Use of confocal laser as light source reveals stomata-autonomous function.
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In most terrestrial plants, stomata open during the day to maximize the update of CO(2) for photosynthesis, but they close at night to minimize water loss. Blue light, among several environmental factors, controls this process. Stomata response to diverse stimuli seems to be dictated by the behaviour of neighbour stomata creating leaf areas of coordinated response. Here individual stomata of Arabidopsis leaves were illuminated with a short blue-light pulse by focusing a confocal argon laser. Beautifully, the illuminated stomata open their pores, whereas their dark-adapted neighbours unexpectedly experience no change. This induction of individual stomata opening by low fluence rates of blue light was disrupted in the phototropin1 phototropin2 (phot1 phot2) double mutant, which exhibits insensitivity of stomatal movements in blue-illuminated epidermal strips. The irradiation of all epidermal cells making direct contact with a given stoma in both wild type and phot1 phot2 plants does not trigger its movement. These results unravel the stoma autonomous function in the blue light response and illuminate the implication of PHOT1 and/or PHOT2 in such response. The micro spatial heterogeneity that solar blue light suffers in partially shaded leaves under natural conditions highlights the physiological significance of the autonomous stomatal behaviour. (+info)
The role of phosphoenolpyruvate carboxylase during C4 photosynthetic isotope exchange and stomatal conductance.
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Phosphoenolpyruvate carboxylase (PEPC; EC 4.1.1.31) plays a key role during C(4) photosynthesis and is involved in anaplerotic metabolism, pH regulation, and stomatal opening. Heterozygous (Pp) and homozygous (pp) forms of a PEPC-deficient mutant of the C(4) dicot Amaranthus edulis were used to study the effect of reduced PEPC activity on CO(2) assimilation rates, stomatal conductance, and (13)CO(2) (Delta(13)C) and C(18)OO (Delta(18)O) isotope discrimination during leaf gas exchange. PEPC activity was reduced to 42% and 3% and the rates of CO(2) assimilation in air dropped to 78% and 10% of the wild-type values in the Pp and pp mutants, respectively. Stomatal conductance in air (531 mubar CO(2)) was similar in the wild-type and Pp mutant but the pp mutant had only 41% of the wild-type steady-state conductance under white light and the stomata opened more slowly in response to increased light or reduced CO(2) partial pressure, suggesting that the C(4) PEPC isoform plays an essential role in stomatal opening. There was little difference in Delta(13)C between the Pp mutant (3.0 per thousand +/- 0.4 per thousand) and wild type (3.3 per thousand +/- 0.4 per thousand), indicating that leakiness (), the ratio of CO(2) leak rate out of the bundle sheath to the rate of CO(2) supply by the C(4) cycle, a measure of the coordination of C(4) photosynthesis, was not affected by a 60% reduction in PEPC activity. In the pp mutant Delta(13)C was 16 per thousand +/- 3.2 per thousand, indicative of direct CO(2) fixation by Rubisco in the bundle sheath at ambient CO(2) partial pressure. Delta(18)O measurements indicated that the extent of isotopic equilibrium between leaf water and the CO(2) at the site of oxygen exchange () was low (0.6) in the wild-type and Pp mutant but increased to 0.9 in the pp mutant. We conclude that in vitro carbonic anhydrase activity overestimated as compared to values determined from Delta(18)O in wild-type plants. (+info)
Stand aside stomata, another actor deserves centre stage: the forgotten role of the internal conductance to CO2 transfer.
