**10. Prospects of applying RNAi in management of Aspergillus species and aflatoxins in grains**

Early genetic studies have identifiedaflatoxin biosynthesis to becontrolled by a cluster of aflatoxin and sterigmatocystin gene in an ~70kb region [85, 13, 69, 76]. A critical analysis of the pathway indicates that their exists three enzymes that catalyse the two rate-limiting steps in aflatoxinbiosynthesis. Two enzymes stcJ and stcKcatalyse the first step that involves the conversion of Acetate and Malonyl-CoA into Hexanoyl-CoA. A further critical examina‐ tion of the pathway identifies another enzyme stcA which catalyses the conversion of Hexa‐ noyl-CoA to Norsolorinic acid. To addstrength to this observation, Brown et al.,[1996] reported that Aspergillus species with mutations in the stcJ and stcK grew normally but could not produce aflatoxin and sterigmatocystin. In the same study addition of Hexanoic acid to growth media restored aflatoxin and sterigmatocystin production. Recent studies have reported the trafficking of molecular cues between hosts and parasites including fungi. Among the molecules are small interfering RNA SiRNA. This targeted downregulation of gene expression bySiRNA has been used to engineer crops against virueses, nematodes and parasitic plants in cross species version. The key steps for this strategy to succeed are;i) iden‐ tifying a key gene to a process, ii) cloning the target sequence of the gene from the parasite, iii) making an RNAi construct with the target sequence of the parasite in sense and antisense direction separated by an intron so as to allow formation of primary small intereferingRNAs (SiRNA) in host (maize), iv) transforming the host (maize) with the construct tailored for RNAi. In this case the rate-limiting steps in aflatoxin biosynthesis are known to be catalysed bystcJ, stcK and stcA [85,]. The strategy is therefore be to make an RNAi construct contain‐ ing either combined partial or full sequences of the stcA, stcK and stcJ in sense and antisense orientation and transform it into maize. Upon colonization with aflatoxigenic fungi in the field, the primary SiRNA molecules will then cross from transgenic maize into Aspergil‐ lussppfungi through the haustoria connection at infection. The siRNAs will then cleave the stcJ, stcK and stcA mRNAs into 20 to 28bp long double molecules hence downregulating or inhibiting aflatoxin and sterigmatocystin biosynthesis.The transformed aflatoxigenic species in the field will be unable to synthesizeaflatoxins both in field and storage. RNAi will suc‐ ceed in this case because it can be both local (cell–cell) and systemic (spread through the vas‐ cular system), hence all parts of the transgenic plant shall remain armed against aflatoxin biosynthesis. The stcJ, stcK and stcA do not exist in maize hence their silencing will not af‐ fect the crop.
