The Bacterial Wilt Organism: The Crop Nemesis with a New Twist

Abstract

Ralstonia solanacearum causes the destructive bacterial wilt of many crops, including banana, tomato, tobacco, potato, eggplant, ginger, and peanuts. It has remained a major nemesis of agricultural crops despite decades of research spent on it. For the past few years, however, a new twist has been introduced in the hope of better understanding the causal organism. This twist is through molecular approaches.

In the late 1990s, the “bugtok” disease, which causes hardening of cooking bananas, was suspected to be caused by R. solanacearum, but its relationship with the causal bacterium in wilt of Cavendish bananas (Moko) was not clear. Molecular probing of the genome of 127 strains of bugtok and Moko-causing strains using different probes and DNA amplification by polymerase chain reaction (PCR) showed that this group of strains is of only one type and thus is monomorphic. A similar study conducted four years later yielded almost the same result. Another approach was taken to study genetic diversity, which was to clone a repetitive element through the construction of a partial genomic library. Using this cloned repetitive element for hybridization, it was shown that the group was indeed monomorphic with just an additional type, but which was not exclusive to either bugtok or Moko. Thus, it was concluded that the causal organisms of bugtok and Moko are one and the same type. In the former, the bacterium infects the plant through the inflorescence, while in the latter, it does so through the roots.

In the course of this study, it was established that only the banana strains harbor the repetitive element. Thus, the flanking regions of the repetitive element were sequenced, and primers were synthesized for PCR. Indeed, the banana strains can be differentiated from the other strains by PCR. This constitutes a fast and efficient method of differentiating the diverse strains. Nonetheless, since PCR requires skill and a special machine, a more user-friendly method, the monoclonal antibody (MAB) technology, was developed. Using the PCR product as the immunogen, an MAB-based technique specific only to R. solanacearum banana strains was developed. However, the sensitivity of this MAB technique was lower than that of PCR.

Since banana strains are almost monomorphic, in contrast to tomato strains, which are polymorphic, the rate of development of polymorphism was determined in a tomato strain as affected by host genotype. Through a series of PCR experiments utilizing a set of tomato varieties with different levels of resistance to bacterial wilt, a substrain, which had undergone changes in genotype and virulence, was isolated. This substrain was eventually able to cause 92% and 100% wilting of plants of the resistant cultivars CI08 and 508, respectively. In contrast, the wild-type strain induced wilting in only 8.3%–16% in the resistant cultivars. The mutant strain was conclusively shown to break down the resistance of the tomato cultivars. This type of strain can be used as a tester in breeding for durable disease resistance.

Other studies conducted by Opina and Natural on the genetic diversities of tomato and potato strains indicated a polymorphic nature. A PCR technique to detect the species R. solanacearum was devised by Opina and collaborators. Natural and colleagues also devised a technique for the selection of bacterial wilt-free potato seed pieces. Balatero and colleagues utilized marker-aided selection in hastening the process of breeding for resistant varieties. Laurena et al. have studied the basis of bacterial wilt resistance in tomatoes.

All of these studies in the Philippines have contributed to a better understanding of the pathogen and more effective management of bacterial wilt.