Comment to Campanoni et al

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• Publié par: Astlio
• Nombre de lectures: 35
• Langue: English

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gelson (pp. 1367–1375)demonstrate Auxin TransportGuard Cell Surface Area that two plant defenserelated chemi Synchronizes Division Guard cells must maintain the in cals, notably jasmonic acid and sali tegrity of the plasma membrane dur Pattern cylic acid, also influence trichome pro ing the large and relatively rapid duction. The results are interesting The tobacco cell line VBI0 (Nicoti changes in volume they undergo. because salicylic acid and jasmonic ana tabacum) provides a simple model Since plasma membranes are only acid are known to play key roles in system to study the role of intercellu about 5% elastic, stretching alone can regulating the induction of other types lar communication in patterning. In not be responsible for maintaining of herbivore resistance. Herbivore this system, singular cells divide axi membrane integrity. In this issue, damage and artificial wounding both ally to produce linear cell files of dis Shope et al. (pp. 1314–1321)explore cause rapid increases in jasmonic acid tinct polarity. A curious feature of the question of what happens to the and, as the authors demonstrate, an these cell files is that they almost al plasma membrane of guard cells dur increase in trichome production. The ways consist of an even number of ing a massive decrease in volume? jar1–1mutant exhibited normal cells. In a strictly binary system of cell Does the guard cell protoplast simply trichome induction following treat n division, files consisting of 2cells (2, shrivel with no reduction area in the ment with jasmonic acid, suggesting 4, 8, 16.. .) should be most frequent. surface area of the plasma membrane? that adenylation of jasmonic acid is In the VBI0 system, however, files Or does the excess plasma membrane not necessary. Salicylic acid, which consisting of 6 cells are highly fre become internalized? To distinguish negatively regulates the jasmonate quent, even more than files consisting between these two possibilities, the dependent pathway in many plants authors determined the surface area of of 8 cells, which is difficult to reconcile including Arabidopsis, had a negative intact guard cells ofVicia fabaas they with a strictly binary system. Thus, effect on trichome production and underwent changes in volume in re Campanoni et al. (pp. 1251–1260) reduced the effect of jascon consistently sponse to changes in the external os jecture the existence of some form of monic acid, suggesting negative motic potential. They also employed a information exchange (coupling) be crosstalk between the jasmonate and membranespecific fluorescent dye to tween the cells that serves to coordi salicylatedependent defense path measure membrane internalization by ways. Interestingly, the effect of sali nate and synchronize cell division. To these cells. Surface area decreased by cylic acid persisted in the non test whether the observed deviation as much as 40% when external os inducible immunity mutantnim1–1, from a binary system could be caused motic potential was increased from suggesting that theNpr1/Nim1gene, by coupling, they developed a simple zero to 1.5 MPa, and surface area var which encodes a nuclear protein that is mathematical model to simulate the ied linearly with volume. Membrane necessary for downstream activity of dynamics of cell number. They report internalization was found to increase the salicylatedependent pathway in that the VBI0 cell culture system can approximately linearly with decreases other systems, isnotdownstream of best be described as a onedimensional in the cell’s surface area. The changes salicylic acid in the negative regulation array of coupled oscillators, where the in surface area, volume, and mem of trichome production. Finally, they number of oscillators is not conserved brane internalization were reversible report that gibberellin and jasmonic over time. Instead, a new oscillator is when the guard cells were returned to acid had a synergistic effect on the in generated and inserted adjacent to the a buffer solution with an osmotic po duction of trichomes, suggesting im “parent” cell, whenever a certain oscil tential near zero. The data show that portant interactions between these two lator has finished one cell cycle. The intact guard cells undergo changes in compounds. model indicates that a given oscillator surface area that are too large to be senses interference with just one neigh accommodated by plasma membrane boring oscillator not two (unilateral stretching and shrinkage, and suggest Urea Transport by coupling). This unilateral or polar in that the plasma membrane is reversibly terference may be related to the polar Aquaporins internalized to maintain cell integrity. transport of a coordinating signal such Although urea is the major form of as auxin. In agreement with this model, nitrogen (N) in most plant fertilizers the authors report that treatment of the WoundingInduced and is also an important N metabolite cell cultures with low concentrations of in plants, the mechanisms that under Trichome Production in 1Nnaphthylphthalamic acid (NPA), lie urea transport in higher plants are Arabidopsis an inhibitor of auxin efflux carriers, not well understood. Transport assays equalizes the frequencies of files with Many plant species, including Arahave previously suggested that two even and uneven cell numbers. These bidopsis, respond to insect damage bymechanisms are at work in characean results show thatintra fileauxin flux increasing the density of trichomes onalgae: At low concentrations, an active, mediates pattern formation in this sim new leaves. Gibberellin has alreadyenergydependent pathway drives the ple system. Auxintransport uptake of urea, whereas at higher exbeen shown to play a role in constitu , not the mere presence of exogenous auxin, tive trichome production in Arabidopternal concentrations, urea uptake fol synchronizes the cell cycle.sis, and in this issue,Traw and Berlows a linear concentration depen 1038Plant Physiology, November 2003, Vol. 133, pp. 1038–1039, www.plantphysiol.org © 2003 American Society of Plant Biologists

