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

Highlights

• Heat stress as elicitor of bioactive compounds in hydroponic Melissa officinalis.

• Rosmarinic acid was elicited by the short-term heat stress.

• Generation/scavenging of reactive oxygen species during rosmarinic acid elicitation.

• Heat shock protein 101 was induced during the heat stress.

• Abscisic and salicylic acids increased during the initial phase of heat stress.

Abstract

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 °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 AsA 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.

Introduction

Heat stress (HS) is defined as a condition of high air temperature (HT; i.e. 10–15 °C above ambient) for a sufficient time to induce a negative impact on plant development, growth and reproduction (Wahid et al., 2007). Plant species and genotypes have several capabilities to cope with HS, and the response depends on the intensity, duration and rate of temperature increase (Wahid et al., 2007). At very HT such as 10–15 °C above the ambient air temperature, severe cellular injury and even cell death occur within minutes, caused by a catastrophic collapse of cellular organization (e.g. protein denaturation/aggregation and increased fluidity of membrane lipids; Schöffl et al., 1999). At moderately HT, injuries or death may occur only after long-term exposure, which could be attributed to reduced cellular function and overall plant fitness (Driedonks et al., 2015).

In the plant-HS interaction, an important role is played by the accumulation of reactive oxygen species (ROS; Mittler, 2006). Usually, ROS production rapidly becomes excessive in plants subjected to HS (Pucciariello et al., 2012; Driedonks et al., 2015; Zhao et al., 2018), causing a cellular damage to membranes, organelles, DNA and denaturation/activation of proteins (Howarth, 2005). To prevent this cell damage and regain redox homeostasis, plants can trigger a heat stress response (HSR) by the hyper-activation of non-enzymatic and/or enzymatic ROS scavenging systems (Apel and Hirt, 2004; Halliwell, 2007; Foyer, 2018). The expression and protein level of genes responsible for ROS scavenging are also induced under HS in several plant species (Panchuk et al., 2002; Qiu et al., 2006; Driedonks et al., 2015), and has been associated to basal heat tolerance (Wahid et al., 2007).

Under HS, similarly to other oxidative stresses, the ROS processing system is not only a simple protection mechanism, but also represents a signal for the modulation of other multiple responses (Berkowitz et al., 2016). Therefore, ROS are thought to be involved in the transduction of intra- and intercellular signals controlling gene expression and activity of anti-stress systems (Singh et al., 2019). Several studies documented that HS together with drought lead to an increase of abscisic acid (ABA) concentration that could regulate the acclimation process through the promotion of heat shock proteins (HSP; Larkindale and Knight, 2002; Liu et al., 2006). Asensi-Fabado et al. (2013) reported that prolonged HS (alone or in combination with water stress) induced the synthesis of ABA, salicylic acid (SA) and α-tocopherol in three Labiatae species.

Changing perspective, short-term stress conditions can also represent natural easy-to-use elicitors of bioactive compound production in plants. In the last years, many attentions have been given to enhance the production of plant secondary metabolites that are unique sources of pharmaceuticals, food additives, flavors and industrially important biochemicals (Ramakrishna and Ravishankar, 2011; Trivellini et al., 2016; Thakur et al., 2018). Among elicitors, several chemical or physical tools (i.e. signal compounds and/or abiotic factors) have been used. Recently, Khaleghnezhad et al. (2019) demonstrated that the combination of ABA application and HS treatment positively influences the accumulation of secondary metabolites in Dracocephalum moldavica. Biotechnological approaches (i.e. shoots, callus, cell suspension and root cultures; Petersen, 2013; D’Angiolillo et al., 2015) can also be used to trigger an array of defense or stress responses that improve the yield of secondary metabolites (Bertoli et al., 2013; Tonelli et al., 2015; Pellegrini et al., 2018; Mosadegh et al., 2018). Hydroponic cultures represent a good approach to investigate the effects of HS alone on plants, in contrast with many reports conducted with standard soil methods where HS was unavoidably combined with other related abiotic stresses (e.g. drought and salinity, Zandalinas et al., 2018).

Melissa officinalis L. (lemon balm) is an aromatic plant from the Mediterranean area, widely cultivated worldwide (Szabó et al., 2016). High quantities of secondary metabolites such as phenolic compounds, tannins and flavonoids (contained both in leaves and essential oils) were identified/quantified in M. officinalis and represent raw material for pharmaceutical, food, beverage, and cosmetic purposes (Moradkhani et al., 2010). Rosmarinic acid (RA), which is constitutively accumulated in field-grown plants as antimicrobial compound and as protection against herbivores (Szabo et al., 1999), is the main phenolic compound found in all organs of M. officinalis, with a level of about 6% of the dry weight (DW) in leaves (Petersen and Simmonds, 2003). For these reasons, as well as for its fast growth, M. officinalis has been exposed to abiotic stress to stimulate some bioactive compounds (e.g. RA, phenols, flavonoids, etc.). Ozone (O₃) exposure caused an alteration in leaf morphology and metabolism both in vitro (Tonelli et al., 2015; D’Angiolillo et al., 2015) and in vivo plants (Pellegrini et al., 2013) with an enhanced pattern of phenylpropanoids (e.g. phenols, anthocyanins, tannins, carotenoids and RA).

The present study pioneering investigated the capability of using a short-term HS as a tool to rapidly increase RA content in leaves of M. officinalis hydroponic cultures, highlighting the cross-talk among antioxidant and signalling molecules involved in the heat acclimation. The soilless cultivation allowed the determination of the heating effect, avoiding the crosstalk with drought stress. It is known that different combinations of stresses seem to influence the transcriptome analyses in Arabidopsis thaliana, while it cannot be predicted the response of one single stress factor (Rasmussen et al., 2013). Specifically, the purpose of this study was to answer the following questions: (i) Does HS elicit the biosynthesis of RA in M. officinalis hydroponic cultures? (ii) What is the behavior of ROS processing systems carrying the potential RA elicitation? (iii) What is the role of hormonal changes in the M. officinalis-HS interaction? We hypothesized that the short-period HS could elicit RA production as part of the heat acclimation consisting of a cross-talk among cellular processes and growth regulators tuned by a partial control of ROS production.