Investigating photosynthesis

In today’s highlighted article, the authors use traditional and far more modern biochemistry to uncover why photosynthesis is inhibited by Streptomyces spp., and characterise a previously unknown step in cyclic electron flow. This is also a good opportunity to point out these great photosynthesis outreach and education resources from Science and Plants for Schools. Admittedly, they don’t have anything on AA-sensitive CEF, because it’s unlikely anyone without a plant biochemistry PhD needs to know about that! But they have brilliant basic photosynthesis teaching resources, including these amazing algae-jelly-balls.

Photosynthesis background: Broadly speaking, photosynthesis is the process by which light energy from the sun is absorbed by Photosystems I and II (PSI and PSII), where it is channelled into electron transport chains and stored in ATP and NADPH. One electron carrier is ferredoxin (Fd).

There are two types of electron flow, cyclic and linear (CEF and LEF), which generate ATP. Though they are different processes, both CEF and LEF require PSI and PSII, two other thylakoid proteins, PGR5 and PGRL1, and electron carriers Fd, plastoquinone (PQ) and plastocyanin.

In CEF there are two processes by which electrons are transferred from Fd to PQ. One is a characterised NADH dehydrogenase-like compex dependent pathway, and all that is known about the other is that it is sensitive to antimycin A (AA), a product of Streptomyces spp. which inhibits CEF.

For background on AA-inhibition and the divergent electron transfer pathways in CEF, see Joët et al. (2001; Plant Phys. 24:1919). For more information on PGR5 and PGRL1, see DalCorso et al. (2008; Cell 132:273).

Gaps in knowledge of CEF:

  • What makes that electron transfer process from Fd to PQ sensitive to AA?
  • How do the electrons from photoreduced Fd get transferred to PQ and back into the electron transport chain?
  • What do functions do PGR5 and PGRL1 perform?

New from Hertle et al.: Titration and Western blotting experiments showed that PGR5 and PGRL1 dimerize to each other. Six cysteine residues were conserved in all PGRL1 proteins. Hertle et al. made mutant PGRL1 proteins in which one or more cysteine residues were substituted for serine, and tested the varients for their capacity to bind PGR5, iron, and to promote AA-sensitive CEF. They worked out that all six cysteines were essential for AA-sensitive CEF, while specific cysteines were involved in binding iron and PGR5.

In vitro assays demonstrated that PGRL1 is capable of transferring electrons between Fd and PQ analogue DMBQ in the presence of PGR5, and this reaction was inhibited in the presence of AA. Hertle et al. show clearly that PGRL1 is physically a fit for a ferredoxin-plastoquinone reductase and make a strong case for it being the mediator between Fd and PQ in CEF, and the AA-sensitive step in the cyclic electron flow.

Highlighted article: Alexander P. Hertle, Thomas Blunder, Tobias Wunder, Paolo Pesaresi, Mathias Pribil, Ute Armbruster, Dario Leister (2013) PGRL1 Is the Elusive Ferredoxin-Plastoquinone Reductase in Photosynthetic Cyclic Electron Flow. Molecular Cell – 03 January 2013, 10.1016/j.molcel.2012.11.030

Plant defence with Katherine Denby

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Published on: December 18, 2012

The second of our video podcasts from PlantSci 2012 is from Katherine Denby, from the University of Warwick. She works on how plants respond to changes in their environment, and in particular in response to pathogens. If you have a slightly cloudy idea of what systems biology is she explains it very well here, including how it can affect future food security. She also explains why she works on Arabidopsis, saying, “It’s just so much quicker to do things in Arabidopsis!”

Overyielding in species mixtures

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Published on: November 22, 2012

Highlighted article: Gerlinde B. De Deyn, Helen Quirk, Simon Oakley, Nick Ostle, Richard D. Bardgett (2012) Increased Plant Carbon Translocation Linked to Overyielding in Grassland Species Mixtures. PLoS ONE 7(9): e45926. doi:10.1371/journal.pone.0045926

Plant biomass yield is often greater in areas where species richness is high than it is in monocultures. This has implications for agriculture, and also the use of non-farmed land as a carbon sink, as more biomass means more carbon assimilation. However, the relationship between growth and species richness on a plot of land is not constant or clear, so a group in Lancaster investigated it. I think their research threw up more questions than it answered, but the authors found intriguing links between lifetime biomass yield and speed of carbon transport from the leaves to other parts of the plants and found that non-legumes and legumes alike benefit from growing alongside one another.

