Exogenous Glutathione Enhances Salt Tolerance in Patchouli by Promoting the Antioxidant Capacity and Activating the Flavonoid Biosynthesis Pathway

Salt stress is a severe threat to medicinal plants, adversely affecting their growth, yield, and quality. As a key antioxidant tripeptide, glutathione (GSH) confers salinity stress resilience in plants. However, how GSH shapes the plant tolerance to salt stress remains a mystery, especially in medicinal plants, including <i>Pogostemon cablin</i>. In this study, we investigated the regulatory effects of exogenous GSH on <i>P. cablin</i> seedlings under salt conditions. The results showed that GSH significantly improved seedling quality under both normal and salt conditions, evidenced by the increased shoot and root dry weight, plant height, and root length. Moreover, GSH effectively protected the photosynthetic system against salt-mediated damage via raised chlorophyll a, chlorophyll b, carotenoids, quantum yield of photosystem II [Y (II)], and PSII maximum efficiency (Fv/Fm). Furthermore, GSH stimulated the antioxidant defense system, including GSH, AsA, SOD, CAT, APX, POD, and GR, to restrain salt-induced malondialdehyde production and ROS burst. In addition, GSH treatment promoted the biosynthesis of secondary metabolites, including total polyphenol and flavonoid. RNA-seq analysis revealed that the activation of the flavonoid biosynthetic pathway significantly enhanced salt tolerance in <i>P. cablin</i>. Notably, several key regulatory genes within this pathway, including <i>PAL</i>, <i>4CL</i>, <i>C4H</i>, <i>CHI</i>, <i>ANS</i>, <i>F3'H</i>, and <i>CYP93</i>, were significantly upregulated 24 h following GSH application under salt conditions. Therefore, exogenous GSH alleviates salt-induced oxidative stress in <i>P. cablin</i> via enhancing the antioxidant defense system and flavonoid biosynthetic activation. These findings provide valuable insights into the dual defense strategies of GSH for conferring salt resistance in plants.