Plant microbiomes have a greater effect than plant size on flowering and growth in Asclepias syriaca

Abstract

Plants strike a balance between growth and defense. The calculus of how to defend against herbivores, and maximize growth and reproduction, can differ between annuals and perennials. Annuals have a live fast, reproduce young strategy; while perennials may maximize fecundity over years. Milkweed, a herbaceous perennial, is a mix of these two strategies. On one hand, a single individual ramet is functionally a herbaceous annual and vulnerable to herbivory. On the other hand, below ground connections allow ramets to share resources and persist for multiple years, perhaps lowering the cost of localized herbivory. Here, we study the impact of herbivory on the leaf and soil microbiome on milkweed growth and flowering in naturally occurring populations. After controlling for plant size and plant genotype, we asked how the microbiome composition was related to flowering time and growth. We find that the leaf microbiome is a strong predictor of plant growth rate, but not flowering. Conversely, we find that the soil microbiome and plant size predict flowering probability. We found no evidence that herbivory altered the leaf or soil microbiome composition. Together these results provide evidence that plant microbiomes also influence growth and fecundity in natural populations of milkweed. These results also provide evidence that the leaf microbiome in milkweed is stable in the face of herbivory. This study expands our knowledge of the contribution of microbiomes to plant demography in a clonal perennial plant.