ESR2–HDA6 complex negatively regulates auxin biosynthesis to delay callus initiation in Arabidopsis leaf explants during tissue culture

Plants exhibit an astonishing ability to regulate organ regeneration upon wounding.Excision of leaf explants promotes the biosynthesis of indole-3-acetic acid(IAA),which is polar-transported to excised regions,where cell fate transition leads to root founder cell specification to induce de novo root regeneration.The regeneration capacity of plants has been utilized to develop in vitro tissue culture technologies.Here,we report that IAA accumulation near the wounded site of leaf explants is essential for callus formation on 2,4-dichlorophenoxyacetic acid(2,4-D)-rich callus-inducing medium(CIM).Notably,a high concentration of 2,4-D does not compensate for the action of IAA because of its limited efflux;rather,it lowers IAA biosynthesis via a negative feedback mechanism at an early stage of in vitro tissue culture,delaying callus initiation.The auxin negative feedback loop in CIM-cultured leaf explants is mediated by an auxin-inducible APETALA2 transcription factor,ENHANCER OF SHOOT REGENERATION 2(ESR2),along with its interacting partner HISTONE DEACETYLASE 6(HDA6).The ESR2–HDA6 complex binds directly to,and removes the H3ac mark from,the YUCCA1(YUC1),YUC7,and YUC9 loci,consequently repressing auxin biosynthesis and inhibiting cell fate transition on 2,4-D-rich CIM.These findings indicate that negative feedback regulation of auxin biosynthesis by ESR2 and HDA6 interferes with proper cell fate transition and callus initiation.

Adenosine monophosphate enhances callus regeneration competence for de novo plant organogenesis

Dear Editor,Plant tissue culture involves callus formation and de novo shoot regeneration.First,explants from differentiated tissues are used to generate a pluripotent cell mass,called callus,on auxin-rich callus-inducing medium(CIM),followed by shoot regeneration on cytokinin-rich shoot-inducing medium(SiM).Callus results from division of pericycle-like cells(Atta et al.,2009;Sugimoto et al.,2010);its cellular identity resembles that of lateral root primordia(Atta et al.,2009;Sugimoto et al.,2010).Callus acquires cellular pluripotency by forming root stem cell niches on CIM(Sugimoto et al.,2010)。