The IAA7-ARF7-ARF19 auxin signaling module plays diverse roles in Arabidopsis growth and development

The IAA7-ARF7-ARF19 regulatory module employs Phox and Bem1 (PB1) domain-mediated interactions to suppress ARF7/ARF19 transcriptional activation, thereby diversely regulating auxin-dependent growth and development. Auxin plays a key role in regulating diverse plant developmental processes through canonical signaling pathways mediated by Auxin Response Factors (ARFs) and Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) proteins. However, the ARF partners and their functional contribution to IAA7-mediated plant growth and auxin response remain poorly understood. Here we show that IAA7 and ARF7 or ARF19 are co-expressed across Arabidopsis development. IAA7 can interact physically with ARF7 and ARF19, and this interaction suppresses the transcriptional activation activity of both ARFs in auxin signaling. Genetic and phenotypic analyses reveal that ARF7 and ARF19 are major contributors to IAA7-mediated growth and development, including auxin sensitivity and gravitropism of root, root hair formation and elongation, and lateral root formation. In contrast, these ARFs only partially regulate IAA7-mediated hypocotyl gravitropism, root and inflorescence stem elongation, and leaf morphology. However, IAA7 regulates hypocotyl auxin sensitivity through ARF activators distinct from ARF7 and ARF19. Our results reveal the diverse degree of contribution by the ARF7 and ARF19, being pairs of IAA7, in regulating plant growth, and highlight the significance of multiple parallel IAA-ARF modules existing in the plant.