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ON THE INSIDE

On the Inside

The increased temperatures and altered soil moistures expected to arise from global climate change during the next 50 years are projected to decrease the yield of food crops. On the other hand, elevated CO₂ concentration ([CO₂]) is predicted to enhance yield and offset these detrimental factors. Maize (Zea mays), a C₄ crop, is predicted to become the world's most important crop, in terms of human food supply, by 2050. However, C₄ photosynthesis is usually saturated at current [CO₂] and theoretically should not be stimulated under elevated [CO₂]. Nevertheless, some controlled environment studies have reported direct stimulation of C₄ photosynthesis and productivity under elevated [CO₂]. Free-Air Concentration Enrichment (FACE) facilities allow researchers to grow plants in the field under elevated CO₂ conditions, thus helping to understand the potential effects of global change on agricultural and natural ecosystems. To test if CO₂ fertilization occurs in the open air and within the American Corn Belt, Leakey et al. (pp. 779–790) grew maize under ambient [CO₂] (376 µmol mol^–1) and elevated [CO₂] (550 µmol mol^–1) using FACE technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO₂] did not stimulate photosynthesis, biomass, or yield. Stomatal conductance, however, was lower (–34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. Therefore, it appears that elevated [CO₂] may increase C₄ crop performance indirectly by the amelioration of drought stress. However, in the absence of any direct stimulation of photosynthesis, it is unclear whether this will be sufficient to override, or even negate, the detrimental effects of increasing temperature and drought on yield. This suggests that rising [CO₂] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

Sucrose-Specific Induction of the Anthocyanin Biosynthesis

Anthocyanins are plant secondary metabolites that are involved in a wide variety of functions in plants, including floral pigmentation, signaling between plants and microbes, plant defense, the modulation of auxin transport, and UV protection. Whole-genome transcript profiling reveals that the flavonoid and anthocyanin biosynthetic pathways are strongly up-regulated following Suc treatment, as are flavonoid and anthocyanin concentrations. It is presently unknown whether sugars induce most of the genes involved in anthocyanin biosynthesis or if only a few genes play a pivotal role. Moreover, little is known about the sugar specificity for the anthocyanin biosynthesis induction in Arabidopsis (Arabidopsis thaliana). Solfanelli et al. (pp. 637–646) have investigated the effects of sugars (Suc, Glu, and Fru) on genes coding for flavonoid and anthocyanin biosynthetic enzymes in Arabidopsis. Their results indicate that the sugar-dependent up-regulation of the anthocyanin synthesis pathway is Suc specific. They also investigated the up-regulation of anthocyanin biosynthesis genes in an Arabidopsis mutant (pgm) that lacks functional plastidial phosphoglucomutase. As a consequence, the pgm mutant lacks starch but accumulates high levels of soluble sugars as a result of photosynthesis. As expected, the authors confirm the induction of several anthocyanin biosynthetic genes in the pgm mutant.

Inositol Trisphosphate: A Universal Role in Gravitropism?

Two types of changes in inositol 1,4,5-trisphosphate (InsP₃) have previously been detected in gravistimulated grass pulvini: a rapid and transient increase that occurs within seconds in both the upper and lower pulvinar halves, and a slower, more sustained increase that occurs exclusively in the lower pulvinar halves and is correlated with the bending response. But how universal is this response? Is it unique to grass pulvini? In this issue, Perera et al. (pp. 746–760) examine the hypothesis that InsP₃ is a universal component involved in establishing gravitropic curvatures in plants. To elucidate the role of InsP₃ in plant gravitropism, transgenic Arabidopsis plants expressing a human inositol polyphosphate 5-phosphatase, an enzyme that specifically hydrolyzes the soluble inositol phosphates InsP₃ and InsP₄, were generated. The transgenic plants show no significant differences in growth and life cycle compared to wild-type plants, although basal InsP₃ levels were reduced by greater than 90% compared to wild-type plants. Gravitropic bending of the roots, hypocotyls, and inflorescence stems of the transgenic plants was reduced by approximately 30% compared with the wild type. Moreover, upon gravistimulation, InsP₃ levels in inflorescence stems of transgenic plants showed no detectable change, whereas in wild-type plant inflorescences InsP₃ levels increased approximately 3-fold within the first 5 to 15 min of gravistimulation, preceding visible bending. The transgenic roots were shown to have a reduction in basipetal indole-3-acetic acid transport, and a delay in asymmetric auxin-induced -glucuronidase expression upon gravistimulation as compared to the controls. The compromised gravitropic responses in all the major axes of growth in these transgenic Arabidopsis plants suggest a universal role for InsP₃ in the gravity signal transduction cascade of higher plants.

