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

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.

Rhizoglomus intraradices Improves Plant Growth, Root Morphology and Phytohormone Balance of Robinia pseudoacacia in Arsenic-Contaminated Soils

“…The root meristem constitutes the main site of HM action and also rapidly synthesizes a great deal of ABA. The inhibition of plant growth, which was induced by HM stress, is also in part a result of the increased ABA concentration (Liu and Wei, 2019). In this study, the contents of IAA and ABA were obviously increased.…”

“…It was suggested that hormones, released by mycorrhizal fungi, may contribute to the enhancement of plant growth (Frankenberger and Arshad, 1995). Phytohormones responded synergistically to HM stress conditions by regulating their synthetic ability, changing the hormone content, and adjusting the balance and ratio of hormones, in which the ratios of the stress-related phytohormones often have more profound implications than does the role of a single phytohormone in plants (Liu and Wei, 2019). Barker and Tagu (2000) verified that changes in the phytohormone ratios affected the different stages of plant development, which were closely related to mycorrhizal symbiosis.…”

“…Zangaro et al (2007) also found that the rootlet diameter of arboreal plants in South Brazil had a positive correlation with AM colonization. More prominent parameters of root diameter also indicated that the plant roots had higher penetration in soils (Liu and Wei, 2019). As toxicity damaged the morphology and configuration of R. pseudoacacia seedlings by forming shorter yet thicker main and lateral roots; however, R. intraradices inoculation improved the plant morphology by increasing the percentage of the thicker root length of R. pseudoacacia seedlings to better adapt to As stress.…”

“…The root length and root surface area, which are determined by the root length and the number of root forks and tips, are essential factors that determine the root volume. A higher number of root forks and tips indicates a better trend of root development (Liu and Wei, 2019). The symbiotic effects of AMF on the root system include the regulatory changes of root morphological characteristics in a textural, dimensional, quantificational, and impermanent manner (Sheng et al, 2009).…”

“…Plants and AMF can form a beneficial symbiotic relationship by establishing an intricate network of signaling and biochemical pathways (Délano-Frier and Tejeda-sartorius, 2008) in which AMF-regulated phytohormones are suitable candidates for continuous signal exchanges between plant cells and the AM hyphae (Liu and Wei, 2019). These phytohormones regulated the plant metabolic and structural genes, whose products, in return, affected the symbiosis between the AMF and plants; ultimately, plant growth is enhanced by AMF-regulated phytohormones (Ludwig-Müller, 2000).…”