Summary: The increase in the ratio of root growth to shoot growth that occurs in response to phosphate (Pi) deprivation is paralleled by a decrease in cytokinin levels under the same conditions. However, the role of cytokinin in the rescue system for Pi starvation remains largely unknown. We have isolated a gene from Arabidopsis thaliana (AtIPS1) that is induced by Pi starvation, and studied the effect of cytokinin on its expression in response to Pi deprivation. AtIPS1 belongs to the TPSI1/Mt4 family, the members of which are specifically induced by Pi starvation, and the RNAs of which contain only short, non‐conserved open reading frames. Pi deprivation induces AtIPS1 expression in all cells of wild‐type plants, whereas in the pho1 mutant grown on Pi‐rich soils, AtIPS1 expression in the root was delimited by the endodermis. This supports the view that pho1 is impaired in xylem loading of Pi, and that long‐distance signals controlling the Pi starvation responses act via negative control. Exogenous cytokinins repress the expression of AtIPS1 and other Pi starvation‐responsive genes in response to Pi deprivation. However, cytokinins did not repress the increase in root‐hair number and length induced by Pi starvation, a response dependent on local Pi concentration rather than on whole‐plant Pi status. Our results raise the possibility that cytokinins may be involved in the negative modulation of long‐distance, systemically controlled Pi starvation responses, which are dependent on whole‐plant Pi status.

Summary: Low phosphorous availability, a common condition of many soils, is known to stimulate phosphatase activity in plants; however, the molecular details of this response remain mostly unknown. We purified and sequenced the N‐terminal region of a phosphate starvation induced acid phosphatase (AtACP5) from Arabidopsis thaliana, and cloned its cDNA and the corresponding genomic DNA. The nucleotide sequence of the cDNA predicted that AtACP5 is synthesised as a 338 amino acid‐long precursor with a signal peptide. AtACP5 was found to be related to known purple acid phosphatases, especially to mammal type 5 acid phosphatases. Other similarities with purple acid phosphatases, which contain a dinuclear metal centre, include the conservation of all residues involved in metal ligand binding and resistance to tartrate inhibition. In addition, AtACP5, like other type 5 acid phosphatases, displayed peroxidation activity. Northern hybridisation experiments, as well as in situ glucuronidase (GUS) activity assays on transgenic plants harbouring AtACP5:GUS translational fusions, showed that AtACP5 is not only responsive to phosphate starvation but also to ABA and salt stress. It is also expressed in senescent leaves and during oxidative stress induced by H₂O₂, but not by paraquat or salicylic acid. Given its bifunctionality, as it displays both phosphatase and peroxidation activity, we propose that AtACP5 could be involved in phosphate mobilisation and in the metabolism of reactive oxygen species in stressed or senescent parts of the plant.

The balance between cell proliferation, cell cycle arrest, and differentiation needed to maintain the organogenetic program depends on the coordination of gene expression, posttranslational modification, and specific proteolysis of cell cycle regulators. The G1/S and G2/M transitions are critical checkpoints controlled, in part, by cyclin-dependent kinases in the retinoblastoma (RBR)/E2F/DP pathway. Arabidopsis thaliana DPB is regulated by phosphorylation and targeted to proteasome-mediated proteolysis by the SCFSKP2A complex. In addition, DPB interacts in vivo with E2FC, because ectopic coexpression of E2FC and DPB produces severe developmental defects. To understand E2FC/DPB heterodimer function, we analyzed the effect of reducing E2FC mRNA levels with RNA interference. The e2fc-R plants developed organs with more but smaller cells and showed increased cell cycle marker gene expression and increased proliferative activity in developing leaves, meristems, and pericycle cells. This last feature produces plants with more lateral roots, consistent with an E2FC role in restricting lateral root initiation. The e2fc-R plants also show marked reductions in ploidy levels of mature leaves. These results indicate that the transition from cell division to the endocycle is sensitive to different pathways, E2FC/DPB being one of them. Our results show that E2FC/DPB is a key factor in controlling the balance between cell proliferation and the switch to the endocycle program.

Arabidopsis Snf1‐related protein kinases (SnRKs) are implicated in pleiotropic regulation of metabolic, hormonal and stress responses through their interaction with the kinase inhibitor PRL1 WD‐protein. Here we show that SKP1/ASK1, a conserved SCF (Skp1‐cullin‐F‐box) ubiquitin ligase subunit, which suppresses the skp1‐4 mitotic defect in yeast, interacts with the PRL1‐binding C‐terminal domains of SnRKs. The same SnRK domains recruit an SKP1/ASK1‐binding proteasomal protein, α4/PAD1, which enhances the formation of a trimeric SnRK complex with SKP1/ASK1 in vitro . By contrast, PRL1 reduces the interaction of SKP1/ASK1 with SnRKs. SKP1/ASK1 is co‐immunoprecipitated with a cullin SCF subunit (AtCUL1) and an SnRK kinase, but not with PRL1 from Arabidopsis cell extracts. SKP1/ASK1, cullin and proteasomal α‐subunits show nuclear co‐localization in differentiated Arabidopsis cells, and are observed in association with mitotic spindles and phragmoplasts during cell division. Detection of SnRK in purified 26S proteasomes and co‐purification of epitope‐ tagged SKP1/ASK1 with SnRK, cullin and proteasomal α‐subunits indicate that the observed protein interactions between SnRK, SKP1/ASK1 and α4/PAD1 are involved in proteasomal binding of an SCF ubiquitin ligase in Arabidopsis .

Read the full review for this F1000Prime recommended article: Arabidopsis E2Fc functions in cell division and is degraded by the ubiquitin-SCF(AtSKP2) pathway in response to light.