Specificity of constitutive and induced resistance: pigment glands influence mites and caterpillars on cotton plants

Key words: Density-dependent induction, genotype-by-environment interaction, pigment glands, herbivory,induced plant defense, plant-insect interactions, plasticityAbstractCotton plants contain suites of phytochemicals thought to be important in defense against herbivores, some ofwhich are localized in pigment glands which contain gossypol and other terpenoid aldehydes. The simple geneticbasis for the expression of these glands has led to the development of near-isogenic glanded and glandless geno-types. Glands may also be phenotypically induced by herbivory. We determined the consequences of constitutiveandinducedgland expressionontwo types ofherbivores, spidermites (cell contentfeeders)andnoctuidcaterpillars(leaf chewers).Induction of glands was strongly dependent on the density of attackers. Spider mite herbivory on cotyledons(1) increased the density (but not total number) of glands on cotyledons linearly, (2) increased the density andtotal number of glands on the ﬁrst true leaf linearly, and (3) affected the density and total number of glands on thesecond true leaf non-linearly, compared to controls. Neither constitutive nor induced expression of glands affectedmite population growth. An equal reductionof mite population size on induced glanded and glandless plants (50%)relative to uninduced controls indicated that factors other than glands were associated with induced resistance tomites. Constitutive gland expression had a strong negative impact on caterpillar performance, reducing growth by45%. Induced resistance to caterpillars was three times stronger in glanded genotypes than in glandless genotypes,indicating that factors associated with induced resistance to caterpillars are strongly associated with glands. Threecotton varieties were highly variable in their constitutive and induced resistance to mites and caterpillars.Thus, defense of cotton plants against herbivores can be roughly categorized as constitutive and induciblefactors associated with terpenoid aldehyde containing pigment glands that are effective against caterpillars, andfactors not associated with glands that are effective against mites.IntroductionPlants are the basal resource for organisms in manyfood webs. As a consequence, most plants are con-sumed by numerous herbivores. Where herbivoresreduce plant ﬁtness, natural selection has presum-ably favored the evolution of defensive mechanismsin plants. Plant defenses can be expressed constitu-tively (all of the time) and facultatively (induced bysome environmental cue) (Karban & Baldwin, 1997;Agrawal, 1998). Genotypically determined constitu-tive and induced expression of phytochemicals canmediate either speciﬁc or general resistance to at-tackers. Furthermore, different resistance mechanismshave the potential to interact with each other to in-crease (or decrease) defense (Berenbaum & Zangerl,1996). Some plant defenses such as the tetracyclictriterpenoid cucurbitacins of the cucurbitaceae are ef-fective against many attackers including spider mites,roaches, beetles, lepidopteran larvae, mice and ver-tebrate grazers (Da Costa & Jones, 1971; Metcalf L Tallamy et al., 1997; Agrawal et al.,