Molecular Responses of Lettuce to Phosphate-Limiting Conditions: Roles of Microbial Communities and Melatonin Regulation

Among the various strategies to improve crop yields and stress resilience in modern agriculture, proper management of macronutrients and plant growth regulators is mandatory. Phosphorus is an essential element involved in many physiological processes in plants, but it mainly exists in the soil as phosphate ions, of which only a small fraction is bioavailable. To prevent phosphorus deficiency, the common practice is the widespread use of phosphate fertilisers, which causes considerable environmental damage. A more sustainable strategy involves using soil microorganisms to enhance phosphate availability to plants.

Melatonin is an indolamine that exhibits hormone-like activity and functions as a plant growth regulator. The current knowledge is based on exogenous administration to plants exposed to abiotic and biotic stresses, where it has shown beneficial effects, while its endogenous roles remain underexplored.

Lettuce (Lactuca sativa L.) is an extensively cultivated crop with a sequenced genome and relatively short life cycle, making it a suitable model for molecular studies, including those involving phosphate-solubilizing microorganisms and endogenous melatonin.

This project aims to investigate: (i) the genetic and metabolic factors involved in the interactions between different lettuce genotypes and a phosphate-solubilizing microbial community under phosphate-limiting conditions, to identify candidate genes and metabolic pathways relevant to low-phosphate resilience and future breeding; (ii) the roles of melatonin in response to this abiotic stress and associated biotic interactions; and (iii) the genes involved in melatonin biosynthesis.

To dissect the metabolic variations occurring under defined biotic and abiotic conditions, we employed an untargeted metabolomics approach using UPLC-HRMS. Investigations into endogenous melatonin levels, its biosynthetic pathway, and its roles under phosphate-limiting conditions and microbial interactions are ongoing. Preliminary results confirm the presence of melatonin at very low levels in lettuce leaves, as well as the identification of the first gene involved in its biosynthesis.