The ABA-INSENSITIVE-4 (ABI4) transcription factor links redox, hormone and sugar signaling pathways

Abstract

The cellular reduction-oxidation (redox) hub processes information from metabolism and the environment and so regulates plant growth and defense through integration with the hormone signaling network. One key pathway of redox control involves interactions with ABSCISIC ACID (ABA). Accumulating evidence suggests that the ABA-INSENSITIVE-4 (ABI4) transcription factor plays a key role in transmitting information concerning the abundance of ascorbate and hence the ability of cells to buffer oxidative challenges. ABI4 is required for the ascorbate-dependent control of growth, a process that involves enhancement of salicylic acid (SA) signaling and inhibition of jasmonic acid (JA) signaling pathways. Low redox buffering capacity reinforces SA- JA- interactions through the mediation of ABA and ABI4 to fine-tune plant growth and defense in relation to metabolic cues and environmental challenges. Moreover, ABI4-mediated pathways of sugar sensitivity are also responsive to the abundance of ascorbate, providing evidence of overlap between redox and sugar signaling pathways.

Figure 1. The role of the redox hub in the integration of environmental and metabolic cues. The redox hub comprising overlapping and interconnected redox signals integrates environmental and metabolic signals and is influenced by the generation of reactive oxygen species (ROS) at a range of spatio-temporal scales. Key modulators of the redox hub are the major soluble antioxidants ascorbate and glutathione whose content and redox status are dependent on both environmental and metabolic conditions. Accumulating evidence links the activity of the redox hub with hormonal signaling resulting in plant resistance to biotic and abiotic stresses.

Figure 2. Common and genotype-specific transcripts that were differentially expressed in the abi4, vtc2 and abi4vtc2 mutants relative to Col0. Whole Arabidopsis rosettes were harvested following six weeks growth in controlled environment chambers, RNA extracted and microarray analysis performed as described. Venn diagrams indicate genes that were commonly or uniquely differentially expressed in the three genotypes in comparison with Col0.

Figure 3. Expression of transcripts encoding defense related proteins responsive to SA (PR1, PR2, PR4, PR5) or JA/ET (PDF1.2a, PDF1.2b) in abi4, vtc2 and abi4vtc2 mutant Arabidopsis relative to Col0. Whole Arabidopsis rosettes were harvested following six weeks growth in controlled environment chambers, RNA extracted and microarray analysis performed as described.

Figure 4. A comparison of the phenotypes of the abi4, vtc2 and abi4vtc2 mutants relative to the wild type (Col0). All plants were grown in a controlled environment chamber for 7.5 weeks with a 10 h photoperiod as previously described.

Figure 5. Heatmap displaying relative metabolite content in Arabidposis Col0, abi4, vtc2 and abi4vtc2. Plants were grown for six weeks under controlled environments and individual leaves harvested from four replicate plants and immediately frozen on liquid nitrogen. Following lyophilisation, leaves were extracted and metabolites were derivatised for GC/MS as previously described. The figure shows metabolite content in each of the four replicates relative to the centered mean across all samples as previously described.