**9. Application of RNA interference in management of biotic challenges in agriculture**

RNAi against crop parasites that include insects, nematodes, viruses and parasitic plants has been demonstrated [3, 9, 10, 11, Day et al., 1991, 22, 31, 82, 83, 88]. For example, the cotton bollworm (Helicoverpaarmiger a; Lepidoptera) and western corn rootworm (Diabroticavir‐ giferavirgiferaLeConte) where dsRNA directed against a gene encoding V-type ATPase A, demonstrated rapid knockdown of endogenous mRNA within 24 hours of ingestion. In ad‐ dition, dsRNAs directed against three target genes (β-tubulin, V-ATPase A and V-ATPase E) in western corn rootworm effectively resulted in high larval mortality [9].

Root-knot nematodes (Meloidogynespp) cause significant crop losses in Africa with the most damaging ones being M. incognita, M. javanica, M. arenariaand M. hapla. Since the discovery that RNAi is active in worms through oral uptake of dsRNA [22], intense studies on the control of parasitic nematodes through targeting essential parasite genes have been carried out [31, 82]. Yadav [82] described almost complete resistance to Meloidogynespp in‐ fection in transgenic tobacco. Geminiviruses, a major problem on crops in tropical and sub‐ tropical countries have been targeted via RNAi [Asadet al., 2003, 10, 11, 19, 83, 88]. With dsRNA and antisense RNA (as RNAi technologies), several regions of the viral genome can be targeted by plants expressing fused viral siRNA or hairpin dsRNA sequences. Multiple targeting of the viral genome provides stable and durable resistance considering that the vi‐ ral genome is highly recombinogenic [12].

Currently it is understood that transcripts can be trafficked from host to parasitic plants [59]. Therefore when the RNAi transformed host plant is attacked by the parasite, the gene spe‐ cific RNAi transcripts can be trafficked into the parasitic plant via the haustorial connection leading to gene silencing. Some studies have proposed the targeting of KNOX genes which are vital in plant development while others have suggested targeting genes that code for aquaporins that aid in loosening of the host plant cell wall during parasite infection [63]. This ability to tap into native pathways has yielded crucial breakthrough in parasitic plant management [23].

proved to have ability to regulate the expression of genes involved in a variety of cell proc‐ esses such as proliferation, apoptosis and differentiation [2]. Moreover, it is thought to play a role in protecting the genome against damage caused by transposons [44]. More recently, these findings have been extended by the observations that siRNA-directed DNA methyla‐ tion in plants is linked to histone modification [89]. In fission yeast, hetero-chromatin forma‐

Application of RNAi in crop improvement has been derived from targeted degradation of gene products with significant homology to the introduced sequence. Successful utilization of RNAi-based resistance effects rely on; (i) identification of a target gene (ii) dsRNA deliv‐ ery, which includes in plantaexpression of dsRNA and (iii) delivery of sufficient amounts of intact dsRNA. RNAi can therefore be an important tool for crop improvement given that the RNAi signal can be both local (cell-cell) and systemic (spread through vascular system) [8, 71]. RNAi mediated silencing for agricultural traits offers the advantage of transmission

**9. Application of RNA interference in management of biotic challenges**

RNAi against crop parasites that include insects, nematodes, viruses and parasitic plants has been demonstrated [3, 9, 10, 11, Day et al., 1991, 22, 31, 82, 83, 88]. For example, the cotton bollworm (Helicoverpaarmiger a; Lepidoptera) and western corn rootworm (Diabroticavir‐ giferavirgiferaLeConte) where dsRNA directed against a gene encoding V-type ATPase A, demonstrated rapid knockdown of endogenous mRNA within 24 hours of ingestion. In ad‐ dition, dsRNAs directed against three target genes (β-tubulin, V-ATPase A and V-ATPase E)

Root-knot nematodes (Meloidogynespp) cause significant crop losses in Africa with the most damaging ones being M. incognita, M. javanica, M. arenariaand M. hapla. Since the discovery that RNAi is active in worms through oral uptake of dsRNA [22], intense studies on the control of parasitic nematodes through targeting essential parasite genes have been carried out [31, 82]. Yadav [82] described almost complete resistance to Meloidogynespp in‐ fection in transgenic tobacco. Geminiviruses, a major problem on crops in tropical and sub‐ tropical countries have been targeted via RNAi [Asadet al., 2003, 10, 11, 19, 83, 88]. With dsRNA and antisense RNA (as RNAi technologies), several regions of the viral genome can be targeted by plants expressing fused viral siRNA or hairpin dsRNA sequences. Multiple targeting of the viral genome provides stable and durable resistance considering that the vi‐

Currently it is understood that transcripts can be trafficked from host to parasitic plants [59]. Therefore when the RNAi transformed host plant is attacked by the parasite, the gene spe‐ cific RNAi transcripts can be trafficked into the parasitic plant via the haustorial connection leading to gene silencing. Some studies have proposed the targeting of KNOX genes which are vital in plant development while others have suggested targeting genes that code for

tion at centromere boundaries is associated with siRNAs [72].

48 Aflatoxins - Recent Advances and Future Prospects

across many cells and application in multigene family silencing.

in western corn rootworm effectively resulted in high larval mortality [9].

ral genome is highly recombinogenic [12].

**in agriculture**
