Final Report Summary - CAMSTAR (Temporal regulation of starch degradation in CAM plants)
Starch plays a pivotal role in human and animal diets. In plants, starch fulfils a multitude of functions, including storage and providing a carbohydrate source to sustain plant growth. In addition to these roles, starch is of particular importance in a range of plants with the specialized mode of photosynthesis known as Crassulacean acid metabolism (CAM). CAM plants have higher (up to 80 %) water use efficiencies than the other modes of photosynthesis (i.e. C3 and C4) and this is accomplished by taking carbon dioxide (CO2) up at night, rather than during the day. Nocturnal CO2 uptake in CAM plants relies on the breakdown of starch to provide the phosphoenol pyruvate building blocks for the enzyme phosphoenol pyruvate carboxylase. Potentially two different routes exist in plants for the nocturnal breakdown of starch, namely the hydrolytic and the phosphorolytic routes. In the model C3 plant Arabidopsis thaliana, the hydrolytic pathway dominates starch breakdown. The aim of this 'Marie Curie Fellowship' was to test the hypothesis that the unique role of starch breakdown in CAM plants is achieved by deploying an alternative, i.e. the phosphorylytic, pathway for nocturnal starch degradation. The fellowship also tested the hypothesis that the circadian clock ensures appropriate partitioning of carbohydrates between export, growth and as substrates for nocturnal CO2 uptake in CAM plants.
To test these hypotheses, a number of genes that encode enzymes and transporters implicated in starch degradation, were genetically modified in the CAM plant Kalanchoë fedtschenkoi. The activities of chloroplastic alpha-glucan phosphorylase (PHS) and the chloroplastic glucose-P translocators (GPT1 and 2) were genetically modified by ribonucleic acid interference (RNAi) to test the involvement of the phosphorylytic pathway of starch degradation. In addition, the activities of beta-amylase 3 (BAM3) which is implicated in the hydrolytic pathway of starch degradation and BAM9, a putative transcription factor were genetically modified in K. fedstchenkoi. The role of the circadian clock in regulating starch degradation was examined using transgenic lines of K. fedtschenkoi over-expressing the central circadian clock gene, Timing of cab expression1 (toc1).
The results showed that a functioning clock is critical for gearing starch breakdown to nocturnal CO2 uptake and organic acid accumulation. The clock ensures that sufficient carbon is available for the nocturnal reactions of CAM and for maintaining growth. Moreover, an impaired clock was shown to restrain K. fedtschekoi from adapting the rate of starch degradation to changing environmental conditions (e.g. shortening / lengthening photoperiods), thereby reducing net carbon gain. Phenotypic characterization of the mutant lines which showed altered expression and activities of enzymes and transporters implicated in the phosphorylytic and hydrolytic pathways of starch degradation, demonstrated that a significant proportion of starch breakdown for CAM occurred via the phosphorylytic pathway. Thus, it would appear that the unique nocturnal requirements for carbohydrate in CAM plants are achieved using a different (to that in C3 plants) route for starch degradation.
The data obtained during this Fellowship have provided fundamental insight on the metabolic requirements for CAM. Understanding the mechanistic and molecular bases of a water-conserving pathway like CAM presents novel opportunities for the bio-engineering of new biological systems that address key challenges pertaining to food and water security, issues of critical importance in the European Union (EU) and beyond.
Contact details:
Johan Ceusters (Marie Curie Fellow), Faculty of Bioscience Engineering, Department of Biosystems, Division of Crop Biotechnics, KU Leuven, Willem De Croylaan 42, B-3001 Heverlee, BELGIUM. E-mail: johan.ceusters@biw.kuleuven.be
Anne Borland (Project Director), School of Biology, Newcastle University, NE1 7RU, UK. E-mail: anne.borland@ncl.ac.uk
To test these hypotheses, a number of genes that encode enzymes and transporters implicated in starch degradation, were genetically modified in the CAM plant Kalanchoë fedtschenkoi. The activities of chloroplastic alpha-glucan phosphorylase (PHS) and the chloroplastic glucose-P translocators (GPT1 and 2) were genetically modified by ribonucleic acid interference (RNAi) to test the involvement of the phosphorylytic pathway of starch degradation. In addition, the activities of beta-amylase 3 (BAM3) which is implicated in the hydrolytic pathway of starch degradation and BAM9, a putative transcription factor were genetically modified in K. fedstchenkoi. The role of the circadian clock in regulating starch degradation was examined using transgenic lines of K. fedtschenkoi over-expressing the central circadian clock gene, Timing of cab expression1 (toc1).
The results showed that a functioning clock is critical for gearing starch breakdown to nocturnal CO2 uptake and organic acid accumulation. The clock ensures that sufficient carbon is available for the nocturnal reactions of CAM and for maintaining growth. Moreover, an impaired clock was shown to restrain K. fedtschekoi from adapting the rate of starch degradation to changing environmental conditions (e.g. shortening / lengthening photoperiods), thereby reducing net carbon gain. Phenotypic characterization of the mutant lines which showed altered expression and activities of enzymes and transporters implicated in the phosphorylytic and hydrolytic pathways of starch degradation, demonstrated that a significant proportion of starch breakdown for CAM occurred via the phosphorylytic pathway. Thus, it would appear that the unique nocturnal requirements for carbohydrate in CAM plants are achieved using a different (to that in C3 plants) route for starch degradation.
The data obtained during this Fellowship have provided fundamental insight on the metabolic requirements for CAM. Understanding the mechanistic and molecular bases of a water-conserving pathway like CAM presents novel opportunities for the bio-engineering of new biological systems that address key challenges pertaining to food and water security, issues of critical importance in the European Union (EU) and beyond.
Contact details:
Johan Ceusters (Marie Curie Fellow), Faculty of Bioscience Engineering, Department of Biosystems, Division of Crop Biotechnics, KU Leuven, Willem De Croylaan 42, B-3001 Heverlee, BELGIUM. E-mail: johan.ceusters@biw.kuleuven.be
Anne Borland (Project Director), School of Biology, Newcastle University, NE1 7RU, UK. E-mail: anne.borland@ncl.ac.uk