Plant genes required for arbuscular mycorrhiza symbiosis
Final Report Abstract
The arbuscular mycorrhiza (AM) symbiosis between the roots of most land plants and fungi of the “Glomeromycota” is improving the phosphate nutrition of the plant. Legumes have evolved a unique ability to form root nodules in symbiosis with nitrogen-fixing rhizobia by recruiting molecular functions from the pre-existing AM symbiosis. Both symbioses are key components of sustainable agriculture by reducing the need for costly fertilizer produced from non-renewable resources. Forward genetic approaches in model legumes such as “Lotus japonicas” unraveled the general molecular network governing rhizobial infection and nodule organogenesis. Network components involved in the activation of the central regulatory calcium and calmodulin-dependent kinase (CCaMK) also control the intracellular accommodation of AM fungi during their passage to the root cortex and thus constitute the so-called common symbiosis pathway. For the present project, a microscopy screen for Lotus japonicus mutants defective in AM was performed and mutants impaired at different stages of AM development were isolated from more than 5600 individuals inspected. In order to identify the causative mutations by map-based cloning, a genotyping setup was established. Using this setup, mutations in already known symbiotic genes were rapidly discerned. A mutant defective in hyphal root colonization (patchy) carried a mutation in the common symbiosis gene POLLUX, encoding an ion channel. Calcium spiking, normally induced by rhizobial Nod factor (NF), was not observed in patchy, although its nodulation phenotype indicated residual activity of the mutant protein. A detailed co-segregation analysis of a mutant with a dimorphic arbuscule phenotype, positioned the red locus on the short arm of chromosome VI. Furthermore, induction of the mycorrhizal phosphate transporter PT4 was abolished in the red mutant line. In nena, fungal infection was aborted in the rhizodermis. NENA encodes a WD40 repeat protein related to the nucleoporins Sec13 and Seh1. Localization of NENA to the nuclear rim and yeast two-hybrid experiments indicated a role for NENA in a conserved sub-complex of the nuclear pore scaffold. Although nena mutants were able to form pink nodules in symbiosis with “Mesorhizobium loti”, root hair infection was not observed. Moreover, NF induction of the symbiotic genes NIN, SbtM4 and SbtS, as well as calcium spiking were impaired. Detailed phenotypic analyses of nena mutants revealed a rhizobial infection mode that overcame the lack of rhizodermal responsiveness and carried the hallmarks of crack entry, including a requirement for ethylene. CCaMK-dependent processes were only abolished in the rhizodermis but not in the cortex of nena mutants.
Publications
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2010. NENA, a Lotus japonicus homolog of Sec13, is required for rhizodermal infection by arbuscular mycorrhiza fungi and rhizobia but dispensable for cortical endosymbiotic development.
Plant Cell vol. 22. 2010, no. 7, pp. 2509-2526.
Groth M, Takeda N, Perry J, Uchida H, Dräxl S, Brachmann A, Sato S, Tabata S, Kawaguchi M, Wang TL, Parniske M.
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2011. Activation of a Lotus japonicus subtilase gene during arbuscular mycorrhiza is dependent on the common symbiosis genes and two cis-active promoter regions. Mol Plant-Microbe Interact. 24:662-70.
Molecular Plant-Microbe Interactions, Vol. 24. 2011, Number 6, pp. 662-670.
Takeda N, Haage K, Sato S, Tabata S, Parniske M.
