Our research program is focused on the molecular basis of biological interactions, particularly of plant-pathogen interactions. Since the outcome of most plant-pathogen interactions is determined by the genotype of both the host and the pathogen, they are model systems for molecular genetic studies of regulatory mechanisms of biological interactions. Disease resistance involves an intricate interaction between the pathogen and the host plant. During all stages of the interaction it is likely that there is a sequential exchange of information between the two partners resulting in expression of specific genes. The overall outcome of an infection depends on the outcome of each of the many steps in the process. We know very little about the regulation of any step in the process.
Our research program currently consists of studies aimed at understanding the molecular genetic basis of two particular plant bacterial-pathogen interactions: (1) Xanthomonas campestris pv. vesicatoria and tomato - functional analysis of avirulence gene avrRxv and (2) X. oryzae pv. oryzae and rice - developmental control of Xa21 resistance.
In the interaction between the bacterial pathogen Xanthomonas campestris pv. vesicatoria and tomato my research focuses primarily on the function of a gene in the pathogen that determines the outcome of the interaction. This so-called avirulence gene is responsible for inducing a resistance response in the host plant tomato. We are interested in studying the function of the avirulence gene avrRxv because it is unusual that a pathogen would continue to carry a gene that prevents it from causing disease. This gene is of special interest because it has sequence similarity to two virulence genes from the mammalian pathogens Yersinia and Salmonella. One of these genes called YopJ, is a secreted protein has been shown to induce programmed cell death in cells of the mammalian immune system. The similarity of the response incited by YopJ in immune system cells and by AvrRxv in plant leaf cells is extremely intriguing and will guide our future research. In addition, avrRxv has sequence similarity to a gene with unknown function from the symbiotic bacterium Rhizobium that forms associations with legumes and fixes nitrogen. This new family of host interaction factors, called the AvrRxv family, will undoubtedly point out common ways in which bacteria have evolved to associate with their hosts. Whether or not this family provides another example of probable conservation of function among plant and mammalian pathogens and symbioants remains to be experimentally dissected by mutagenesis studies that are underway. Another approach to identifying the function of a gene is to understand the environmental conditions under which it is expressed. We have used several approaches to study regulation of avrRxv, reporter gene analysis, RNA gel blot analysis, primer extension and site-directed mutagenesis of promoter domains. Our studies of regulation of expression of avrRxv have indicated that it is regulated without regard for environmental conditions and not by a putative plant-inducible promoter sequence. It is expressed constitutively suggesting that its function is crucial for all aspects of its existence and not just when it has invaded host tissues. Further research will dissect the regulatory pathways.
In the interaction between rice and the bacterial pathogen Xanthomonas oryzae pv. oryzae (abbreviated Xoo) we are studying the developmental control of the resistance response in rice. The overall goal of this research is to understand the molecular genetic mechanisms controlling the integration of a cell-surface receptor mediated response with the developmental program of an organism. Recent molecular genetic studies of interactions between plants and pathogens have indicated that resistance responses are controlled by resistance genes that appear to be members of cellular signal transduction cascades. The focus of my work is the rice resistance gene Xa21 which confers stable resistance to the bacterial pathogen Xoo. Xa21 encodes a serine-threonine kinase receptor-like protein. Interestingly, the Xa21-specific resistance appears to be developmentally controlled. Until the plants reaches a certain developmental stage, full Xa21-specific disease resistance is not exhibited. Understanding the molecular genetic basis of the developmental regulation of disease resistance in this model system will pave the way to designing strategies to increase the usefulness of stable resistance that are only expressed later in development. We have been using six different approaches to understand the developmental control of Xa21 resistance in rice: ribonuclease protection assay, reverse transcriptase PCR, differential mRNA display, RNA gel blot analysis, precocious mutant screen and analysis of phase change. What we have found is that although another member of the Xa21-multigene family is highly expressed, Xa21 itself is a very rare message and it appears to be constitutively expressed at all stages and in the absence of infection. We have found that there are differentially expressed messages that correspond to the transition and these are presently under analysis.
Century, K., Morlan, J., Adkisson, M., Lagman, R. A., Trudeau, R., Smith, A., Love, J., Ronald, P. C., and Whalen, M. C. 1998. Developmental control of Xa21 resistance. (manuscript in preparation)
Ciesiolka, L. D., Hwin, T., Gearlds, J. D., Minsavage, G. V., Saenz, R., Bravo, M., Handley, V., Conover, S. M., Zheng, H., Caporgno, J., Phengrasamy, N. B., Toms, A. O., Stall, R. E., and Whalen, M. C. 1998. Regulation of expression of avirulence gene avrRxv and identification of a family of host interaction factors by sequence analysis of avrBsT. (manuscript accepted at Molecular Plant-Microbe Interactions)
Wang, G.-L., Ruan D-.L., Song, W-.Y., Sideris, S., Chen, L.-L., Pi, L.-Y., Zhang, S., Zhang, Z., Fauquet, C., Gaut, B.S., Whalen, M. C., and Ronald, P. C. 1998. The LRR domain encoded by the rice gene Xa21D, an Xa21 receptor-like gene family member, determines race specific recognition and is subject to adaptive evolution. The Plant Cell 10: 765-779.
