SummaryLow 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 puri®ed 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 H 2 O 2 , 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.

Circadian clock performance during winter dormancy has been investigated in chestnut by using as marker genes CsTOC1 and CsLHY, which are homologous to essential components of the central circadian oscillator in Arabidopsis. During vegetative growth, mRNA levels of these two genes in chestnut seedlings and adult plants cycled daily, as expected. However, during winter dormancy, CsTOC1 and CsLHY mRNA levels were high and did not oscillate, indicating that the circadian clock was altered. A similar disruption was induced by chilling chestnut seedlings (to 4°C). Normal cycling resumed when endodormant or cold-treated plants were returned to 22°C. The behavior of CsTOC1 and CsLHY during a cold response reveals a relevant aspect of clock regulation not yet encountered in Arabidopsis.chilling ͉ winter dormancy ͉ ecodormancy ͉ cold response W inter dormancy is an important adaptive strategy that enables plants to persist during periods of stressful environmental conditions (1). Dormancy parameters are key determinants in woody plants in agriculture and forestry. Dormancy determines to what degree fruit crops will survive winter and early spring without shoot and flower bud damage, and, in long-lived forest species, the length of rest limits the growing season and thus affects wood production and quality. The onset of winter deep dormancy (endodormancy) is preceded by a stage of ecodormancy. Endodormancy is caused by plant endogenous factors, and, once established,

A small heat-shock protein (sHSP) that shows molecular chaperone activity in vitro was recently purified from mature chestnut (Castanea sativa) cotyledons. This protein, renamed here as CsHSP17.5, belongs to cytosolic class I, as revealed by cDNA sequencing and immunoelectron microscopy. Recombinant CsHSP17.5 was overexpressed in Escherichia coli to study its possible function under stress conditions. Upon transfer from 37°C to 50°C, a temperature known to cause cell autolysis, those cells that accumulated CsHSP17.5 showed improved viability compared with control cultures. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis of cell lysates suggested that such a protective effect in vivo is due to the ability of recombinant sHSP to maintain soluble cytosolic proteins in their native conformation, with little substrate specificity. To test the recent hypothesis that sHSPs may be involved in protection against cold stress, we also studied the viability of recombinant cells at 4°C. Unlike the major heat-induced chaperone, GroEL/ES, the chestnut sHSP significantly enhanced cell survivability at this temperature. CsHSP17.5 thus represents an example of a HSP capable of protecting cells against both thermal extremes. Consistent with these findings, high-level induction of homologous transcripts was observed in vegetative tissues of chestnut plantlets exposed to either type of thermal stress but not salt stress.

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