Rice is the staple food for over 50% of the world’s population, but yields are threatened by rice blast, blight and other diseases caused by microbes. To protect our rice crops against these diseases, it is important that we understand how they can defend themselves against infection.
Plants have two defence systems that to allow them to identify and defend against microbes. The first system is a general response to the detection of molecules that are produced by many different microbes. For example, plants can detect chitin, which is the major component of the cell walls of fungi. So if a plant detects chitin it knows that there is a fungus lurking around and it can activate general defences.
The second system is much more specific to individual species of microbe. For example, the fungus that causes rice blast (Magnaporthe oryzae) produces particular molecules that help it to infect the plant, known as effectors. If the plant is able to detect any of these molecules it is able to identify the fungus as a threat and can activate more specific defence responses. For example, the plant may trigger the death of cells in the area around the fungus to halt the spread of the fungus.
Many proteins involved in plant defence responses have been identified, but it is not clear how they are all controlled to make sure that they are only active at the right times. One way to control proteins is to target them for degradation when they are not needed. A protein called SPL11 suppresses programmed cell death and defence responses in rice plants. It belongs to a family of proteins—called the U-box E3 ligases—that can tag other proteins with ubiquitin proteins to target them for destruction. However, it was not known which proteins SPL11 can target.
In a paper recently published in PLOS Pathogens, Liu, Park, He et al. found that SPL11 can add ubiquitin to a protein called SPIN6 (1). Rice plants lacking SPIN6 have higher levels of programmed cell death (see image) and increased resistance to M. oryzae and the bacteria that causes blight (Xanthomonas oryzae pv. Oryzae). The plants also had higher levels of expression of genes involved in defence responses and elevated levels of reactive oxygen species (another defence response).
SPIN6 is a member of a family of proteins—called the RhoGAPs—that are able to control the activity of members of another family of proteins called the ROP GTPases. The researchers found that SPIN6 can inactivate a ROP GTPase called OsRAC1, which is a key protein in rice defense responses that is activated when rice cells detect chitin.
OsRAC1 is involved in both general and more specific rice defines responses so SPIN6 may be involved in linking these systems to enable the plants to mount an effective defence. The equivalent of the SPIN6 protein in the model plant Arabidopsis thaliana also suppresses programmed cell death and defence responses in plants, so it is likely that other plants have proteins with similar roles. The next challenge is to find out how the activity of SPIN6 is controlled when a microbe invades to better understand what role it plays in defense.
Reference: 1) Liu, J., Park, C.H. He, F., Nagano, M., Wang, M., Bellizzi, M., Zhang, K., Zeng, X., Liu, W., Ning, Y., Kawano, Y. and Wang, G. (2015) The RhoGAP SPIN6 associates with SPOL11 and OsRac11 and negatively regulates programmed cell death and innate immunity in rice. PLOS Pathogens. http://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1004629