O-GlcNAcylation of master growth repressor DELLA by SECRET AGENT modulates multiple signaling pathways in Arabidopsis.

The DELLA family of transcription regulators functions as master growth repressors in plants by inhibiting phytohormone gibberellin (GA) signaling in response to developmental and environmental cues. DELLAs also play a central role in mediating cross-talk between GA and other signaling pathways via antagonistic direct interactions with key transcription factors. However, how these crucial protein-protein interactions can be dynamically regulated during plant development remains unclear. Here, we show that DELLAs are modified by the O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT) SECRET AGENT (SEC) in Arabidopsis. O-GlcNAcylation of the DELLA protein REPRESSOR OF ga1-3 (RGA) inhibits RGA binding to four of its interactors-PHYTOCHROME-INTERACTING FACTOR3 (PIF3), PIF4, JASMONATE-ZIM DOMAIN1, and BRASSINAZOLE-RESISTANT1 (BZR1)-that are key regulators in light, jasmonate, and brassinosteroid signaling pathways, respectively. Consistent with this, the sec-null mutant displayed reduced responses to GA and brassinosteroid and showed decreased expression of several common target genes of DELLAs, BZR1, and PIFs. Our results reveal a direct role of OGT in repressing DELLA activity and indicate that O-GlcNAcylation of DELLAs provides a fine-tuning mechanism in coordinating multiple signaling activities during plant development.

Genome-wide identification of nodule-specific transcripts in the model legume Medicago truncatula.

The Medicago truncatula expressed sequence tag (EST) database (Gene Index) contains over 140,000 sequences from 30 cDNA libraries. This resource offers the possibility of identifying previously uncharacterized genes and assessing the frequency and tissue specificity of their expression in silico. Because M. truncatula forms symbiotic root nodules, unlike Arabidopsis, this is a particularly important approach in investigating genes specific to nodule development and function in legumes. Our analyses have revealed 340 putative gene products, or tentative consensus sequences (TCs), expressed solely in root nodules. These TCs were represented by two to 379 ESTs. Of these TCs, 3% appear to encode novel proteins, 57% encode proteins with a weak similarity to the GenBank accessions, and 40% encode proteins with strong similarity to the known proteins. Nodule-specific TCs were grouped into nine categories based on the predicted function of their protein products. Besides previously characterized nodulins, other examples of highly abundant nodule-specific transcripts include plantacyanin, agglutinin, embryo-specific protein, and purine permease. Six nodule-specific TCs encode calmodulin-like proteins that possess a unique cleavable transit sequence potentially targeting the protein into the peribacteroid space. Surprisingly, 114 nodule-specific TCs encode small Cys cluster proteins with a cleavable transit peptide. To determine the validity of the in silico analysis, expression of 91 putative nodule-specific TCs was analyzed by macroarray and RNA-blot hybridizations. Nodule-enhanced expression was confirmed experimentally for the TCs composed of five or more ESTs, whereas the results for those TCs containing fewer ESTs were variable.