Deyn et al. planted seedlings in monocultures or in a mixture and sampled soil and plant matter at 2, 24, and 48 hours, and finally at 8 days, after labeling carbon in the system using a 13CO2 pulse (Ostle et al., 2003). They assessed carbon assimilation and carbon/nitrogen ratio. Two years later, all the above-ground vegetation was harvested and weighed to obtain ‘yield’ data. The species used were common grassland species Trifolium repens and Lotus cornicalatus (both leguminous species), Plantgo lanceolata, Anthoxanthum odoratum, Achillea millefolium, and Lolium perenne. (more…)

Molecular Plant Pathology’s Top Ten

Keeping with last week’s plant pathology theme, I thought I’d highlight a paper that came out in the spring in case any plant pathologists missed it. This may also help people in other fields of plant science out there who might need to hold their own in a pathology-based conversation occasionally. In April 2012, Molecular Plant Pathology published ‘The top 10 fungal pathogens in molecular plant pathology,’ as voted for by 495 readers of the journal.

Quesadillas made with corn infected with Ustilago maydis, which is called huitacoche.

The pathogens chosen are:

  1. Magnaporthe oryzae, the cause of rice blast disease.
  2. Botrytis cinerea, also known as grey mold – probably the cause of the mold on the strawberries in the back of the fridge that you bought when they were offer.
  3. Puccinia spp., the cause of an unpleasant range of rust diseases that occur on wheat.
  4. Fusarium graminearum, a cereal pathogen commonly known variously as head blight, ear blight, or, delightfully, head scab. Infected grain can be poisonous.
  5. Fusarium oxysporum, a ubiquitous soil-borne pathogen that can infect many species, including important fruit species, and humans.
  6. Blumeria graminis, powdery mildew, which infects wheat and barley.
  7. Mycosphaerella graminicola is also called Septoria tritici and causes blotch disease in wheat.
  8. Colletotrichum spp. can infect a large range of crops, and latent infections can destroy stores of fruits post-harvest.
  9. Ustilago maydis, or corn smut, is actually cultured on corn cobs by farmers in Mexico, where the infected corn is called huitacoche and is a common recipe ingredient.
  10. Melampsora lini, or flax rust, the classic model plant pathogen.

The paper gives a ‘resume’ of each one, written by an expert in that particular species. Is your favourite pathogen missing? What other ‘Top 10’ would be interesting to put together?

Paper: DEAN, R., VAN KAN, J. A. L., PRETORIUS, Z. A., HAMMOND-KOSACK, K. E., DI PIETRO, A., SPANU, P. D., RUDD, J. J., DICKMAN, M., KAHMANN, R., ELLIS, J. and FOSTER, G. D. (2012), The Top 10 fungal pathogens in molecular plant pathology. Molecular Plant Pathology, 13: 414–430. doi: 10.1111/j.1364-3703.2011.00783.x

Teaching resources: This is an exercise easily recreated in a tutorial group or classroom. Groups of students could each make a pitch for their plant pathogen (or crop species, or organelle, etc …) and the whole group would vote for which one deserves the ‘top’ spot.

Image credit: Lesley Téllez, via her blog The Mija Chronicles

Arabidopsis basics

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Published on: November 2, 2012

We added a new page to this blog to show how basic Arabidopsis research works. This is for anyone who is on the periphery of plant molecular biology but doesn’t work directly on Arabidopsis thaliana – teachers, undergraduates, environmental scientists … whoever.

