The plant that tells you when danger is coming...

"Synthetic Biology" is a term bandied about with increasing frequency but what does this actually mean? How is it different to standard procedures that experimentally alter organisms?


The key difference is that synthetic biology seeks to re-design or introduce complete biological processes. This is done by introducing artificial systems through a "bottom up" approach using DNA "modules" or "parts", a bit like the building blocks found in Lego kits. This is only possible where there is a sound knowledge of the processes concerned, and where they can be introduced without interrupting the normal functioning of the organism.


Plants are attractive candidates for synthetic biology as their cells are split into defined compartments (chloroplasts, mitochondria, etc.), and have well-understood pathways that determine the flow of metabolites. Hence, new functions can be packaged into organelles, where they cannot disturb other vital processes. This concept is highly exciting and has prompted a wealth of inspired "blue sky thinking" projects. Just imagine - what functions would you introduce into a plant? "Naturally fluorescent" Christmas Trees? (Just think - no more tangled wires or broken bulbs on the fairy lights!). "Odour-removing" houseplants that exude pleasing fragrances? Foodstuffs that combined novel or unusual flavours? Perhaps these are more "cosmetic" than useful, but one idea with great potential is the use of plants as biosensors.


Carbon monoxide poisoning is a constant threat from appliances that burn fuel, particularly as this lethal gas cannot be seen or smelt. The gas is produced when there in insufficient oxygen for the combustion process to be complete. Besides causing death, symptoms of carbon monoxide poisoning include headaches, breathlessness, nausea and dizziness. Hence, gas boards and product manufacturers advise clients to be vigilant in checking their homes and appliances regularly. But what if there was a "natural sensor" - a plant capable of giving out a clear signal when CO levels crept dangerously high?


An attractive idea and not as outlandish as it sounds! Professor June Medford from Colorado State University and her group have introduced a similar idea into the model plant, Arabidopsis thaliana (thale cress). Their aim was to design a sensory system for
Synthetic reporter systems are already well developed in plants and include introducing the gene for Green Fluorescent Protein (GFP) from Jellyfish, producing a green glow under standard conditions. The problem with these systems is that they are difficult to reset, and often require specialist equipment to detect, making them unsuitable for constant monitoring. Medford's group decided to use chlorophyll loss, a natural process which we are familiar with during Autumn, when the leaves die (senesce). Normally this occurs very slowly, over the course of several days; for chlorophyll loss to act as a useful biosensor, however, this would have to happen much faster. Outside of Autumn, the chlorophyll pigment is constantly turned over, with levels maintained by balancing synthesis and degradation. To induce rapid chlorophyll loss, the team introduced two synthetic gene circuits that would respond to an input signal by both inhibiting chlorophyll synthesis and promoting breakdown. In this case, the input signal was the synthetic steroid hormone 4-hydroxytamoxifen (4-OHT). To prevent chlorophyll from being manufactured, this gene circuit contained double stranded interfering RNA (diRNA) constructs specific for two key enzymes for chlorophyll manufacture, protochlorophyllide oxidoreductase (POR) and GENOMES UNCOUPLED 4 (GUN4). Interfering RNA constructs work by binding to the mRNA products of target genes (the coding molecules which allow DNA sequences to be translated into proteins) and causing them to be degraded by the RNA Induced Silencing Complex (RISC). This effectively silences the genes, whilst leaving the nuclear DNA intact. The group also introduced a gene circuit which activated the enzymes which break down chlorophyll. Interestingly, when the circuits were introduced to Arabidopsis separately, there was no detectable change in "greenness", suggesting that the plants could compensate by inhibiting degradation or upregulating synthesis. However when both synthetic circuits were introduced, the plants rapidly lost chlorophyll, becoming white within 48 hours. This is a distinct phenotype from the yellow colour which is revealed when the leaves senesce in Autumn - very useful to avoid confusion between a natural process and an alarm system! In addition, when 4-OHT was removed, the plants regained their green colour, effectively "resetting" the system. This is crucial to allow multiple detection and for the plants to act as "long-term sentinels".


Intriguing science! And what would you do with it? Please leave your comments below!




Reference: "A synthetic de-greening gene circuit provides a reporting system that is remotely detectable and has a re-set capacity", Antunes et al. 2006. Plant Biotechnology Journal, Volume 4, Issue 6.