Interplant communication of tomato plants through underground common mycorrhizal networks

Abstract

Plants can defend themselves to pathogen and herbivore attack by responding to chemical signals that are emitted by attacked plants. It is well established that such signals can be transferred through the air. In theory, plants can also communicate with each other through underground common mycorrhizal networks (CMNs) that interconnect roots of multiple plants. However, until now research focused on plant-to-plant carbon nutrient movement and there is no evidence that defense signals can be exchanged through such mycorrhizal hyphal networks. Here, we show that CMNs mediate plant-plant communication between healthy plants and pathogen-infected tomato plants (Lycopersicon esculentum Mill.). After establishment of CMNs with the arbuscular mycorrhizal fungus Glomus mosseae between tomato plants, inoculation of 'donor' plants with the pathogen Alternaria solani led to increases in disease resistance and activities of the putative defensive enzymes, peroxidase, polyphenol oxidase, chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase and lipoxygenase in healthy neighbouring 'receiver' plants. The uninfected 'receiver' plants also activated six defence-related genes when CMNs connected 'donor' plants challenged with A. solani. This finding indicates that CMNs may function as a plant-plant underground communication conduit whereby disease resistance and induced defence signals can be transferred between the healthy and pathogen-infected neighbouring plants, suggesting that plants can 'eavesdrop' on defence signals from the pathogen-challenged neighbours through CMNs to activate defences before being attacked themselves.

Figure 1. Levels of six defence-related enzymes in leaves of tomato ‘receiver’ plants in response to common mycorrhizal networks (CMNs) connected with Alternaria solani-infected neighbouring tomato.

Glomus mosseae was used to established the CMNs. Six defence-related enzymes are peroxidase (POD), polyphenol oxidase (PPO), chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase (PAL) and lipoxygenase (LOX). Four treatments included: A) a healthy tomato ‘receiver’ plant was connected with a neighboring A. solani-challenged tomato ‘donor’ plant through CMNs; B) a healthy ‘receiver’ plant was grown near A. solani-challenged ‘donor’ plant but no mycorrhiza was applied; C) a healthy mycorrhizal ‘receiver’ plant was grown near the pathogen-challenged mycorrhizal ‘donor’ plant but the two tomato plants separated by a water-proof membrane and D) a healthy ‘receiver’ plant was connected with the neighbouring plant by CMNs without pathogen inoculation. Values are means ± standard error from three sets of independent experiments with three pots per treatment for each set of experiments. Significant differences among treatments were tested at P = 0.05 by Tukey post-hoc test (Supporting Information Table S3).

Figure 2. Expression of six defence-related genes in leaves of tomato ‘receiver’ plants in response to common mycorrhizal networks (CMNs) connected with Alternaria solani-infected neighbouring tomato.

Glomus mosseae was used to established the CMNs. Quantitative real time RT-PCR was used to detect the transcripts of six defence genes encoding the pathogen-related proteins (PR1), basic type PR-2 (β-1,3-glucanase) and PR-3 (chitinase), phenylalanine ammonia-lyase (PAL), lipoxygenase (LOX) and allene oxide cyclase (AOC). A) a healthy tomato ‘receiver’ plant was connected with a neighboring A. solani challenged tomato ‘donor’ plant through CMNs; B) a healthy ‘receiver’ plant was grown near A. solani-challenged ‘donor’ plant but no mycorrhiza was applied; C) a healthy mycorrhizal ‘receiver’ plant was grown near the pathogen-challenged mycorrhizal ‘donor’ plant but the two tomato plants separated by a water proof membrane and D) a healthy ‘receiver’ plant was connected with the neighbouring plant by CMNs without pathogen inoculation. Values are means + standard error from three sets of independent experiments with three pots per treatment for each set of experiments. Significant differences (P<0.05 using Tukey post-hoc test) among treatments in a group are indicated by different letters above bars.

Figure 3. Experimental design.

(a) Four treatments included: A) a healthy tomato ‘receiver’ plant was connected with a neighboring Alternaria solani-challenged tomato ‘donor’ plant through common mycorrhizal networks (CMNs) of Glomus mosseae (GM); B) a healthy ‘receiver’ plant was grown near A. solani-challenged ‘donor’ plant but no mycorrhiza was applied; C) a healthy mycorrhizal ‘receiver’ plant was grown near the pathogen-challenged mycorrhizal ‘donor’ plant but the two tomato plants separated by a water-proof membrane and D) a healthy ‘receiver’ plant was connected with the neighbouring plant by CMNs without pathogen inoculation. (b) Tomato plants with the four treatments. In Figure 3a +GM refers to inoculation with G. mosseae in the compartment, and -GM refers to non-inoculation in the compartment. White fine filamentous networks refer to hyphal networks of G. mosseae. The hyphal networks across the two fine stainless steel screens in treatment A and D indicate the establishment of CMNs between ‘donor’ and ‘receiver’ plants.