Abstract
Arbuscular mycorrhiza (AM) is one of the most spread symbiosis established between 80% of land plants and soil fungi belonging to the Glomeromycota. Molecular determinants involved in the formation of arbuscular mycorrhizas are still poorly understood. It has been demonstrated that in both Legumes and rice plants, several GRAS transcription factors are directly involved in both mycorrhizal signaling and colonization, namely NSP1, NSP2, RAM1, DELLA, DELLA-interacting protein (DIP1) and RAD1. Here, we focused on a rice GRAS protein, named Arbuscular Mycorrhizal 18 (OsAM18), previously identified as specifically expressed in rice mycorrhizal roots, and considered as an AM-specific gene. Phylogenetic analysis revealed that OsAM18 had a peculiar amino acid sequence, which clustered with putative SCARECROW proteins, even though it formed a separate branch. Allelic osma18 mutant displayed a drastic reduction in mycorrhizal colonization in-tensity and in arbuscule abundance, as mirrored by OsPT11 expression level. Non-mycorrhizal osam18 plants displayed a comparable plant development and root apparatus compared with the WT, while mycorrhizal osam18 mutants showed a reduction of plant biomass compared with mycorrhizal WT plants. The results suggest that OsAM18 is a rice protein, which is likely to have an impact not only on the colonization process and AM functionality, but also on the systemic effects of the AM symbiosis.
Highlights
The arbuscular mycorrhizal (AM) symbiosis is established between fungi of the Glomeromycota phylum and more than 80% of land plant species, being the most widespread terrestrial symbiosis [1]
On the basis of a RNAseq analysis, Xue and colleagues showed that 45 genes encoding transcriptional factors (TFs) or transcriptional regulators were significantly up-regulated in Lotus japonicus mycorrhizal roots when compared with non-mycorrhizal roots [15]
Recent works demonstrated a direct involvement of several GRAS transcription factors in both mycorrhizal signaling and colonization, namely NODULATION SIGNALING PATHWAY 1 (NSP1), NSP2, Required for Arbuscular Mycorrhization 1 (RAM1), DELLA, DELLA-interacting protein (DIP1) and RAD1 [15] [18] [29] [30] [32] [33] [50]
Summary
The arbuscular mycorrhizal (AM) symbiosis is established between fungi of the Glomeromycota phylum and more than 80% of land plant species, being the most widespread terrestrial symbiosis [1]. On the basis of a RNAseq analysis, Xue and colleagues showed that 45 genes encoding transcriptional factors (TFs) or transcriptional regulators were significantly up-regulated in Lotus japonicus mycorrhizal roots when compared with non-mycorrhizal roots [15] These genes belong to the main TFs families and include GRAS, AP2/ERF, NO APICAL MERISTEM, ARABIDOPSIS TRANSCRIPTION ACTIVATION FACTOR, CUP-SHAPED COTYLEDON (NAC)-domain, MADS, MYB, C2H2 zinc finger, bZIP, ARF, WRKY DNA binding domain, NIN-like protein and LOB domain. These results indicate how mycorrhizal symbiosis leads to deep change in transcription regulation. Some of these TFs have already been reported to be induced during the AM symbiosis [10] [16] [17] and in some cases have been characterized [18]-[20]
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