LAUSR

Nutrient sensing and signaling are critical for plants to coordinate growth and development in response to nutrient availability. Plant ACT DOMAIN REPEAT (ACR) proteins have been proposed to serve as nutrient sensors, but their functions remain largely unknown. Here, we showed that Arabidopsis (Arabidopsis thaliana) ACR9 might function as a repressor in glucose (Glc) signaling pathways. ACR9 was highly expressed in the leaves, and its expression was down-regulated by sugars. Interestingly, the acr9-1 and acr9-2 T-DNA insertion mutants were hypersensitive to Glc during seedling growth, development, and anthocyanin accumulation. Nitrogen deficiency increased the mutants' sensitivity to Glc. The expression of sugar-responsive genes was also significantly enhanced in the acr9 mutants. By contrast, the 35S:ACR9 and 35S:ACR9-GFP overexpression lines were insensitive to Glc during early seedling development. The Glc signaling pathway is known to interact with the plant hormone abscisic acid (ABA). Notably, the acr9 mutants were also hypersensitive to ABA during early seedling development. The Glc sensor HEXOKINASE1 (HXK1) and the energy sensor SUCROSE NON-FERMENTING1 (SNF1)-RELATED PROTEIN KINASE1 (SnRK1) are key components of the Glc signaling pathways. The acr9-1/hxk1-3 and acr9-1/snrk1 double mutants were no longer hypersensitive to Glc, indicating that functional HXK1 and SnRK1 were required for the acr9-1 mutant to be hypersensitive to Glc. Together, these results suggest that ACR9 is a repressor of the Glc signaling pathway, which may act independently or upstream of the HXK1-SnRK1 signaling module.