Figure 10

A proposal of the GhPEPC 1 as a core enzyme to regulate carbon turnover and eventually improve seeds oil content in plant.

GhPEPC1 works as a core enzyme not only involved in photosynthesis but also regulated the inflowing of carbon turnover to fatty acid biosynthesis and finally contributed to the increase of cottonseed oil content. In this report, the carboxylation pathway of PEP to OAA was blocked through RNAi of GhPEPC1 and resulted in a decline in OAA concentration. Under this background, more proteins would be converted to aspartate involved in anaplerotic reactions to offset the OAA deficiency in mitochondrial and this hypothesis has been confirmed by our RNA-seq data. Among the DEGs, the glutamine-dependent asparagine synthase 1 was found to be down-regulated in the RNAi lines. Simultaneously, more pyruvate will be transported into mitochondria due to the acceleration of glycolysis, through mitochondrial pyruvate carrier (MPC) located in the mitochondrial inner membrane. The pyruvate located in mitochondria was then involved into two metabolism branches: conversion into acetyl-CoA through pyruvate decarboxylation with pyruvate dehydrogenase complex (PDC) and other irreversible carboxylation to form OAA by pyruvate carboxylase (PC) ligase to serves as an anaplerotic reaction for TCA. The excessive acetyl CoA and relative lack of OAA forced chloroplasts to the heighten light-dependent reactions based on photosynthetic electron transport chains and which produced the ATP and NADPH by using Calvin cycle, where the fixed CO2 was converted as sucrose to provide substrate for glycolysis. However, the RNAi cotton plant was in a state of ‘starvation’ because of the down-regulation of GhPEPC and the TCA were confined. Moreover, the expression levels of ACC in transgenic lines were significantly increased, which indicates that superfluous acetyl-CoA could combine with OAA and form citrate and then transported to cytoplasm via citrate transport protein (CTP). These citrates have participated into biosynthesis of fatty acids and finally stored in cottonseed in the form of TAG. The red marker region indicated that relevant genes exhibiting rising trend in RNA-seq data. The blue marker indicated down-regulated genes.