Functional analysis of the pH responsive pathway Pal/Rim in the phytopathogenic basidiomycete Ustilago maydis

The most important mechanism for fungal response to the environmental pH is the Rim or Pal pathway. Details on its operation are known through the analysis of ascomycete fungi. In this study we analyzed whether this pathway is conserved in a basidiomycete, Ustilago maydis. We could identify only five homologues of the seven known components of the pathway in the U. maydis as well as in other basidiomycete genomes. We determined that only genes encoding Rim20/PalA, Rim13/PalB and Rim23/PalC, that constitute the endosomal membrane complex, and Rim9/PalI of the complex located at the plasma membrane are conserved, but this latter lacked a detectable role in signal transduction. Mutants in this pathway showed a pleiotropic phenotype, but dimorphism and virulence were not affected. Our data reveal that the Rim/Pal pathway is conserved in basidiomycetes, but with notable differences to the ascomycete systems.

Unraveling the network: Novel developments in the understanding of signaling and nutrient exchange mechanisms in the arbuscular mycorrhizal symbiosis

The arbuscular mycorrhhiza (AM) symbiosis involves an intricate network of signaling and biochemical pathways designed to ensure that a beneficial relationship is established between the plant and fungal partners as a result of a mutual nutrient exchange. Emerging data has been recently published to explain why the relationship is not always fair, as observed in prevalent parasitic AM relationships in which the plant host receives no phosphorus (P) in exchange for carbon (C) delivered to the fungus. The theory behind this unorthodox view of the AM relationship, together with the description of other recent developments in nutrient mobilization as well as in key aspects of the bi-directional signaling that culminates in the symbiotic association, is the subject of this review.

Jasmonic acid influences mycorrhizal colonization in tomato plants by modifying the expression of genes involved in carbohydrate partitioning

The role of jasmonic acid (JA) on mycorrhizal colonization by Glomus fasciculatum in tomato plants was examined using mutant plants overexpressing prosystemin (PS) or affected in the synthesis of JA (suppressor of prosystemin-mediated responses 2, spr2). The degree of mycorrhizal colonization was determined by measuring frequency (F%) and intensity (M%) of colonization and arbuscule abundance (A%). Gene expression and biochemical analyses were also performed in roots to detect changes in carbon (C) partitioning. Colonization was similar in mycorrhizal PS and wild-type roots, except for a higher A% in the former. Conversely, colonization was severely reduced in roots of spr2 mutants. No association was found between levels of expression of genes coding for systemic wound responsive proteins (or SWRPs) and other defense-related proteins in roots and mycorrhization levels in these plants. On the other hand, the degree of mycorrhizal colonization correlated with changes in the transcriptional regulation of a number of genes involved in sucrose hydrolysis and transport, cell wall invertase activity and mycorrhizal-specific fatty acid content in roots. The results obtained suggest that one of the mechanisms by which JA might operate to modulate the mycorrhization process could be through its influence on the regulation of C partitioning in the plant. The significant colonization increase observed in mycorrhizal spr2 plants supplied with exogenous methyl jasmonate supports its role as a positive regulator of the symbiosis.