When insect pests begin chewing on a tomato leaf, the plant pulls essential nutrients out of that very leaf, relocating it to the roots — out of the insect’s reach. The result is a leaf that is less nutritious, less palatable, and ultimately less capable of supporting the pests’ growth and survival.
“We demonstrated that upon S. littoralis [cotton leafworm] attack, tomato plants removed sodium from the feeding site by relocating [..] from the leaf to the root,” the researchers write in the paper. The findings, they say, provide new evidence on a “novel mechanism” of plant response to herbivore insect damage.
Sodium, in insect diets, plays a surprisingly central role, often acting as a limiting nutrient that drives specific foraging behaviors. “This element has been recently defined as the seventh macronutrient,” say researchers. Most plants, however, contain very little of it — and that scarcity, the researchers argue, may be no accident.
Valerio Cirillo and his colleagues grew tomato plants (Solanum lycopersicum) under two conditions: one with normal sodium levels and one with reduced sodium. They then introduced Spodoptera littoralis larvae — a widespread agricultural pest — and tracked what happened over 72 hours.
In plants under attack, leaf sodium concentrations dropped by more than 27 percent, while sodium in the roots climbed by nearly 31 percent. Concentrations of other ions — potassium, calcium, and nitrate — showed no significant change, suggesting the plant was running a sodium-specific operation.
The researchers found two sodium transporter genes, SlHKT1;1 and SlHKT1;2. When larvae began feeding, expression of SlHKT1;2 increased substantially over the following 24 to 72 hours. This gene is involved in moving sodium out of the water-conducting vessels in leaves and loading it into the phloem for downward transport toward the roots. Simultaneously, SlHKT1;1, which helps load sodium into those same vessels, was dialed down. Working together, the two transporters appear to redirect sodium away from the feeding site.
Those larvae fed on low-sodium plants were lighter, slower to develop, and far less likely to survive. By the end of laboratory feeding experiments, survival on low-sodium leaves had fallen to around 40 percent, compared with full survival on normal plants. Those that did survive took longer to pupate, produced lighter pupae, and experienced roughly 60 percent pupal mortality. When given a choice, larvae consistently and strongly preferred high-sodium leaves.
The study “demonstrates the existence of a new plant defense pathway based on sodium translocation activated in response to insect infestation. This finding opens new perspectives on the study of the crosstalk between abiotic (e.g., salt stress) and biotic (e.g., herbivore attacks) stress responses in plants and provides novel insights into the coevolutionary dynamics between plants and herbivores,” researchers write.
Journal Reference: New Phytologist. DOI: 10.1111/nph.71150
