No abstract

A bscisic acid (ABA) plays an important role in plant growth, development, and responses to environmental stresses, such as drought, salinity, and low temperature (1). Reversible protein phosphorylation is involved in the early events of ABA signal transduction (2, 3). Specific protein kinases are activated in response to ABA and have been proposed to play a positive role in ABA signaling (2, 4). On the other hand, protein phosphatases 2C (PP2C), such as ABI1, ABI2, and AtPP2CA, are negative regulators in ABA responses (5). The loss of ABI1 or ABI2 PP2C activity in the intragenic revertants of abi1-1 or abi2-1 leads to an enhanced response to ABA (6, 7), whereas overexpression of ABI1 or AtPP2CA blocks the expression of ABAinducible genes in Arabidopsis protoplasts (3). Antisense (AS) inhibition of AtPP2CA results in the enhancement of cold-and ABA-induced gene expression (8). Recently, ABI1 was shown to interact with ATHB6, a transcriptional regulator (9), and with PKS18, a SOS2-like protein kinase (10), whereas ABI2 and AtPP2CA were found to regulate SOS2 kinase and K ϩ channels, respectively (10-12). However, how the activity and function of PP2Cs are regulated in plant cells is still unclear.Recent studies indicate that phospholipase D (PLD) is involved in ABA responses. PLD activity has been implicated in mediating the ABA inhibition of ␣-amylase secretion (13), ABA-regulated stomatal movement (14), and ABA-induced gene expression (15,16). Arabidopsis has 12 PLDs, and multiple types of PLDs display distinctly different catalytic and regulatory properties (17). Molecular and genetic data have indicated that a specific PLD participates in signaling the ABA response. AS inhibition of PLD␣1 diminished stomatal closure induced by ABA or drought and increased water loss in Arabidopsis, whereas overexpression of PLD␣1 resulted in an increased sensitivity to ABA (18). In addition, specific PLDs have been shown to regulate many other plant functions, including cell patterning, programmed cell death, and stress tolerance (19-21). However, the direct target of PLD action is unknown. In this study, we present evidence for a direct link between PLD␣1 and PP2C in the ABA signaling response. Materials and Methods Plant Materials and Growth.A T-DNA-insertional mutant of PLD␣1 (PLD␣1-KO) was identified from SALK 053785 seeds obtained from the Arabidopsis Biological Resource Center (Columbus, OH). The site of T-DNA insertion was confirmed by DNA sequencing. The generation of PLD␣1-AS plants was described in ref. 22. Seeds of PLD␣1-KO, PLD␣1-AS, and wild type of Arabidopsis thaliana (ecotype Columbia) were sown in soil and kept at 4°C for 2 days. Plants were grown in a growth chamber with cool white fluorescent light of 100 mol m Ϫ2 ⅐s Ϫ1 under 14-h light͞10-h dark and 23°C͞18°C cycles.Water Loss and Stomatal Aperture Measurements. Detached leaves from 6-week-old plants were exposed to cool white light (125 mol m Ϫ2 ⅐s Ϫ1 ) at 23°C. Leaves were weighed at various time intervals, and the loss of fresh weight (%) was used to in...

We determined the role of Phospholipase Da1 (PLDa1) and its lipid product phosphatidic acid (PA) in abscisic acid (ABA)-induced production of reactive oxygen species (ROS) in Arabidopsis thaliana guard cells. The plda1 mutant failed to produce ROS in guard cells in response to ABA. ABA stimulated NADPH oxidase activity in wild-type guard cells but not in plda1 cells, whereas PA stimulated NADPH oxidase activity in both genotypes. PA bound to recombinant Arabidopsis NADPH oxidase RbohD (respiratory burst oxidase homolog D) and RbohF. The PA binding motifs were identified, and mutation of the Arg residues 149, 150, 156, and 157 in RbohD resulted in the loss of PA binding and the loss of PA activation of RbohD. The rbohD mutant expressing non-PA-binding RbohD was compromised in ABA-mediated ROS production and stomatal closure. Furthermore, ABA-induced production of nitric oxide (NO) was impaired in plda1 guard cells. Disruption of PA binding to ABI1 protein phosphatase 2C did not affect ABA-induced production of ROS or NO, but the PA-ABI1 interaction was required for stomatal closure induced by ABA, H 2 O 2 , or NO. Thus, PA is as a central lipid signaling molecule that links different components in the ABA signaling network in guard cells.

No abstract

No abstract

No abstract

Terrestrial plants lose water primarily through stomata, pores on the leaves. The hormone abscisic acid (ABA) decreases water loss by regulating opening and closing of stomata. Here, we show that phospholipase Dalpha1 (PLDalpha1) mediates the ABA effects on stomata through interaction with a protein phosphatase 2C (PP2C) and a heterotrimeric GTP-binding protein (G protein) in Arabidopsis. PLDalpha1-produced phosphatidic acid (PA) binds to the ABI1 PP2C to signal ABA-promoted stomatal closure, whereas PLDalpha1 and PA interact with the Galpha subunit of heterotrimeric G protein to mediate ABA inhibition of stomatal opening. The results reveal a bifurcating signaling pathway that regulates plant water loss.

Nanoporous metal ribbons including Au, Pd, Pt, Ag, and Cu can be fabricated through chemical dealloying of rapidly solidified Al-based alloys under free corrosion conditions. The formation and microstructure of these nanoporous metals have been investigated using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, transmission electron microscopy, and high-resolution transmission electron microscopy. All metal ribbons exhibit an open, three-dimensional bicontinuous interpenetrating ligament-channel structure with nanometer length scales. For a given dealloying solution, the length scale of ligaments/channels in these nanoporous metals is associated with surface diffusion of more noble atoms, and increases with increasing diffusion coefficients in sequence: Pt/Pd < Au < Ag < Cu. In addition, the length scale of ligaments/channels of these nanoporous metals can be modulated by simply changing the dealloying solution. Nanoindentation tests show that Young’s modulus and hardness of nanoporous gold are dependent upon the length scale of ligaments/channels. The electrical resistivity of these nanoporous metals is one to two orders of magnitude higher than that of their bulk counterparts. These nanoporous metals can be good candidates to probe the mechanical, physical, and chemical properties associated with random porous structures of nanoporous solids and will find wide applications in catalysis, sensors, actuators, fuel cells, and so forth.