Water stress alters indirect defenses in tomato and wheat

Water stress altered morphological characteristics, expression of defense genes, and olfactory responses of natural enemies in tomato and wheat plants. The result comes from a study in three water regimes and plants with or without pest infestation.

The research evaluated "bottom-up" effects, in which changes in the plant influence higher trophic levels. This type of response can interfere with the interaction between crop, pest, and biological control agent. According to the scientists, the topic gains relevance in climate change scenarios, with greater water restriction and pressure on integrated pest management programs.

The researchers worked with tomato plants (Solanum lycopersicum) and wheat (Triticum aestivum). The plants received three irrigation regimes: optimal regime, moderate water stress, and severe water stress. Some of the plants were infested with phytophagous insects. In the tomato plants, the researchers used Bemisia tabaci and Phenacoccus solenopsis. In wheat, they used Sitobion fragariae.

The study measured height, stem diameter, root length, fresh mass, and dry mass. It also analyzed genes linked to abscisic acid, jasmonic acid, and salicylic acid pathways. The team evaluated the olfactory response of three natural enemies: encarsia formosa, Cryptolaemus montrouzieri and Aphidius colemani.

Water deficit

In tomato plants, water deficit reduced height, stem diameter, fresh weight, and dry weight. Moderate stress increased root length compared to the optimum regime and severe stress. In wheat, severe stress reduced height and dry weight. Stem diameter also decreased under moderate and severe stress.

The molecular response varied between crops. In tomatoes, moderate and severe regimes increased the expression of ASR1, a gene associated with the abscisic acid pathway, and PIN2, a marker related to jasmonic acid. In plants infested by Bemisia tabaci, ASR1 increased in tomato plants under severe stress. In plants infested with Phenacoccus solenopsis, ASR1 increased under moderate and severe stress.

In wheat, the PR1 gene, linked to the salicylic acid pathway, showed higher expression under severe water stress, with or without infestation by Sitobion fragariae. The LOX gene, associated with jasmonic acid, did not show activation under the same conditions. The authors suggest a possible antagonism between the salicylic acid and jasmonic acid pathways.

natural enemies

The responses of natural enemies have also changed. Encarsia formosa preferred volatiles from tomato plants under moderate water stress when there was no infestation. Cryptolaemus montrouzieri also showed a preference for tomato plants under moderate stress without the presence of pests. However, when there was an infestation by Phenacoccus solenopsis, the predator preferred well-watered plants compared to plants under severe stress.

In wheat, Aphidius colemani showed a preference for plants under optimal irrigation conditions in most comparisons. The exception occurred in the comparison with plants under moderate stress and infested by Sitobion fragariae.

Plant defense

Scientists conclude that water deficit and pest infestation activate plant defense mechanisms with distinct effects on natural enemies. The combination of abiotic and biotic stress altered volatile signals and influenced the attraction of biological control agents.

The results may support biological management strategies within integrated pest management. The researchers highlight the need for further studies. They suggest analyses of volatile compounds emitted under different stress combinations and field trials to validate the use of induced plant defenses in biological control programs.

More information at doi.org/10.1002/ps.70900