Being tough, being toxic, or being a salad? Understanding the dynamics of plant defense mechanisms against herbivores

Young wheat plants are continuously exposed to insect herbivorous attack. To maintain their fitness, plants have evolved different defense mechanisms, including the biosynthesis of toxic compounds and/or trichome formation that provides physical barriers. It is unclear whether both of these mechanisms are equally critical in providing an efficient defense for wheat seedlings against the bird cherry-oat (Rhopalosiphum padi) aphid—an economically costly pest in cereal production. In our research, we compared the transcriptomic, metabolomic, chemical, and physical defenses of three selected wheat genotypes, two tetraploid wheat genotypes, domesticated durum ‘Svevo’ and wild emmer ‘Zavitan,’ and one hexaploid bread wheat, ‘Chinese Spring.’ The full transcriptomic analysis revealed a major difference between the three genotypes, while the clustering of significantly different genes suggested a higher similarity between the two domesticated than the wild wheat. Measurement the levels of toxic compounds, benzoxazinoid, at the three time-points (11, 15 and 18 days-after-germination) revealed high abundance levels in the two domesticated genotypes, while the levels were very low in the wild emmer wheat. In contrast to the benzoxazinoid levels, the trichome density was dramatically higher in the wild emmer than in the domesticated wheat. Evaluation of aphid reproduction indicated that the domesticated bread wheat is more resistant than the tetraploid genotypes. Overall, the results suggested that although wheat seedlings possess both chemical and physical defense mechanisms, the chemical defense plays a more significant defensive role than the physical barriers.