Auxin Response Factors MediateArabidopsisOrgan Asymmetry via Modulation of KANADI Activity

Abstract

Members of the KANADI gene family in Arabidopsis thaliana regulate abaxial identity and laminar growth of lateral organs. Promoter APETALA3-mediated ectopic expression of KANADI restricts petal expansion and was used in a genetic screen for factors involved in KANADI-mediated signaling. Through this screen, mutations in ETTIN (ETT; also known as Auxin Response Factor3 [ARF3]) were isolated as second site suppressors and found to ameliorate ectopic KANADI activity throughout the plant as well. Mutant phenotypes of ett are restricted to flowers; however, double mutants with a closely related gene ARF4 exhibit transformation of abaxial tissues into adaxial ones in all aerial parts, resembling mutations in KANADI. Accordingly, the common RNA expression domain of both ARFs was found to be on the abaxial side of all lateral organs. Truncated, negatively acting gene products of strong ett alleles map to an ARF-specific, N-terminal domain of ETT. Such gene products strongly enhance abaxial tissue loss only when ARF activities are compromised. As KANADI is not required for either ETT or ARF4 transcription, and their overexpression cannot rescue kanadi mutants, cooperative activity is implied. ARF proteins are pivotal in mediating auxin responses; thus, we present a model linking transient local auxin gradients and gradual partitioning of lateral organs along the abaxial/adaxial axis. INTRODUCTIONFormation of lateral organs, such as leaves and floral organs at the flanks of shoot and floral apices, involves the specification of a group of initial cells patterned distinctively relative to the meristem. Organ primordium cells adjacent to the meristem will develop adaxial characteristics and give rise to the upper layers, while cells away from the meristem will have abaxial characteristics and give rise to the lower organ layers. The physical continuum of the primordial adaxial domain with the meristem is also manifested in the expression and role of the PHABULOSA (PHB)-like class III HD-ZIP proteins in apical meristem establishment and maintenance as well as adaxial organ identity (McConnell and Barton, 1998;Emery et al., 2003;Prigge et al., 2005). Elaboration of the lateral organs into flat, laterally expanding organs requires concerted exclusion of the PHB-like activities from the abaxial domain and of the abaxial factors from the adaxial domain. The abaxial repression of the PHB-like adaxial factors is mediated by activities of the GARP-type transcription factors, KANADIs, and by the activities of a group of microRNAs (miR165/6; Eshed et al., 2001Eshed et al., , 2004McConnell et al., 2001;Emery et al., 2003;Juarez et al., 2004;Kidner and Martienssen, 2004;Mallory et al., 2004;McHale and Koning, 2004). Whether the activities of the KANADI proteins and miR165/6 are connected is presently unknown. RNA in situ analyses suggest that initial restriction of the adaxial factors takes place between the onset of primordium initiation (P0) and outgrowth (P1) reflected by abaxial exclusion of PHB and REV Kerstetter et al., 2001;McCo...

RiceLHS1/OsMADS1Controls Floret Meristem Specification by Coordinated Regulation of Transcription Factors and Hormone Signaling Pathways

“…ett arf4 double mutants have several lateral organ defects, such as short sepals with abaxial protrusions, narrow petals, sterile stamens, and gynoecia with distal abaxial and adaxial ovules, thus showing that these genes act in concert. Interestingly, these phenotypes are also seen in kan1 kan2 double mutants (Pekker et al, 2005). Recently, physical interactions between KANADI4 (KAN4/ ATS) and ETTIN were shown to coordinate ovule and integument development (Kelley et al, 2012).…”

RNA-Dependent RNA Polymerase 6 Is Required for Efficient hpRNA-Induced Gene Silencing in Plants

“…1C). Auxin response factors ARF3 and ARF4 are known to regulate a subset of the traits associated with the vegetative-toreproductive transition and leaf heteroblasty (Pekker et al, 2005). It has also been reported that the production of tasiR-ARF, a trans-acting siRNA that targets both ARF3 and ARF4, requires P-labeled full-length GFP cDNA probe.…”

The Function of RNAi in Plant Development

“…Strong ett/arf3 mutations simplify the shape of spongy mesophyll cells in a manner similar to that of kan1, and ett/arf3 arf4 double mutants resemble kan1 kan2 double mutants. Furthermore, ett/arf3 is able to suppress the floral phenotype of KAN1 overexpression (Pekker et al 2005). Because many of the traits that change during vegetative development are polarized (e.g., trichome production and leaf curvature), it may be that temporal changes in the regulation of leaf polarity are the basis for the phase-specific expression pattern of these traits.…”

“…Mutations in ETT/ARF3 were originally identified because of their effects on carpel polarity (Sessions and Zambryski 1995;Sessions et al 1997). Subsequent studies showed that ETT/ARF3 and ARF4 regulate leaf polarity as well, probably in conjunction with the KAN family of transcription factors (Pekker et al 2005;Hunter et al 2006). Strong ett/arf3 mutations simplify the shape of spongy mesophyll cells in a manner similar to that of kan1, and ett/arf3 arf4 double mutants resemble kan1 kan2 double mutants.…”