Scientists investigate an innovative protein family in plants

By using the plant thale cress (Arabidopsis thaliana), the team was able to show how these proteins are essential for building watertight micro-nets at the root of the plant. This allows the plant to filter nutrients from the soil and protect itself from dangerous microorganisms.

Scientists from Belgium, Germany, the Netherlands and Switzerland came together to work on the study, which was funded in part by the PLANT-MEMB-TRAFF ('Plant endomembrane trafficking in physiology and development') project, which received a European Research Council (ERC) Starting Grant worth EUR 1 199 889 under the 'Ideas' Theme area of the EU's Seventh Framework Programme (FP7).

The proteins found by the scientists are a group of transmembrane proteins that the team are calling 'CASPs' (Casparian strip membrane domain proteins), due to their location on the Casparian Strips - belts of specialised cell wall material present in the root endodermis that generate an extracellular diffusion barrier.

The main role of the root endodermis is to manage nutrient uptake and stress resistance.

The team were able to identify these CASPs thanks to a fluorescent marking technique. They found that these proteins are coded by five different genes and play a central role in the formation of the Casparian Strips.

'These structures can be compared to the joints which make the spaces between the root endodermis airtight,' explains Niko Geldner, one of the researchers on the project. 'The CASPs form a sort of trellis on which other proteins then come to fix themselves to in order to form a sequence which leads to the creation of an extremely effective three-dimensional 'roadblock'. This fascinating discovery will allow us to better understand how the roots are capable of selecting good nutrients and eliminating the bad ones. In other words, how plants feed themselves.'

Given that most plants function along relatively similar lines, these results have implications for research in sustainable agriculture, in terms of how rice, maize, flour and even tomato plants get their daily nourishment.

The researchers behind the PLANT-MEMB-TRAFF project also state that as current comparisons between yeast and animals do not give us any reliable and coherent idea about what is truly fundamental or derived in eukaryotic membrane organisation, unbiased research on plant membrane trafficking is essential and can provide insight into an additional, divergent type of eukaryotic cell and allow a better appreciation of the evolution of eukaryotic membrane organisation.

'Ultimately, the idea would be to improve the uptake of nutrients by developing plants which need less water and fertiliser, from the perspective of a more sustainable type of agriculture,' states Niko Geldner.

Understanding the structure and function of endomembrane compartments is central to a mechanistic understanding of eukaryotic cell behaviour. Multi-cellular organisms show an increased complexity and specialisation in their endomembrane trafficking pathways. Higher plants have independently developed multi-cellularity and show a differently structured, but highly complex endomembrane system that regulates numerous fundamental processes, such as cell wall composition, plant nutrition or immune responses.

The specifics of the plant endomembrane's functioning are only really being understood now and for a long time were insufficiently addressed by homology-based approaches, which are inherently biased and limited to modules and pathways that are conserved between animals/yeast and plants.

This research is therefore particularly poignant as it marks a significant breakthrough in scientific knowledge. Although the first description of Casparian Strips was made more than 150 years ago by the botanist Robert Caspary in 1865, the mechanisms and inner workings of its proteins have remained a mystery until today.

More information: Roppolo, D., et al. (2011) A novel protein family mediates Casparian strip formation in the endodermi. Nature. DOI: 10.1038/nature1007

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