No abstract

SummaryEctomycorrhizas are mutalistic symbiotic associations formed between fine roots of higher plants, mostly trees, and a wide range of soil ascomycetes and basidiomycetes. It is commonly accepted that there is mutual benefit to the partners, due to the exchange of plant-derived carbohydrates for amino acids and nutrients supplied by the fungus. While the major concepts of mycorrhizal functioning (exchange of nutrients and metabolites) were proposed in the 1960s, their verification at the molecular level started approximately 10 years ago. This review covers concepts at the molecular level concerned with the fungal carbohydrate supply in symbiosis. We discuss: strategies used by host plants to compensate (and perhaps restrict) carbohydrate drain to the fungal partner; fungal mechanisms that generate strong monosaccharide sinks in colonized plant roots (the formation of a strong carbohydrate sink is a prerequisite for efficient fungal carbohydrate support by the plant partner); and the impact of apoplastic hexose concentration on the regulation of fungal metabolism in symbiosis, since monosaccharides not only serve as nutrients but also as a signal that regulates gene expression.

B chromosomes (Bs) are dispensable components of the genomes of numerous species. To test whether the transcriptome of a host is influenced by Bs, we looked for differences in expression in response to additional Bs. Comparative complementary DNA amplified fragment length polymorphism experiments resulted in the identification of 16 putative B-chromosome-associated transcripts. This comprises 0.7% of the total transcript number and indicates a low activity of Bs. We also provide evidence that B chromosome influences in trans the transcription of A chromosome sequences. The B-specific transcribed sequences B1334, B8149, and B2465 belong to high-copy families with similarity to mobile elements. For all analyzed B-chromosome-derived transcripts, similar A chromosome-encoded sequences were found which supports an A-derived origin of rye B chromosomes.

The tissue-specific and development-dependent accumulation of secondary products in roots and mycorrhizas of larch (Larix decidua Mill.; Pinaceae) was studied using high-performance liquid chromatography and histochemical methods. The compounds identified were soluble catechin, epicatechin, quercetin 3-Ocr-rhamnoside, cyanidin-and peonidin 3-Op-glucoside, 4-OP-hydroxybenzoyl-OP-glucose, 4-hydroxybenzoate 4-OP-glucoside, maltol 3 -0 4 -glucoside, and the wall-bound 4-hydroxybenzaldehyde, vanillin, and ferulate. I n addition, we partially identified a tetrahydroxystilbene monoglycoside, a quercetin glycoside, and eight oligomeric proanthocyanidins. Comparison between the compounds accumulating in the apical tissue of fine roots, long roots, and in vitro grown mycorrhizas (L. decidua-Suillus tridentinus) showed elevated levels of the major compounds catechin and epicatechin as well as the minor compound 4-hydroxybenzoate 4-OP-glucoside specifically in the root apex of young mycorrhizas. l h e amounts of wall-bound 4-hydroxybenzaldehyde and vanillin were increased in all of the mycorrhizal sections examined. During the early stages of mycorrhization the concentrations of these compounds increased rapidly, perhaps induced by the mycorrhizal fungus. I n addition, studies of L. decidua-Boletinus cavipes mycorrhizas from a natural stand showed that the central part of the subapical cortex tissue and the endodermis both accumulate massive concentrations of catechin, epicatechin, and wall-bound ferulate compared with the outer part of the cortex, where the Hartig net is being formed.