In today's post, I will share an image that I took when I was still a postdoc. It is a confocal microscopy image of the interaction between two transgenic organisms: Nicotiana benthamiana and Phytophthora capsici. P. capsici loves infecting its plant host N. benthamiana (the plant doesn't to be fair).
In fact, many pathogens like to infect N. benthamiana, most notably, the natural genetic engineer Agrobacterium tumefaciens. I took advantage of this by doing the following:
1. Express an Endoplasmatic reticulum (ER) localized marker protein (Cyan Fluorescent protein or CFP) in N. benthamiana epidermal cells. I did this by ectopically expressing this protein through A. tumefaciens infiltration into leaf panels.
2. I took a transgenic P. capsici strain that expressed the red fluorescent protein TdTomato and used it to infect the leaf panels I prepared as described above.
3. I waited 48 hours and then used confocal microscopy to image plant cells (seen as these puzzle shapes) that were being infected by P. capsici. For this, I used three channels: 1-TdTomato (red), CFP (cyan coloured) and bright field.
Confocal image of P. capsici infected N. benthamiana (Source: own collection)
OK, the image. What does it show? On the image, you can see how a red structure pushes into a 'balloon' coloured in cyan. The balloon is the plant ER, labelled with the marker, pressed against the host cell membrane. The red structure is the pathogen (remember, it expresses TdTomato) and pushes into the host cell to make the balloon. Alternatively, it is pushing against the host cell without breaking it (similar to someone putting on a glove). This structure is called the haustorium and is a specialized "organ" that we now think is important for the interaction between host and pathogen. Mostly communication and exchange of molecules.
Why is this important?
Well, in the case of Phytophthora, haustorium formation is a critical step in infection. When this fails, infection stops or the disease cycle cannot be completed (Avrova et al., 2008). The formation of the haustorium and establishment of infection is, therefore, a stage that is targeted by plants. We know plants do this by deploying receptors that can detect effectors that are secreted from the haustorium into host cells (see my recent posts on this). It also means that if plants deem this stage to be important, we can also try to hit this stage of the infection cycle and try to prevent infection or its progression. That, in turn, should reduce disease pressure!
What is next?
While these interactions are extremely intimate and important, we know fairly little about the things that happen inside the pathogen during infection. How do filamentous pathogens build these haustoria? How does it develop such intricate structures whilst keeping the plant defence response at bay? Is the Haustorium required for infection? what are the plant signals that inform the pathogens' next steps? Gene expression, metabolomics and functional analyses are currently being done to answer these and many other questions. By answering these questions, we can not only start to shed light on the fundamental processes that underpin plant infection, but we can also come up with new methodologies or chemistries to inhibit these processes and disable pathogenesis. These advances, in turn, will limit crop diseases and increase yields.
To read more about how pathogens and plant slug it out in the quest for survival, please find links to other episodes and articles below:
Avrova, A. O., Boevink, P. C., Young, V., Grenville‐Briggs, L. J., Van West, P., Birch, P. R., & Whisson, S. C. (2008). A novel Phytophthora infestans haustorium‐specific membrane protein is required for infection of potato. Cellular microbiology, 10(11), 2271-2284.