Plants that rely mostly on C4 carbon fixation

Phaitun Bupphada
University of Liverpool
“Functional genomics of CAM in the monocot biofuels feedstock crop Agave sisalana”
Phaitun Bupphada
Crassulacean acid metabolism (CAM) is a metabolic adaptation of photosynthesis in which primary CO2 fixation occurs in the dark yielding and storing malate. Stomata close during the light due to malate decarboxylation enhancing water use efficiency 6-20 fold relative to C3 photosynthesis. Certain CAM crops have been recognised as having great potential for the production of renewable biomass from seasonally dry lands. In this project we are studying the functional genomics of CAM development and light/ dark regulation in Agave sisalana. The A. sisalana transcriptome has been de novo sequenced and assembled using 454 sequencing. Semi-quantitative RT-PCR analysis was employed to study the regulation of CAM and circadian clock-related genes in leaf tissues. The transcript level of CAM genes was highest in the mature leaf tip, lower in the young, expanding leaf base and very low to undetectable in the most basal white leaf tissue. Several CAM and clock genes were found to display robust light dark oscillations in transcript abundance. Illumina RNA-seq analysis is now underway using these different parts of the A. sisalana leaf sampled in the light and dark. After obtaining RNA-seq data, comprehensive analysis to identify novel CAM regulatory genes will be performed.

All types of carbon fixation eventually use the Calvin cycle to make CO2 and sugar.

Paula Natália Pereira
University of Sao Paulo
“Regulation of PEPC and NR activities by endogenous cytokinins in Guzmania monostachia CAM-induced leaves”
Pereira, P.N., Kerbauy, G.B., Mercier, H.
Guzmania monostachia, an epiphytic bromeliad, can shift from C3 photosynthesis to crassulacean acid metabolism (CAM) in response to changes in the environmental conditions. Based on the limited literature data available on the endogenous signaling networks controlling carbon and nitrogen metabolisms and taking advantage of the high photosynthetic plasticity exhibited by G. monostachia plants, this study investigated a possible influence of endogenous cytokinins in the regulation of phosphoenolpyruvatecarboxylase (PEPC) and nitrate reductase (NR) activities. To achieve this aim, detached leaves of G. monostachia young plants were subjected to a treatment with 30% polyethylene glycol (PEG) for 7 days or were maintained in water for a similar period (control). Subsequently, the PEPC and NR activities and endogenous levels of four cytokinin types (zeatin, Z riboside, isopenteniladenine and iP riboside) were determined throughout the diel cycle in the apical and basal leaf portion. Interestingly, the day/night pattern of NR and PEPC activities revealed that both occur mainly during the night either in leaves performing CAM (water deficit) or in control ones. However, these enzymes had a spatial separation along the leaf, since NR had the highest activity in the base, whereas PEPC in the apical portion. In relation to the regulation of NR and PEPC activities by cytokinins, it was observed a hormonal peak before the highest NR activity during the night in the basal part of the leaves, whereas for PEPC activity, the peak of cytokinins occurred in the apical region during the day when there was the lowest activity of this enzyme. Financial Support: CAPES/FAPESP.

What is a common feature of all forms of carbon fixation?1.

These are Oxygen Evolution and CarbonFixation.

To minimize the oxygenase activity of rubisco and the concurrent loss of carbon through the photorespiratory cycle, C4 photosynthesis appears to have evolved as one of the major carbon-concentrating mechanisms used by land plants to compensate for limitations associated with the low level of atmospheric CO2. M.D. Hatch and C.R. Slack elucidated what is now named the C4 photosynthetic carbon cycle (also known as the Hatch-Slack cycle or the C4 cycle). They established that malate and aspartate are the first stable, detectable intermediates of photosynthesis in leaves of sugarcane. This novel metabolic pathway takes place in two morphologically distinct cell types, the mesophyll and bundle sheet cells. In the C4 cycle, the enzyme phosphoenolpyruvate carboxylase (PEPCase), rather than rubisco, catalyzes the primary carboxylation in a tissue that is close to the external atmosphere. The resulting 4-carbon acid flows across the diffusion barrier to the vascular region, where it is decarboxylated, releasing CO2 that is refixed by rubisco via the Calvin-Benson cycle.