Annexins are Ca2+--and phospholipid-binding proteins that form an evolutionarily conserved multigene family throughout the animal and plant kingdoms. Two annexins, AnnAt1 and AnnAt4, have been identified as components in osmotic stress and abscisic acid signaling in Arabidopsis. Here, we report that AnnAt1 and AnnAt4 regulate plant stress responses in a light-dependent manner. The single-mutant annAt1 and annAt4 plants showed tolerance to drought and salt stress, which was greatly enhanced in double-mutant annAt1annAt4 plants, but AnnAt4-overexpressing transgenic plants (35S:AnnAt4) were more sensitive to stress treatments under long day conditions. Furthermore, expression of stress-related genes was altered in these mutant and transgenic plants. Upon dehydration and salt treatment, AtNCED3, encoding 9-cis-epoxycarotenoid dioxygenase, and P5CS1, encoding Δ-1-pyrroline-5-carboxylate synthase, which are key enzymes in ABA and proline synthesis, respectively, were highly induced in annAt1annAt4 plants and to a lesser extent in annAt1 and annAt4 plants, but not in 35S:AnnAt4 plants. While annAt1 plants were more drought sensitive, annAt4 plants were more tolerant in short days than in long days. In vitro and in vivo binding assays revealed that AnnAt1 and AnnAt4 bind to each other in a Ca2+-dependent manner. Our results suggest that AnnAt1 and AnnAt4 function cooperatively in response to drought and salt stress and their functions are affected by photoperiod.

To elucidate the cellular functions of phospholipase A 2 in plants, an Arabidopsis cDNA encoding a secretory low molecular weight phospholipase A 2 ( At sPLA 2 ␤ ) was isolated. Phenotype analyses of transgenic plants showed that overexpression of At sPLA 2 ␤ promotes cell elongation, resulting in prolonged leaf petioles and inflorescence stems, whereas RNA interferencemediated silencing of At sPLA 2 ␤ expression retards cell elongation, resulting in shortened leaf petioles and stems. At sPLA 2 ␤ is expressed in the cortical, vascular, and endodermal cells of the actively growing tissues of inflorescence stems and hypocotyls. At sPLA 2 ␤ then is secreted into the extracellular spaces, where signaling for cell wall acidification is thought to occur. At sPLA 2 ␤ -overexpressing or -silenced transgenic plants showed altered gravitropism in inflorescence stems and hypocotyls. At sPLA 2 ␤ expression is induced rapidly by auxin treatment and in the curving regions of inflorescence stems undergoing the gravitropic response. These results suggest that At sPLA 2 ␤ regulates the process of cell elongation and plays important roles in shoot gravitropism by mediating auxin-induced cell elongation.

When root temperature dropped below 25 degrees C, there was a sharp drop in the root pressure (P(r)) and hydraulic conductivity of excised roots (Lp(r)) of young cucumber (Cucumis sativus L.) seedlings as measured with the root pressure probe. A detailed analysis of root hydraulics provided evidence for a larger reduction in the osmotic component of Lp(r) (77%) in comparison with the hydrostatic component (34%) in response to the exposure of the root system to 13 degrees C. The activity of the plasma membrane H(+)-ATPase (EC 3.6.1.35) was reduced from 30 to 16 micro mol Pi mg(-1) protein h(-1) upon exposure to 8 degrees C for 1 day. Ultrastructural observations showed no evidence of loosening of the microstructure of endodermal cell walls in low temperature (LT)-treated roots. It is concluded that the rapid drop in the P(r) in response to LT is largely caused by a reduction in the activity of the plasma membrane H(+)-ATPase rather than by loosening of the endodermal wall which would cause substantial solute losses. On the other hand, water permeability of root cell membrane at LT was related to changes in the activity (open/closed state) of water channels.

Antarctic hairgrass (Deschampsia antarctica Desv.) is the only natural grass species in the maritime Antarctic. It has been studied as an extremophile that has successfully adapted to marginal land with the harshest environment for terrestrial plants. However, limited genetic research has focused on this species due to the lack of genomic resources. Here, we present the first de novo assembly of its transcriptome by massive parallel sequencing and its expression profile using D. antarctica grown under various stress conditions. Total sequence reads generated by pyrosequencing were assembled into 60,765 unigenes (28,177 contigs and 32,588 singletons). A total of 29,173 unique protein-coding genes were identified based on sequence similarities to known proteins. The combined results from all three stress conditions indicated differential expression of 3,110 genes. Quantitative reverse transcription polymerase chain reaction showed that several well-known stress-responsive genes encoding late embryogenesis abundant protein, dehydrin 1, and ice recrystallization inhibition protein were induced dramatically and that genes encoding U-box-domain-containing protein, electron transfer flavoprotein-ubiquinone, and F-box-containing protein were induced by abiotic stressors in a manner conserved with other plant species. We identified more than 2,000 simple sequence repeats that can be developed as functional molecular markers. This dataset is the most comprehensive transcriptome resource currently available for D. antarctica and is therefore expected to be an important foundation for future genetic studies of grasses and extremophiles.

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