Perturbation Of Auxin Homeostasis By Manipulating AtIAA15Expression Results In Impaired Root Stem Cell Development And Gravitropism

Quiescent Center (QC), together with its neighboring cells form root stem cell niche which functions as the reservoir of all root cells. The maintaince and differentiation of root stem cell niche reach a balance under the control of phytohormone Auxin, peptides hormones and several transcription factors, namely SHORT-ROOT (SHR), SCARECROW (SCR), WUSCHEL-RELATEDHOMEOBOX5(WOX5) and PLETHORA (PLT). Here we show that AtIAA15, which belongs to auxin signaling repressor AUX/IAAs family, participates in the maintaince-differentiation balance of root stem cell niche when ectopicly expressed. Besides, gain-of-function mutation of AtiAA15failed to response to gravity possibly results from perturbation of Auxin homeostasis. The main results are:1Overexpression of AtIAA15led to several developmental defects. On the aerial parts, mutants produced small curled leaves, short inflorescences with reduced apical dominance, and short crinkled siliques with fewer seeds. On the underground parts, ectopic expression of AtIAA15gave rise to agravitropic roots with small meristem size, additionally, there were more undifferentiated columella stem cell (CSC) layers and reduced columella cell layers in root cap. On the other hand, knock down of AtIAA15resulted in no significant developmental changes in our obserbation.2The staining of auxin marker line DR5::GUS in mutant roots expanded to proximal meristem and elongation zone compared to control, indicates the impairation of Polar Auxin Transport (PAT). Indeed, Auxin influx carrier AUX1::YFP and efflux carrier PIN2::GFP, PIN3::GFP, PIN7::GFP were substantially decreased while no changes found in PIN1::GFP expression level. In addition, when mutant seedlings were placed horizontally, auxin could not effectively distribute to the new bottom of root tip to turn physically signal to physiological signal, which is vital to gravitropic response.3In line with less differentiated root columella, WOX5::GFP, which marks Quiescent Center (QC), spreaded to adjacent cells. Meanwhile columella stem cell marker J2341showed irregular expression, often with expansion to additional cell layers, suggests the failure of differentiation of CSC. More importantly, the meristem cell determinant, AP2-domain transcription factor PLETHORA (PLT), decreased dramatically as marked by the PLT1::CFP fluorescence and resulted in short meristem in AtIAA15overexpression lines.4Using GUS as report gene, we found AtIAA15expressed in root meristem and root cap, lateral root primordium, shoot apical meristem and leaf veins. In reproductive organs, weak expression of AtIAA15could be seen in stigma and the tip of siliques. At the subcellular level, GFP-IAA15protein localized in nuclear as most AUX/IAAs did.5In yeast two hybrid assays, among all auxin response factors (ARFs) tested, IAA15only interacted with ARF5and ARF6.