SRF3-bHLH Crosstalk in Arabidopsis Iron Homeostasis: Receptor Kinase Signaling, Transcriptional Regulation and Genetic Epistasis

Abstract

In this review, the available evidence that connects the SRF3 receptor kinase with the bHLH-driven Fe-deficiency regulatory network in Arabidopsis plants is summarized. The data currently available suggests that SRF3 and bHLH-mediated regulation are functionally convergent in the regulation of IRT1, but there is currently no evidence for direct genetic epistasis or physical interaction between these regulators. Three possible models are considered: SRF3 could act upstream of bHLH regulators; SRF3 could be active alongside the bHLH pathway; or SRF3 could have indirect effects on bHLH activity such as by phosphorylation, protein stability or subcellular localization. Based on these models, genetic interaction experiments between SRF3 and important bHLH mutants, as well as transcriptional, protein-level and phosphoproteomic experiments, are suggested to determine the pathway hierarchy and molecular connectivity. Further research is also needed to uncover the role of SRF3–bHLH crosstalk in coordinating iron homeostasis with plant immunity and how insights can be used to develop strategies for enhancing iron acquisition and nutritional quality in crops. Overall, the SRF3-bHLH relationship is a hypothesis to be tested, not a pathway