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Plant Physiology 136:3851-3852 (2004) © 2004 American Society of Plant Biologists On the InsidePlasmodesmatal Regulation of Fiber Elongation
Cotton (Gossypium hirsutum or G. barbadense) fibers are single cells that emerge from the epidermis of the outer integument of ovules near anthesis. Depending upon the genotype, the final length attained by cotton fibers can range from 1.8 to 5.0 cm. Previous work has shown that the plasmodesmata on the basal side of the fiber close transiently during elongation. To examine the importance of plasmodesmatal closure in fiber elongation, Ruan et al. (pp. 4104–4113) compared the duration of the plasmodesmata closure among different cotton genotypes differing in fiber length. Confocal imaging of the symplastic movement of a membrane-impermeant fluorescent molecule clearly revealed genotypic differences in the duration of plasmodesmatal closure that were positively correlated with the average fiber lengths in the genotypes studied. In all cases, the closure occurred during the most rapid phase of elongation. Aniline blue staining and immunolocalization studies revealed that callose deposition and degradation at the fiber base were correlated with the timing of plasmodesmata closure and reopening, respectively. A fiber-specific Metabolomics of Temperature Shock Temperature acclimation is a complex process involving a number of physiological and biochemical changes, including changes in membrane structure and function, tissue water content, global gene expression, and protein, lipid, and primary and secondary metabolite composition. Kaplan et al. (pp. 4159–4168) have performed metabolite-profiling analysis using gas chromatography and mass spectrometry (GC-MS) to determine metabolite temporal dynamics associated with the induction of acquired thermotolerance in response to heat shock (HS) and acquired freezing tolerance in response to cold shock (CS). Eighty-one identified metabolites and 416 unidentified mass spectral tags (MSTs), characterized by retention time indices and specific mass fragments, were monitored. CS influenced metabolism far more profoundly than heat shock. The steady-state pool sizes of 143 and 311 metabolites or MSTs were altered in response to heat and cold shock, respectively. The majority of metabolites responsive to CS were specific to CS. In contrast, a very large proportion of the HS metabolite response (about two-thirds) seemed to be shared with that of CS. Only a very small proportion of heat responsive metabolites were HS specific. The present results perhaps offer some insight into a number of paradoxical early observations that some cold hardened plants were also more heat stress tolerant. The metabolite profiling analyses also indicate more dynamic and complex changes in compatible solute concentrations than previously recognized. This may explain why attempts to engineer overproduction of single compatible solute compounds have not produced plants with high levels of stress tolerance. Extracellular Calmodulin and Stomatal Function
Calmodulin (CaM) is an intracellular Ca2+-binding protein found in all eukaryotic cells. Recently, it has been found that extracellular CaM (ExtCaM) is physiologically active. In animals, ExtCaM is present in many body fluids (saliva, urine, and milk) and has been found to stimulate cell proliferation of cultured hepatocytes, melanoma cells, and fibroblasts. ExtCaM has also been found in many plant species and has been reported to stimulate proliferation of suspension-cultured cells of Angelica dahurica, Fenistum typhoides, and Sataria italica and to accelerate pollen germination and tube growth. Previous studies have also demonstrated that ExtCaM exists in the walls of guard cells and that its exogenous application promotes stomatal closure. In this issue, Chen et al. (pp. 4096–4103) explore the intracellular signaling mechanism by which ExtCaM mediates stomatal movement in Vicia faba using a combination of pharmacological, physiological, and genetic approaches. They provide evidence that ExtCaM induces an increase in both H2O2 levels and [Ca2+]cyt, leading to a reduction in stomatal aperture. Pharmacological evidence suggests that heterotrimeric G proteins transmit the ExtCaM signal, and act upstream of [Ca2+]cyt elevation and H2O2 generation in guard cell responses. Moreover, the stomata of an Arabidopsis mutant (gpa1) lacking a functional G-protein Nitrogen Metabolism in Developing Maize Cobs Nitrogen is necessary for the normal growth and development of maize (Zea mays) kernels, and is an essential component of enzymes, nucleic acids, and regulatory proteins. However, most studies examining kernel abortion and kernel set have focused on sugars and carbon supply, even though a deficiency of nitrogen can also severely limit kernel development. Unfortunately, it is not known how the cob and developing kernel sense the plant's nitrogen status and then determine the number of kernels to set and fill. To fill this gap in our knowledge, Seebauer et al. (pp. 4326–4334) have examined amino acid metabolism in cob and spikelet tissues during the critical 2-week period following silking when the reproductive sink capacity of