Unlike animals, plants do not have a circulating blood system containing cell capable of fighting off bacterial invasion. Instead, they have to rely on various other techniques, which I covered in detail way back on my old Field of Science blog. One method they use is to kill off cells that are close to a bacterial or fungi infection in order to starve the oncoming infection and prevent it spreading.
This technique can work very well, killing off approaching pathogens before they cause too much damage and then just growing more leaves or branches to replace the ones left. It's an intriguing technique and so is completely alien to anything a human body would think of doing! I was fascinated the first time I wrote about it, which was why I was particularly interested to find an article in the latest PLoS Pathogens exploring the death proteins in barley.
The proteins the paper looks at are called R-proteins, short for "disease resistant proteins". When the barley recognises an approaching pathogen, such as a mildew fungi, it activates the R-proteins in the cells closest to the fungi. The idea is that faced with a barren landscape of dead cells the fungi will simply die from starvation. From the plant's point of view the trick is to coordinate these R-proteins, quickly and efficiently killing off the required cells without causing any damage to the rest of the plant.
In barley the relevant R-proteins are called the MLA proteins, and numbered 1-10. These proteins can either hang out in the nucleus (where the DNA is) or in the cytoplasm (the rest of the cell) It had been previously shown that a pool of MLA10 in the nucleus was required for resistance against a certain strain of fungi. In this paper the researchers wanted to examine this further. What the found was that while MLA10 in the nucleus was important for disease resistance, it alone was not enough to cause cell-death. In order for the plants cells to successfully die, MLA10 had to be present in the cytoplasm. Not only that but cytoplasmic MLA10 alone was enough to cause cell death.
In order to get a better understanding of how MLA10 works, the researchers created various proteins that were fragments of the MLA10, containing different combinations of the domains shown in the diagram above. For example they created a protein that was just the CC domain (the turquoise box), or the CC-NB domain. They then tested for cell-death by using a blue stain. One of the first things they found was that the CC domain is vital for cell death. Using any fragments that did not contain a CC domain completely removed the death response. They also rather interestingly found that the fragment that produced the strongest death response was the CC-NB fragment. This produced a stronger death than the CC-NB-ARC fragment which suggests that the ARC may have a negative regulatory effect.
To take a closer look at the importance of the CC fragment they created proteins with mutations in the CC part. Each of the 17 mutants had just one amino-acid changed. All except 1 of these mutants decreased the cell-death ability of the protein, some very significantly. The image below is taken from the paper, and shows the areas of the leaves with the different mutations stained for cell-death activity. There's one strong blue blob for the wild type (labelled CC in the top left) and one other mutant also showing cell-death ability down at the bottom.
When they looked at the location of the different fragment mutants, it was found that rather than different fragments locating to different places, the CC-NB fragment was found both in the nucleus and the cytoplasm. This was a little surprising as the LRR section of the protein is the part that carries out the binding to the DNA. However forcing the CC-NB fragment to stay only in the nucleus blocked the death response completely. Forcing it to stay in the cytoplasm increased the death response. The same results were seen with the full length wild-type MLA10.
In order to fit in with earlier research showing high localisation of MLA10 in the nucleus (and to explain what the hell the LRR domain is doing if it's floating around in the cytoplasm) the researchers propose an integrated model. They suggest that MLA10 in the cytoplasm is needed in order to initiate and amplify the death-signal, while MLA10 in the nucleus helps to trigger further disease resistance. They also imply that looking at "nuclear localisation" might be a bit simplistic, as responses can differ depending on exactly where in the nucleus the MLA10 hangs out. Around the edges of the nuclear envelope might allow one portion of the MLA10 to maintain sneakily cytoplasmic while the rest is inside the nucleus.
This research builds up a picture of an incredibly sophisticated plant defence system which, while it might not seem as complex as the human immune system, nevertheless depends on organised and structured intracellular behaviour. As the human population keeps expanding finding ways of protecting our crop species against disease, and to maximise the yield of what we can get out of the ground, will become more and more vital.
Credit link for T. Voekler
Reference: Bai S, Liu J, Chang C, Zhang L, Maekawa T, et al. (2012) Structure-Function Analysis of Barley NLR Immune Receptor MLA10 Reveals Its Cell Compartment Specific Activity in Cell Death and Disease Resistance. PLoS Pathog 8(6): e1002752. doi:10.1371/journal.ppat.1002752