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Internal conductance describes the movement of CO(2) from substomatal cavities to sites of carboxylation. Internal conductance has now been measured in approximately 50 species, and in all of these species it is a large limitation of photosynthesis. It accounts for somewhat less than half of the decrease in CO(2) concentrations from the atmosphere to sites of carboxylation. There have been two major findings in the past decade. First, the limitation due to internal conductance (i.e. C(i)-C(c)) is not fixed but varies among species and functional groups. Second, internal conductance is affected by some environmental variables and can change rapidly, for example, in response to leaf temperature, drought stress or CO(2) concentration. Biochemical factors such as carbonic anhydrase or aquaporins are probably responsible for these rapid changes. The determinants of internal conductance remain elusive, but are probably a combination of leaf anatomy, morphology, and biochemical factors. In most plants, the gas phase component of internal conductance is negligible with the majority of resistance resting in the liquid phase from cell walls to sites of carboxylation. The internal conductance story is far from complete and many exciting challenges remain. Internal conductance ought to be included in models of canopy photosynthesis, but before this is feasible additional data on the variation in internal conductance among and within species are urgently required. Future research should also focus on teasing apart the different steps in the diffusion pathway (intercellular spaces, cell wall, plasmalemma, cytosol, and chloroplast envelope) since it is likely that this will provide clues as to what determines internal conductance. (+info)
Comparison of several models for calculating ozone stomatal fluxes on a Mediterranean wheat cultivar (Triticum durum Desf. cv. Camacho).
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Ozone stomatal fluxes were modeled for a 3-year period following different approaches for a commercial variety of durum wheat (Triticum durum Desf. cv. Camacho) at the phenological stage of anthesis. All models performed in the same range, although not all of them afforded equally significant results. Nevertheless, all of them suggest that stomatal conductance would account for the main percentage of ozone deposition fluxes. A new modeling approach was tested, based on a 3-D architectural model of the wheat canopy, and fairly accurate results were obtained. Plant species-specific measurements, as well as measurements of stomatal conductance and environmental parameters, were required. The method proposed for calculating ozone stomatal fluxes (FO(3_3-D)) from experimental gs data and modeling them as a function of certain environmental parameters in conjunction with the use of the YPLANT model seems to be adequate, providing realistic estimates of the canopy FO(3_3-D), integrating and not neglecting the contribution of the lower leaves with respect to the flag leaf, although a further development of this model is needed. (+info)
The role of stomatal acclimation in modelling tree adaptation to high CO2.
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Carbon dioxide enrichment changes the balance of photosynthetic limitations due to water, nitrogen, and light. This paper examines the role of stomata in these changes by comparing enrichment responses predicted by an optimality-based tree growth model, DESPOT, using three alternative 'setpoints' for stomatal acclimation: leaf water potential (psi(l)-setpoint), the ratio of intercellular to ambient CO(2) mole fraction (c(i)/c(a)-setpoint), and the parameters in a simple model in which stomata are controlled by H(2)O and CO(2) supply and demand (linked feedback). In each scenario, stomatal conductance (g(s)) and photosynthetic capacity (V(m)) declined, productivity and leaf area index (LAI) increased, and c(i)/c(a) remained within 5% of its pre-enrichment value. Height growth preceded the LAI response in the psi(l)-setpoint and linked feedback scenarios, but not in the c(i)/c(a)-setpoint scenario. These trends were explained in terms of photosynthetic resource substitution using the equimarginal principle of production theory, which controls carbon allocation in DESPOT: enrichment initially increased the marginal product for light, driving substitution towards light; height growth also drove substitution towards N in the psi(l) and feedback scenarios, but the inflexibility of c(i)/c(a) prevented that substitution in the c(i)/c(a) scenario, explaining the lack of height response. Each scenario, however, predicted similar behaviour for c(i)/c(a) and carbon and water flux. These results suggest that 'setpoints' may be robust tools for linking and constraining carbon and water fluxes, but that they should be used more cautiously in predicting or interpreting how those fluxes arise from changes in tree structure and physiology. (+info)
Lysigenous aerenchyma formation in Arabidopsis is controlled by LESION SIMULATING DISEASE1.
