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Subgroup 4 (Sg4) members of the R2R3-MYB are generally known as negative regulators of the phenylpropanoid pathway in plants. Our previous research showed that a R2R3-MYB Sg4 member from Camellia sinensis (CsMYB4a) inhibits expression of some genes in the phenylpropanoid pathway, but its physiological function in the tea plant remained unknown. Here, CsMYB4a was found to be highly expressed in anther and filaments, and participated in regulating filament growth. Transcriptome analysis and exogenous auxin treatment showed that the target of CsMYB4a might be the auxin signal pathway. Auxin/indole-3-acetic acid 4 (AUX/IAA4), a repressor in auxin signal transduction, was detected from a yeast two-hybrid screen using CsMYB4a as bait. Gene silencing assays showed that both CsIAA4 and CsMYB4a regulate filament growth. Tobacco plants overexpressing CsIAA4 were insensitive to exogenous α-NAA, consistent with overexpression of CsMYB4a. Protein-protein interaction experiments revealed that CsMYB4a interacts with N-terminal of CsIAA4 to prevent CsIAA4 degradation. Knock out of the endogenous NtIAA4 gene, a CsIAA4 homolog, in tobacco alleviated filament growth inhibition and α-NAA insensitivity in plants overexpressing CsMYB4a. All results strongly suggest that CsMYB4a works synergistically with CsIAA4 and participates in regulation of the auxin pathway in stamen. [ABSTRACT FROM AUTHOR]