Maize Transformation to Obtain Plants Tolerant to Viruses by RNAi Technology

Abstract

Plants represent the principal source of human foods and livestock feeds and efforts to improve them in many agronomic aspects have focused on plant breeding. The biotechnology revolution in the past decade made possible for plant breeders access new sources of genetic variability for the development of superior cultivars. It has been possible to define additional strategies for crop improvement through the introduction and stable integration of heterologous genes in plant cells with the knowledge of the regulation of the important agronomic characteristics. The genetic manipulation of plants allows their adaptation to different environmental stresses, whether biotic or abiotic. Currently, the production of genetically modified plants occupies a prominent place in both, basic and applied plant research. Genetically modified crops are generated through a process known as genetic engineering, in which genes of interest are transferred to plants without the need of natural crossing. The most widely used methods for introducing transgenes into the genome of plants are Agrobacterium mediated transformation and microprojectile bombardment. In the first case, scientists took advantage of the natural ability of Agrobacterium to transfer some of its wild genes to plant cells causing the diseases known as crown gall or hairy roots, and replace them by other genes expressing traits of agronomic interest. However, Agrobacterium is not able to infect all plants in a very efficient way, as a consequence, new systems for direct transfer of genes to plants emerged. The microprojectile bombardment system is a direct transfer of genes that involves an equipment known as gene gun. The DNA to be introduced into plant cells is physically attached to metal microparticles that are then propelled against the plant cells, using the gene gun. DNA that penetrates the plant cell can be integrated into the plant genome. Maize is one of the most cultivated cereals in the world. The main maize producer’s countries are the United States, China, and Brazil, followed by Mexico, France, Argentina and India. Among the big losses faced by agriculture are the attacks of pests and diseases. For maize, these problems have worsened since 1990 because of the increase of the cultivated areas in both the normal growing season and the off season, mainly due to intensive cultivation of maize in the irrigated areas, and lack of adoption of crop rotation in certain fields. In recent years, diseases that were not a problem, increased in importance such as the viruses. Among the strains of the virus complexes, potyviruses cause significant losses in grain and forage of maize susceptible genotypes. Plants have different mechanisms for protection against invasion by pathogens, and different genes directly related to