Mustards, mustard oils and mycorrhizas

Abstract

Intact, living roots of Brassica kaber (DC.) Wheeler and Brassica nigra. L. (Brassicaceae) were inhibitory to the germination of spores of the mycorrhizal fungi Glomus intraradices Schenck & Smith and Glomus etunicatum Becker & Gerd. Roots from two similarly non-mycotrophic species, spinach (Chenopodiaceae) and Amaranthus retroflexus L. (Amaranthaceae), had no effect on the germination of spores of G. intraradices. The roots of two out of three mycotrophic species examined were stimulatory to germination of G. intraradices. To test the hypothesis that the inhibition of VAM fungus spore germination by the roots of B. kaber and B. nigra was due to isothiocyanates (mustard oils), lysine, arginine and glutathione were added separately to the nutrient solutions applied to the plants. These compounds were expected to react with isothiocyanates released into the rhizosphere. In one experiment, lysine, arginine and glutathione restored the germination of G. intraradices to control levels for B. kaber, and nearly to the control levels for B. nigra. In a second experiment employing lower concentrations of lysine, arginine and glutathione, germination of spores of G. etunicatum in the presence of B. kaber roots was restored nearly to the levels of controls. In the same experiment, these treatments had no significant effects on spore germination in the presence of B. nigra roots. Lysine and arginine also protected spores of G. etunicatum from in vitro reactions of B. kaber root glucosinolate extract with myrosinase. The involvement of isothiocyanates in the resistance of mustards to VAM fungi is discussed.

Plant–soil feedbacks contribute to an intransitive competitive network that promotes both genetic and species diversity

“…Measures of sinigrin content in the leaves of the field grown plants confirmed that the two selection lines maintained their differences in sinigrin expression in the field conditions (Lankau & Strauss 2007). While the toxic properties of glucosinolate breakdown products are well documented (Schreiner & Koide 1993;Kliebenstein 2004), the proximate mechanism by which the genetic differences in sinigrin expression in leaves caused the observed changes in soil microbial communities is not clear from this research. Sinigrin could have leached from leaves or litter, or leaf sinigrin levels could be genetically correlated with root sinigrin levels or some other unmeasured trait.…”

“…We used three other annual species (Amsinckia menziesii, Sonchus oleraceus, and Malva parviflora) as heterospecific competitors; all three commonly co-occur with B. nigra and form connections with AMF (observed and documented below). Brassica nigra, like most members of the Brassicaceae, does not form AMF associations (Schreiner & Koide 1993).…”

“…Arbuscular mycorrhizal fungal (AMF) infection potential was significantly lower in soils from underneath randomly assigned, experimentally planted patches of high sinigrin B. nigra compared with low sinigrin or heterospecific patches (Lankau & Strauss 2007). Glucosinolates, including sinigrin, break down into by-products toxic to a wide range of organisms (Kliebenstein 2004), including mycorrhizal fungi (Schreiner & Koide 1993;Roberts & Anderson 2001). Most flowering plants benefit from associations with AMF, in which, among other benefits, the fungus aids in nutrient acquisition in exchange for fixed carbon from the plant (Smith & Read 1997;Sikes, Cottenie & Klironomos 2009;Sikes, Powell & Rillig 2010).…”

Exogenous glucosinolate produced by Arabidopsis thaliana has an impact on microbes in the rhizosphere and plant roots

“…We measure effective production of the exogenous glucosinolate in root tissues of A. thaliana CYP79A1 (Choesin and Boerner 1991) and mustard roots (Schreiner and Koide 1993) and directly impact microbe living on roots. Populations closely associated with plant roots directly exposed to these compounds but also rhizospheric populations are impacted by this glucosinolate profile change.…”

Coevolution between invasive and native plants driven by chemical competition and soil biota

“…These allelochemicals appear to act as "novel weapons" sensu Callaway and Ridenhour (25), because they are more toxic to AMF strains from North America (introduced range) versus Europe (native range) (38). A. petiolata, like all members of the Brassicaceae, does not form mycorrhizal connections (40). Tissue concentrations of sinigrin, one of the major allelochemicals of A. petiolata, have evolved rapidly during this invasion, with higher sinigrin concentrations in populations on the expanding range edge compared with older populations (41).…”