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Subalpine conifers host core endophytic bacteria conserved across sites and tree species


The overall aim of my dissertation was to determine if subalpine conifers host a consistent core of endophytic bacteria. Endophytic bacteria, symbiotic bacteria found within healthy plant tissue, have been found in all plants studied to date. Endophytic bacteria can influence plant healthy through growth promotion, resistance to pests and pathogens, and confer plasticity to abiotic and biotic stress. Despite the role they can play in plant health, they have largely been examined in agricultural crops with little exploration of endophytic bacteria in long-lived forest trees. Forest trees may rely on bacterial endophytes to confer robustness to variable and environmental conditions they face over their lifetime. Subalpine conifers are long-lived trees that inhabit nutrient limited soils and harsh environments and therefore a good candidate to examine endophytic bacteria that may confer host robustness. In animals, core bacterial symbionts with important roles in host health are often consistently associated with individuals of a host species. To uncover potential beneficial endophytic bacteria, I used next-generation sequencing of the 16S rRNA bacterial gene to determine if subalpine conifers were host to a consistent core of endophytes. I found Pinus flexilis and Picea engelmannii at Niwot Ridge, CO were consistently dominated by a potential N2 fixing phylotype (AAB1). Next, to determine if the consistent AAB1 association was a result of shared environment (Niwot Ridge) or shared ancestry (Pinus and Picea), I characterized the needle endophytic community of P. flexilis and Pinus contorta at two distance sites: Niwot Ridge, CO and Horseshoe Meadows, CA. I found AAB1 to dominate the endophytic bacterial communities across both species and sites, suggesting the association was not dependent only on environment but may be a long-term association of AAB1 and Pinus and Picea. To further examine the distribution of AAB1 I examined the endophytic community of P. contorta at Tuolumne Meadows, CA. AAB1was present but not in high abundance, indicating the AAB1 association is facultative. I found that the remaining endophyte community (i.e. excluding AAB1) of P. flexilis, P. engelmanni and P. contorta was structured by geographic location. In contrast to the high elevation pines, a core and consistent endophytic bacteria taxa was not identified in Sequoia sempervirens and Sequoiadendron giganteum in the family Cupressaceae . In these species, the endophyte communities were highly variable within and across tree individuals and species. These results suggest that subalpine conifers in the Pinaceae associate with facultative but symbiotic N2 fixing endophytic bacteria, and that the symbiosis may have originated after the divergence of Pinaceae and the Cupressaceae.

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