Accumulation of rosmarinic acid and behaviour of ROS processing systems in Melissa officinalis L. under heat stress

Heat stress (HS) due to increased air temperature is a major agricultural problem. On the other hand, short-term HS can represent a natural easy-to-use elicitor of bioactive compounds in plants. Similar elicitations can be induced by biotechnological approaches such as hydroponic cultures. The present study pioneering investigated the capability of using a short-term HS (38 degrees C, 5 h) as a tool to rapidly elicit rosmarinic acid (RA) content in leaves of Melissa officinalis L. (a species for which RA is the dominant active phenolic compound) hydroponic cultures, highlighting the cross-talk among antioxidant and signalling molecules involved in the heat acclimation. During HS treatment, we found an elicitation of RA biosynthesis associated with (i) an imbalance in reactive oxygen species (ROS) production and scavenging, (ii) an involvement of reduced ascorbate (AsA) in maintaining a high normal reduced state of cells, (iii) an induction of heat shock proteins (i.e. HSP101-like), and (iv) a stimulation of phytohormones. The RA biosynthesis lasted also during the recovery, although plants activated cellular processes to partially control ROS production, as confirmed by the increased activity of MA regenerating enzymes, the accumulation of total carotenoids and the stimulation of total antioxidant capacity. The unchanged values of abscisic acid, ethylene and salicylic and jasmonic acids during the recovery phase also documented a reduced demand for protection. The present study represents a wide-ranging investigation of the potential use of HS (without drought interaction) as a technological application for improving bioactive compound production.