Molecular and Genetic Analysis of Plant-Microbe Interactions.
The research in my lab focuses on the early stages of the interaction
between nitrogen-fixing Rhizobium bacteria and legumes such as alfalfa, pea, and soybean in order to determine why this
interaction occurs exclusively with leguminous plants. Root nodules are formed following the perception of signal
molecules (lipochitooligosaccharides) from a compatible Rhizobium . The nodules house the bacteria, which convert atmospheric
nitrogen into ammonia (Fig.1).
Our investigations focus mainly on
the earliest stages in the symbiosis. We have been studying the plant's role in establishing the symbiosis by
investigating the involvement of both lectin and early nodulin (ENOD) genes. We have approached the study of lectin
function in nodulation by doing both gain-of-function and loss-of-function (antisense technology) experiments.
The plant's role can also be studied by looking at single-gene plant mutants. We are studying Melilotus alba , an autogamous diploid with a relatively small genome. A number of mutants, which are defective in both the nitrogen-fixing and mycorrhizal symbioses, have already been identified and characterized. One of these mutants, sym3 , is the focus of most of our studies. It forms neither root nodules nor mycorrhizae (Fig. 2).
On the Rhizobium side, we are studying rhizobial attachment by using an in
vitro assay for biofilm formation. Biofilms are structured communities of bacterial cells enclosed in a self-produced polymeric matrix and
adherent to an inert or living surface; they have clinical, industrial and environmental impacts. The in vitro assay allows us to screen a large
number of mutants and thus serves as an excellent model system for studying root colonization. (Fig. 3)
We are also studying other plant-microbe interactions. One of these is the Frankia -actinorhizal plant symbiosis. Frankia is a
Gram-positive actinomycete that fixes nitrogen both within the nodule and ex planta (Fig. 4). We have a long-standing
collaboration with Dr. Maria Valdés of IPN in Mexico City investigating a novel group of diazotrophs isolated from
nodules of Casuarina growing in Mexico. These bacteria have nifH , but they are not Frankia . Phylogenetic
studies indicate that some of the isolates belong to Micromonosporaceae.
Dietary Supplements and Medicinal Plants 
The Hirsch lab is developing DNA-based methods to validate the authenticity of ascertain whether contaminants are present in dietary supplements. We are also examining the effects of environmental parameters such as light, temperature and nutrition on the production of phytochemicals in plants used for human health (Fig. 5). In addition, we are analyzing the effects of plant extracts, particularly the flavonoidsand other phytochemicals within the extracts, on bacterial biofilm formation. Biofilms are responsible for urinary tract infections and other chronic infections. They also foul surfaces such as catheters, contact lenses, and the hulls of ships