Plant domestication decreases both constitutive and induced chemical defences by direct selection against defensive traits

Abstract

Studies reporting domestication effects on plant defences have focused on constitutive, but not on induced defences. However, theory predicts a trade-off between constitutive (CD) and induced defences (ID), which intrinsically links both defensive strategies and argues for their joint consideration in plant domestications studies. We measured constitutive and induced glucosinolates in wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties (B. oleracea var. acephala and B. oleracea var. capitata) in which the leaves have been selected to grow larger. We also estimated leaf area (proxy of leaf size) to assess size-defence trade-offs and whether domestication effects on defences are indirect via selection for larger leaves. Both CD and ID were lower in domesticated than in wild cabbage and they were negatively correlated (i.e. traded off) in all of the cabbage lines studied. Reductions in CD were similar in magnitude for leaves and stems, and CD and leaf size were uncorrelated. We conclude that domestication of cabbage has reduced levels not only constitutive defences but also their inducibility, and that reductions in CD may span organs not targeted by breeding. This reduction in defences in domesticated cabbage is presumably the result of direct selection rather than indirect effects via trade-offs between size and defences.

Figure 1

Effects of plant domestication on constitutive leaf chemical defences. Constitutive concentrations of (a) aliphatic glucosinolates and (b) indolic glucosinolates in leaves of wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties of this species (B. oleracea var. acephala and B. oleracea var. capitata). Bars are least square means ± s.e.m. (N = 60 plants per plant line). Different letters indicate significant (P < 0.05) differences between plant lines (see Table 1 for statistics). d.w. = dry weight.

Figure 2

Correlations between constitutive glucosinolates and leaf area. Genetic correlations between leaf constitutive concentrations (in µmol g⁻¹ d.w.) of (a–c) aliphatic glucosinolates and (d–f) indolic glucosinolates and leaf area (in cm²) in wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties of this species (B. oleracea var. acephala and B. oleracea var. capitata). R² coefficients and associated P-values are shown. Each point represent a genotype (N = 15 genotypes).

Figure 3

Effects of plant domestication on the inducibility of leaf chemical defences. Inducibility (i.e., the ability of plants to increase their defence levels in response to herbivory or some other type of stressor) of (a) aliphatic and (b) indolic glucosinolates in leaves of wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties of this species (B. oleracea var. acephala and B. oleracea var. capitata). Plants were induced by Mamestra brassicae larvae which fed on the plants for one week. Bars are least square means ± s.e.m. (N = 60 plants per plant line). Different letters indicate significant (P < 0.05) differences between plant lines (see Table 2 for statistics). d.w. = dry weight.

Figure 4

Correlations between constitutive glucosinolates and their inducibility. Genetic correlations between leaf constitutive concentrations (in µmol g⁻¹ d.w.) of (a–c) aliphatic glucosinolates and (d–f) indolic glucosinolates and their corresponding inducibility by Mamestra brassicae damage in wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties of this species (B. oleracea var. acephala and B. oleracea var. capitata). Inducibility was measured as the genetic mean values in induced minus control plants. Negative significant correlations denote a genetic trade-off between constitutive and induced defences. R² coefficients and P-values are shown. Each point represent a genotype (N = 15 genotypes).

Figure 5

Effects of plant domestication on leaf vs. stem constitutive chemical defences. Constitutive concentrations of (a) aliphatic and (b) indolic glucosinolates in leaves and stems of wild cabbage (Brassica oleracea ssp. oleracea) and two domesticated varieties of this species (B. oleracea var. acephala and B. oleracea var. capitata). Bars are least square means ± s.e.m. (N = 36 replicate plants per plant line). Different letters indicate significant (P < 0.05) differences between plant lines (see Table 3 for statistics). d.w. = dry weight.