dency, indicating a second passive or diffusioncontrolled transport path way. Higher plants have been shown  to transport urea actively by the H urea cotransporter AtDUR3, which is preferentially expressed in roots under N deficiency Also, in some systems, aquaporins facilitate urea transport al though this is not a general property of all aquaporins. To better understand the molecular basis for urea transport in higher plants,Liu et al. (pp. 1220– 1228)adopted a yeast (Saccharomyces cerevisiae) complementation approach to isolate genes encoding urea trans port proteins in Arabidopsis. Here, they report that this approach led ex clusively to the isolation of tonoplast intrinsic protein (TIP)related genes. The ureatransporting properties of these aquaporins were characterized in yeast andXenopus laevisoocytes and found to differ fundamentally from the recently characterized secondary active urea transport mediated by AtDUR3.  In contrast to the highaffinity H / urea symporter AtDUR3, these AtTIPs provide a less concentration and pH dependent transport pathway for urea. The identified AtTIPs could potentially facilitate urea transport either from the external growth medium into the cy tosol or from the cytosol into the vac uole, for example, for the storage or detoxification of excessive urea. Tran scriptional upregulation of the iso lated AtTIPs under N deficiency in roots further supports a role for aqua porins in urea transport.

Arabidopsis Transcriptome Responses to 2,4,6 Trinitrotoluene (TNT) The manufacture, processing, and storage of explosives, such as 2,4,6 trinitrotoluene (TNT) during the past

Plant Physiol. Vol. 133, 2003

century has led to soil and groundwa ter contamination in some areas. Un like many other nitroaromatic com pounds, including pesticides and various feedstock chemicals, the ener getic nitroaromatics are highly resis tant to degradation and may persist in the environment for decades. Certain plant species have the ability to accu mulate TNT from their surroundings and, thus, offer a potential means for removing these compounds from the environment by phytoremediation. Few of these species, however, are ca pable of tolerating the high contami nation levels typically encountered in those sites most in need of remedia tion. Unfortunately, a lack of informa tion on the biochemical mechanisms involved in TNT uptake and metabo lism limits our ability to genetically modify plants specifically for this task. In this issue,Ekman et al. (pp. 1397– 1406)report on their use of Serial Analysis of Gene Expression (SAGE) to profile transcript levels in Arabi dopsis roots and assess their re sponses to TNT exposure. Among the proteins that were most highly tran scribed in response to TNT exposure included a glutathione Stransferase, several cytochrome P450 enzymes, an ABC transporter and a probable ni troreductase. Analyses also revealed an oxidative stress response upon TNT exposure as well as the repres sion of some transcript levels. Al though many of these findings were expected based on current models of xenobiotic metabolism in plants, evi dence for an unsuspected anthranilate conjugation pathway was also noted. Identifying transcriptomelevel re sponses to TNT exposure will better define the metabolic pathways plants use to detoxify this xenobiotic com pound, which should help improve phytoremediation strategies directed

at TNT and other nitroaromatic compounds.

Does the Krebs Cycle Reduce Photosynthesis?

Although the operation and loca tion of the Krebs cycle was demon strated in plant cells decades ago, many fundamental questions remain concerning how its activity is inte grated with other plant processes. For example, controversy exists over whether the Krebs cycle operates in illuminated photosynthetic tissue and if it contributes to the energy require ments for the synthesis of Suc in pho tosynthetic tissues. In this issue,Car rari et al. (pp. 1322–1335)describe the molecular and genetic analysis of Aco1, aLycopersicon pennelliiaccession that is deficient in aconitase, the Krebs cycle enzyme that catalyzes the re versible interconversion of citrate and isocitrate. As expected, the mutation resulted in lowered expression of the Aco1transcript and lowered levels of both cytosolic and mitochondrial ac onitase protein and activity. Biochem ical analysis of leaves of theAco1ac cession suggested that they exhibited a restricted flux through the Krebs cy cle and reduced levels of Krebs cycle intermediates but were characterized by elevated adenylate levels and an enhanced rate of CO assimilation. 2 Furthermore, the analysis of both steady state metabolite levels and met abolic fluxes revealed that this acces sion also exhibited elevated rates of photosynthetic Suc synthesis and a corresponding increase in fruit yield. The enhancement of photosynthesis and fruit yield in this aconitase deficient mutant is somewhat stun ning given the limited success in achieving similar results through di rect modification of the Calvin cycle.

Peter V. Minorsky Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522

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