Ward M. Tingey

Glycoalkaloids as pest resistance factors

Potato glycoalkaloids, a class of steroid glycosides, possess antimicrobial and pesticidal properties. These phytochemicals have been implicated as natural resistance factors inSolanum species against the Colorado potato beetle,Leptinotarsa decemlineata Say, and the potato leafhopper,Empoasca fabae (Harris). Adverse effects of these secondary metabolites on insect behavioral and developmental biology are described, as well as considerations in manipulation of foliar glycoalkaloids in breeding for varietal resistance to insects.ResumenLos glicoalcaloides de la papa, una clase de glicósidos esteroides, tienen propiedades antimicrobianas y de pesticida. Estos elementos fitoquímicos han sido incorporados en las especies deSolanum como factores naturales de resistencia al escarabajo de la papa,Leptinotarsa decemlineata Say, y a la cigarrita verde,Empoasca fabae (Harris). Se describen los efectos adversos de estos metabolitos secundarios en la biología del comportamiento y del desarrollo de los insectos, como también las consideraciones respecto a la manipulación de los glicoalkaloides foliares en el mejoramiento para resistencia varietal a insectos.

Aphid deterrence by glucose esters in glandular trichome exudate of the wild tomato,Lycopersicon pennellii

Settling of the potato aphid,Macrosiphum euphorbiae, on feeding membranes was deterred by methanolic leaf rinses ofLycopersicon pennellii, or of its F1 with tomato,L. esculentum. The active compounds in theL. pennellii rinsates were identified as 2,3,4-tri-O-acylglucoses bearing short to medium chain length fatty acids. These compounds are localized in the glandular exudate of the type IV trichomes and may accumulate to levels in excess of 400 μg/cm2. In choice assays, purified glucose esters fromL. pennellii reduced aphid settling at concentrations as low as 25 μg/cm2; at concentrations of 150 μg/cm2 or more, all aphids avoided treated areas. Glucose esters were also active in deterring aphid settling in no-choice assays. At 100 μg/ cm2, these esters resulted in increased levels of mortality after 48 hr.

Sucrose esters of carboxylic acids in glandular trichomes ofSolanum berthaultii deter settling and probing by green peach aphid

Removal of type B trichome exudate fromSolanum berthaultii leaflets leads to a decrease in tarsal gumming and mortality and an increase in feeding by the green peach aphid,Myzus persicae. Type B trichome exudate of theS. berthaultii accession PI 473331 is composed of a complex of 3′,3,4,6-tetra-O-acyl sucroses containing primarily short-chain branched carboxylic acids. The acyl constituents are primarily derived from 2-methylpropanoic, 2-methylbutyric, and 8-methylnonanoic acids but constituents derived fromn-decanoic and dodecanoic acids are also present. Sucrose esters inhibit settling and probing by aphids in glass feeding cages.

Phenolic oxidase activities in glandular trichomes of Solanum berthaultii

Abstract Polyphenol oxidase and peroxidase activities were detected in foliar glandular trichomes of the wild, insect-resistant potato species, Solanum berthaultii . These enzyme activities may provide the basis for conversion of clear, viscous trichome exudate into a hard, brown substance which is formed rapidly after insect attack.

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.ResumenBajo condiciones de campo e infestaciones naturales, quince entradas deSolatium berthaultii Hawkes yS. berthaultii x S. tarijense Hawkes fueron evaluadas para determinar su resistencia al áfido verde del melocotonero,Myzus persicae (Sulzer), cigarrita de la papa,Empoasca fabae (Harris) y pulguilla de la papa,Epitrix cucumeris (Harris). Las entradas que poseían ambos tipos de tricomas glandulares, tipo A y tipo B, fueron mucho más resistentes al áfido verde del melocotonero y a la cigarrita de la papa que las entradas que poseían solamente el tipo A de pelos. Todas las entradas tuvieron poblaciones significativamente menores de estas tres pestes que los cultivares deS. tuberosum. El contenido total de glycoalcaloides (TGA) en follaje y tubérculos no tuvo correlación con las poblaciones de insectos. Los niveles de TGA en el follaje de plantas desarrolladas en el campo variaron entre las entradas, desde < 2–240 mg/100 g peso fresco. Solasonine y solamargine fueron los alcaloides de follaje en major cantidad mientras que solamarines predominó en los tubérculos.

