Abstract
Plants are able to sense and mediate the balance between carbon (C) and nitrogen (N) nutrient availability to optimize metabolism and growth, described as the C/N response. To clarify the C/N signalling mechanism, C/N-insensitive plants were obtained from an Arabidopsis FOX hunting population, which over-expresses full-length cDNAs for individuals. The resulting cni2-D (carbon/nitrogen insensitive 2-dominant) plant was found to overcome the post-germination growth checkpoint and to expand green cotyledons in disrupted high C/low N stress conditions. The CNI2 gene encodes ABI1, a phosphatase type 2C protein, which negatively regulates abscisic acid (ABA) signal transduction. Over-expressors of ABI1 were found to be insensitive to disrupted C/N stress, whereas the loss-of function mutant abi1-2 was hypersensitive, suggesting that ABI1 plays an essential role in the plant C/N response. By contrast, the C/N-dependent growth phenotype observed in wild-type plants was not associated with endogenous ABA content. Accordingly, the ABA-insensitive mutant abi1-1, which could not bind to the ABA-ABA receptor complex, was not insensitive and restored normal sensitivity to high C/low N stress. The canonical ABA signalling mutants abi4 and abi5 were also sensitive to disrupted C/N stress. Further gene expression analysis demonstrated that several genes in the SnRK2s and SnRK1s pathways are transcriptionally affected by high C/low N stress in wild-type plants regardless of the lack of increased endogenous ABA contents, whereas the expression of these genes were significantly suppressed in ABI1 over-expressors. Taken together, these results suggest direct cross-talk between C/N and non-canonical ABA signalling pathways, regulated by ABI1, in plants.
Highlights
Plant growth and development are controlled by many environmental factors and stresses, including nutrition, light, drought, and osmotic stress
Further gene expression analysis demonstrated that several genes in the SnRK2s and SnRK1s pathways are transcriptionally affected by high C/low N stress in wild-type plants regardless of the lack of increased endogenous abscisic acid (ABA) contents, whereas the expression of these genes were significantly suppressed in ABI1 over-expressors
In abi1-1, the Gly180 residue is replaced by Asp, with the mutated ABI1 protein unable to bind to the ABA-receptor complex, resulting in constitutive inactivation of SnRK2 proteins (Santiago et al, 2012)
Summary
Plant growth and development are controlled by many environmental factors and stresses, including nutrition, light, drought, and osmotic stress. Carbon and nitrogen are essential for plants, being constituents of nutrients and metabolites that provide energy and serve as constitutive molecular backbones. These constitutive molecules possess hormone-like functions, transducing signals to regulate plant growth and development (Krouk et al, 2010; Smeekens et al, 2010; Stitt et al, 2010). Several genome-wide investigations have shown that carbon and nitrogen metabolites and signalling co-operatively control various pathways involved in plant growth and development, such as glycolysis/gluconeogenesis, the pentose-phosphate pathway, protein synthesis, protein degradation, protein targeting, and the regulation of protein activity (Palenchar et al, 2004; Gutiérrez et al, 2007). Despite the physiological importance of the C/N response, the molecular mechanisms mediated by C/N signals remain unclear
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