Fitness Costs of Disease Resistance Genes in Arabidopsis thaliana
Plants use resistance genes to recognize pathogens and activate the defense system. So far, the products of cloned disease resistance genes have been detected in the plant even in the absence of the pathogen, which suggest that resistance genes are on constant surveillance for potential intruders.
In the plant genome, there are many such kinds of resistance genes. The relatively large number of resistance genes conserved during the evolution process indicates that the plants need to maintain a collection of different resistance genes to deal with the predators or opportunists including viruses, fungi, bacteria, and insects that coexist with the plant in the soil, water, or the air. However these resistance genes segregate for resistance and susceptibility alleles. ‘Why does nature preserve those susceptible alleles given the obvious advantage to an individual of being disease resistant?’. One long-standing hypothesis is that the resistance allele has fitness penalty in the plant. A group of scientists in University of Chicago and Nanjing University of China (Tian et al., fitness costs of R-gene mediated resistance in Arabidopsis thaliana, Nature, May 1, 2003, pages 74-77) tested this hypothesis by comparing the fitness of stains with and without a resistance gene in Arabidopsis thaliana, a member of the woody mustard family.
Tian et al. used a RPM1 gene that confers resistance to a bacterial pathogen of Arabidopsis thaliana. The susceptible individuals without this RMP1 gene do not have the ability to code for any protein products. They engineered a pair of plants, which are genetically different except that one plant carries the resistance allele, the other plant does not carry the resistance allele. Those plants were planted in the field. At the end of the season, the number of siliques per plant and number of seeds per silique, and the dry biomass of each plant were counted. The research shows that there is a cost of resistance in plants. The plants with resistance gene did not set as much seed as the plants without the resistance gene.
So why should a plant so well prepared against infection appears so sickly? Tian et al. explained that the cost of the resistance gene ‘is unlikely explained by the metabolic cost of RPM1 synthesis’, as the level of RPM1 product is very low. The precise molecular mechanism of the phenomena is not clear. They proposed two hypotheses to explain the phenomena. First, the resistance gene could activate the plant defense pathways even in the absence of the pathogen. Environmental stresses could be recognized by the resistance gene and trigger the plant’s defense system. Second hypothesis is that the basal levels of the resistance genes interact with other genes in the plant, and induce some level of plant defense system.
At the end of the article, the authors cautioned that a 9% fitness penalty of the resistance genes is an awfully high load for a considering more than 100 resistance genes located across the Arabidopsis genome. Theoretically the resistance alleles with the largest fitness penalty will be less likely fixed in a species. In the nature environment, the proportion of resistance and susceptible allele can be fluctuated according to the pressure of the disease. The co-existence of resistance alleles and susceptible alleles in nature colonies is a balance of the costs and benefits of resistance.