Virulence in the rice pathogen Xanthomonas oryzae pv

Virulence in the rice pathogen Xanthomonas oryzae pv. oryzae

UN Millenium Goals • Cut extreme poverty in half by 2015 • Total number living in extreme poverty estimated at 1. 1 billion • Disease resistance and strategies for resistance deployment are major contributions to agricultural security and agricultural development • Contribution to this goal would be one motivation for many agricultural experiments in the tropics

Virulence and aggressiveness, in the world of plant pathologists • Virulence: ability to infect plants with a particular R gene • Aggressiveness: relative ability to infect plants for which a pathogen is virulent R 1 X X Pathogen genotype with virulence corresponding to R 1 R 2 R 1 X R 2 X Pathogen genotype with virulence corresponding to R 1 and more aggressive R 1 R 2 X Pathogen genotype lacking virulence but still more aggressive

Costs of virulence and costs of avirulence Parameterization of host-pathogen interactions based on work of Kurt Leonard

9 30 Virulence Detected 20 7 5 10 3 1 0 1 2 3 4 5 6 7 8 9 10 Years after initial planting Resistance rating Virulence frequency (%) Boom and Bust Cycle

Re lea se Br ea kd ow n Blast R gene For rice blast, single R genes do not offer long-lasting protection 1965 1970 1975 1980 1985 1990 Adapted from Lee and Cho, International Rice Research Conference, Seoul, 1991

Analysis of virulence in populations of Xoo between 1993 and 2003 • This is work from Kim Webb’s dissertation, advised by Jan Leach, and in collaboration with others including I. Ona, J. Bai, and C. Vera Cruz • Data for virulence over such a long time period is quite rare for a bacterial plant pathogen • The manuscript describing this work is currently in preparation

Race frequency over time Race 9 A/C “overcomes resistance”

Disease incidence and severity over time

Increases in virulence and aggressiveness over time

Temperature alters the effects of the R gene

Homogeneous and heterogeneous rice populations • In this experiment, experimental plots of rice were adjacent to each other • The likely rate of inoculum between plots is not known precisely, but is probably important • Effectively, selection is based on fitness in multiple host genotypes • For more information on rice research, see the website of the International Rice Research Institute: http: //www. irri. org/

Rice blast management through variety mixtures Here advances in resistance are combined with concepts about host mixtures to produce an effective solution to a disease problem Zhu et al. 2000 Nature

Effects of susceptible host abundance on disease severity for two pathogens with different life histories Cox et al. 2004 Phytopathology

Example 3 - SIR model (sort of) • Suppose a pathogen is transmitted by root-to-root contact • Resistance of a root might be root-age specific (thus, roots can move from one class to another with age) • For a real example, see the work of Chris Gilligan and colleagues Photo courtesy of Scott Bontz