Hooked trichomes: a physical plant barrier to a major agricultural pest

Hooked epidermal appendages (trichomes) on leaves of field bean cultivars effectively capture nymph and adult leafhoppers. Frequency of capture and capture mortality are highly correlated with trichome density. Hooked trichomes inserted at angles less than 30� are ineffective in capture.

Aphid repellent sesquiterpenes in glandular trichomes of Solanum berthaultii and S. tuberosum

Exudate from glandular trichomes of the wild potato species, Solanum berthaultii Hawkes, and the cultivated potato, S. tuberosum L., contained volatile substances including sesquiterpenes when examined with gas chromatography/mass spectrometry procedures. Sesquiterpenes were the major volatile constituents of Type A trichome exudate but were a minor constituent of Type B exudate. Sesquiterpene mixtures of the two potato species differed qualitatively and quantitatively. β‐Caryophyllene and E‐β‐Farnesene were major components identified in Type A trichome exudate of the cultivated potato. Sesquiterpene mixtures of both potato species were repellent to the green peach aphid, Myzus persicae (Sulzer). The role of trichome sesquiterpenes as a determinant of aphid behavior on the two potato species and in the expression of host resistance is discussed.

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.

Glandular trichomes of Solatium berthaultii and resistance to the Colorado potato beetle

The defensive mechanisms of the wild potato, Solanum berthaultii Hawkes, to larvae of the Colorado potato beetle, Leptinotarsa decemlineata (Say), were studied by selective removal of glandular trichomes and trichome exudates from leaflets, and by comparing performance on S. berthaultii and on the cultivated potato, S. tuberosum L., which lacks defensively active type A and B glandular trichomes. Removal of type A trichomes increased the proportion of larvae that fed on S. berthaultii. Removal of the exudate from type B trichomes increased the proportion of larvae that fed and led to a decrease in mortality. The predominant active compounds in type B exudate, i.e. fatty acid esters of sucrose, were only effective in the presence of type A trichomes. Sucrose esters did not affect larval feeding on S. tuberosum leaflets or on S. berthaultii leaf discs from which the type A trichomes had been removed. Growth of surviving larvae was not significantly affected by removing type A trichomes or type B exudate. Growth of larvae was significantly increased when S. berthaultii leaflets were presented in artificial diet which eliminated the physical barrier of the type B stalks. Growth was no different on artificial diet containing either S. berthaultii or S. tuberosum leaf material (fresh or lyophilized powder) but was poorer on these diets than on S. tuberosum leaflets. The presence of type A trichomes is a fundamental requirement for expression of S. berthaultii resistance to L1 L. decemlineata. Type B droplets containing sucrose esters increase the expression of resistance in the presence of defensively‐active type A trichomes.

Glandular trichomes of Solanum berthaultii alter host preference of the Colorado potato Beetle, Leptinotarsa decemlineata

Choice and no‐choice studies were conducted to determine how the glandular trichomes of the wild potato, Solanum berthaultii Hawkes, affect host preference of the Colorado potato beetle, Leptinotarsa decemlineata (Say). Given a feeding choice between S. tuberosum and S. berthaultii, larvae and adults preferred the foliage of S. tuberosum, but adults were more discriminating. When foliage of S. berthaultii was appressed to S. tuberosum leaflets, fewer adults fed on the appressed leaflets. When given a choice between ‘trichome‐intact’ and ‘trichome‐removed’ S. berthaultii foliage, adults preferred to feed on the latter. The preference for ‘trichome‐removed’ foliage and the percent of adults initiating feeding, increased with the degree of trichome removal. These studies provide evidence that the resistance of S. berthaultii is associated with feeding deterrents localized in the glandular trichomes, that S. berthaultii possesses more than one mechanism of resistance to the Colorado potato beetle, and that the expression of resistance is dependent on the developmental stage of the insect.