Mild High Concentrations of Boric Acid Repress Leaf Formation and Primary Root Growth in Arabidopsis Seedlings While Showing Anti-apoptotic Effects in a Mutant with Compromised Cell Viability

Boron (B) has been considered either as a nutrient or as a toxic non-essential element for plants and considerable debate arose recently regarding its functions and mechanisms of action. To gain further detail on the roles of boron in growth, development, and cell viability, Arabidopsis wild-type and mediator18 mutants, these later showing genetically fixed cell death in the pro-vasculature of roots, were germinated and grown side by side in agar-solidified plates with a standard nutrient solution supplemented with increasing concentrations (0.25-8 mM) of boric acid (BA). In the WT and med18-1 mutants, BA exerted a dose-dependent inhibition of leaf formation and primary root growth, but did not significantly promote root branching in the WT or med18-1 mutants, these later manifesting an enhanced lateral root formation capacity under a wide range of BA concentrations. Although the overall effect of BA in roots was growth repressing, no signs of cell death in meristems of primary roots of WT seedlings could be appreciated, instead, it appears to diminish the cell death manifested in pro-vasculature of med18 mutants, which correlates with reduced expression of the ERF115 transcription factor, which is induced in inner tissues upon damage, and recovery of auxin-inducible gene expression within the root tip. Irrespective of the overall growth-repressing effects in the shoot and root systems, it seems clear that important protective functions are orchestrated by BA, which supports cell viability in a mutant with spontaneous tissue damage.