Salicaceae Endophytes: Growth Promotion Potential in Rice (Oryza sativa L.) and Maize (Zea mays L.) and Bio-Control of Plant Pathogen

Thesis (Ph.D.)--University of Washington, 2016-08

Abstract Salicaceae plants; poplar (Populus trichocarpa) and willow (Salix sitchensis) are hosts of many endophyte species. Salicaceae endophytes colonize the plant endosphere and confer various growth benefits to host plants. First three studies were conducted focusing on how these endophytes colonize and support the growth of major food crops; rice and maize under nutrient limited conditions. Fourth study was conducted to investigate the biocontrol and other growth promoting traits of these endophytes. The first study was conducted to determine the growth promoting potential of Salicaceae endophytes to rice under nitrogen (N) limited conditions. Rice seedlings were inoculated with endophytes and grown in the N limited conditions in the greenhouse for about four months. Endophyte inoculated rice plants were taller, and had higher biomass and tiller numbers over mock inoculated control plants. Furthermore, colonizing performance of these endophytes in rice seedlings was verified through fluorescent microscopy, and counting in planta endophyte density. Rice seedlings were considerably colonized by these endophytes. The second study was conducted to determine the growth potential of Salicaceae endophytes in maize and rice plants in N limited conditions. Endophyte inoculated plants were grown in the greenhouse, and plant physical characters such as plant height and biomass were recorded as growth response. Endophyte inoculated plants outperformed the mock inoculated plants but response was variable depending on crop genotypes or inoculated endophytes. In addition, through 15N dilution assay, evidence of N fixing activity was observed in rice. The third study was conducted to determine the colonization performance of poplar bacterial and yeast endophytes in rice and maize. Bacterial strains; WP5 (Rahnella sp.), and WP9 (Burkholderia sp.) labeled with green fluorescent protein, and yeast strain, WP1 were introduced in rice and maize seedlings aseptically. The in planta density of endophytes were determined by counting colony forming units and colonization pattern was observed using microscopy. These endophytes were found competent to colonize both rice and maize seedlings. They were observed in leaves and roots, and localized mostly in the intercellular spaces of root cortex and leaf mesophyll tissues. Higher in planta population of endophytes were observed in leaves and stems in majority of the colonization assays. Positive growth response was observed in endophytes inoculated rice and maize plants as compared to mock-inoculated control plants. The fourth study was conducted to investigate the biocontrol potential of Salicaceae endophytes over a soil borne plant pathogen, Rhizoctonia solani AG-8. These endophytes were also examined to delineate their other plant growth promoting features including N fixing activity, indole-3- acetic acid (IAA) and siderophore biosynthesis, and phosphate solubulization. Endophyte strains; Burkholderia, Rahnella, Pseudomonas, and Curtobacterium displayed antagonistic activity against R. solani AG-8. Burkholderia spp. showed relatively stronger antagonistic effect than other endophytes, perhaps very useful to explore as biocontrol measures to manage different soil borne plant pathogens. From nucleotide sequence analysis of Burkholderia spp., a 56-kb ofc gene cluster responsible for biosynthesis of anti-fungal glycolipopeptide, occidiofungin was detected in all species. Furthermore, these endophytes were found potential to support plant growth through multiple mechanisms such as N fixation, IAA and siderophore production, and phosphate solubilization besides protection from invading plant pathogens.