PLANT PHYSIOLOGY WHOLE PLANT AND ECOPHYSIOLOGY

[WHOLE PLANT AND ECOPHYSIOLOGY] An External {delta}-Carbonic Anhydrase in a Free-Living Marine Dinoflagellate May Circumvent Diffusion-Limited Carbon Acquisition

Lapointe, M., MacKenzie, T. D.B., Morse, D. — 2008-07-08

The oceans globally constitute an important sink for carbon dioxide (CO₂) due to phytoplankton photosynthesis. However, the marine environment imposes serious restraints to carbon fixation. First, the equilibrium between CO₂ and bicarbonate (HCO₃^–) is pH dependent, and, in normal, slightly alkaline seawater, [CO₂] is typically low (approximately 10 µm). Second, the rate of CO₂ diffusion in seawater is slow, so, for any cells unable to take up bicarbonate efficiently, photosynthesis could become carbon limited due to depletion of CO₂ from their immediate vicinity. This may be especially problematic for those dinoflagellates using a form II Rubisco because this form is less oxygen tolerant than the usually found form I enzyme. We have identified a carbonic anhydrase (CA) from the free-living marine dinoflagellate Lingulodinium polyedrum that appears to play a role in carbon acquisition. This CA shares 60% sequence identity with -class CAs, isoforms so far found only in marine algae. Immunoelectron microscopy indicates that this enzyme is associated exclusively with the plasma membrane. Furthermore, this enzyme appears to be exposed to the external medium as determined by whole-cell CA assays and vectorial labeling of cell surface proteins with ¹²⁵I. The fixation of ¹⁴CO₂ is strongly pH dependent, suggesting preferential uptake of CO₂ rather than HCO₃^–, and photosynthetic rates decrease in the presence of 1 mm acetazolamide, a non-membrane-permeable CA inhibitor. This constitutes the first CA identified in the dinoflagellates, and, taken together, our results suggest that this enzyme may help to increase CO₂ availability at the cell surface.

[WHOLE PLANT AND ECOPHYSIOLOGY] Nitrogen Recycling and Remobilization Are Differentially Controlled by Leaf Senescence and Development Stage in Arabidopsis under Low Nitrogen Nutrition

2008-07-08

Five recombinant inbred lines (RILs) of Arabidopsis (Arabidopsis thaliana), previously selected from the Bay-0 x Shahdara RIL population on the basis of differential leaf senescence phenotypes (from early senescing to late senescing) when cultivated under nitrogen (N)-limiting conditions, were analyzed to monitor metabolic markers related to N assimilation and N remobilization pathways. In each RIL, a decrease of total N, free amino acid, and soluble protein contents with leaf aging was observed. In parallel, the expression of markers for N remobilization such as cytosolic glutamine synthetase, glutamate dehydrogenase, and CND41-like protease was increased. This increase occurred earlier and more rapidly in early-senescing lines than in late-senescing lines. We measured the partitioning of ¹⁵N between sink and source leaves during the vegetative stage of development using ¹⁵N tracing and showed that N remobilization from the source leaves to the sink leaves was more efficient in the early-senescing lines. The N remobilization rate was correlated with leaf senescence severity at the vegetative stage. Experiments of ¹⁵N tracing at the reproductive stage showed, however, that the rate of N remobilization from the rosettes to the flowering organs and to the seeds was similar in early- and late-senescing lines. At the reproductive stage, N remobilization efficiency did not depend on senescence phenotypes but was related to the ratio between the biomasses of the sink and the source organs.

[WHOLE PLANT AND ECOPHYSIOLOGY] A Diurnal Component to the Variation in Sieve Tube Amino Acid Content in Wheat

2008-06-04

We have used high-sensitivity capillary electrophoresis coupled to a laser-induced fluorescence detection method to quantify 16 amino acids in wheat (Triticum aestivum) sieve tube (ST) samples as small as 2 nL collected by severing the stylets of feeding aphids. The sensitivity of the method was sufficient to determine a quantitative amino acid profile of individual STs without the need to bulk samples to produce larger volumes for analysis. This allowed the observation of the full range of variation that exists in individual STs. Some of the total concentrations of amino acids recorded are higher than those reported previously. The results obtained show variation in the concentrations of phenylalanine (Phe), histidine/valine (His/Val), leucine/isoleucine (Leu/Ile), arginine, asparagine, glutamine, tyrosine (Tyr), and lysine (Lys) across the ST samples. These could not be explained by plant-to-plant variation. Statistical analyses revealed five analytes (Tyr, Lys, Phe, His/Val, and Leu/Ile) that showed striking covariation in their concentrations across ST samples. A regression analysis revealed a significant relationship between the concentrations of Tyr, Lys, Phe, Leu/Ile, His/Val, asparagine, arginine, and proline and the time of collection of ST samples, with these amino acids increasing in concentration during the afternoon. This increase was confirmed to occur in individual STs by analyzing samples obtained from stylet bundles exuding for many hours. Finally, an apparent relationship between the exudation rate of ST sap and its total amino acid concentration was observed: samples containing higher total amino acid concentrations were observed to exude from the severed stylet bundles more slowly.

[WHOLE PLANT AND ECOPHYSIOLOGY] The Stomata of the Fern Adiantum capillus-veneris Do Not Respond to CO2 in the Dark and Open by Photosynthesis in Guard Cells

Doi, M., Shimazaki, K.-i. — 2008-06-04

The stomata of the fern Adiantum capillus-veneris lack a blue light-specific opening response but open in response to red light. We investigated this light response of Adiantum stomata and found that the light wavelength dependence of stomatal opening matched that of photosynthesis. The simultaneous application of red (2 µmol m^–2 s^–1) and far-red (50 µmol m^–2 s^–1) light synergistically induced stomatal opening, but application of only one of these wavelengths was ineffective. Adiantum stomata did not respond to CO₂ in the dark; the stomata neither opened under a low intercellular CO₂ concentration nor closed under high intercellular CO₂ concentration. Stomata in Arabidopsis (Arabidopsis thaliana), which were used as a control, showed clear sensitivity to CO₂. In Adiantum, stomatal conductance showed much higher light sensitivity when the light was applied to the lower leaf surface, where stomata exist, than when it was applied to the upper surface. This suggests that guard cells likely sensed the light required for stomatal opening. In the epidermal fragments, red light induced both stomatal opening and K⁺ accumulation in guard cells, and both of these responses were inhibited by a photosynthetic inhibitor, 3-(3,4-dichlorophenyl)-1,1-dimethylurea. The stomatal opening was completely inhibited by CsCl, a K⁺ channel blocker. In intact fern leaves, red light-induced stomatal opening was also suppressed by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. These results indicate that Adiantum stomata lack sensitivity to CO₂ in the dark and that stomatal opening is driven by photosynthetic electron transport in guard cell chloroplasts, probably via K⁺ uptake.