Transcription Factors WOX11/12 Directly Activate WOX5/7 to Promote Root Primordia Initiation and Organogenesis

Abstract

De novo organogenesis, which gives rise to adventitious roots and shoots, is a type of plant regeneration for survival after wounding. In Arabidopsis (Arabidopsis thaliana), two main cell fate transition steps are required to establish the root primordium during de novo root organogenesis from leaf explants. The first step from regeneration-competent cells to root founder cells involves activation of WUSCHEL-RELATED HOMEOBOX11 (WOX11) and WOX12 (WOX11/12) expression by auxin. However, the molecular mechanism controlling the second step of fate transition from root founder cells to root primordium is poorly understood. In this study, we show that the expression levels of WOX11/12 decrease while those of WOX5 and 7 (WOX5/7) increase during the transition from root founder cells to the root primordium. WOX11/12 function genetically upstream of WOX5/7, and the WOX11/12 proteins directly bind to the promoters of WOX5/7 to activate their transcription. Mutations in WOX5/7 result in defective primordium formation. Overall, our data indicate that the expression switch from WOX11/12 to WOX5/7 is critical for initiation of the root primordium during de novo root organogenesis.

Regulation of Hormonal Control, Cell Reprogramming, and Patterning during De Novo Root Organogenesis

“…WUSCHEL-RELATED HO-MEOBOX11 (WOX11) expression has been associated to first cell-fate transition from regenerationcompetent cells to root founder cells during leaf blade rooting Hu and Xu, 2016). WOX11 activates WOX5 during root formation in leaf blades; however, we did not find WOX5 expression in endogenous callus during de novo regeneration from whole leaves, suggesting that WOX11 could be involved in an earlier step in the reprograming process.…”

“…Thus, we hypothesized that root founder cells (RFCs) could be specified within the endogenous callus to de novo form a root. It has been proposed that RFCs during de novo root formation in leaf blades could be marked by Pro WOX11 :GUS expression Hu and Xu, 2016). We observed discrete Pro WOX11 :GUS signals early detected, at 1 dae, in a few xylemassociated cells at the petiole base (Fig.…”

“…We found severe impairment in de novo primordia initiation in shr-2 plt1-4 plt2-2, and WOX5 expression during de novo primordia formation requires auxin input through an unknown pathway (Hu and Xu, 2016). Therefore, it is possible that PLT1 and PLT2 are required for specification of WOX5-marked cells downstream of auxin during de novo organ initiation, which could occur in combination with activity of WOX5 transcription factor itself and/or SHR, in turn acting in an auxin-independent pathway.…”

“…To follow primordia formation, we used Pro WOX5 :GUS. WOX5 is expressed early during lateral root primordia formation (Tian et al, 2014;Goh et al, 2016), and also after root primordia initiation during de novo organogenesis Hu and Xu, 2016). Expression of Pro WOX5 :GUS was first detected at 4 dae in clusters of cells within the endogenous callus (Fig.…”

“…WOX11 and its homolog WOX12 can in addition promote callus formation and up-regulate the callus formation factors LBD16 and LDB29 (Fan et al, 2012;Liu et al, 2014), suggesting that de novo root formation might share similar regulatory mechanisms with callus formation. Subsequently in leaf blade rooting, WOX11 and WOX12 activate WOX5 and WOX7 factors, which are expressed in dividing cells forming root primordia, whereas WOX11/12 expression quickly decreases in dividing cells Hu and Xu, 2016). Activation and maintenance of WOX5/7 expression also requires auxin signaling in an unknown pathway different from WOX11/12.…”