The relationship between thiamine and two symbioses: Root nodule symbiosis and arbuscular mycorrhiza

摘要

Lotus japonicus THIC is expressed in all organs, and the encoded protein catalyzes thiamine biosynthesis. Loss of function produces chlorosis, a typical thiamine-deficiency phenotype, and mortality. To investigate thiamine's role in symbiosis, we focused on THII, a thiamine-biosynthesis gene expressed in roots, nodules, and seeds. The thil mutant had green leaves, but formed small nodules and immature seeds. These phenotypes were rescued by THII complementation and by exogenous thiamine. Thus, THII is required for nodule enlargement and seed maturation. On the other hand, colonization by arbuscular mycorrhiza (AM) fungus Rhizophagus irregularis was not affected in the thil mutant or by exogenous thiamine. However, spores of R. irregularis stored more thiamine than the source (host plants), despite lacking thiamine biosynthesis genes. Therefore, disturbance of the thiamine supply would affect progeny phenotypes such as spore formation and hyphal growth. Further investigation will be required to elucidate thiamine's effect on AM.