The Use of African Indigenous Genes in the Development of Transgenic Maize Tolerant to Drought and Resistant to Maize Streak Virus

Abstract

When developing a plant with resistance to biotic and abiotic stresses and utilizing genetic engineering, why should scientists limit themselves to genes from known organisms? Why not test those from indigenous species that might have unique properties? In this chapter we describe the use of indigenous genes for the development of crops important to Africa. The first is maize tolerant to drought, a situation which appears to be worsening on the continent, and the second is maize resistant to the African-endemic maize streak virus. The genes for drought tolerance were derived from the resurrection plant, Xerophyta viscosa, which survives even when it contains only 5 % of its relative water content. The plant can be ‘resurrected’ within 80 h of receiving moisture. Two methods were used to identify potential genes of interest. The first was complementation by functional sufficiency in Escherichia coli, resulting in the isolation of XvSap1 (which was found to code for a membrane-associated signalling protein) and XvAld, coding for aldose reducatase which converts glucose to sorbitol, an osmoprotectant. The second method was differential screening of expression libraries resulting in the isolation of XvPrx2, which codes for an antioxidant peroxiredoxin, and XvG6, which codes for a stress-responsive regulatory protein. Other genes isolated, tested, and not used further are also mentioned. For resistance to maize streak virus, the approach of pathogen derived resistance was used, resulting in the isolation of dominant negative mutants of the viral replication associated protein gene, rep. In a refinement of this approach, a virus-inducible version of the mutants was developed as well as an siRNA approach. As the development of transgenic maize is a lengthy process, the genes were first tested in model systems. For drought tolerance the model plants were Arabidopsis and tobacco while for virus resistance black Mexican sweetcorn in tissue culture and transgenic Digitaria sanguinalis, an MSV sensitive grass, were used. Cassettes of the genes shown to be effective, including inducible systems for both drought and virus resistance, were introduced into maize and results are presented. The paper concludes with a discussion on how to bring these products to the farmers’ market, in Africa.

When scientists decide to develop a crop with a specific trait, using genetic engineering, they can choose the relevant genes from a variety of different sources. The question arises, does one stick to tried and tested genes from known organisms, or does one consider those from indigenous species that might be able to solve the problem more efficiently? In this chapter, we look at two examples of how that latter approach can be used. The first is drought tolerance in maize where the resurrection plant, X. viscosa, was chosen for its ability to withstand the loss of 95 % of its water content and readily ‘resurrect’ upon watering. The second is an already proven approach, namely that of using a pathogen to derive resistance to it, and the example is the African indigenous maize streak virus.