Integrated Physiological, Transcriptomic, and Metabolomic Analyses of Chrysanthemum 'Boju' Under Excessive Indole-3-Acetic Acid Stress

Yuqing Wang ^1,2 Yingying Duan ^1,3 Na Chen ² WanYue Ding ⁴ YaoWu Liu ^2* Shihai Xing ^1,5,6*

• ¹ Anhui University of Chinese Medicine, Hefei, China
• ² Joint Research Center for Chinese Herbal Medicine of Anhui of IHM, Bozhou Vocational and Technical College, Bozhou, China
• ³ Institute of Traditional Chinese Medicine Resources Protection and Development, Anhui Academy of Chinese Medicine, Hefei, China
• ⁴ Joint Research Center for Chinese Herbal Medicine of Anhui of IHM, Anhui University of Chinese Medicine, Hefei, China
• ⁵ MOE-Anhui Joint Collaborative Innovation Center for Quality Improvement of Anhui Genuine Chinese Medicinal Materials, Hefei, China
• ⁶ Anhui Province Key Laboratory of Research & Development of Chinese Medicine, Hefei, China

Indole-3-acetic acid (IAA) plays a vital role in regulating plant development and responses to abiotic stress. However, prolonged and excessive IAA treatment may induce oxidative stress, hinder growth, and even lead to plant death. This study investigates the effects of excessive IAA on the growth of Chrysanthemum morifolium (Boju) to elucidate the underlying molecular mechanisms. We examined the impact of a 10 mg/L IAA treatment over nine consecutive days from various perspectives, including physiological, biochemical, transcriptomic, and metabolomic analyses. The results revealed that IAA treatment significantly increased chlorophyll a, chlorophyll b, and carotenoid levels by 37%, 46%, and 25%, respectively, compared to pre-treatment levels. These changes suggest that C. morifolium was experiencing stress. Furthermore, the malondialdehyde (MDA) content was 1.79 times higher than the pre-treatment level, supporting this conclusion. In response to stress, the plant heightened its antioxidant defense, illustrated by a 93.8% increase in peroxidase (POD) activity and a 45% increase in superoxide dismutase (SOD) activity. Moreover, exogenous IAA treatment resulted in a significant reduction in endogenous levels of gibberellins (GA3 and GA4), abscisic acid (ABA), and IAA, with decreases of 93%, 45%, 99%, and 99%, respectively. These findings indicate that Boju is experiencing stress and is likely limiting growth by reducing endogenous hormone levels to mitigate the oxidative stress induced by excessive IAA. Transcriptomic and metabolomic analyses identified 263 differentially expressed metabolites and 144 differentially expressed genes. These findings suggest that under IAA-induced stress, plants enhance their stress tolerance by upregulating proB(Glutamate 5-kinase), proA(Glutamate-5-semialdehyde dehydrogenase), GAD(Glutamate decarboxylase), and peroxidases, while downregulating the PK(Pyruvate kinase) gene, thereby regulating amino acid biosynthesis, coumaric acid metabolism, starch and sucrose metabolism, and pyruvate metabolism. In conclusion, this study unveils the complex response of C. morifolium to excessive IAA treatment, indicating that prolonged exposure to IAA may inhibit plant growth and emphasizing the nonlinear effects of IAA.