PARASITIC PLANTS! A thread on why cheating beats competing, for the dodder parasite @jianqiang_wu @MikeAxtell @RunoSteven @kevinfolta @LynnSosnoskie @plantae_org @ASPB @PlantTeaching @QUBIGFS @FrontYoungMinds #Plants #PlantSci #Parasite
For the younger readers out there, a plain language summary on how a plant, commonly called the ‘dodder’ can steal its food and water from other plants, courtesy of the fantastic resource at @FrontYoungMinds doi.org/10.3389/frym.2… I am also super proud to be an editor here!
The dodder (Cuscuta campestris, or any of the other Cuscuta species) is a parasite, it needs to steal all its food and resource from other plants. The adaptations required to live like this make for a lot of really interesting behavioural contrasts!
These parasites, like the nematodes that I introduced earlier in the week, have a serious impact on agricultural productivity. Control options are limited, and often, the only way to deal with this issue is to burn everything - crop and parasite.
Most ‘normal’ plants respond to certain stimuli in a characteristic way. For example, when plants are in shaded conditions, they will try to escape that shade, by investing all their resource in growing up as fast as possible - shading is a negative stimulus...
...for the dodder parasite, shading is actually a positive stimulus, and tells it that there it could be underneath a plant leaf canopy, and so it grows in a helical fashion to try and grab hold of anything nearby! It also tries to ‘smell’ the host plant. Not even kidding.
Here’s the paper demonstrating that parasitic plants can sniff out suitable host plants to steal from: doi.org/10.1126/scienc…
The dodder parasite uses shading, smell and touch to discover a host plant, and when it makes contact, as you’ll see in the opening Gif, it winds around it using a behaviour called ‘thigmorphogenesis’ It alters its growth pattern to wind around the host plant stem. 
The parasite also sticks itself to the host plant stem by releasing a biological glue, rich in a substance called pectin. This is where it gets really interesting! We’re now at the start of biological siege. The parasite starts to develop an invasive organ called the ‘haustorium’
The haustorium uses enzymes and brute force to mechanically invade the host plant stem, as in the pictures below, and will develop specialised cells that will go ‘foraging’ for the host plants’ xylem and phloem cells, which contain all the water and nutrients the parasite wants.
Now the host plant isn’t taking this lying down - it is doing its best to prevent invasion from occurring, and will fortify plant cell walls with a super tough polymer called lignin, to create a mechanical barricade, and slow the parasite’s progress.
It will also be hammering the parasite with toxic radical oxygen compounds. Some plants are successful, and have the capacity to shut the parasite down before it gets in - when this happens you can see a patch of brown / dying cells around the invasion site.
The host plant kills all the cells around the invading parasite, knowing that it requires live cells to interact with and feed from. As I said yesterday in a different context, if the host plant can’t have those cells, the parasite can’t have them either!
If the parasite is successful, it will fuse to host phloem and xylem cells, creating a graft that allows transfer of molecules...IN BOTH DIRECTIONS! This is significant, because the parasite can now manipulate the host, but the host can also try to manipulate the parasite!
The host plant will try to send lots of toxic compounds through the haustorium into the parasite, and likewise, the parasite will be trying to calm the host down. They are both engaged in a chemical dialogue, trying to wrestle control from each other.
There is a lot of really interesting research in this space, but I’d like to point you in particular to @MikeAxtell & colleagues, who are looking at how the parasite uses small non-coding RNAs to manipulate host plant genes *REALLY* fascinating stuff! doi.org/10.1016/j.pbi.…
There is so much more to say on the basic biology, and we may well come back to that, but I’d like to talk briefly about some of our research in this area. I mentioned RNAi yesterday in the #GMO thread, it is a pathway that allows plant to reduce the amount of specified genes...
We’re using it to rejuvenate a well established herbicide target in the dodder - a protein called PDS (Phytoene DeSaturase)! Phytoene is a carotenoid pigment that is a substrate for hormone synthesis, and is involved in mopping up metabolomic waste in the cell.
As I said, PDS is already a target of established herbicides, but when you put organisms under extreme selective pressure, you can absolutely expect resistance to follow! We’re trying to target the same protein/gene using a very different approach...
...not by interfering with protein function, but by reducing the amount of gene and protein. You can see in the images below what happens when we use RNAi to reduce the amount of PDS in the model plant Arabidopsis thaliana!
We’ve used a variety of approaches to induce RNAi in the dodder parasite, including delivering the RNAi trigger to the parasite, through the host plant - below! We’re still collecting data, but in short, it works - the parasite gene is reduced, and it grows much less than normal!
The endgame here could be #GMO plants expressing the RNAi trigger, or it could be an RNAi spray that is applied exogenously, like any other herbicide. This is certainly a great deal of interest in this kind of approach - it is thought to be much cleaner and safer than other chems
Ok, so I’ve kids to feed, and football / rugby matches to watch them play in shortly, so we’ll jab the pause button on this. Hopefully I can revisit later. Thanks everyone for following along so far. BIOLOGY IS BANANAS, right?!
