Characterization of genes/proteins involved in mineral nutrient acquistion during the arbuscular mycorrhiza symbiosis
Final Report Abstract
Most land plants are capable to engage into the mutualistic symbiosis called Arbuscular Mycorrhiza (AM) thereby improving their mineral nutrition in regards of phosphorous, nitrogen and sulfur. Even though the basic factor governing the establishment of the association with the arbuscular mycorrhizal fungi is the plants need for phosphorous, recent reports highlight the importance of the parallel delivery of nitrogen towards the plant. Based on our previous studies we set out to invetigate further genes which might be involved in the symbiotic acquisition or distribution of nitrogen, i.e. a mycorrhizaspecific plant ammoniumtransporter LjAMT2;2 and two mycorrhiza-induced amino acid transporters LjCAT1/LjLHT1;2 of the model plant Lotus japonicus. With promoter-reporter constructs and fluorescent reporter gene fusions we could show clearly that the LjAMT2;2 transporter is expressed in cortical root cells haboring functional arbscules, the key features of the symbiosis. At this the AMT is located in the periarbuscular membrane, which is considered the main site for the nutrient exchange between the fungus and the plant. This confirms the previous biochemical characterization of the transporter and makes a role in the symbiotic acquisition of inorganic nitrogen more likely. However Ljamt2;2 plant mutants lacked a visible mycorrhizal phenotype developing morphological normal arbscules. Quantitative phenotyping has been hampered by technical problems and the question whether and how much nitrogen is transported via this transporter in the symbiosis remains open. Interestingly the expression pattern of the LjAMT2;2 showed a good correlation to that of the mycorrhiza marker LjPT4, even though no common regulatory motifs have been found in the promoters. A putative new regulatory element is currently analyzed for its transcriptional response to mycorrhization and thus might shed light into the mechanisms regulating the expression of this gene. The two mycorrhiza-induced genes LjCAT1/LjLHT1;2 encode for high-affinity amino acid transporters and thus present the first characterized AATs active in the AM. They exhibit different substrate specificities and based on expression pattern and regulation the LjLHT1.2 transporter might be involved in recycling/reuptake processes during arbuscule development and/or degradation. The results for the LjCAT1 transporter however indicate a function in fully developed/active arbuscules. Further the activity of the LjCAT1 promoter in the infection zone of young root nodules and in the meristematic cells of root tips suggests an additional, more general function in symbioses and/or plant development. So far it has not been possible to localize the AAT proteins on a subcellular level and no mycorrhizal phenotypes in mutants or RNAi plants have been observed. Therefore the exact roles of the two transporters and the long standing question whether amino acids might be also a symbiotically transferred N-form in the AM remain to be elucidated.
Publications
- (2011) LjLHT1.2—a mycorrhiza-inducible plant amino acid transporter from Lotus japonicus. Biol. Fertil. Soils. 47(8): 925-936
Guether M, Volpe V, Ballestrini R, Requena N., Wipf D and Bonfante P
- Two putative-aquaporin genes are differentially expressed during arbuscular mycorrhizal symbiosis in Lotus japonicus. BMC Plant Biol, 12:186, 2012
Giovannetti M, Ballestrini R, Volpe V, Guether M, Straub D, Costa A, Ludewig U, and Bonfante P
(See online at https://doi.org/10.1186/1471-2229-12-186) - (2013) An AM-induced, MYB-family gene of Lotus japonicus (LjMAMI) affects root growth in an AM-independent manner. Plant Journal. 73(3):442-55
Volpe V, Dell’Aglio E, Giovannetti M, Ruberti C, Costa A, Genre A, Guether M, Bonfante P
(See online at https://doi.org/10.1111/tpj.12045)
