Fig. 6

The simplified schema and functions of SAUR genes in plant evolution. During the process of plant evolution, SAUR gene family from bryophyta to anthophyta expended largely, and every clade experienced expansion at different degrees (left panel). Right panel of Fig. 6 showed the function of SAUR genes in response to hormonal and environmental signals: based on acid growth mediated by plasma membrane H⁺-ATPase, auxin-responsive SAUR proteins (AtSAUR 19- and AtSAUR 26-subclades in clade 1) activate plasma membrane H⁺-ATPase by inhibiting PP2C-D phosphatases to promote cell expansion [31]. The AtSAUR76-subclade in clade 9 mediates ethylene signaling via interaction with Ethylene Insensitive 4 (EIN4) and Ethylene Receptor 2 (ETR2) to promote plant growth [32]. The SAUR41-subclade is inducible by abscisic acid to modulate cell expansion and salt tolerance, and maintain calcium homeostasis [24]. Auxin Response Factor 6 (ARF6), Brassinazole Resistant1 (BZR1) and Phytochrome Interacting Factor 4 (PIF4) may directly regulate transcription of AtSAUR19-subclade and AtSAUR26-subclade independently or as a complex [33] sensing hormonal and environmental signals including auxin, brassinosteroids (BR), gibberellin (GA), jasmonic acid (JA), light, heat and shade [25, 28, 33,34,35,36,37,38,39,40]. Arabidopsis zinc-finger protein 1 (AZF1) and AZF2 in the ABA-response pathway can repress the expression of AtSAUR63 subclade [41]