Maize Domestication and Anti-Herbivore Defences: Leaf-Specific Dynamics during Early Ontogeny of Maize and Its Wild Ancestors

Abstract

As a consequence of artificial selection for specific traits, crop plants underwent considerable genotypic and phenotypic changes during the process of domestication. These changes may have led to reduced resistance in the cultivated plant due to shifts in resource allocation from defensive traits to increased growth rates and yield. Modern maize (Zea mays ssp. mays) was domesticated from its ancestor Balsas teosinte (Z. mays ssp. parviglumis) approximately 9000 years ago. Although maize displays a high genetic overlap with its direct ancestor and other annual teosintes, several studies show that maize and its ancestors differ in their resistance phenotypes with teosintes being less susceptible to herbivore damage. However, the underlying mechanisms are poorly understood. Here we addressed the question to what extent maize domestication has affected two crucial chemical and one physical defence traits and whether differences in their expression may explain the differences in herbivore resistance levels. The ontogenetic trajectories of 1,4-benzoxazin-3-ones, maysin and leaf toughness were monitored for different leaf types across several maize cultivars and teosinte accessions during early vegetative growth stages. We found significant quantitative and qualitative differences in 1,4-benzoxazin-3-one accumulation in an initial pairwise comparison, but we did not find consistent differences between wild and cultivated genotypes during a more thorough examination employing several cultivars/accessions. Yet, 1,4-benzoxazin-3-one levels tended to decline more rapidly with plant age in the modern maize cultivars. Foliar maysin levels and leaf toughness increased with plant age in a leaf-specific manner, but were also unaffected by domestication. Based on our findings we suggest that defence traits other than the ones that were investigated are responsible for the observed differences in herbivore resistance between teosinte and maize. Furthermore, our results indicate that single pairwise comparisons may lead to false conclusions regarding the effects of domestication on defensive and possibly other traits.

Fig 1. Pairwise comparison of constitutive BX levels in Balsas teosinte and maize.

Total 1,4-benzoxazin-3-one (BX) concentrations were calculated as the sum of seven individual BX glucosides in one population of Balsas teosinte (left) and one maize inbred line (B73, right) at three different growth stages and are given as μg / g fresh weight (FW) (mean ± SE; N = 5–6). BX levels were monitored in old (blue) and young (red) leaves as indicated in the drawing to the left-hand side for growth stage L4. Note that at L2 old and young leaf were the same. Significant effects of growth stage and leaf identity on total BXs are indicated by asterisks: * P<0.05; ** P <0.01; *** P <0.001; ns: not significant; two-way ANOVA without growth stage L2. Chemical data for B73 has been published before [45].

Fig 2. Constitutive BX levels in multiple representatives of teosinte and cultivated maize.

Total levels of 1,4-benzoxazin-3-ones (BXs) at different growth stages of two teosinte accessions (T62 and T77), a Tuxpeño landrace variety from two collection sites (Talpitita and El Cuyotomate) and two maize hybrid lines (Pactol and Delprim) were calculated as the sum of seven individual BX glucosides and are given as μg / g fresh weight (FW) (mean ± SE; N = 3–5). At each growth stage old (blue) and young (red) leaves were sampled. Note that at L2 old and young leaf were the same. The specific positions of harvested leaves are highlighted in the maize drawings. Significant effects of plant genotype and leaf identity on total BXs are denoted by asterisks: * P<0.05; ** P <0.01; *** P <0.001; ns: not significant; two-way ANOVA without growth stage L2. Significant differences between plant lines are indicated by different letters and derive from Holm-Sidak tests for the respective leaf type. The concentrations of Ejutla and B73 from the previous figure are included for comparison (black bars).

Fig 3. Maysin accumulates in newly developing leaves of maize and teosinte at later growth stages.

Maysin concentrations were determined in old (blue) and young (red) leaves of two teosinte accessions (T62 and T77), a Tuxpeño landrace variety from two collection sites (Talpitita and El Cuyotomate) and two maize hybrid lines (Pactol and Delprim) at three early vegetative growth stages (L2, L4 and L6). Concentrations are given as μg / g fresh weight (FW) (mean ± SE; N = 3–5). Statistically significant effects of plant growth stage or leaf identity are denoted by asterisks (*P < 0.05; **P < 0.01; ***P < 0.001; ns: not significant; two-way ANOVA without growth stage L2).

Fig 4. Leaf toughness in maize and teosinte increases in a leaf-specific manner during early ontogeny.

Leaf toughness was measured in old (blue) and young (red) leaves of two teosinte accessions (T62 and T77), a Tuxpeño landrace variety from two collection sites (Talpitita and El Cuyotomate) and two maize hybrid lines (Pactol and Delprim) at three early vegetative growth stages (L2, L4 and L6). Leaf toughness is expressed as the force needed for penetration of the leaf blade and given in Newton (N) (mean ± SE; N = 3–5). Statistically significant effects of plant growth stage or leaf identity on leaf toughness are denoted by asterisks (*P < 0.05; **P < 0.01; ***P < 0.001, ns: not significant; two-way ANOVA without growth stage L2).