Water stress alleviation by polyamines and phenolic compounds in Scrophularia striata is mediated by NO and H₂O₂

Highlights

• Stress signal transduction is bestirred by NO and PAs in response to water stress.

• NO-evoked PAs alter phenolic compounds pathways in S. striata to respond water scarcity.

• The activation of PAs pathway induced the release of H₂O₂ and associated with oxidative stress-induced responses.

• Shift of echinacoside toward acteoside elevates drought tolerance in S. striata.

Abstract

Plants respond to water stress through a variety of mechanisms, depending on metabolites preferences and their available resources. This work was performed to elucidate the cross-talk between signaling molecules (polyamines (PAs), hydrogen peroxide (H₂O₂) and nitric oxide (NO)), phenolic compounds and osmolytes (phenylethanoid glycosides (PhGs), phenolic acids, flavonoids, soluble sugars and amino acids) under water stress in Scrophularia striata plants. The results revealed that PAs, NO levels were enhanced in the plants, earlier in response to polyethylene glycol-induced water stress. The antioxidative mechanisms with increased activity of catalase (CAT), guaiacol peroxidase (GPX) and superoxide dismutase (SOD) and also phenylalanine ammonia-lyase (PAL), tyrosine ammonia-lyase (TAL), as key enzymes in phenolic pathway were deployed in response to the stress. Mannose, glucose, xylose/rhamnose which are involved in PhGs biosynthesis as well as in serving osmotic adjustment were modulated. The elevated content of arginine and methionine as PAs precursors and tyrosine and phenylalanine as PhGs precursors was enhanced by water stress and was significantly associated with PAs and PhGs accumulations. Metabolic profiling revealed new information about relationship between stress signal molecules; PAs, NO and H₂O₂, osmolytes (sugers, PhGs) and phenolic compounds which involved in the improvement of water stress tolerance in S. striata.

Introduction

Water stress is a major abiotic challenge affecting plant metabolism, growth and development. Responses of plants to water stress may be assigned as either injurious change or tolerance index. This stress often leads to the production of reactive oxygen species (ROS), in particular hydrogen peroxide (H₂O₂) and superoxide anion radicals. Production of ROS associated with oxidative damage react with lipids, DNA, proteins and disturbing the normal functions of cells (Foyer and Fletcher, 2001; Munné-Bosch and Penuelas, 2003). Plants have a number of defense mechanisms to cope with water stress (Chaves and Oliveira, 2004). Plant ROS are removed by antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and guaiacol peroxidase (GPX) and non-enzymatic scavenger components (Hasegawa et al., 2000). It has been shown that amino acids, sugars and ions accumulated under water stress to contribute to osmotic adjustments (Hanson and Smeekens, 2009; Chen and Jiang, 2010). On the other hand, polyamines (PAs) such as putrescine (Put), spermidine (Spd), cadaverine (Cad) and spermine (Spm) are molecules that participated in plant growth, development and maintaining osmotic adjustment and protection of membranes, nucleic acids and proteins (Gill and Tuteja, 2010; Li et al., 2015). The endogenous PAs contribute significantly in enhancing plants defense strategies in response to any stress (Groppa and Benavides, 2008). More recent evidences have been proposed that nitric oxide (NO) as another bioactive molecule involved in signaling pathway within plants, increases in biotic and abiotic stresses and plays a central role in a variety of physiological and biochemical functions such as protection against oxidative damage induced by stress (Singh et al., 2008, 2013). To control the level of ROS and to protect the cells under stress condition, NO activates antioxidant enzymes such as SOD, CAT, GPX and APX (Singh et al., 2013). It has been shown that PAs are involved in the regulation of NO and H₂O₂ production in plants under stress (Tun et al., 2006; An et al., 2008; Arasimowicz-Jelonek et al., 2009). Moreover, secondary metabolites such as phenolic compounds, are involved in the defense against biotic and abiotic stresses and contribute significantly to the antioxidant activity of plant tissues (Bharti et al., 2013; Pourcel et al., 2007; Zhao et al., 2010). Water stress can be a major factor in increasing concentration of secondary metabolites in some medicinal plants (Moinuddin et al., 2012). Phenylethanoid glycosides (PhGs) are present in medical plants such as Scrophularia striata (Monsef-Esfahani et al., 2010) which grows in arid and semi-arid regions of southwestern Iran (Khanpour-Ardestani et al., 2014). PhGs including acteoside and echinacoside are characterized by their bioactivities including antioxidant, osmo-protectant and hypertensive effects (Xue and Yang, 2016). We recently reported that water stress induced accumulation of phenolic compounds (such as acteoside and echinacoside) in roots of S. striata, which was associated with oxidative stress-induced responses (Falahi et al., 2018). However, it is not clear cross-talk between different signal molecules (such as NO, H₂O₂ and PAs) and phenolic pathways. Therefore, we generated a global study to elucidate changes on stress signal transductions; PAs, NO and H₂O₂ levels and cross-talk between different signal molecules and metabolites preferences, especially the ROS-dependent ones under water stress. This study will be useful for understanding the mechanisms of drought tolerance in S. striata and will provide an effective pathway for the exploration of the role of PhGs in S. striata that might improve drought tolerance in this plant. For making a water stress condition we used polyethylene glycol 6000 (PEG 6000). PEG 6000 is a high molecular weight solute, which cannot penetrate cell wall pores and the apoplastic space. So adding PEG to hydroponic culture induces osmotic (drought) stress and this solution mimic dry soil (Yang et al., 2010).