Martha C. Hawes. Associate Professor, Departments of Plant Pathology and Molecular & Cellular Biology. Ph.D., University of Kentucky. Plant genes controlling programmed separation of living cells from roots; role of the cells in regulating microbial populations colonizing roots.

Plants of many species shed thousands of healthy cells from each of their root tips daily. Because these cells provide a living interface between the root and the soil, we refer to them as root ìborderî cells. This release of living somatic cells into the environment is unique among higher organisms, and its function is unknown. My hypothesis is that border cells provide a selective advantage to the plant by releasing specific chemicals that regulate levels of root-associated microorganisms. Because soil-borne bacteria and fungi comprise not only pathogens that can destroy roots but also beneficial organisms that are crucial for normal plant development, regulating the balance of such microbial populations is crucial to survival. Border cells act as a reservoir of chemicals that can specifically attract or repel microorganisms, as well as activate microbial genes needed for root infection. Mutants of soil-borne microorganisms have been used to demonstrate that, in some cases, recognition of such chemicals from the cells is required for the establishment of root-microbe associations.

The release of living border cells from the growing tip could help plants in a number of ways. For example, specific chemicals in border cells could selectively attract beneficial microorganisms like nitrogen-fixing bacteria to the root. Alternatively, border cell-specific chemicals could repel or be toxic to pathogenic organisms. We are in the process of identifying border cell-specific genes that give this cell population its unique properties. Also of interest are the genes that control separation of border cells from the root. These plant genes are being used to genetically engineer plants with specific alterations in the production of border cells, and in their biological properties. These transgenic plants can be used to test the hypothesis that border cells control the susceptibility of plants to infection by symbiotic and pathogenic bacteria and fungi.

Brigham LA, Woo HH, Hawes MC 1995. Differential expression of proteins and mRNAs from border cells and root tips of Pisum sativum L. Plant Physiol 109:457-463.

Brigham LA, Woo HH, Hawes MC 1995. Root border cells as tools in plant cell biology studies. Methods in Cell Biology 26:377-387.

Nicoll SM, Brigham LA, Wen F, Hawes MC 1995. Expression of transferred genes during development of transgenic hairy roots in pea. Plant Cell Tissue Organ Culture 42:57-66.

Hawes, MC, Brigham LA, Woo HH, Zhu Y, WenF 1996. Root border cell. Biology of Plant-Microbe Interactions Vol 8:509-514.

Woo HH, Brigham LA, Hawes 1997. Cloning of genes whose expression is correlated with mitosis and is localized in dividing cells of pea root caps. Plant Molecular Biology (In press)

Zhu Y, Pierson LS III, Hawes MC 1997. Release of nodulation gene inducing chemicals from root border cells. Plant Physiology (In press)

Hawes MC et al 1997 Root border cells: Phenomenology of signal exchange. International Symposium on Root Biology Vol 8: (In press)

Hawes MC, Brigham LA, Wen F, Woo HH, Zhu Y 1998. The role of root border cells in regulating the ecology of the rhizosphere. Ann Rev Phytopathol (In press)