Tomato lipocalins mediate ABA (abscisic acid)- and ethylene-dependent regulation of stress tolerance and fruit ripening.

Plant temperature-induced lipocalins (TILs) have been shown to be responsive to heat stress. The expression of TIL in wheat and Arabidopsis is induced by heat shock treatment and cold acclimation, but other responses and functions of lipocalins remain unknown. In this study, we focused on the response of lipocalins to phytohormones in tomato, as cis-element analysis revealed the presence of multiple phytohormone-responsive elements involving ethylene, abscisic acid (ABA), and jasmonic acid (JA). The expression levels of SlTIL1 and SlTIL2 increased after ABA treatment in young leaves, and tomato plants exhibited enhanced drought stress tolerance 24 h after ABA application. In addition, SlTIL1 expression increased in tomato fruits at the yellow stage following ABA treatment from the orange stage, thereby accelerating fruit ripening. We compared wild-type plants with overexpressing lines of SlTIL1, SlTIL2, and SlCHL and found that both ethylene gas production and expression of the ethylene synthesis gene SlACS2 were elevated from the yellow stage in SlTIL1-OX and SlTIL2-OX lines compared to wild-type. We suggest that ethylene and ABA treatments induce reactive oxygen species (ROS) in tomatoes, to which lipocalins respond by not only contributing to ROS accumulation scavenging but also promoting leaf senescence and fruit ripening.