuber-bearing species constitute an important source of genes for resistance to diseases and pests as well as abiotic factors. Seven interspecific hybrid populations were generated from crosses between diploidTuberosum clones and four wild species:Solanum berthaultii, S. gourlayi, S. tarijense andS. vernei, and evaluated in a field experiment in Burgos (Spain) as a way to broaden the genetic base of the cultivated potato. Good tuberization ability and great variability for yield within the different families were detected. Hybridization with the wild species resulted in high levels of PVY resistance inS. berthaultii andS. vernei hybrids and a large increase of dry matter content in all populations except theS. berthaultii hybrids.     

Glycoalkaloid content in foliage of hybrid and backcross populations from aSolanum tuberosum X S. chacoense cross

The glycoalkaloid contents of foliage were measured in populations derived from a cross ofSolanum chacoense Bitter (PI 472810) with a composite sample of genotypes from a randomly matedSolanum tuberosum L. population. The mean total glycoalkaloid contents of the parent PI 472810 andtuberosum populations, and the F₂ and F₄ populations were 856, 121, 286, and 279 mg/100g fresh weight, respectively. The total glycoalkaloid content of the first backcross population, F₂ xtuberosum, was 290 mg/100 g, close to the content of the F₂. In a second backcross totuberosum, the mean total content was 148 mg/100 g, close to the content of the parenttuberosum population. Only the glycoalkaloids α-solanine and α-chaconine were found in PI 472810, whereas in the parenttuberosum, hybrid, and backcross populations, the glycoalkaloids α & β-solamarine also were found in a small number of genotypes. The ratios of chaconine to solanine contents were significantly (P≤.05) different in the parent PI 472810 andtuberosum populations, 2.55 and 2.12, respectively. The ratios in the F₂ and F₄ populations were similar to that of PI 472810, whereas the ratios in the backcross populations were closer to that of thetuberosum parent. The levels of foliage glycoalkaloids in the hybrid and backcross populations paralleled the levels of potato leafhopper,Empoasca fabae Harris, resistance measured in a previous study.     

Resistance to late blight and soft rot in six potato progenies and glycoalkaloid contents in the tubers

Glycoalkaloids are anti-nutritional compounds commonly found in wildSolanum species used as resistance sources to major potato pathogens. It is therefore important for breeding purposes to know whether selecting for resistance using such species necessarily selects also for high glycoalkaloid contents in the tubers. To test this hypothesis, we used six partial progenies from crosses betweenSolanum tuberosum and accessions ofS. andigena, S. berthaultii, S. phureja, andS. vernei to investigate the possible correlation between resistance toPhytophthora infestans and/or toErwinia carotovora subsp.atroseptica and the concentration of glycoalkaloids in tubers. Concentrations of α-solanine and α-chaconine in the tubers segregated in each progeny, as did resistance to each pathogen. Some, but not all, clones from each progeny showed hypersensitive reactions to the isolate ofP. infestans used. Furthermore, clones within each progeny also differed for components of partial resistance toP. infestans, suggesting that all four wild species could be used as sources of both race-specific and partial resistance to late blight. With the exception of low, but statistically significant, correlations between concentration of α-solanine and two late blight resistance components (incubation period and spore production per unit lesion area) in progenies derived fromS. vernei, and despite a trend towards higher glycoalkaloid concentrations in the tubers of the clones most resistant to soft rot within progenies derived fromS. berthaultii andS. vernei, no consistent relationship between resistance to either disease and concentrations of α-solanine and/or α-chaconine was observed. These results indicate that neither race-specific nor partial resistance to late blight and soft rot in the accessions used as progenitors of resistance depend on high solanine or chaconine concentrations. These resistance sources could thus prove useful in breeding programs for improved behaviour againstP. infestans and/orE. carotovora.