# Environmental Drivers Override Host Phylogeny in a Locoweed–Endophyte Symbiosis

## TL;DR

This study shows that environmental factors, not plant ancestry, mainly control the symbiotic relationship between locoweeds and a toxin-producing endophyte.

## Contribution

The study reveals that environmental drivers, not host phylogeny, dominate the distribution and function of a locoweed–endophyte symbiosis.

## Key findings

- Alternaria sect. Undifilum colonized 11 species among 32 surveyed, with swainsonine concentrations varying widely.
- Environmental factors, not host evolutionary history, primarily determine the geographical distribution and function of the symbiosis.
- Low temperature and drought stress influence the symbiosis and chemical defense through direct and indirect pathways.

## Abstract

Plant endophytes, often termed the “second genome”, critically shape host adaptability. However, the complexity of their interactions, regulated by microbial traits, host species, and environment, has limited both our understanding of symbiosis and the application of beneficial endophytes. The symbiosis between locoweeds (Oxytropis and Astragalus species) and the endophyte Alternaria sect. Undifilum, which produces the neurotoxin swainsonine, serves as an ideal model for investigating these relationships. Through extensive national surveys (2021–2023) across China’s major locoweed habitats, combining field sampling with cultivation, molecular, quantitative, and modeling approaches, a central question emerged: To what extent are the distribution and function of this symbiosis shaped by the contemporary environment versus host evolutionary history? The results showed that: (1) Among 32 surveyed species of Oxytropis, Astragalus, and Sphaerophysa, the endophyte Alternaria sect. Undifilum colonized 11 species. In colonized plants, endophyte loads ranged from 0.02 to 58.87 pg/ng total DNA, and swainsonine concentrations varied from 0.00003% to 1.00%. (2) Environmental factors, rather than host phylogeny, were the key driver governing the geographical distribution and expression of the symbiosis. (3) Low temperature and drought stress regulated the symbiotic relationship and chemical defense through both direct effects on the symbionts and indirect pathways involving grazing pressure. This study demonstrates that the environment is the core force dominating the geographical pattern and functional expression of the locoweed–endophyte symbiosis at ecological scales. These findings provide new perspectives for understanding the general principles of plant–endophyte symbiosis and establish a scientific foundation for predicting and utilizing endophyte resources in changing environments.