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Cell wall integrity control in plants involves multiple signaling modules that are mostly defined by genetic interactions. The putative co-receptors FEI1 and FEI2 and the extracellular glycoprotein FLA4 present the core components of a signaling pathway that acts in response to environmental conditions and insults to cell wall structure to modulate the balance of various growth regulators and, ultimately, to regulate the performance of the primary cell wall. Although the previously established genetic interactions are presently not matched by intermolecular binding studies, numerous receptor-like molecules that were identified in genome-wide interaction studies potentially contribute to the signaling machinery around the FLA4-FEI core. Apart from its function throughout the model plant Arabidopsis thaliana for the homeostasis of growth and stress responses, the FLA4-FEI pathway might support important agronomic traits in crop plants.

We previously suggested that At-FLA4 and ABA signaling act in synergy. Reactive oxygen species generated from the NADPH oxidases At-RBOHD and At-RBOHF play an important role in cell wall integrity control and ABA signaling and here we investigate their role for the At-FLA4 pathway. We find that in the At-fla4 At-rbohD At-rbohF triple mutant the root phenotype of At-fla4 is enhanced. Moreover, the abnormally high level of reactive oxygen species in At-fla4 mutant does not depend on AtRBOHD and -F. Likewise, suppression of the At-fla4 phenotype by ABA does not depend on the 2 oxidases. Consistent with their lack of effect on ROS level in At-fla4, transcript level of AtRBOHD and -F is reduced in the At-fla4 mutant background. Taken together, our findings suggest that neither At-RBOHD nor At-RBOHF is involved in the synergism between ABA and At-FLA4. Consistently, the oxidases and At-FLA4 act independently of each other in ROS control.

Background and Aims: The putative FASCICLIN-LIKE ARABINOGALACTAN PROTEIN 4 (At-FLA4) locus of Arabidopsis thaliana has previously been shown to be required for the normal growth of wild-type roots in response to moderately elevated salinity. However, the genetic and physiological pathway that connects At-FLA4 and normal root growth remains to be elucidated., Methods: The radial swelling phenotype of At-fla4 was modulated with growth regulators and their inhibitors. The relationship of At-FLA4 to abscisic acid (ABA) signalling was analysed by probing marker gene expression and the observation of the At-fla4 phenotype in combination with ABA signalling mutants., Key Results: Application of ABA suppresses the non-redundant role of At-FLA4 in the salt response. At-FLA4 positively regulates the response to low ABA concentration in roots and is required for the normal expression of ABA- and abiotic stress-induced genes. The At-fla4 phenotype is enhanced in the At-abi4 background, while two genetic suppressors of ABA-induced gene expression are required for salt oversensitivity of At-fla4. Salt oversensitivity in At-fla4 is suppressed by the CYP707A inhibitor abscinazole E2B, and salt oversensitivity in At-fla4 roots is phenocopied by chemical inhibition of ABA biosynthesis., Conclusions: The predicted lipid-anchored glycoprotein At-FLA4 positively regulates cell wall biosynthesis and root growth by modulating ABA signalling., (© The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company.)