Impact of the combined application of β-cyclocitric acid and mycorrhizal inoculum on tomato response to water deficit

Apocarotenoids are metabolites derived from the oxidative breakdown of carotenoids and are known to modulate molecular and biochemical responses that help plants overcome the adverse effects of abiotic stresses. Particularly, β-cyclocitric acid (β-CCA), a derivative of β-carotene, enhances plant tolerance to drought conditions. However, there is currently rare information regarding the impact of β-CCA on the interaction with root-associated microorganisms. In this study, we investigated the effects of exogenous β-CCA (1.15 mmol·L⁻¹) on mycorrhizal colonization of tomato roots and on the plant responses to water deficit conditions, using an RNA-seq approach performed on both root and shoot samples. We found that β-CCA negatively influenced the regulation of genes related to root growth, while acting as a priming agent in well-watered conditions by activating genes encoding scavenging enzymes. Under stress conditions, β-CCA positively affected the mycorrhizal status in terms of frequency of mycorrhization. The expression of genes involved in pathways related to ethylene, abscisic acid, and reactive oxygen species in mycorrhizal plants contributed to enhanced drought tolerance. In conclusion, β-CCA can be considered a putative priming agent for increasing plant stress resilience in agricultural applications in open fields, without disrupting beneficial organisms in symbiosis with roots. However, targeted fungal analyses and the use of different fungal partners are necessary to validate the findings from the fungal side. Additionally, it would be interesting to assess the impact of the apocarotenoids on soil native microbial communities.