Auxin fluctuation and PIN polarization in moss leaf cell reprogramming.

Auxin and its PIN-FORMED (PIN) exporters are essential for tissue repair and regeneration in flowering plants. To gain insight into the evolution of this mechanism, we investigated their roles in leaves excised from Physcomitrium patens, a bryophyte known for its remarkable cell reprogramming capacity. We used various approaches to manipulate auxin levels, including exogenous application, pharmacological manipulations, and auxin biosynthesis mutants. We observed no significant effect on the rate of cell reprogramming. Rather, our analysis of auxin dynamics revealed a decrease in auxin levels upon excision, which was followed by a local increase before the reprogramming process began. Mutant analysis revealed that PpPINs are required for effective cell reprogramming, and endogenously expressed PpPINA-GFP accumulates polarly at sites that will develop into future filamentous stem cells. In addition, hyperpolarized PpPINA variants carrying mutated phosphorylation sites showed a marked delay in reprogramming, whereas endogenous or non-polar versions do not have this effect. These results underscore that both, the levels and the polarity of PpPINA are important for efficient cell reprogramming. Overall, these findings highlight the pivotal role of PIN polarity in plant regeneration. Furthermore, they suggest that understanding polarity mechanisms could have broader implications for improving regenerative processes across various plant species.