Characterization of the life cycle and cellular interactions of AM fungi with the reduced mycorrhizal colonization (rmc) mutant of tomato (Solanum lycopersicum L.)

Abstract

The broad aim of the work described in this thesis was to use the arbuscular mycorrhizal (AM) defective rmc tomato to explore the development and function of different types of fungus-plant interfaces (phenotypes) and to characterize the cellular modifications preceding colonization of rmc by a range of different AM fungi. Three main patterns of colonization with rmc have been described: 1) Pen- phenotype in which the AM fungus is restricted to the root surface with several attempts to penetrate the epidermal cells without success; 2) Coiphenotype where AM fungi penetrate the epidermis but cannot develop cortical colonization; and 3) Myc+ phenotype (with G. intraradices WFVAM23), where the AM fungus penetrates the cortex and forms a “normal” colonization after a delayed penetration of the epidermal cells (Review of literature). Little is known about cellular interactions, nutrient transfer or the ability of the fungi to complete their life cycles in the different phenotypes. These aspects were the main foci of this work. In addition further fungal isolates were screened to asses their ability to colonize rmc. The first experiments involved compartmented pots to follow the fungal life cycle, production of external mycelium and spores in the different rmc phenotypes (Chapter 3). The results showed that in the Pen- and Coiphenotypes, AM fungi are unable to form spores to complete the life cycle. However, in the Coi-phenotype, the fungus remained alive up to week 18, suggesting that some C transfer occurred. The fungus forming the Myc+ phenotype, G. intraradices WFVAM23, was able to produce spores, although they were significantly smaller than those produced with the wild-type tomato. The results suggested that arbuscules are essential for completion of the fungal life cycle. Labeled 32P was used to determine whether arbuscules are also essential for P transfer (Chapter 4). A compartmented pot system was used in which only fungal hyphae but not roots could obtain 32P. 32P was found in the shoots of rmc inoculated with S. calospora (Coi- phenotype), indicating that interfaces other than arbuscules can be involved in transfer of P. A nurse pot system was used to obtain synchronized colonization to determine how long AM fungi stay alive during the interactions with rmc and to elucidate the cellular modifications preceding colonization of rmc by a range of different AM fungi (Chapter 5). The results showed that rmc did attract the AM fungi, that the plant nucleus moved to the middle of the plant cell only after fungal penetration of plant roots and that callose deposition in rmc was not involved in blocking the AM fungi. Fourteen AM fungi with different taxonomic affiliations and fourteen different G. intraradices isolates were screened to try to relate phylogeny of AM fungi with phenotypes in rmc (Chapter 6). There were a large number of interactions, depending on the inoculated AM fungi, and although there were some similarities in the rmc phenotypes within phylogenetic groups, there was no clear relationship between phylogeny and development of interactions with rmc. This study showed the following. 1) Arbuscules/arbusculate coils are necessary for the completion of the AM fungal cycle. However, intraradical hyphae also participate in transfer of both P and C as demonstrated with the Coi- phenotype. 2) rmc clearly attracted AM fungi and the fungi stay alive and induce plant cellular responses such as nuclear movement only after penetrating rmc roots. 3) Plant defense responses such as callose deposition are not involved in blocking AM fungi in rmc; and 4) there was no relationship between the phenotypes described in rmc and phylogeny of the Glomeromycota.