Dark Septate Endophyte Improves Drought Tolerance of Ormosia hosiei Hemsley &amp; E. H. Wilson by Modulating Root Morphology, Ultrastructure, and the Ratio of Root Hormones

Yan Liu,Xiaoli Wei

doi:10.3390/f10100830

Abstract

Dark septate endophytes (DSEs) are known to help host plants survive drought stress; however, how DSEs enhance host plant drought resistance under water stress conditions remains unclear. The objective of this study was to inoculate Ormosia hosiei seedlings with a DSE strain (Acrocalymma vagum) to investigate the effects of DSE inoculation on root morphology, ultrastructure, and the endogenous hormone content under drought stress conditions and to elucidate the drought resistance mechanism involved in the DSE–host-plant association. The inoculated seedlings were grown under three different soil water conditions (well watered—75% field water capacity, moderate water—55% field water capacity, or low water—35% field water capacity) for 114 days. Fresh root weight, root volume, root surface area, root fork, and root tip number were significantly higher in inoculated seedlings than in noninoculated seedlings. Furthermore, the root architecture of the inoculated seedlings changed from herringbone branching to dichotomous branching. Mitochondria and other organelles in root cells of inoculated seedlings remained largely undamaged under water stress, whereas organelles in root cells of noninoculated seedlings were severely damaged. The abscisic acid (ABA) and indole-3-acetic acid (IAA) content and IAA/ABA ratio of inoculated seedlings were significantly higher than those of noninoculated seedlings, whereas the content of gibberellic acid (GA) and the ratios of GA/ABA, zeatin riboside (ZR)/ABA, and ZR/IAA in inoculated seedlings were lower than those of noninoculated seedlings. DSE inoculation could help plants adapt to a drought stress environment by altering root morphology, reducing ultrastructural damage, and influencing the balance of endogenous hormones, which could be of great significance for the cultivation and preservation of the O. hosiei tree.

Highlights

Drought stress restricts the growth and yield of plants and causes changes in plant structure and function [1]
Drought stress damaged root cell ultrastructure, A. vagum inoculation effectively reduced the damage of cell ultrastructure caused by drought stress
The cytoplasmic membrane of Non-AV seedlings had severely separated from the cell wall, most of the chromatin was degraded, the nucleus was not present, the cytoplasm had leaked out of the cell, and many vacuoles were observed in the cells (Figure 2e; Figure S1e)

Summary

Introduction

Drought stress restricts the growth and yield of plants and causes changes in plant structure and function [1]. The root system is the first part of the plant to perceive drought stress. Drought stress directly causes root cells to lose water, which damages the cell membrane structure, disintegrates organelles, hinders cell metabolism, accelerates the aging process, and eventually leads to plant death [2]. Mitochondria in plant root cells are sensitive to drought stress. Maintaining the normal structure and function of mitochondria plays an important role in plant drought resistance [3,4]. Under a drought stress environment, with the aggravation of drought stress, the mitochondrial matrix gradually becomes thinner and the internal cristae decrease; the mitochondrial membrane breaks

Objectives

Methods

Results

Discussion

Conclusion