UC Merced

Plant microbiomes are an essential component of the plant host and are widely known to ameliorate environmental stress response. In the California Sierra Nevada, the endemic cutleaf monkeyflower (Mimulus laciniatus) is experiencing and adapting to various levels of stress associated with climate change, however, it is unclear to what extent the M. laciniatus microbiome is engaged in responding to this stress. This thesis aims to determine the monkeyflower microbiome along with their growing substrate (moss or soil) across a steep elevation gradient in the species range during the 2019 growing season. Specifically, we collected monkeyflower plants and divided those collections into compartment subsamples (roots, shoots, soil, and moss substrate) to assess the bacterial communities of each compartment by 16S rRNA gene sequencing to answer the following: (i) Do the M. laciniatus endophyte communities differ in composition and diversity across the M. laciniatus species range, and does this composition vary by elevation? (ii) Does the M. laciniatus endophyte community differ between plant compartments (e.g. root and shoot)? (iii) Do soil and moss, from which M. laciniatus grows, serve as a source of M. laciniatus endophytes, and do soil and moss communities vary in structure across the M. laciniatus species range? We found that the bacterial community structure of both plant compartments (roots and shoots) and the substrate (soil, and moss) differed across the range of M. laciniatus. Compartment strongly influenced bacterial communities, supporting the idea that plant compartments provide micro-habitats for bacterial communities. Elevation played a role in community variation when all samples were analyzed together. When compartment subsets were analyzed separately, population and site (nested in population) were significant drivers of variation in all four compartments. The differences between sites and population may be explained by the effect on bacterial communities by local climate, which could in turn reflect that the plant host selects beneficial microbes to support plant responses to the local environment, leading to habitat-adapted symbiosis. Alternatively, differences in bacterial community structure across the M. laciniatus species range could be due to differences in the metacommunity of locally available bacterial taxa to colonize compartments. This thesis highlights the importance of plant compartment and local climate or metacommunity in shaping bacterial communities along the elevation gradient of plant species.