Interactive effects of arbuscular mycorrhizal fungi and intercropping with sesame (Sesamum indicum) on the glucosinolate profile in broccoli (Brassica oleracea var. Italica)
Introduction
Glucosinolates are a group of secondary plant metabolites found almost exclusively in plants of the order Brassicales, which include important horticultural crops such as broccoli of the Brassicaceae family (Verkerk et al., 2009). Glucosinolates consist of a sulphur-linked β-D-glucopyranose moiety and an amino acid-derived side chain. The side chain structure determines whether they are classified as aliphatic, aromatic or indole glucosinolates (Halkier and Gershenzon, 2006). Breakdown products of glucosinolates hydrolysed by endogenous thioglucosidase enzymes (myrosinase) induced by herbivore (Textor and Gershenzon, 2009) or fungal penetration (Matsumura et al., 2007) activate the plants’ defence response as these hydrolysis products act as deterrents due to their toxicity.
The association between plant roots and arbuscular mycorrhizal (AM) fungi has proved to be an evolutionary successful strategy for more than 80% of all land plant species (Smith and Read, 2008). This mutualistic association provides the fungus with carbohydrates and in return, the plant gains the benefits of mineral nutrients. However, there are some families of plants—which have been called ‘non-host’ plants—that do not form AM symbiosis (Francis and Read, 1994). Brassicaceae is one of these families that have long been known as AM non-host which neither release AM fungi-stimulating compounds (Giovannetti et al., 1994) nor promote appressoria formation (Nagahashi and Douds, 1997). The incompatibility between non-host Brassica species and AM fungi has been suggested to be at least partly due to the formation of fungitoxic or fungistatic glucosinolate hydrolysis products (Fahey et al., 2001, Mayton et al., 1996). In non-host Brassica species, AM fungi induce changes in endogenous glucosinolates concentrations in the roots, probably indicating an intrinsic defence reaction (Vierheilig et al., 2000). The aromatic 2-phenylethyl glucosinolate or some indole glucosinolate, e.g. 1-methoxy-3-indolylmethyl glucosinolate and 4-methoxy-3-indolylmethyl glucosinolate, has been observed to increase in AM non-host plant roots when inoculated with AM fungi depending on the species (Vierheilig et al., 2000). It has also been suggested that the effect of phytohormones may be involved in this incompatibility. After the foliar application of jasmonic acid and salicylic acid to Tropaeolum majus and Carica papaya, both the aromatic benzyl and 3-indolylmethyl glucosinolate concentrations in both plants roots were increased. In all plants treated with jasmonic acid a reduction of root colonization by the AM fungus Glomus mosseae was observed. No such effect occurred after the salicylic acid treatment (Ludwig-Müller and Cohen, 2002).
The response of an AM non-host plant to AM fungal inoculation might be modified in the presence of a mycorrhizal AM host plant. Even though a functional mycorrhization does not occur in an AM non-host plant, the unavoidable invasion of hyphae deriving from the intercropped mycorrhizal host plants to the non-host plant roots is persistent (Fontenla et al., 1999). These invading AM hyphae may lead to a continuous induction of a defence response (Matsumura et al., 2007), and therefore sustain an increased glucosinolate concentration in the non-host Brassica species.
By intercropping an AM non-host plant (broccoli) with an AM host plant (sesame) and applying AM fungi (Rhizophagus irregularies) in all combinations, we specifically addressed the question of whether interactive effects of intercropping and AM fungi on the glucosinolate status in broccoli occur. It was hypothesised that when inoculated with AM fungi, glucosinolate concentration will be systemically increased in broccoli plants and this effect will be more distinct when intercropped with an AM host plant. By analysing glucosinolate concentrations and profile in roots and leaves of broccoli, we were able to determine whether the glucosinolate induction by AM fungi is root-localised or results in a systemic increase as seen for controlled drought stress (Tong et al., 2014). In addition to the glucosinolate status of the entire broccoli plant, we also determined changes in the biomass of both intercropped species, which may indicate potential costs involved in the production of defence compounds and in the nutritional support by AM fungi.
Section snippets
Plant material, three-compartment split-root system and growth conditions
The study involved a pot-based experiment, carried out at the Leibniz-Institute of Vegetable and Ornamental Crops Grossberen/Erfurt e. V., in Grossbeeren (Germany), from 16th September to 27th November. Each treatment was replicated four times and set up completely at random. Plants were grown in a greenhouse with a day/night cycle of 16 h/8 h at average air temperatures of 22 °C (day) and 18 °C (night). Relative humidity was in the range of 60–70% with a daily mean photosynthetic photon flux
Intra- and extraradical am fungal development
The roots of both broccoli and sesame plants in the non-mycorrhizal treatment [−M] were free from AM fungal colonisation. The colonisation rate of [+M] sesame roots in the solo compartment was maintained at a high level and was not affected by the neighbouring plant species. The roots of [+M] sesame plants in the combi compartment differed remarkably in the extent of AM fungal colonisation depending on the neighbouring plant species. The colonised root length of [+M] sesame in combi
Discussion
This study shows that presence of AM fungi in soil and/or intercropping with another plant species can lead to pronounced changes in the concentration of glucosinolate in the roots of broccoli, especially when part of the root system of both species share the same soil volume. This effect is not only local, but also systemic: the concentration of total glucosinolates in broccoli leaves was also increased by AM fungi and intercropping, mainly due to higher concentrations of 3-indolylmethyl
Acknowledgements
This work was supported by the Ministries of Consumer Protection, Food and Agriculture of the Federal Republic of Germany, of the states of Brandenburg and Thüringen. We appreciate technical assistance in laboratory work by Andrea Jankowsky, Susanne Jeserigk and Kerstin Bieler.
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