the maize plant is determined. The effects of the reproductive sink on cob nitrogen metabolism were examined by comparing pollinated to unpollinated earshoots. Major amino acids in the cob were Gln, Asp, Asn, Glu, and Ala. Gln concentrations dropped dramatically from 2 to 14 d after silking in both pollinated and unpollinated cobs, whereas all other measured amino acids accumulated over time in unpollinated spikelets and cobs, especially Asn. Nitrogen supply had a variable effect on individual amino acid levels in young cobs and spikelets, with Asn being the most notably enhanced. They report that the cob performs significant enzymatic interconversions among Gln, Ala, Asp, and Asn during early reproductive development, which may precondition the nitrogen assimilate supply for sustained kernel growth. The measured amino acid profiles and enzymatic activities suggest that the Asn/Gln ratio in cobs may be part of a signal transduction pathway that indicates plant nitrogen status during kernel development. Actin Regulation of Pollen Ca2+ Channel Both cytosolic free Ca2+ ([Ca2+]i) and actin microfilaments play crucial roles in regulating pollen germination and tube growth. Contrary to some early reports that reported no actin in the tube tip, more recent evidence suggests the existence of an actin ring in the tube tip. Since there is increasing evidence that actin microfilaments can regulate ion channel activity, and since ion channel activity, particularly Ca2+ channel activity, is critical for pollen tube elongation, Wang et al. (pp. 3892–3904) have hypothesized that actin microfilaments may also play an important role in regulating plasma membrane Ca2+ permeable channels during pollen germination and tube growth. Using the patch-clamp technique, they have identified a hyperpolarization-activated, inward Ca2+channel in the plasma membrane of Arabidopsis pollen protoplasts. The activity of this Ca2+ channel was stimulated by cytochalasin B (CB) or cytochalasin D (CD). Increasing external Ca2+ enhanced the inhibitory effects of CD or CB on pollen germination and tube growth. Moreover, Ca2+ fluorescence imaging showed that the addition of actin depolymerization reagents significantly increased cytoplasmic Ca2+ levels in pollen protoplasts and pollen tubes, and that cytoplasmic Ca2+ increases induced by CD or CB were abolished by the addition of Ca2+ channel blockers. These results support the idea of a tight and finely controlled connection between actin structure and [Ca2+]i dynamics at the pollen tube tip. Inhibition of Blue Light Effect in Guard Cells by Abscisic Acid Blue light (BL)-dependent H+ pumping by guard cells, which drives stomatal opening, is inhibited by abscisic acid (ABA). Phototropins have been identified as the BL receptors that lead to the enhancement of H+-ATPase activity in the plasma membranes of stomatal guard cells during BL-induced stomatal opening. Recent investigations have demonstrated that BL activates the H+-ATPase through the phosphorylation of threonine residues in the C terminus. Phosphorylation, in turn, induces the binding of 14-3-3 protein to a specific threonine in the H+-ATPase, which acts as a positive regulator for the H+-ATPase. In this issue, Zhang et al. (pp. 4150–4158) examine the effects of ABA, an inhibitor of BL-induced stomatal opening, on BL-dependent H+ pumping in Vicia faba guard cell protoplasts. ATP hydrolysis by the plasma membrane H+-ATPase, phosphorylation of the H+-ATPase, and the binding of 14-3-3 protein to the H+-ATPase were all stimulated by BL and all inhibited by 10 µM ABA. These results suggest that ABA may inhibit H+ pumping by suppressing BL-dependent phosphorylation of the plasma membrane H+-ATPase. All of these ABA-induced inhibitions were similarly inhibited by hydrogen peroxide (H2O2) at 1 mM and partially restored by ascorbate, an intracellular H2O2 scavenger. A single-cell analysis of the cytosolic H2O2 using 2',7'-dichlorofluorescein revealed that H2O2 was generated by ABA in guard cell protoplasts. These results indicate that the inhibition of BL-dependent H+-pumping by ABA is due to a decrease in the phosphorylation levels of H+-ATPase and that H2O2 might be involved in this response.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900132. Related articles in Plant Physiol.:
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-1,3-glucanase gene, GhGluc1, was undetectable when callose was deposited at the fiber base but became evident at the time of callose degradation. Genotypically, the level of GhGluc1 expression was high in the short fiber genotype and weak in the longer fiber genotypes. The data provide genotypic and developmental evidence that (1) plasmodesmata closure appears to play an important role in elongating cotton fibers, (2) callose deposition and degradation may be involved in the plasmodesmata closure and reopening, respectively, and (3) the expression of GhGluc1 could play a role in this process by degrading callose. 
-subunit was insensitive to ExtCaM. Taken together, these results suggest that ExtCaM activates an intracellular signaling pathway involving activation of a heterotrimeric G protein, H2O2 generation, and changes in [Ca2+]cyt, leading to stomatal closure.