Mycorrhizal Colonisation in Glucosinolate Containing Plants

GS-containing plants are widely recognised as being nonmycorrhizal, or at best they show an extremely low incidence of colonisation by AM fungi (Harley and Harley 1987; Wang and Qiu 2006). The nonmycorrhizal status of these plants has been attributed to:

• In situ root GS hydrolysis products inhibiting AM fungal spore germination (Schreiner and Koide 1993).

• The structural-chemical properties of the root cell wall that hinder or inhibit fungal growth (Glenn et al. 1988).

• The differences in phosphate acquisition/ scavenging systems compared to mycorrhizal species (Murley et al. 1998).

• The lack of communication signals between the symbionts (Akiyama et al. 2005; Vierheilig et al. 2000).

• The relationship between the plant developmental stages and nutrient demands during the plant life cycle (Pongrac et al. 2007; Vogel-Mikus et al. 2006, 2007).

Recent discoveries of AM colonisation in Biscutella laevigata (Orlowska et al. 2002) and Thlaspi sp. (Regvar et al. 2003, 2006; Vogel-Mikus et al. 2005, 2006) have posed new questions about AM development in GS-containing species and the role of GS in AM formation. An increase in GS concentrations and a distinct change in the GS profile after inoculation are frequently seen in several GS-containing plant species, regardless of whether colonisation has occurred or not (Vierheilig et al. 2000). As the concentrations of different GS vary widely across the developmental stages and organs of plants (Rask et al. 2000), this may account

Fig. 7.5 Root glucosinolate profiles and arbuscular mycorrhiza (AM) through the different developmental phases of field-collected Thlaspi praecox. VP, vegetative; FI, flower induction; FP, flowering; SP, seeding; SC, senescence phases. Redrawn from Pongrac et al. (2008)

for AM formation during particular plant development periods. A decrease in total GS concentrations and a distinct change in GS profile were seen to coincide with the highest AM colonisation in field-collected T. praecox, whereas in the AM hosts Tropaeolum majus and Carica papaya, reduced AM colonisation did not correlate with the concentrations of GS induced in these plants (Fig. 7.5) (Ludwig-Müller et al. 2002; Pongrac et al. 2007, 2008).

Not only the total GS, but also the GS profile and individual GS in particular may be decisive in AM formation. Most of the non-AM plants contain gluconastur-tiin in their roots, and therefore gluconasturtiin was proposed to act as a general AM inhibitory factor (Vierheilig et al. 2000). Hence gluconasturtiin was not found in T. praecox (Fig. 7.5) (Pongrac et al. 2008). In contrast, the main GS of the AM host T. majus is glucotropaeolin, and its concentration was seen to increase during AM colonisation (Vierheilig et al. 2000; Ludwig-Müller et al. 2002). In T. praecox, the peak glucotropaeolin concentrations are concomitant with the peak AM colonisation (Pongrac et al. 2008). This GS is known as an important metabolite for root growth regulation and a potential precursor of phenylacetic acid, another important naturally occurring auxin in plants (Ludwig-Müller 1999, Ludwig-Müller and Cohen 2002; Davies 2004). AM colonisation has also been shown to enhance auxin levels and alter auxin biosynthesis in T. majus during its early stages (Jentschel et al. 2007). Another individual GS disappears along with AM colonisation of T. praecox, namely glucobrassicanapin, which has previously been shown in the AM nonhost B. napus (Fig. 7.5) (Pongrac et al. 2008; Vierheilig et al. 2000). These observed changes in GS levels may also be salicylate mediated, as root colonisation by AM fungi is affected by the salicylic acid content of the plant (Herrera Medina et al. 2003). However, the effects of jasmonic acid also appear to provide a likely candidate here

(Regvar et al. 1996). Sinalbin and its hydrolysis product p-OH-benzylisothiocyanate were identified as the predominant antifungal compounds in Brassica kaber (Schreiner and Koide 1993). Sinalbin is also the predominant GS induced by inoculation with Glomus mossae in Sinapis alba (Vierheilig et al. 2000). Although it was the only GS found in all of the plant organs and all of the developmental stages of T. praecox, its role in AM formation remains to be established (Pongrac et al. 2008).

From the currently available literature data, we can conclude that not only total GS levels but also the GS pattern - or more likely the presence and/or absence of specific GS - appear to be decisive for AM formation in GS-containing plant species (Pongrac et al. 2008; Vierheilig et al. 2000). In addition, the factors that have previously been established as important drivers of AM formation should also be taken into account, including plant mineral demands, the seasonal dynamics of mycorrhization, pollution (Smith and Read 1997; Vogel-Mikus et al. 2006; Pongrac et al. 2007; Regvar et al. 2006), and plant hormones and diverse signalling molecules (Gogala 1991; Akiyama et al. 2005).

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