textlesssectextgreater textlesstitletextgreaterBackgroundtextless/titletextgreater textlessptextgreaterMutations in the textlessitalictextgreaterFRY1/SAL1textless/italictextgreater Arabidopsis locus are highly pleiotropic, affecting drought tolerance, leaf shape and root growth. textlessitalictextgreaterFRY1textless/italictextgreater encodes a nucleotide phosphatase that textlessitalictextgreaterin vitrotextless/italictextgreater has inositol polyphosphate 1-phosphatase and 3′,(2′),5′-bisphosphate nucleotide phosphatase activities. It is not clear which activity mediates each of the diverse biological functions of FRY1 textlessitalictextgreaterin plantatextless/italictextgreater.textless/ptextgreater textless/sectextgreater textlesssectextgreater textlesstitletextgreaterPrincipal Findingstextless/titletextgreater textlessptextgreaterA textlessitalictextgreaterfry1textless/italictextgreater mutant was identified in a genetic screen for Arabidopsis mutants deregulated in the expression of textlessunderlinetextgreaterPtextless/underlinetextgreateri textlessunderlinetextgreaterHtextless/underlinetextgreaterigh affinity textlessunderlinetextgreaterTtextless/underlinetextgreaterransporter 1;4 (textlessitalictextgreaterPHT1;4textless/italictextgreater). Histological analysis revealed that, in roots, textlessitalictextgreaterFRY1textless/italictextgreater expression was restricted to the stele and meristems. The textlessitalictextgreaterfry1textless/italictextgreater mutant displayed an altered root architecture phenotype and an increased drought tolerance. All of the phenotypes analyzed were complemented with the textlessitalictextgreaterAHLtextless/italictextgreater gene encoding a protein that converts 3′-polyadenosine 5′-phosphate (PAP) into AMP and Pi. PAP is known to inhibit exoribonucleases (XRN) textlessitalictextgreaterin vitrotextless/italictextgreater. Accordingly, an textlessitalictextgreaterxrntextless/italictextgreater triple mutant with mutations in all three XRNs shared the textlessitalictextgreaterfry1textless/italictextgreater drought tolerance and root architecture phenotypes. Interestingly these two traits were also complemented by grafting, revealing that drought tolerance was primarily conferred by the rosette and that the root architecture can be complemented by long-distance regulation derived from leaves. By contrast, textlessitalictextgreaterPHT1textless/italictextgreater expression was not altered in textlessitalictextgreaterxrntextless/italictextgreater mutants or in grafting experiments. Thus, textlessitalictextgreaterPHT1textless/italictextgreater up-regulation probably resulted from a local depletion of Pi in the textlessitalictextgreaterfry1textless/italictextgreater stele. This hypothesis is supported by the identification of other genes modulated by Pi deficiency in the stele, which are found induced in a textlessitalictextgreaterfry1textless/italictextgreater background.textless/ptextgreater textless/sectextgreater textlesssectextgreater textlesstitletextgreaterConclusions/Significancetextless/titletextgreater textlessptextgreaterOur results indicate that the 3′,(2′),5′-bisphosphate nucleotide phosphatase activity of FRY1 is involved in long-distance as well as local regulatory activities in roots. The local up-regulation of textlessitalictextgreaterPHT1textless/italictextgreater genes transcription in roots likely results from local depletion of Pi and is independent of the XRNs.textless/ptextgreater textless/sectextgreater