Abstract

Plants colonization and growth on different environments is usually limited by a variety of stressful conditions existing in available land. Adverse environmental factors such as drought, high soil salinity, and soil contamination by pollutants, due to anthropic activity, are widespread in culturable land, and continue to expand rapidly to new areas worlwide. Microorganisms are able to provide key functions that improve the growth and development of plants in different environments by establishing a mutualistic relationship, that can enhance the ability of the host to take up nutrients and protect itself against pathogens or adverse conditions in the surrounding soil. Colonization of the internal tissues of plants by endophytic bacteria is a very frequent phenomenon, and has been recently shown to influence several plant physiological processes that include growth promotion, by the modification of plant hormone levels, and could lead to enhanced resistance to some abiotic challenges such as osmotic stress. Furthermore, plant resistance to soil pollution by xenobiotics has also been associated with endophytic bacteria, since the presence of specific strains enhances plant growth when exposed to toxic aromatic compounds, as well as the ability to eliminate these compounds from their environment. However, only limited data is currently available on the basis of endophyte-host molecular interactions. Burkholderia phytofirmans PsJN is a bacterium with outstanding abilities for colonization of different plant hosts, including tomato potato and grape. This strain is also able to promote the growth of Vitis vinifera plants and improve their resistance to osmotic stress and chilling, by modifying metabolite levels and hormone synthesis within the plant tissue. In the laboratory of Bernardo González, at P. Universidad Católica de Chile, strain PsJN has been shown to colonize a new plant host: Acacia caven, a plant with drought resistant properties, able to grow in eroded soils in central Chile. Little is known about the host specificity of growth promotion effects of B. phytofirmans, as well as the bacterial genes encoding the functions responsible for these beneficial effects. The aim of this work is to study the ability of B. phytofirmans PsJN to improve the growth of different colonized plant hosts under conditions of water, saline and pollutant stress, and to assess the role of specific bacterial functions in these growth promotion effects. The central hypothesis of this project is that B. phytofirmans PsJN colonization is able to improve the growth of Vitis vinifera and Acacia caven under abiotic stress due to the expression of key bacterial functions. In order to assess the ability of the three B. phytofirmans PsJN colonized plants to grow in the presence or absence of water, saline, or pollutant-induced stress, plants will be grown in both gnotobiotic in vitro cultures, and in plant/soil microcosms. These will be subjected to abiotic stress, and plant growth parameter will be evaluated and compared to non stressed conditions. Abundance of the bacterium will be assessed in plant tissues, to determine if the population of bacteria inside the plant is influenced by abiotic stress. These experiments are expected to show the ability of the bacterium to produce stress resistance and growth promotion in different plant hosts. Additionally, expression of bacterial genes encoding key bacterial functions that could be responsible for the beneficial effects of PsJN, will be assessed in bacteria colonizing each plant. B. phytofirmans mutant strains will be obtained for genes putatively related to growth promotion and their infuence on the host, as well as their ability to colonize and survive within plant tissues will be assessed. This will allow identification of relevant genes and mechanisms of plant-bacteria interaction. This project will contribute to a groundbreaking area of research, which is receiving considerable attention worldwide. It will contribute to the understanding of bacteria-plant interaction with emphasis on host specificity, and the bacterial functions required for its establishment. Conclusions of this project are expected to be relevant for the planning of land recovery strategies and the enhancement of the stress resistance on different plants by microbial stimulation of growth and fitness, without requiring a genetic modification of the plant host.