Mechanism Of Resistance To Aflatoxin Production In Peanut(Arachis Hypogaea L.)

Peanut(Arachis hypogaea L.) is an important oilseed and cash crop and a key source of vegetable oil and protein worldwide. China is the largest country worldwide in terms of annual peanut production, consumption, and international trade. Among the major oilseed crops in China, peanut has obvious advantages in terms of crop yield, oil yield, crop value to the farmer and competitiveness of export trade as compared with the soybean and rapeseed. However, aflatoxin contamination caused by Aspergillus fungi, especially A. flavus and A. parasiticus, is a serious constraint to peanut industry, affecting food safety of peanut products and increasing health risk of consumers because aflatoxins are toxic, teratogenic and carcinogenic mycotoxins associated with both acute and chronic toxicity in animals and humans. Breeding and utilization of peanut varieties with desirable resistance to aflatoxin contamination is the most cost-effective approach for aflatoxin management. Resistance to aflatoxin production in peanut is a very complex biotic stress-responsive mechanism. It is highly necessary to investigate the molecular mechanisms of peanut resistance to aflatoxin production in order to develop elite resistant peanut varieties and more effective management strategies. In this study, we analyzed the transcript profiling of resistant(Zhonghua 6) and susceptible(Zhonghua 12) peanut seeds in response to aflatoxin production by A. flavus, the functional genomics of the A. flavus in its interacting with resistant and susceptible peanut genotypes, and transcriptomes in A. flavus in response to resveratrol. The main results are as following.1. The study provided the first comprehensive report of transcriptomes of post-harvest peanut seed in response to aflatoxin production by A. flavus.The resistant(R) and susceptible(S) peanut genotypes both underwent a large transcriptional modulation representing various metabolic processes involved in defense against aflatoxin production, but more DEGs were up-regulated in the resistant genotype than in the susceptible at each time point. A number of putative candidate genes such as NBS-LRR, LRR-RLK, MAPK, PR, TF, LOX, PGIP, ARF, PAL, C4 H, 4CL, STS and CHS related with aflatoxin production resistance in post-harvest seeds were identified. These transcriptional modulations could eventually result in the synthesis of resistance-related proteins, secondary metabolites and signaling molecules that provide defensive advantages to the peanut. Furthermore, many more resistance-related DEGs were significantly up-regulated and enriched in the reistant genotype, which suggested that Zhonghua 6 possessed comprehensive and prompt responses to the biotic stress.2. For the first time, an RNA-seq approach was employed to investigate molecular events involved in the development and metabolism of A. flavus during the fungus interaction with the peanut. The research demonstrated that the global transcriptional analysis provided an exhaustive view of genes involved in development of mycelia and asexual spores, controlling of biosynthesis and activities of enzymes, conidial pigments and secondary metabolites processes, which were coordinately influenced in A. flavus by its host peanut(R and S genotypes). The transcriptome comparisons revealed that DEGs associated with mycelial penetration(face, abfB, mndA and pgxC), conidial formationand development(RodA/RolA, AtfA, Con-6, Con-10 and PksP) and aflatoxin biosynthesis and accumulation(aflD, aflX, aflNa) were up-regulated in af_S compared with af_R. This differential transcription could explain why aflatoxin accumulation was much higher in susceptible Zhonghua 12 seeds than in the resistant Zhonghua 6.3. Treatment of resveratrol in A. flavus resulted with decreased aflatoxin production and conidia formation, and the treatment also caused abnormal mycelia development. Resveratrol directly inhibit the expression of aflatoxin biosynthetic pathway cluster genes, the aflA and aflB expression at significantly low transcriptional levels could result in an insufficient amount of the starter unit hexanoate for aflatoxin biosynthesis. Resveratrol could also significantly enhance the activity of antioxidative enzymes, which destroy radicals produced within the cells that are toxic to biological systems, leading to decreased aflatoxin production. In addition, decreased transcription of stuA, fluG and flbC by resveratrol could affect the mycelial(colony) and conidial development. The above observations demonstrated that the high resveratrol content in Zhonghua 6 was a key molecular and biochemical mechanism for its resistance to aflatoxin production.