Author response: Cell type boundaries organize plant development

Stable establishment of organ polarity occurs several plastochrons before primordium outgrowth in Arabidopsis

“…Although results in tomato and potato suggested that the dorsoventral patterning was not very robust during the initiation process itself (Sussex, 1951;Reinhardt et al, 2005), we show here that, even at a stage when major regulators, such as FIL or PRS, are not yet activated, polarity is already robustly established and maintained. Indeed, although the ablations cause major changes in local signaling and even can affect gene expression (Heisler et al, 2010;Caggiano et al, 2017;Zhao et al, 2019), we did not see any effect on leaf polarity, suggesting that an external signaling gradient does not have a role once pDRNL is activated. Further work is now required using additional markers to show that there are no moresubtle effects on ad/abaxial polarity.…”

“…To understand this early establishment of polarity, the results reported by Caggiano et al (2017) are particularly relevant. They showed the existence of concentric expression zones around the meristem center of genes involved in organ polarity, a domain weakly expressing the adaxial determinant REV in the meristem center, surrounded by cells expressing the abaxial gene KAN1 at the meristem periphery.…”

“…The network also involves regulation via small RNAs (Kuhlemeier and Timmermans, 2016). Some of the polarity genes are expressed very early, and REV and KAN1 in particular might define a prepattern (Caggiano et al, 2017) even before the upregulation of organ founder cell markers, such as DORNROESCHEN-LIKE (DRNL) (Chandler et al, 2011) or ARABIDOPSIS PHOSPHOTRANSFER PROTEIN 6 (AHP6) (Besnard et al, 2014). If the regulatory network controlling polarity is impaired, leaf and leaf-like organs lose their bilateral symmetry and tend to become axisymmetric (Yamaguchi et al, 2012;Kuhlemeier and Timmermans, 2016).…”

“…Therefore, Snow and Snow suggested that cuts leading to a loss in polarity inadvertently caused a reduction in size of the primordia in the radial direction, which then led to abnormal development. More recently, several, more elaborated hypotheses have been brought forward to explain the processes leading to dorsoventrality (Husbands et al, 2009;Caggiano et al, 2017), although all of them require some form of meristem-based signal or information. From a conceptual point of view, Koch and Meinhardt (1994) proposed a model that combined the existence of an organ-inhibiting gradient around the meristem center, with more local, self-organizing patterning processes within the initium itself as soon as the cells leave the inhibiting field.…”

“…From a conceptual point of view, Koch and Meinhardt (1994) proposed a model that combined the existence of an organ-inhibiting gradient around the meristem center, with more local, self-organizing patterning processes within the initium itself as soon as the cells leave the inhibiting field. Although this has remained purely hypothetical, findings of Caggiano et al (2017) in Arabidopsis suggest a more complex network of interactions involving auxin transport and pre-existing gradients of several transcription factors, such as KAN1 and REV. The same authors performed a number of laser ablations between the meristem and incipient primordia and showed that the wounds inflicted by cutting can locally induce the expression of certain polarity genes, such as KAN1.…”