Photosynthetic Inhibition and CO₂-Induced Changes in Stomatal Conductance

Although the responses of guard cells to light and CO₂ have traditionally been considered independently of each other, it has recently been proposed that the balance between electron transport capacity and the Calvin cycle may control stomatal response to CO₂. To shed light on this controversial area, Messinger et al. (pp. 771–778) have examined the role of photosynthetic processes in the response of the stomatal conductance of cocklebur (Xanthium strumarium) to changes in CO₂. They examined the responses of stomatal conductance and net CO₂ assimilation rate to CO₂ in light and darkness, in the presence and absence of the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and at 2% and 21% ambient oxygen. They show that the response of stomatal conductance to intercellular CO₂ in intact leaves is qualitatively different when photosynthetic electron transport is eliminated, either by removal of light or by the addition of DCMU. In the presence of photosynthetic electron transport, the slope of the changes in stomatal conductance in response to CO₂ changes markedly at values of CO₂ very close to the transition of whole-leaf photosynthesis from Rubisco limitation to photosynthetic electron transport limitation. In contrast, the response in darkness or under DCMU is relatively small and does not show a distinct change in slope. These data suggest that there are at least two mechanisms by which stomata respond to CO₂. One of these depends on photosynthetic electron transport, and is therefore sensitive to the balance between the light and dark reactions of photosynthesis; the other is independent of photosynthetic electron transport and is therefore present in darkness. Both mechanisms may contribute to "normal" stomatal responses to CO₂ in the light.

Metabolic Effects of Nickel Deficiency

Nickel (Ni) at long last has become recognized as an essential mineral nutrient for higher plants. The long-delayed recognition of Ni as an essential mineral largely stems from the low levels needed by plants (approximately 1–100 ng g^–1 dry weight) compared to the relative abundance of Ni in most soils (>5 kg ha^–1). The existence of field-level Ni deficiency in crops was only recently discovered; mouse-ear, a century-old malady of pecan (Carya illinoinensis) trees, was found to arise from a deficiency in Ni. Relatively little is known about the physiological role(s) of Ni in plant function. There are several enzyme systems in bacteria and lower plants that are activated by Ni; however, the activation of urease appears to date to be the only enzymatic function of Ni in higher plants. In an effort to gain more insight into the physiological perturbations caused by Ni deficiency, Bai et al. (pp. 433–443) evaluated the concentrations of ureides, amino acids, and organic acids in photosynthetic foliar tissue from Ni-sufficient versus Ni-deficient pecan seedlings. Their studies suggest that Ni deficiency disrupts several metabolic pathways in pecan, including carbon respiration and nitrogen metabolism. Disruption of carbon metabolism apparently leads to an accumulation of lactic and oxalic acids. The authors suggest that mouse-ear, a key morphological symptom of Ni deficiency in pecan, is probably linked to the toxic accumulation of oxalic and lactic acids in the rapidly growing tips and margins of leaflets. Most importantly, the large extent of metabolic disruption raises the possibility that Ni exerts an active role in pecan metabolism other than that of being a cofactor of urease. The results therefore point to the likelihood of undiscovered roles for Ni in plant nutritional physiology.

A Transgenic Potato with Enhanced Blight Resistance

Late blight, caused by the pathogen Phytophthora infestans, is the infamous disease that led to the Irish potato (Solanum tuberosum) famine of the 1840s. Earlier resistant cultivars used disease resistance (R) genes that confer immunity only to those strains of the pathogen that harbored corresponding avirulence (Avr) gene. Unfortunately, defenses involving specific R-mediated immunity as well as chemical controls have tended to become rapidly breached in the field as a result of new pathogen strains. Thus, currently grown potato cultivars lack adequate blight tolerance. Many lines of evidence suggest that the modulation of mitogen-activated protein kinase (MAPK) cascades may enable plants to resist pathogen invasion. NtMEK2, a tobacco (Nicotiana tabacum) MAPK kinase, is known to activate both salicylic acid-induced protein kinase and wound-induced protein kinase. Expression of NtMEK2^DD, a constitutively active allele of NtMEK2, induced hypersensitive response-like cell death, defense gene expression, and generation of reactive oxygen species. Yamamizo et al. (pp. 681–692) examine the possibility that potato MEK1^DD (StMEK1^DD) might be engineered into plants to accelerate MAPK signal transduction efficiently and enable plants to resist pathogen invasion. However, since the constitutive activation of the defense mechanism might be lethal to a plant, they have developed transgenic potato plants that carry a constitutively active form of MAPK kinase driven by a pathogen-inducible promoter of potato. The transgenic potato plants showed high resistance to P. infestans as well as to the early blight pathogen Alternaria solani. Pathogen attack provoked defense-related MAPK activation followed by induction of the gene expression of NADPH oxidase, which plays a role in reactive oxygen species production, and the hypersensitive response. The authors propose that enhancing disease resistance through altered regulation of plant defense mechanisms should be more durable and publicly acceptable than engineering the overexpression of antimicrobial proteins.

Peter V. Minorsky

Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522

FOOTNOTES

www.plantphysiol.org/cgi/doi/10.1104/pp.104.900185.

Related articles in Plant Physiol.:

Nickel Deficiency Disrupts Metabolism of Ureides, Amino Acids, and Organic Acids of Young Pecan Foliage

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Plant Physiol. 2006 140: 433-443. [Abstract] [Full Text]  

Photosynthesis, Productivity, and Yield of Maize Are Not Affected by Open-Air Elevation of CO₂ Concentration in the Absence of Drought

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