Low temperature stress modulates the biochemical, metabolic, and molecular behavior of the Trans-Himalayan medicinal herb Rheum spiciforme Royle (spiked rhubarb)

The study aimed to evaluate the effects of low temperature (LT) stress on different aspects of the perennial medicinal herb commonly called spiked rhubarb (Rheum spiciforme Royle) from North West Himalayas. LT is one of the most significant abiotic variables influencing plant growth, distribution, and productivity. It leads to oxidative damage by the overproduction of reactive oxygen species (ROS) that activate a complex plant defense mechanism involving primary and specialized metabolism. Based on its temperature-fluctuating habitat, R. spiciforme is hypothesized as a system of choice to study plant responses against LT stress. In this context, the present study analyzed the impact of LT stress on ROS accumulation, antioxidant defense system, accumulation of different classes of phenolic constituents, and associated regulatory gene expression in cold-subjected R. spiciforme. With the progressive LT treatment, the stress indicators viz hydrogen peroxide (H2O2) and malondialdehyde (MDA) showed a significant accumulation. It also exhibited a substantial impact on the antioxidant defense system by augmenting the activities of ROS scavenging enzymes including guaiacol peroxidase (RsPOX), superoxide dismutase (RsSOD), ascorbate peroxidase (RsAPX), and glutathione reductase (RsGTR) correlated with respective mRNA levels. LC-QTOF-MS analyses depicted the accumulation of anthraquinone (rhein, aloe-emodin, physcion, and emodin), stilbenoid (resveratrol, piceatannol), and flavonoid compounds (quercetin, rutin) that showed a positive correlation with specific stress-related gene transcripts. Additionally, the principal component analysis (PCA) verified that the examined metabolites in control and stress treatments are distinct suggesting that each stress treatment considerably changed the metabolite profile. Similarly, heatmap and clustering analysis showed considerable changes in targeted metabolome vis-à-vis long-term LT stress. Overall, this study forms the basis for understanding LT stress mechanics that may act as a prelude to enhancing desired metabolite content under controlled conditions.