Our previous work in yeast has demonstrated that overexpression of FPR1, and others FKBP immunophilins, conferred tolerance to weak organic acids such as acetic and sorbic acid. FK506 binding proteins (FKBP) where originally identified as the cellular targets of the immunosuppressant drugs rapamycin an FK506. FKBPs are peptidyl-prolyl cis-trans isomerases (PPase EC 5.1.2.8) that catalize the isomerization of peptidyl prolyl bonds between cis and trans configuration. FKBP are ubiquitous proteins that can be found either as a single catalytic proteins or being part of more complex proteins. To assess the implication of FKBP proteins in weak acid tolerance in plants we have generated lines of Arabidopsis thaliana overexpressing two different proteins: yeast FPR1 an Arabidopsis FKBP65 (ROF2). We isolated knock-out mutant rof2 and rof1 from T-DNA mutant seeds collection of Salk institute. Finally we crossed the single mutants to get the double mutant rof1 x rof2. In presence of acetic acid transgenic lines overexpressing any of these genes grew better than wild type plant. On the other hand an AtFKBP65 loss-of-function mutant line showed weak acid sensitivity. We Observed a similar behavior in presence of toxic cations (Norspermidine, Hygromycim B) suggesting a role in K+ transport and we have got the confirmation with the growth al low levels of K+. We excluded the direct participation of plasma membrane ATPase because its activity in rof2 knock out mutant is higher. Furthermore ROF2 overexpression lines show a lower activity than wild-type. With 35S:: ROF2-GFP construction it was possible to see a cytosolic and nuclear cellular distribution that in presence of weak acid condition change: the ROF"-GFP leave the nuclear region. In absence of stress we have observed a gain of apical dominance in 35S::AtFKBP65 mutants and its loss in FKBP65 knock-out line. The roots of AtFKBP65 knock-out mutants have reduced number of lateral roots and exogenous application of IAA was abl