Methyl Jasmonate (MeJA) is also a well-known mediator of biotic and abiotic stress responses, it can significantly and broadly regulate the synthesis of defensive metabolites and initiate the expression of genes associated with systemically acquired and localized resistance(Wasternack et al., 2013; Zhu, 2014). The jasmonin pathway is an important signaling pathway in plants. The biologically active jasmonic acid derivative jasmonoyl-isoleucine (JA-Ile) could binds to the F-box protein COI1, resulting in the protein ubiquitination of JAZ protein, the negative regulator of the jasmonin pathway, and release transcription activators, thereby activating the expressions of functional genes downstream. Jasmonin’s functions involved in the regulation of plant growth and development such as root growth, plant fertility, tuber formation, fruit ripening and senescence and other life activities, even play important roles in plant biotic and abiotic stress(Pollier et al., 2013; WasternackHause, 2013). Meanwhile, MeJA could promote the accumulation of other phytohormones (Bai et al., 2019; Garza et al., 2009; Post et al., 2012). Crosstalk usually exists between phytohormones, in some cases the crosstalk was controlled by those shared components during signal transduction pathways, which reflected the regulation of the synthesis of another by a given hormone (Du et al., 2013; Gutierrez et al., 2012; Hentrich et al., 2013). For example, MeJA could regulate the expression of ANTHRANILATE AYNTHASE α1/β1 (ASA1/ASB1) to achieve metabolic effects on the tryptophan pathway (Dombrecht et al., 2007; Sun et al., 2009), and promote the accumulation of IAA by increasing tryptophan, the formation of IAA precursor. And MeJA could even directly regulate the YUCCA gene in the IAA synthesis pathway to regulating the synthesis of IAA in roots(Hentrich et al., 2013). MeJA could also affect the polar transport of auxin and promote the function of auxin, such as, MeJA could participate in the transport of auxin by affecting the distribution and expression of the polar auxin transporter genes PIN2 (Sun et al., 2011). Otherwise, MeJA and IAA could also interact through the action of auxin response factor (ARF) to regulate the transcription of downstream auxin-dependent genes (Fattorini et al., 2018). Through these ways, but not only these, MeJA closely cooperates with IAA in regulating the formation and growth of adventitious roots formation (Gutjahr et al., 2005; Sharma et al., 2022; Wang et al., 2022). IAA in plants could affect the early division of plants, root elongation and the formation of tissues (Rui et al., 2020). MeJA could regulates a variety of secondary metabolites, especially terpenoids (Rui et al., 2020; Sirhindi et al., 2020). MeJA could not only significantly up-regulated the transcription level of genes in MVA pathway, MEP pathway and the triterpenoid synthesis pathway, but also regulated the synthesis of triterpenes through the endoplasmic reticulum-related degradation quality control system (Bai et al., 2019; Pollier et al., 2013), thereby promoting the accumulation of triterpenoids in plants and achieving plant resistance to adverse environments.
In order to adapt to different environmental niches, plants had evolved the ability to produce large, diverse, and specific secondary metabolites, at the meanwhile, specific metabolites of plants often have certain ecological functions and mediate the interaction between plants and the environment (Chae et al., 2014; Huang et al., 2019). Triterpenes are one of plant-specific metabolites, which are almost the largest and most structurally diverse natural production family, with plant defense and signal transduction functions (Augustin et al., 2011; Papadopoulou et al., 1999). Triterpenes are widely distributed in various parts of plants and play an important role during plant growth, such as, the expression of triterpenes in roots can trigger the "super-hairy" root phenotype (Kemen et al., 2014), and the marginal cells of alfalfa heel secrete large amounts of triterpenoids to protect the fragile root cap and ensure root development and growth (Field et al., 2008; Watson et al., 2015). Triterpenes in stems can effectively protect plants against dehydration, they defended against potential herbivore damage together with triterpenes secreted in leaves (Thimmappa et al., 2014). Compared with traditional chemical pesticides, triterpenes are greener and healthier as pesticides. At present, many triterpenoids containing C-28 carboxyl groups have been developed and applied as biological pesticides (Tian et al., 2021). In terms of defense function, the synthesis of triterpenes can increase the tolerance of plants to external stress and alleviate the damage of UV-B to plants(Jiang et al., 2020). In addition to the defense function, triterpenoids also have good antibacterial activity. In plants such as Arabidopsis (Huang et al., 2019) and Schisandra(Jacoby et al., 2020), triterpenoids could play as signal molecules, nutrient sources, antibiotics, etc., regulating endophytes which are beneficial to growth (You et al., 2021), and promoting plant growth and metabolism (van Damme et al., 2006).
