A. thaliana hybrids develop growth abnormalities through integration of stress, hormone and growth signalling

Hybrids between A. thaliana accessions are important in revealing the consequences of epistatic interactions in plants. F1 hybrids between the A. thaliana accessions displaying either defence or developmental phenotypes have been revealing the roles of the underlying epistatic genes. The interaction of two naturally occurring alleles of the OUTGROWTH ASSOCIATED KINASE (OAK) gene in Sha and Lag2-2, previously shown to cause a similar phenotype in a different allelic combination in A. thaliana, was required for the hybrid phenotype. Outgrowth formation in the hybrids was associated with reduced levels of SA, JA, and ABA in petioles and application of these hormones mitigated the formation of the outgrowths. Moreover, different abiotic stresses were found to mitigate the outgrowth phenotype. The involvement of stress and hormone signalling in outgrowth formation was supported by a global transcriptome analysis, which additionally revealed that TCP1, a transcription factor known to regulate leaf growth and symmetry, was downregulated in the outgrowth tissue. These results demonstrate that a combination of natural alleles of OAK regulates growth and development through the integration of hormone and stress signals and highlight the importance of natural variation as a resource to discover the function of gene variants that are not present in the most studied accessions of A. thaliana.