Differential gene expressions in arbuscular mycorrhizal-colonized tomato grown under heavy metal stress

When tomato was grown in either Breinigerberg soil, which has a high content of Zn and of other heavy metals or in non-polluted soil enriched with upto 1 mM CdCl2, plants colonized with the arbuscular mycorrhizal fungus (AMF) Glomus intraradices grew distinctly better than non-mycorrhizal controls. An analysis of differential mRNA transcript formations was performed on several plant genes coding for products potentially involved in heavy metal tolerance. Northern blot analyses indicated that the mRNA from either roots or Leaves was not differentially expressed in the case of LePCS1 (coding for phytochelatin synthase), Lemt1, Lemt3 and Lemt4 (for metallothioneins) or LeNramp2 (for a broad range heavy metal transporter) in both mycorrhizal and non-mycorrhizal plants, grown either with or without heavy metals. In contrast, Lemt2 was strongly expressed only in non-AMF-colonized roots, and only after growth in the Breinigerberg soil or in the presence of high CdCl2-concentrations. AMF colonization distinctly reduced the Level of Lemt2 transcripts. This was also the case for the root specific LeNramp1 transporter, however, only after growth in the Breinigerberg soil, but not under Cd-stress. Likewise, the levels of LeNramp3 transcripts were reduced by the AMF colonization in roots, but not in Leaves. Quantitative Real-Time RT-PCR-experiments performed with Lemt2, LeNramp1 and LeNramp3 largely corroborated the Northern analysis data. In situ hybridization experiments with Lemt2 and LeNrampl showed that both genes were strongly expressed throughout the plant cells in non-colonized roots, whereas colonized roots revealed only few signals restricted to some parenchyma cells. All the data suggest that the transcript levels of some, but not all genes of the Nramp or mt family are elevated under heavy metal stress. AMF colonization results in a down-regutation of these genes, presumably due to the fact that the content of heavy metals is tower in mycorrhizal than in non-colonized roots. A suppression subtractive hybridization (SSH) library from hyphae of the AMF G. intraradices grown in high versus tow Zn++ provided none of the genes which were down-regutated at the plant side (mt or Nramp genes). In contrast, several gene sequences coding for enzymes potentially catalysing the detoxification of reactive oxygen species were found. Thus the fungal cells in the symbiosis may primarily have to cope with the heavy metal.-induced oxidative stress. (c) 2005 Elsevier GmbH. All rights reserved.