The N-end rule pathway controls plant response to drought

In order to maintain food security for a growing population under climate change, there is a pressing need to develop crops that use less water and that are more tolerant to environmental stresses, particularly drought. Our recent research at the Universities of Nottingham and Sheffield using the laboratory model Arabidopsis thaliana (the plant equivalent of a lab-rat) has identified plants that are extremely tolerant to drought stress. The drought tolerant plants lack the ability to recognise particular proteins inside their cells and send them for destruction by a biochemical process called the N-end rule pathway. From our analysis so far, we already have an important clue as to the identity of the 'drought tolerance protein' that is not destroyed and is therefore stabilised in our drought-tolerant plants. We know that this protein has the amino acid residues methionine and cysteine at one end. In this project we shall identify which specific protein(s) are stabilised in our plants providing us with important information on how we could, in the future, make plants more drought-tolerant. We will also find out if the same N-end rule drought tolerance system works in an important UK crop, barley.

Many parts of the UK have experienced unusual and extended periods of drought over the past year which has included the driest 12 months since records began in 1910. This has led to a high level of crop failure with 2011 grain yields being particularly affected. Farmers have reported failure of 10% to 50% of their barley crop, and the surviving barley grain has often been of poor quality, only suitable for low value animal feed rather than for beer-making. As this is a problem for both farmers and brewers, SABMiller a major UK brewing company have agreed to fund part of our research project. SABMiller are committed to reducing the amount of water that they use, and believe that a better understanding of how barley responds to drought may help them to achieve this aim.

Our research falls directly within the remit of Global Food Security and Living with Environment Change cross-council priorities identified by the UK Research Councils. The project will help to address the BBSRC strategic research priority Food Security (Crop Science) and our collaboration with SABMiller addresses the Building Partnerships (Collaborative Research with Users) agenda.

The N-end rule pathway is an evolutionarily conserved process that selects proteins for degradation by the proteasome, depending on the amino acids at their N-terminus. For example, proteins with an N-terminal arginine residue are particularly unstable as they are rapidly ubiquitinated by PROTEOLYSIS 6 (PRT6) E3 ligase. We recently identified the first plant substrates for the N-end rule pathway and showed that they regulate seed dormancy, and plant responses to waterlogging. In our recent unpublished research we have discovered that the N-end rule pathway also regulates the plant drought response. Arabidopsis prt6 knockout mutants are highly drought-tolerant. They appear to be 'primed' to react to drought signals as prt6 stomatal closure is significantly more sensitive to ABA than wild-type plants, and prt6 plants reduce transpirational water loss much more rapidly under water stress. In this project we shall characterise, in both Arabidopsis and barley, the protein substrates for PRT6 which when stabilised in the mutant confer drought tolerance. We have already studied other mutants to identify which branch of the N-end rule pathway the drought-tolerant substrates are prepared by. We found that when arg tRNA transferase or methionine amino-peptidase activity is removed, stomata are also ABA-hypersensitive. Because of the substrate specificity of these enzymes, this indicates that the protein(s) that confer drought tolerance must have methionine and cysteine residues at the N-terminus. The major group of 'MC-proteins' in plant genomes are the MC-ERF transcription factors. We know that these regulate ABA-responses in seed and that other AP2/ERFs affect drought stress responses. The main focus of our research will be to characterise the specific roles of the five MC-ERFs and their downstream targets in drought responses. We will also study the regulation of MC-ERF proteolysis via cysteine oxidation, and the possibility of MC-ERF activation by nuclear translocation.

Who will benefit from this research?

Beyond academic audiences where the impact is obvious in terms of increased understanding of how a model plant cell signalling system works, the major beneficiaries of the work described in this proposal will be the commercial private sector and the wider public, and perhaps ultimately the farming and brewing sectors.

How will they benefit?

If successful our research could in the future lead to the production of crops that have increased drought tolerance and/or require less water during cultivation. We are aware that water use efficiency in crops is a key strategic target for agricultural sustainability. The work described in this application has the potential to be exploited by plant breeders aiming to produce more drought tolerant crops tailored to an increasingly unstable global climate scenario. The commercial arms of both of our Universities recognise the importance of our findings and are already engaged in applying for a patent to protect the results arising from our work on the role of the N-end rule pathway in regulating abiotic stress in plants. We have already begun to discuss the potential of the results arising from this project with the several commercial user groups. We have presented our proposed experiments to the brewing company SABMiller who offered to help support our research through this IPA application. SABMiller agreed to part-fund our research because of its potential to reduce water use in barley production. We will continue to discuss our results through regular progress meetings with SABMiller, and with other appropriate commercial end users whenever possible to inform them of our findings. For example, we will have a display at Cereals, the leading technical event for the UK arable industry which attracts approximately 30,000 visitors annually.
A wider commercial user group have the potential to benefit as our project will train highly skilled workers for the workforce. We will produce scientifically and computer literate researchers, with excellent oral and written presentational skills.
Engaging with the wider public will be undertaken by all of the investigators and researchers employed on the project. We believe that through our efforts the general public will benefit from a greater knowledge of how plants work and an understanding of how tax payers' money is being used to help contribute towards global food security. To engage with this wider sector of end users, the aims of our work will be presented to visitors to our institutions (for example during science week events, university open days and graduation celebrations). We will produce a series of display boards aimed at introducing the general public to the requirement for food security, the need to produce more water use efficient crops, and to outline how modifying ABA signalling and stomatal behaviour is likely to impact on this process. We will also make full use of existing CPIB infrastructure in Nottingham, and the Project Sunshine initiative in Sheffield to access further scientific engagement events both inside and outside of the university environment.