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Sent: 04 July 2002 08:51
Subject: 107: Transgenic trees may cause problems with mycorrhiza
A wide range of plant systems form symbiotic associations with fungi. The associations are called arbuscular mycorrhizae. The symbiosis develops in the roots where the fungus colonizes cells of the cortex to access carbon compounds supplied by the plant. The fungus, in return, contributes to the mineral nutrition of the host plant and defends the plant from microbial pathogens. The symbiosis is believed to have originated at least 500 million years ago, persisting for as long as plants have colonized the land (1). Symbiosis that has lasted for so long should not be ignored when evaluating the impact of transgenic plants on the environment, particularly the forest trees that strongly depend on the symbiosis to maintain a long life in a challenging environment.
Mycorrizal colonization was studied in a field trial of transgenic aspen. The modification included the rol-C gene from the T-DNA of the bacterium Agrobacterium rhizogenes. The study focused on 35S-rolC transgenic aspen trees characterised by dwarfed phenotype, precocious bud break/leaf development, and smaller wrinkled leaves as compared with non-transformed control trees. Transgenic plants carrying the 35S-rolC gene construct show primarily alterations in the content of cytokinins. [Cytokinins are plant growth regulators characterized as substances that induce cell division and cell differentiation...Moderator]. However, in leaves, buds, apices and stems of different 35S-rolC transgenic aspen lines the levels of different other plant hormones, such as abscisic acid (ABA) and indole-3-acetic acid (IAA)..
Wood formation of the 35S-rolC transgenic aspen trees shows stable alterations as compared with non-transformed controls, such as delayed formation of cells, the occurrence of thin-walled and less-lignified fibres and the lack of typical latewood. The transgenic aspen was deficient in colonization of one of four fungal symbiont strains. The study showed that the transgenic aspen influenced the establishment of a mycorrhizal symbiont with a potential fungal partner(2). There is little doubt that other transgenic trees, such as those deficient in lignin, may exert an influence on the mycorrizal symbiosis. It seems imperative that transgenic trees should not be rushed to commercialization until important characteristics are fully evaluated under stringent field conditions.
1.Harrison,M. "Molecular and cellular aspects of the arbuscular mycorrizal symbiosis" 1999 Ann Rev. Plant Mol. Biol. 50,361-89
2. Kaldorf,M,Fladung,M,Muhs,H and Buscot,F "Mycorrizal colonization of transgenic aspen in a field trial" 2002 Planta 214,653-60
[The abstract from paper 2, referred to above, reads as follows: Mycorrhizal colonization of genetically modified hybrid aspen (Populus tremula x P. tremuloides Michx.) was investigated over 15 months in a field experiment. The aspen carried the rolC gene from Agrobacterium rhizogenes under control of either the constitutive cauliflower mosaic virus 35S promoter or the light-inducible rbcS promoter. Arbuscular mycorrhizas (AMs) were rare in all root samples, while fully developed ectomycorrhizas (EMs) were found in all samples. No significant differences in the degree of mycorrhizal colonization between aspen lines were seen with either AMs or EMs. The EM community on the release area was dominated by four fungal species that formed more than 90% of all mycorrhizas, while eleven EM types were found occasionally. Mycorrhizal diversity did not differ between transgenic and non-transgenic trees. The structure of mycorrhizal communities was similar for most aspen lines. The sole significant difference was found in the abundance and development of one of the four common EM morphotypes, which was rare and poorly developed on roots from the transgenic aspen line Esch5:35S-rolC-#5 compared with non-transgenic controls. This effect is clone specific as the formation of this EM type was not affected by the transgene expression in the other transgenic line, Esch5:35S-rolC-#1. This is the first demonstration of a clonal effect influencing the ability of a transgenic plant to form a mycorrhizal symbiosis with a potential fungal partner...Moderator]
Professor Joe Cummins,
University of Western Ontario.
jcummins (at) uwo.ca