Plant Defense Mechanisms

By: Nicholas Panopoulos and Milton Schroth

Scientists want to understand how plants ward off diseases caused by bacteria, fungi, and viruses since they do not have specialized immune cells or antibodies as do humans.  But plants do have structural barriers preventing pathogens from entering their tissues. They also have antimicrobial substances naturally present that slow down or stop multiplication of pathogens at an early stage.  Reaction of plants to internal bacteria depend if they are pathogenic or harmless. Early research showed that harmless bacteria in leaves could inhibit multiplication of a pathogen by causing a disease defense response. Other bacteria also may enhance the growth of certain pathogens resulting in increased disease.   

Often, plant cells at or near the infection sites trigger a response where cells die, a process called a hypersensitive response (HR).  Thus, those pathogens that feed on live cells are deprived of the rich supply of plant nutrients they need for growth. Infected tissues also generate multiple proteins and other molecules with hormone-like functions that are transported to distant tissues causing genes to establish what is called systemic acquired resistance (SAR).

Classical and modern genetics and molecular research have uncovered secrets that may seem like science fiction. For example, it is known that molecules produced by many microbes, including non-pathogens, trigger the plant immune response. It was exciting to find parallels how plants and animals perceive the presence of pathogens and alert defense signaling mechanisms. A rich array of tools, such as molecular mutagenesis, DNA sequencing and small molecule identification technologies now enable researchers to identify resistance genes in breeding lines, and even transfer such genes from other species in ways that improves plant resistance.

Selected References

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3) Spoel, S. H. & Dong, X. 2012. How do plants achieve immunity? Defense without specialized immune cells. Nature Reviews Immunology 12, 89–100.

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6). Mansfield, J., Genin, S., Magori, S., Citovsky, V., Sriariyanum, M., Ronald, P., Dow, M., Verdier, V., Beer, S. V., Machado, M. A., Toth, I., Salmond, G., & Foster, G. D. 2012. Top 10 plant pathogenic bacteria in molecular plant pathology. Molecular Plant Pathology 13, 614–629.

7). Maino, A.L., Schroth, M. N., and Vitanza, V. B. Synergy between Achromobacter sp. and Pseudomonas phaseolicola resulting in increased  disease. Phytopathology Vol. 64, P. 277-283. 1974. Nickolas Panopoulos, Plant Pathology Department, UCB, emeritus.  University of Crete, emeritus.