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  • In Hevea brasiliensis both the soluble PPase and the

    2020-09-21

    In Hevea brasiliensis, both the soluble PPase and the membrane-bound V-PPase are present in the latex; the soluble PPase is in the C-serum (cytosol) of the latex [6] and the V-PPase is on the tonoplast of the lutoid, a special vacuole in the laticifers of rubber trees [7]. The V-PPase on the lutoid has two main functions: firstly, hydrolysis of PPi to inorganic phosphorus (Pi), which maintains the concentration of PPi below 0.1mmolL−1 and prevents the accumulation of PPi in the latex; secondly, it acts as a H+ pump and translocates H+ into the lutoid lumen (called B-serum) so as to prevent the acidification of the cytoplasm. These two functions keep the lutoids in homeostasis and regulate the cytoplasmic pH, which provides an optimum pH environment for rubber biosynthesis in the latex. Thus, the lutoid V-PPase promotes the latex metabolism and the synthesis of rubber in the laticifers of Hevea[8]. In the process of rubber biosynthesis, large amounts of PPi are released into the latex of the laticifers, which can result in feedback inhibition of rubber synthesis via the accumulation of PPi [6]; therefore, prompt hydrolysis in situ of the PPi released from the rubber particles is very important. The synthesis of rubber takes place on the rubber particles in some rubber-producing plants, such as H. brasiliensis, etc. The crucial enzymes and protein factors involved in the synthesis of rubber are tightly associated with the rubber particles. Great efforts have been made to isolate and identify these fundamental enzymes and proteins related to the synthesis of rubber in rubber-producing plants [9], [10]. Among these enzymes and proteins, perhaps the most important is rubber transferase, a cis-prenyl transferase (EC 2.5.1.20). Together with other enzymes and proteins, such as the rubber HBX 41108 mg factor (REF), etc. [11], rubber transferase could catalyse the cis-condensation of isopentenyl pyrophosphate (IPP) molecules to form the rubber molecule [12], [13]. In addition, rubber transferase was considered as the key factor that determines the rubber-producing ability of rubber trees [14]. So far, little is known about the molecular mechanisms of rubber biosynthesis. It was presumed that rubber transferase and other related proteins or enzymes form a rubber transferase complex [15] that might be the elemental unit of rubber biosynthesis. If the rubber transferase complex is located on the rubber particles, the challenge is to unravel the components of the rubber biosynthesis-related protein complex. Rubber biosynthesis of rubber trees is a typically plant secondary metabolism of isoprenoid pathway [16]. Some studies reported that jasmonic acid (JA) was an effector that could regulate the secondary metabolism of plants [17], [18], [19]. To elucidate the molecular mechanisms that plant hormones, such as JA and ethylene, etc., regulate the rubber biosynthesis of H. brasiliensis is very important in the natural rubber production. JA has been considered to be a wound signal that can induce many physiological responses [20]. In H. brasiliensis, JA could induce the laticifer differentiation [21] and might be a very important signal molecule that regulates the synthesis of rubber [22], [23]; however, it is unclear how JA might achieve these. Our earlier investigation demonstrated that some genes in the latex are regulated by exogenous JA, and Hbvp1 is one of the JA-responsive genes in the latex of rubber trees [23]. The aim of the present study is further to reveal the regulatory mechanisms of the Hbvp1 expression, and the involvement of Hbvp1 in the rubber biosynthesis of H. brasiliensis. In this research, we demonstrate that Hbvp1 is a JA-responsive V-PPase located on the rubber particles, which could imply that PPi released from the biosynthesis of rubber might be hydrolyzed in situ to Pi mainly by Hbvp1 on the rubber particles, rather than by V-PPases on the lutoids or the soluble PPases in the C-serum. These data provide a new insight into the function and the localization of V-PPases in plants, and could contribute to understanding the relationship between the transduction of JA signal and the biosynthesis of rubber in the laticiferous cells of H. brasiliensis.