# Effects of Temperature on Growth and Isoprene Metabolism Pathway in _Eucomnia ulmoides_ [PERSON]
## 1 Introduction  
The isoprene metabolic pathway is important for the synthesis of secondary metabolites in plants. The intermediate isoprenyl pyrophosphate is generated by two pathways: the mevalonate (MVA) pathway, which occurs in the cytoplasm, and the pyruvate/phosphoglyceraldehyde (MEP) pathway, which occurs in the plastids [1]. According to a previous study, the isopentenyl pyrophosphate (IPP) that is used as a precursor for sesquiterpene and triterpene synthesis is produced by the MVA pathway [2], whereas IPP that is used as a precursor for monoterpene,iterpene, and carotenoid synthesis is produced by the MEP pathway [3; 4]. Based on the number of isoprene residues, terpen can be classified into monoterpenes, sesquiterpenes, diterpenes, and triterpenes [5]. Tertpenoids play a crucial role in regulating plant growth, development, and environmental adaptation [6]. When environmental temperatures increase, plants adapt by increasing the production and emission of isoprene [7; 8]. Geranyl pyrophosphate (GPP), frangvel pyrophosphate (FPP), and geranylegenyl pyrophosphate (GGPP) are direct precursors used in the synthesis of monoterpenes, sesquiterpenes, and diterpenes, respectively, using terpene synthase (TPS) [9]. The TPS family is categorized into seven subfamilies: TPS-a, TPS-b, TPS-c, TPS-d, TPS-e/_f_, TPS-g, and TPS-h [10]. This family is classified into monoterpene synthases, sesquiterpene syntheses, diterpene syntheses, and triterpene syntheses based on terpene products. Isoprene metabolites in plants encompass artemisinin, gibberellins [11], abscisic acid, chlorophyll, terpenoids, sterols, perfumes, flavors, and rubbers [12]. _Eucommia ulmoides (E. ulmoides)_, a tertiary relict tree species unique to China, is not only a precious traditional medicinal herb but also a high-quality source of rubber [13; 14; 15]. The pericarp, bark, leaves, and roots of _E. ulmoides_ are rich in rubber filaments. The rubber content in the fruit ranges from 12% to 17%, and the leaf rubber content is between 2% and 5% [15]. _E. ulmoides_ rubber is a natural polymer compound with the molecular structure of trans-polyisoprene. It possesses a shape memory function [16], and a unique \"rubber-plastic\" duality [17]. This property makes it suitable for development into high-elasticity rubber materials, low-temperature plasticity materials, and thermoelastic memory materials. Accordingly, _E. ulmoides_ is widely used in the rubber industry. For instance, it is blended with chloroprene rubber or polypropylene to enhance the characteristics of the composite materials and to prepare shape memory materials [18; 19; 20]. Rubber is one of the end products of the isoprene metabolic pathway in rubber-producing plants. IPP is a precursor for terpenoids, and rubber synthesis, with rubber synthesis primarily sourced from the MVA pathway and terpenoids, compounds from the MEP pathway [21]. In the MVA pathway, HMGR determines the direction of \"carbon flow.\" When HMGR activity increases, sesquiterpene products increase [22]. However, _HMGR_ expression is affected by environmental factors, such as light, temperature, and humidity [4; 23; 24]. Global temperature extremes are becoming more frequent, impacting plant health via cold stress (0-18 \({}^{\circ}\)C), freezing (<0 \({}^{\circ}\)C), and heat stress (>25 \({}^{\circ}\)C), ultimately affecting crop production [25; 26]. The growth, development, and accumulation of secondary metabolites in _E. ulmoides_ are inevitably influenced by climate change [18]. Prior research has demonstrated that rubber trees yield more rubber within a temperature range of 18-28 \({}^{\circ}\)C [27]. To understand the effect of temperature on the growth and isoprene metabolic pathway of _E. ulmoides_ seedlings, this study analyzed the transcriptomes of _E. ulmoides_ seedlings after 40 days of treatment at the control temperature and high temperature (HT) using RNA sequencing (RNA-Seq).