Gene Expression Profiles in a Grass-endophyte Symbiosis

Many temperate grasses used for forage or conservation have seed-transmissible fungal endophytes that enhance fitness and stand longevity of the grasses. Among the fitness enhancements are anti-insect, antinematode and anti-mammalian activities, enhanced growth and seed set, improved drought tolerance, and improved nutrient acquisition and utilization. Some endophytes induce also limited disease (choke) on their hosts. The remarkable ability of an endophyte to efficiently colonize tillers, seeds and embryos, implies developmental coordination with the host. This project is aimed at addressing the hypothesis that different tissues and stages in host plant development entail different host responses to endophytes and vice versa, and that many such responses are manifested at the level of gene expression. A clone library of normalized cDNAs will be constructed for endophyte-symbiotic inflorescences at various stages of development. Another normalized cDNA library will be from inflorescences at the earliest visible manifestation of choke disease. From each of the two libraries, 22,560 clones will be sequenced at the cDNA 5'-ends (45,120 sequence runs in total). We expect to identify approx. 20,000 unigenes, including more than 3,500 unigenes from the endophyte. The unigene sets will be arrayed in 40 replicate microarrays. These will be hybridized to probes from various tissues of endophytesymbiotic and aposymbiotic plants. The results will indicate profiles of plant gene expression in response to endophyte, and of endophyte gene expression in response to developmental changes in the plant, and differences in plant and endophyte gene expression associated with the benign (seed-transmission) versus pathogenic (choke disease) states. Such results should provide clues as to the mechanisms of host benefits conferred by the endophytes, and the remarkable stability of the grass-endophyte symbioses. This proposal addresses USDA Goal 3 (enhancement of protection and safety of the Nation's agriculture and food supply), because it entails a functional genomic study with the following potential benefits: (1) Elucidating mechanisms of benign endophytic growth in different plant tissues, which may improve their exploitation in pasture and forage grasses and in creation of synthetic bio-protective endophytes; (2) Elucidating the mechanism of endophyte seed transmission to provide clues to seed-transmission of pathogenic fungi and possible preventive actions in worldwide seed trade; (3) It also addresses USDA Goal 5 (protection and enhancement of the Nation's natural resource base and environment), because the host grasses are important for soil conservation/erosion control, and their persistence under stress conditions is enhanced by endophytes.