Whalen, M. C., Wang, J. F., Carland, F. M., Heiskell, M. E., Dahlbeck, D., Minsavage, G. V., Jones, J. W., Scott, J. B., Stall, R. E., and Staskawicz, B. J., 1993. Avirulence gene avrRxv from Xanthomonas campestris pv. vesicatoria specifies resistance on tomato line Hawaii 7998. Molecular Plant-Microbe Interactions 6: 616-627.
Whalen, M. C. 1991. Breeding for resistance to bacterial diseases. In: Advanced methods in plant breeding and biotechnology, ed., Murray, D. R., CAB International, Wallingford, U.K.
Whalen, M. C., Innes, R. W., Bent, A. F., and Staskawicz, B. J. 1991. Identification of Pseudomonas syringae pathogens of Arabidopsis and a bacterial locus determining avirulence on both Arabidopsis and soybean. The Plant Cell 3: 49-59.
Minsavage, G. V., Dahlbeck, D., Whalen, M. C., Kearney, B., Staskawicz, B. J., and Stall, R. E. 1990. Gene-for-gene relationships specifying disease resistance in Xanthomonas campestris pv. vesicatoria - pepper interactions. Molecular Plant-Microbe Interactions 3: 41-47.
Whalen, M. C., Stall, R. E. and Staskawicz, B. J. 1988. Characterization of a gene from a tomato pathogen determining hypersensitive resistance in non-host species and genetic analysis of this resistance in bean. Proc. Natl. Acad. Sci., USA 85: 6743-6747.
Century, K., Lagman, R. A., Smith, A., Trudeau, R., Whalen, M. C. 1998. Molecular genetic analysis of developmentally regulated disease resistance in rice. Sixth International Symposium on Rice Molecular Biology, Shanghai and Hangzhou, China.
Al-Azzeh, D. N. and Whalen, M. C. 1998. Molecular analysis of the transcription start site of avrRxv. Annual Northern California American Society for Microbiology Conference, Palo Alto, CA.
Thompson, C. L. and Whalen, M. C. 1998. AvrRxv and YopJ: the search for homologous function. Annual Northern California American Society for Microbiology Conference, Palo Alto, CA.
Smith, A., Trudeau, R., and Whalen, M. C. 1997. Physiological and molecular characterization of host responses to bacterial pathogens. National Minority Research Symposium, New Orleans, LA.
Love, J., and Whalen, M. C. 1997. Developmental control of defense gene induction. National Minority Research Symposium, New Orleans, LA.
Lagman, R. A., and Whalen, M. C. 1997. Correlation of development and disease resistance in rice. National Minority Research Symposium, New Orleans, LA.
Gearlds, J. D., and Whalen, M. C. 1996. Interaction between bacterial pathogen Xanthomonas campestris pv. vesicatoria and tomato: role of the PIP-box in the expression of the avirulence gene avrRxv. National Minority Research Symposium, Miami, FL.
Love, J., and Whalen, M. C. 1996. Identification of differentially expressed genes in the developmental transition from disease susceptibility to resistance in rice. National Minority Research Symposium, Miami, FL.
Macneale, E. and Whalen, M. C. 1996. Identification of a locus in Xanthomonas campestris pv. vesicatoria producing a symptom of bacterial spot disease. 13th Annual Northern California American Society for Microbiology Conference, San Jose, CA.
Gearlds, J. D., and Whalen, M. C. 1996. Interaction between bacterial pathogen Xanthomonas campestris pv. vesicatoria and tomato: role of the PIP-box in the expression of the avirulence gene avrRxv. SACNAS Conference, Los Angeles, CA.
Bravo, M. and Whalen, M. C. 1996. Interaction of bacterial pathogen Xanthomonas campestris pv. vesicatoria and tomato plants: effects of mutations in hrp loci on avrRxv expression. SACNAS Conference, Los Angeles, CA.
Trudeau, R., and Whalen, M. C. 1996. Developmental control of Xa21 disease resistance phenotype in rice. Eighth International Symposium on Molecular Plant-Microbe Interactions, Knoxville, TN.
Hwin, T., Ciesiolka, L. D., Gearlds, J. D., Handley, V., and Whalen, M. C. 1996. Expression analysis of avrRxv in the interaction between Xanthomonas campestris pv. vesicatoria and tomato. Eighth International Symposium on Molecular Plant-Microbe Interactions, Knoxville, TN.
Macneale, E. and Whalen, M. C. 1996. Identification of a locus in Xanthomonas campestris pv. vesicatoria producing a symptom of bacterial spot disease. Molecular Biology of Tomato Conference, Berkeley, CA.
Ciesiolka, L. D., Hwin, T., Gearlds, J. D., Handley, V., Lopez, M., Bravo, M. and Whalen, M. C. 1996. Interaction between bacterial pathogen Xanthomonas campestris pv. vesicatoria and tomato: expression analysis of avirulence gene avrRxv. Molecular Biology of Tomato Conference, Berkeley, CA.
Last Modified: August 25, 1998 by Regina A. Lagman