Rosa Roxburghii (also named Cili in Chinese) is a unique medicinal and edible plant that grows in southwest China, which has a long history and belongs to the Rosaceae family (Xu et al., 2019). It was first recorded in the Compendium of Materia Medica. People make use of its advantages of rich in vitamin C and antioxidant enzymes like SOD and POD (Liu et al., 2016). The fruit of R. Roxburghii were mass-produced as functional foods, such as Cili wine, preserved fruits and juice, etc. With the deepening of research, the mysterious veil of R. Roxburghii has been unraveled: R. Roxburghii was not only rich in vitamin C, but also rich in bioactive secondary metabolites, especially the specific triterpenoids including kajiichigoside F1, roxburic acid, rosamultin, pomonic acid, and tormentic acid, as well as Oleanolic acid, ect (Xu et al., 2019). These triterpenoids rich in active hydroxyl and carboxyl groups make the excellent performance of R. Roxburghii as a functional food in anti-tumor, anti-oxidation, anti-virus and radiation protection (Alkhatib, 2020; Jo et al., 2020; Wang et al., 2020; Xu et al., 2017; Xu et al., 2020; Xu et al., 2018). Even Academician Zhong Nanshan proposed that Cili has high nutritional value and health care effects to play a positive effect on the prevention of Corona Virus Disease 2019, at the Guizhou Cili Industry Development Forum held in Guangzhou, On April 28, 2020.
In this study, R. Roxburghii were treated with different hormones, and various growth trends were observed, which MeJA promoted the most obvious growth phenotype among them. In order to explore the possible reasons how MeJA promotes the growth of R. Roxburghii seedings, we monitored some physicochemical indexes, including contents of MAD, peroxidases, soluble sugar, soluble protein and proline. We found that the contents of IAA, JA-Ile and triterpenoids in R. Roxburghii were significantly increased after MeJA treated. In order to found out the possible mutual regulation relationship among MeJA, IAA and triterpenes, the JA receptor protein gene COI1, and one of the SCFCOI1complex proteins gene ASK1, the IAA synthesis pathway genes ANTHRANILATE AYNTHASE α1/β1 (ASA1/ASB1), The IAA receptor protein gene TIR1, the IAA polar transport protein gene PIN2 and key genes in the triterpene synthesis pathway incuding 3-hydroxy-3-methylglutaryl coenzyme-A reductase (HMGR), 1-Deoxy-D-xylulose 5-phosphate synthase (DXS), 1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR), 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase (HDR), squaleme synthetase (SQS), squalene epoxidase (SQE) and amyrin synthetase (AS) were quantified by qPCR. Finally, the qPCR results were consistent with our hypothesis. Our experimental results preliminarily explain that MeJA promoted the JA signal transduction pathway, as well as the synthesis and transport of IAA. While, MeJA could also accelerated the accumulation of triterpenoids which could not only regulate the root growth of R. Roxburghii but also endow R. Roxburghii with certain defense ability. In addition, we speculate that JA and IAA might also be involved in affecting the synthesis and metabolism of triterpenes, they promoted the growth of plants together, however, all these changes might cause by the only variate value, exogenous MeJA.