The interaction with arbuscular mycorrhizal fungi or Trichoderma harzianum alters the shoot hormonal profile in melon plants

Abstract

Arbuscular mycorrhizal fungi (AMF) and Trichoderma harzianum are known to affect plant growth and disease resistance through interaction with phytohormone synthesis or transport in the plant. Cross-talk between these microorganisms and their host plants normally occurs in nature and may affect plant resistance. Simultaneous quantification in the shoots of melon plants revealed significant changes in the levels of several hormones in response to inoculation with T. harzianum and two different AMF (Glomus intraradices and Glomus mosseae). Analysis of zeatin (Ze), indole-3-acetic acid (IAA), 1-aminocyclopropane-1-carboxylic acid (ACC), salicylic acid (SA), jasmonic acid (JA) and abscisic acid (ABA) in the shoot showed common and divergent responses of melon plants to G. intraradices and G. mosseae. T. harzianum effected systemic increases in Ze, IAA, ACC, SA, JA and ABA. The interaction of T. harzianum and the AMF with the plant produced a characteristic hormonal profile, which differed from that produced by inoculation with each microorganism singly, suggesting an attenuation of the plant response, related to the hormones SA, JA and ethylene. These results are discussed in relation to their involvement in biomass allocation and basal resistance against Fusarium wilt.

Introduction

Beneficial rhizosphere microorganisms that improve plant nutrition and health include arbuscular mycorrhizal fungi (AMF) and Trichoderma spp. Alleviation of damage caused by soil-borne pathogens, such as Phytophthora, Fusarium, Pythium, Rhizoctonia, Sclerotium and Verticillium has been reported widely in mycorrhizal plants (Barea et al., 1997, Bi et al., 2007, Whipps, 2004). The AM establishment and functioning result from a complex molecular dialogue between the plant and the AM fungus (Harrison, 2005, Parniske, 2008, Paszkowski, 2006a, Requena et al., 2007). Some processes occurring in this dialogue are known to be mediated by phytohormones on the plant side (Hause et al., 2007). The establishment of the AM symbiosis has been reported to induce changes in the phytohormone balance in the roots of the host plants, with respect to cytokinins, gibberellins, ethylene, abscisic acid (ABA) and jasmonates (Allen et al., 1980, Allen et al., 1982, Drüge and Schönbeck, 1992, Hause et al., 2002, Hause et al., 2007, López-Ráez et al., 2010, Ludwig-Müller, 2000, Ludwig-Müller et al., 2002, Riedel et al., 2008), but there is only limited evidence about the systemic effects of this particular symbiosis in the shoots of AM plants (Pozo et al., 2009, Toussaint, 2007). In this regard, Taylor and Harrier (2003) observed that AM-tomato plants showed different gene expression patterns in leaf and root tissues, which could have been the result of an alteration of the hormonal balance in the host plants. There have been also several studies related to the induction of resistance by AMF, focusing especially on the activation of plant defence mechanisms in roots (Avis et al., 2008, Garcia-Garrido and Ocampo, 2002, Pozo and Azcón-Aguilar, 2007, Pozo et al., 2009). However, among the studies that have explored the role of phytohormones in AM–plant interactions, the results are inconsistent (Hause et al., 2007, Ludwig-Müller, 2000, Pozo and Azcón-Aguilar, 2007) and are focused on understanding the biology of the AM symbiosis, mainly at the root level. Therefore, there is still a lack of information on the physiological implications of the symbiosis in the shoot of the host plant (Toussaint, 2007). In this regard, accumulation of insect anti-feedant compounds (Gange, 2006) and transcriptional up-regulation of defence-related genes (Liu et al., 2007) have been described recently in the shoots of mycorrhizal plants.

Trichoderma (teleomorph Hypocrea) is a genus of asexual fungi found in the soils of all climatic zones. These fungi are opportunistic, avirulent plant symbionts and function as parasites and antagonists of many phytopathogenic fungi, thus protecting plants from diseases (Benítez et al., 2004, Harman et al., 2004, Howell, 2003, Vinale et al., 2008). Some strains can penetrate plant roots and colonise the epidermis and outer cortex, causing substantial changes in plant metabolism. It is well documented that some strains promote plant growth, increase nutrient availability, improve crop production and enhance resistance to pathogens, even in the shoot (Elad, 2000, Harman et al., 2004, Korolev et al., 2008, Shoresh et al., 2005, Vinale et al., 2008, Yedidia et al., 2003). It has been proposed that the phytohormones jasmonic acid (JA), ethylene and salicylic acid (SA) play a major role in the resistance induced by several Trichoderma isolates. Trichoderma asperellum isolate T203 has been shown to induce resistance in cucumber to Pseudomonas syringae pv. Lachrymans, through the pathway mediated by the signal phytohormones JA and ethylene in the plant (Shoresh et al., 2005). Further evidence for a role of JA and ethylene in Trichoderma-induced resistance was provided by Korolev et al. (2008), who demonstrated that mutants of Arabidopsis with a defect in ethylene or JA signalling were unable to elevate their systemic resistance levels to Botrytis cinerea after colonisation by Trichoderma harzianum isolate T39. Moreover, a number of mechanisms have been proposed to explain the growth enhancement by Trichoderma spp. (Benítez et al., 2004, Harman et al., 2004), among them, fungal interactions with phytohormonal signalling and induction of resistance against pathogens (Vassilev et al., 2006). A possible role of indoleacetic acid (IAA) in the growth stimulation of tomato plants produced by inoculation with Trichoderma aureoviride was proposed by Gravel et al. (2007).

We hypothesised that this wide range of plant responses induced by each of these beneficial microorganisms could be modified by their simultaneous co-inoculation, and that this could be reflected in the overall metabolism of melon plants. This study was conducted to investigate the alteration of the hormonal profile of melon shoots as a consequence of the plant response to two AMF (Glomus intraradices and Glomus mosseae), which showed different colonisation patterns and functionalities in previous studies, and to the beneficial fungus T. harzianum, following their individual and co-inoculation. Such studies are crucial, not only to the development of an integrated understanding of complex plant-beneficial microorganism relationships, but also for the development of rational strategies for improving pathogen resistance using beneficial microorganisms.