Rhizobial diversity impacts soybean resistance, but not tolerance, to herbivory during drought

Abstract

Biodiversity within ecological communities has been shown to influence plant resistance and tolerance to herbivory, and may strongly interact with climate change. However, the effects of diversity within microbial symbionts on their plant hosts’ responses to herbivores is less well documented, particularly within the context of agricultural systems. Here we examine the interactive effects of rhizobial diversity and drought in promoting soybean resistance and tolerance to insect and mammalian herbivores in an outdoor pot experiment. We demonstrate that soybean plants provided with a diverse mixture of rhizobial strains experience 30% less damage by chewing insect herbivores under both drought (reduced watering and increased temperature) and ambient weather conditions, as well as half as much colonization by aphids under drought conditions. This effect was not due to the presence of any particularly beneficial rhizobial strain (i.e., not a selection effect), but rather due to an enhanced benefit provided by the rhizobial strains in combination (i.e., a complementarity effect). No effects of rhizobial diversity were observed on the rate of defoliation by rabbits. While rhizobial diversity did affect soybean resistance to insect herbivores, these differences in insect damage did not carry through to affect soybean growth or final yield. Thus, soybean plants exhibited high tolerance to herbivory regardless of our experimentally imposed rhizobial diversity or drought treatments. With future projections for increased pest outbreaks and drought, understanding the factors that can sustainably enhance agricultural yield is critical. Our results suggest that promoting rhizobial diversity within soybean agriculture may be one nature-based solution to promote the resistance of this critical crop to insect pests.

Keywords

• Climate change
• Insect herbivore
• Legume
• Mammalian herbivore
• Mutualism
• Rabbit