Abstract

There is increasing evidence that root exudates play important roles in plant disease resistance. Black shank, caused by Phytophthora nicotianae, is a destructive soil-borne disease in tobacco (Nicotiana tabacum L.). The aim of the present study was to investigate the activity and composition of the root exudates from resistant and susceptible tobacco cultivars. The root exudates of the resistant cultivar Gexin 3 showed inhibitory activity against P. nicotianae, while the exudates of susceptible cultivar Xiaohuangjin 1025 stimulated the colony growth but had no effect on spore germination. Metabolic profiling using liquid chromatography/electrospray ionization-quadrupole-time-of-flight mass spectrometry depicted differing metabolic patterns of root exudates between Gexin 3 and Xiaohuangjin 1025. The activity and composition of root exudates was altered by P. nicotianae inoculation. Multivariate analysis showed that root exudates (including organic acids, alkaloids, fatty acids, and esters) were different between the two varieties. The defense substances in root exudates, such as tartaric acid, ferulic acid, and lauric acid, may represent a pre-infection prevention strategy for tobacco. Phenylpropanoids as well as inducers of salicylic acid, fatty acids, 6-hydroxyhexanoic acid, and hydrojasmonate may be involved in tobacco defense responses. Compared to the susceptible cultivar, the roots of the resistant cultivar exhibited high enzyme activities of phenylalanine ammonia-lyase, cinnamate-4-hydroxylase and 4-coumarate-CoA ligase, which may prompt the synthesis and secretion of phenylpropanoids. Our results indicated that the root exudates not only provide a pre-infection prevention strategy by exuding antimicrobial substances, but also increase tobacco disease resistance by eliciting plant defense responses. In addition, some defense compounds as well as compounds that play a role in inducing plant defense responses, showed potential for disease control application. This study provides an insight into possible disease resistance mechanisms of root exudates, and attempts the beneficial utilization of these secondary metabolites of plants.

Highlights

  • Stevenson et al (1995) reported that the root exudates of two wilt-resistant chickpea (Cicer arietinum) cultivars significantly inhibited the spore germination of Fusarium oxysporum f. sp. ciceri as well as the hyphal growth of the germinated spores, while another two susceptible cultivars showed no antifungal activity

  • In accordance with the previous studies, our results showed that root exudates of black shank-resistant tobacco cultivar Gexin 3 significantly inhibited the zoospore germination and mycelial growth of P. nicotianae compared to the control, whereas the root exudates of the susceptible cultivar Xiaohuangjin 1025 stimulated the colony growth but had no effect on spore germination

  • The current study revealed differing metabolic patterns and functions between root exudates of a disease-resistant tobacco cultivar and a susceptible tobacco cultivar

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Summary

Introduction

P. parasitica Dastur) is a typical soil borne pathogen with great economic significance and academic research value (Panabières et al, 2016). It can infect 255 species of plants in 90 families, and cause serious disease in a variety of crops, such as tobacco (Nicotiana spp.), tomato (Lycopersicum esculentum), and citrus (Citrus spp.) (Cline et al, 2008). The significance of root exudates associated with plant disease resistance has long been underestimated. Previous studies have indicated that root exudates of resistant and susceptible crop cultivars had different effects on pathogens (Whalley and Taylor, 1973; Stevenson et al, 1995). Root exudates of some resistant varieties had an inhibitory effect on pathogen growth, whereas those of some susceptible varieties exhibited a stimulatory effect (Wu et al, 2008; Schalchli et al, 2012), indicating a relationship between the activity of root exudates and plant disease resistance

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