Rice Genetics IV - IRRI books - International Rice Research Institute

proteins, (2) the expression of dysfunctional virus proteins, and (3) the expression ofvirus RNA or virus-associated RNA.PDR using wild-type virus proteinsIt is logical to assume that deliberately crippled viruses can be complemented bytransgenic expression of the wild-type gene as shown by Osbourn et al (1990). Inmost cases, however, virus-derived protein expression has either inhibited or had noeffect on virus multiplication. This could be due to the viral protein being expressedinappropriately in terms of the regulated expression of viral genes during a normalinfection.Coat proteins. Coat protein–mediated resistance has been the most widely usedform of genetically engineered resistance against plant virus infection (Fitchden andBeachy 1993). It is achieved by incorporating a gene sequence encoding the viral coatprotein into a plant gene expression cassette and transferring the cassette into theplant genome. Expression of the virus coat protein in the plant usually gives resistanceagainst not only the virus from which the coat protein was isolated but alsoagainst other related viruses. Since the first report of this type of resistance for TMV(Powell-Abel et al 1986), coat protein transgenes have been used to engineer virusresistance for 23 plant virus groups. Although this approach has been widely effective,the mechanisms by which it operates are not well understood. The resistanceseems to operate by interfering with virus particle un-coating (Wilson and Watkins1986), virus replication (Osbourn et al 1989), and virus spread (Wisniewski et al1990). There are inconsistencies, however, between the attributes of TMV coat protein–mediatedresistance and those of other viruses. Many of the examples of “coatprotein–mediated resistance” have attributes more consistent with RNA-mediatedmechanisms (discussed later).Replicase. Replicase-mediated resistance was first reported for TMV (Golemboskiet al 1990) and was shown to be protein-mediated by Carr and Zaitlin (1991) using amutated replicase. Replicase-mediated resistance has been shown to be effective withother viruses (see Waterhouse and Upadhyaya 1998). However, in plants carrying atransgene derived from the replicase genes of potato virus X (Mueller et al 1995),cowpea mosaic virus (Sijen et al 1996), and pepper mild mottle virus (Tenllado et al1995), there is clear evidence that resistance is mediated at an RNA level.Movement protein. The suggestion that the expression of a heterologous movementprotein, which is not adapted to a particular plant species, can interfere withviral spread comes from TMV resistance in transgenic tobacco (Nicotiana tabacum)expressing the brome mosaic virus (BMV) movement protein gene (Malyshenko et al1993).PDR using dysfunctional virus proteinsA mutated protein may be able to compete with, and disrupt, the function of a wildtypeprotein. Mutant replicase and movement protein transgenes have been shown tobe effective PDR genes.Engineering for virus resistance in rice 407