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Aerenchyma tissues form gas-conducting tubes that provide roots with oxygen under hypoxic conditions. Although aerenchyma have received considerable attention in Zea mays, the signaling events and genes controlling aerenchyma induction remain elusive. Here, we show that Arabidopsis thaliana hypocotyls form lysigenous aerenchyma in response to hypoxia and that this process involves H(2)O(2) and ethylene signaling. By studying Arabidopsis mutants that are deregulated for excess light acclimation, cell death, and defense responses, we find that the formation of lysigenous aerenchyma depends on the plant defense regulators LESION SIMULATING DISEASE1 (LSD1), ENHANCED DISEASE SUSCEPIBILITY1 (EDS1), and PHYTOALEXIN DEFICIENT4 (PAD4) that operate upstream of ethylene and reactive oxygen species production. The obtained results indicate that programmed cell death of lysigenous aerenchyma in hypocotyls occurs in a similar but independent manner from the foliar programmed cell death. Thus, the induction of aerenchyma is subject to a genetic and tissue-specific program. The data lead us to conclude that the balanced activities of LSD1, EDS1, and PAD4 regulate lysigenous aerenchyma formation in response to hypoxia. (+info)
The contribution of photosynthesis to the red light response of stomatal conductance.
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To determine the contribution of photosynthesis on stomatal conductance, we contrasted the stomatal red light response of wild-type tobacco (Nicotiana tabacum 'W38') with that of plants impaired in photosynthesis by antisense reductions in the content of either cytochrome b(6)f complex (anti-b/f plants) or Rubisco (anti-SSU plants). Both transgenic genotypes showed a lowered content of the antisense target proteins in guard cells as well as in the mesophyll. In the anti-b/f plants, CO(2) assimilation rates were proportional to leaf cytochrome b(6)f content, but there was little effect on stomatal conductance and the rate of stomatal opening. To compare the relationship between photosynthesis and stomatal conductance, wild-type plants and anti-SSU plants were grown at 30 and 300 micromol photon m(-2) s(-1) irradiance (low light and medium light [ML], respectively). Growth in ML increased CO(2) assimilation rates and stomatal conductance in both genotypes. Despite the significantly lower CO(2) assimilation rate in the anti-SSU plants, the differences in stomatal conductance between the genotypes were nonsignificant at either growth irradiance. Irrespective of plant genotype, stomatal density in the two leaf surfaces was 2-fold higher in ML-grown plants than in low-light-grown plants and conductance normalized to stomatal density was unaffected by growth irradiance. We conclude that the red light response of stomatal conductance is independent of the concurrent photosynthetic rate of the guard cells or of that of the underlying mesophyll. Furthermore, we suggest that the correlation of photosynthetic capacity and stomatal conductance observed under different light environments is caused by signals largely independent of photosynthesis. (+info)
Overexpression of AtMYB44 enhances stomatal closure to confer abiotic stress tolerance in transgenic Arabidopsis.
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AtMYB44 belongs to the R2R3 MYB subgroup 22 transcription factor family in Arabidopsis (Arabidopsis thaliana). Treatment with abscisic acid (ABA) induced AtMYB44 transcript accumulation within 30 min. The gene was also activated under various abiotic stresses, such as dehydration, low temperature, and salinity. In transgenic Arabidopsis carrying an AtMYB44 promoter-driven beta-glucuronidase (GUS) construct, strong GUS activity was observed in the vasculature and leaf epidermal guard cells. Transgenic Arabidopsis overexpressing AtMYB44 is more sensitive to ABA and has a more rapid ABA-induced stomatal closure response than wild-type and atmyb44 knockout plants. Transgenic plants exhibited a reduced rate of water loss, as measured by the fresh-weight loss of detached shoots, and remarkably enhanced tolerance to drought and salt stress compared to wild-type plants. Microarray analysis and northern blots revealed that salt-induced activation of the genes that encode a group of serine/threonine protein phosphatases 2C (PP2Cs), such as ABI1, ABI2, AtPP2CA, HAB1, and HAB2, was diminished in transgenic plants overexpressing AtMYB44. By contrast, the atmyb44 knockout mutant line exhibited enhanced salt-induced expression of PP2C-encoding genes and reduced drought/salt stress tolerance compared to wild-type plants. Therefore, enhanced abiotic stress tolerance of transgenic Arabidopsis overexpressing AtMYB44 was conferred by reduced expression of genes encoding PP2Cs, which have been described as negative regulators of ABA signaling. (+info)