Research in the Lum lab revolves around understanding the beneficial association microbes have with plants. Projects range from dissection of the mechanisms involved in the interaction between specific bacterial species and plants, to getting a global view of the diversity in the microbial community associated with different plant species.
Paraburkholderia functional genomics
Nitrogen deficiency is the most frequent cause of limited plant productivity. The production and use of nitrogen fertilizer agriculturally is both costly in terms of energy and its impact on the environment, resulting in an ongoing effort to improve nitrogen-fixing associations. The symbiosis that occurs between legumes and bacteria of the Rhizobiaceae is one of the best-studied interactions between plants and microbes. Under conditions of nitrogen deficiency, an intricate molecular signaling cascade begins, whereby the bacteria are attracted to the roots of specific host plants and trigger the production of a novel plant organ, the nodule, in which the bacteria are housed and fix atmospheric nitrogen in exchange for carbohydrates.
Until relatively recently, it was thought that only members of the alpha-proteobacteria nodulated plants. However, it is now clear that there is an extraordinary diversity in rhizobial species that associate with legumes. Members of the beta-proteobacteria, including at a number of Paraburkholderia species, such as P. tuberum, nodulate legumes. Furthermore, many species of Paraburkholderia have been found to be plant-associated endophytic bacteria, including P. unamae.
We have been using forward and reverse genetics approaches to generate mutants in P. unamae and P. tuberum in order to better understand the mechanisms in these bacteria that are important for their association with plants. We have been generating transposon insertion mutants, screening for defects in processes such as motility and exopolysaccharide production that are known in other bacteria to be important for plant-microbe interactions. In the case of P. tuberum, we are also screening for mutants defective in their ability to nodulate legumes. A number of mutants in each of these processes have been identified, the mutated genes identified using molecular methods, and further characterization being done, investigating attachment, biofilm formation, and plant growth promotion. We are also using directed mutagenesis to generate in-frame deletions of genes we hypothesize are critical for the interaction with plants.
Identification and characterization of plant growth promoting bacteria
Plant Growth Promoting Bacteria (PGPB) are bacteria that reside in the
rhizosphere (region associated with the surface of the root) of plants and have a beneficial
effect on plant growth. The bacteria do this either by directly promoting plant growth by making nutrients available, producing phytohormones, etc., or indirectly by controlling plant pathogens.
We are characterizing the microbial community associated with several local California native plant species, including Eschscholzia california (California poppy), Camissoniopsis cheiranthifolia (beach evening primrose), Abronia maritima (red sand verbena) and nodulating legumes such as Lupinus chamissonis (dune lupine) and L. bicolor (miniature lupine). Dune lupine is a California native prominent in the nearby Ballona wetlands and El Segundo sand dunes that has thrived despite the heavy impacts of urbanization on these locations. Beach evening primrose and sand verbena are both critical to the restoration and stabilization of sand dunes and part of the revegetation strategy by the Los Angeles World Airports and The Bay Foundation for the El Segundo dunes.
Students in the spring semester of the General Biology Laboratory (BIOL112) are heavily involved in this project, isolating, identifying and characterizing bacteria found in the rhizosphere of local native plants, with additional analyses continuing in the Lum lab to develop some of these strains for pretreatment of seeds, with the goal of improving plant establishment in local restoration activities.