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Glutamate triggers defense responses and promotes root hair elongation in Arabidopsis
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DOI：10.1093/plphys/kiag484.png)
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Abstract
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Glutamate (Glu), a key amino acid in nitrogen (N) metabolism, also functions as a signaling molecule in plants. When supplied as the sole N source, Glu acts both as an N nutrient and as a signaling molecule in Arabidopsis, depending on the physiological context. Compared with NH4NO3, Glu inhibited primary root growth, which was associated with reduced auxin responses and diminished stem cell niche activity in the root apex, likely reflecting Glu’s signaling role. In contrast, Arabidopsis seedlings grown on Glu accumulated more anthocyanins than those grown on NH4NO3, possibly reflecting its metabolic function. Glu differentially regulates Gln synthetase (GS) genes, inducing cytosolic GS1 while repressing chloroplastic GS2, suggesting transcriptional regulation independent of substrate availability. Glu also altered amino acid profiles, increasing Glu and Glu-dependent amino acids while decreasing Gln and Gln-derived amino acids, consistent with limited NH4+ availability in Arabidopsis seedlings grown on Glu as the sole N source. Transcriptome analysis revealed that Glu preferentially induced defense-related genes and repressed photosynthesis-associated genes relative to NH4NO3. Notably, Glu rapidly induced defense-related transcripts within 30 min in N-starved seedlings, supporting Glu’s signaling role associated with induced defense responses. In addition, Glu promoted root hair elongation by repressing GLABRA2 and activating the ROOT HAIR DEFECTIVE6 signaling pathway. This effect involved ethylene and salicylic acid signaling and persisted in the presence of NH4NO3. Together, our findings demonstrate that extracellular Glu functions both as an N nutrient and as a signaling molecule that coordinates growth, defense, and root hair development in Arabidopsis.
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