To go with the new page, here is a time-lapse video of growing Arabidopsis thaliana plants. Although it is a little out-of focus at the beginning, I chose it to show you because it shows how frustrating Arabidopsis research can be! At around 1.20, the plant on the left starts to bolt, which is when the stem begins to grow. It grows so quickly after all those days of watching the rosette leaves get bigger. The plant on the right, which has the same size rosette, doesn’t start to bolt until about 1.34 – it probably won’t catch up with the first plant until they both have siliques that are drying out. Developmental stage is important in a lot of experiments, so it is often necessary to grow far more plants than you expect to use so that you have a good selection of plants at the same growth stage when you start the experiment.


The genetics of broad-spectrum resistance

Downy mildew infection of Arabidopsis thaliana seedlings

Highlighted article: Dmitry Lapin, Rhonda C. Meyer, Hideki Takahashi, Ulrike Bechtold, Guido Van den Ackerveken (2012) Broad-spectrum resistance of Arabidopsis C24 to downy mildew is mediated by different combinations of isolate-specific loci. New Phytologist DOI: 10.1111/j.1469-8137.2012.04344.x

It is a mark of how effective plant immune systems are that most bacteria, fungi, and viruses do not affect plants at all either because plant tissues are not suitable for them to live in, or they are fended off. Of course there are pathogens that are compatible with plants – and within species that share compatibility, there are pockets of resistance. Some sub-groups are resistant to specific pathogen isolates, and this is caused by dominant resistant genes. A much broader, more complicated, and less common form of resistance occurs when a particular accession is resistant to a whole pathogen species, or several species. This is broad-spectrum resistance, and it can be caused by a simple dominant gene or multiple genes. Natural broad-spectrum resistance is not simple to transfer from its origin to a commercial crop because it can come from a complex set of genes which are not necessarily all dominant. (more…)

Friday Film: Powerful Plants

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Published on: October 12, 2012

If you thought that plants were stationary, lazy beings, think again. Bladderworts are water-dwelling carnivorous plants that trap prey by storing elastic energy in the trap body and releasing it by very fast opening and closing of a water-tight trap door. This video was made by Phillippe Marmottant and his research group from Grenoble, who published the mechanism of bladderwort action in their 2011 paper.

Another super fast plant is the dogwood species, Cornus canadensis, whose flowers explode faster than a rifle shot as they disperse their pollen.

While exploding plants and super-suction make for exciting viewing, they happen too fast for the human eye to see without the benefit of slow motion footage. It is possible to show slightly slower plant reflexes to students in schools, though – SAPS have a carnivorous plants information page and worksheets.

The cost of glucosinolate biosynthesis

Highighted article: Michaël Bekaert, Patrick P. Edger, Corey M. Hudson, J.Chris Pires, Gavin C. Conant (2012) Metabolic and evolutionary costs of herbivory defense: systems biology of glucosinolate synthesis. New Phytologist 196:596–605.

Research published in a current New Phytologist paper uses a systems biology approach to demonstrate the metabolic and evolutionary costs of producing glucosinolates for defence.  Bekart et al. used AraGEM (Oliveira Dal’Molin et al., 2010) as a starting point. They collected data on Arabidopsis glucosinolate genes by scouring published papers and downloading their expression patterns from AtGenExpress. This information was integrated into the basic dataset from AraGEM. The complete list of genes involved in glucosinolate reactions, including references, is in Supplementary Table S1 of the paper.

The team performed flux balance analysis on the integrated database to estimate metabolic and energy flux through reactions in the system both with glucosinolate biosynthesis activity and with none. They found that glucosinolate biosynthesis affected flux incidentally through 241 reactions in addition to the 196 reactions which are only active when glucosinolates are being produced.

The main finding of the research is the heavy cost of glucosinolate biosynthesis. Sulphur import dramatically increased when glucosinolates were being synthesised, and demand for water, carbon dioxide, ammonia, and photons increased too. Despite the increase in substrate import, biomass synthesis fell by around 15% during glucosinolate production. This cost is reflected in other studies demonstrating that the evolutionary competitive edge glucosinolates give to plants is a disadvantage when there are no predators around (Mauricio, 1997), and reduces the number of seeds and flowers produced per plant compared to non-producers (Stowe and Marquis, 2011). (more…)

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