Tracing the roots of an ancient friendship

 

An AM fungus (yellow) contacts the surface of a plant root. The nuclei of the plant cells are visible as blue spots. Image adapted from ref 3. Credit: Andrea Genre and Mara Novero (CC BY 3.0).

Plants need nutrients to be able to grow. Unfortunately, many of these nutrients can be scarce in the soil and therefore hard to get hold of. To get around this problem, most plants are able to form friendly relationships – known as symbioses – with soil microbes that can provide them with certain nutrients in exchange for sugars.

Today, around 80% of land plants form symbioses with a group of fungi known as arbuscular mycorrhizal (AM) fungi (1). Fossil evidence suggests that this symbiosis first emerged around 450 million years ago. This is around the same time that plants first started to colonise land. The transition from water to the dry and harsh environments on land would have presented many challenges to the early land plants, for example, how to avoid losing too much water. Another challenge would have been how to access essential nutrients that their ancestor (a type of green algae) would have gained directly from the water.

The liverworts, hornworts and mosses are thought to be the earliest groups of land plants (2). Since the AM symbiosis is widespread in these groups, it has been suggested that this symbiosis is one of the innovations that helped these primitive plants to survive on land.

Previous studies have identified many plant genes that are needed for AM symbiosis in legumes and other land plants. These genes can be split into two main groups: some are in a signalling pathway needed for the plant and fungus to communicate with each other, and others are activated later to allow the fungus to infect into the roots of the plant. Recently, Pierre-Marc Delaux and colleagues used a technique called phylogenetics to analyse genetic material from many different algae, liverworts, hornworts and mosses with the aim of finding out when the AM symbiosis genes first appeared (2).

Delaux et al. show that these plant genes emerged in stages, starting from before earliest plants colonised land. The signalling pathway genes appeared first, and are present in the algae that are thought to be the closest relatives of land plants, the Charophytes (2). On the other hand, the infection genes appear to be missing from the algae, but are present in the liverworts, hornworts and mosses.

These findings suggest that the algal ancestors of land plants were pre-adapted to interact with fungi. Currently, there is no evidence to suggest that the Charophytes are able to form AM symbioses themselves. Therefore, it is possible the signalling pathway evolved to allow algae to interact with other microbes and was later altered to allow the early land plants to interact with AM fungi.

Reference:

1. Parniske, M. (2008). Arbuscular mycorrhiza: the mother of plant root endosymbioses. Nat Rev Microbiol, 6, 763-75.
(Good review of AM symbiosis, but unfortunately this article is hidden behind a paywall…)
2. Delaux P, Radhakrishnan GV, Jayaraman D, Cheema J, Malbreil M, Volkening JD, Sekimoto H, Nishiyama T, Melkonian M, Pokorny L, Rothfels CJ, Sederoff HW, Stevenson DW, Surek B, Zhang Y, Sussman MR, Dunand C, Morris RJ, Roux C, Wong GK-S, Oldroyd GED, Ané JM. 2015. Algal ancestor of land plants was preadapted for symbiosis. Proceedings of the National Academy of Sciences of the United States of America. 2015, DOI: 10.1073/pnas.1515426112, PMID: 26438870
3. Corradi N, Bonfante P. 2012. The Arbuscular Mycorrhizal Symbiosis: Origin and Evolution of a Beneficial Plant Infection. PLoS Pathog 8(4): e1002600. doi:10.1371/journal.ppat.1002600

How to live with a legume

Peas, beans and other members of the legume family of plants can form friendly relationships (symbioses) with nitrogen fixing bacteria from the soil. Guided by the plant, the bacteria infect into the root and colonise plant organs called nodules, which supply sugars to the bacteria. Nodules also provide conditions that enable the bacteria to efficiently convert nitrogen gas (N₂) into a form of nitrogen that plants can use to grow.

Endophytic bacteria (red) in surrounded by nitrogen-fixing bacteria (green) in a nodule from the legume Lotus japonicas. Scale bar = 50 um. Image from Figure 1 by Zgadzaj et al (2015) (Licenced under CC BY 4.0)

To set up a symbiosis, the plant and bacteria exchange signals to enable them to identify each other. The plants release molecules called flavonoids into the soil and, in return, the nitrogen-fixing bacteria produce molecules called Nod factors. These Nod factors activate signalling pathways that trigger many responses in the plant and allow the bacteria to enter. The bacteria need to produce the correct Nod factors to gain admittance and so most legumes are only able to team up with a few species of bacteria. Other bacterial molecules such as exopolysaccharides also play important roles in establishing the symbioses.

Sharing plant genes to fight crop disease

Rice suffering from bacterial rice blight caused by Xanthomonas oryzae pv. oryzae. Image by International Rice Research Institute (IRRI) via Flickr (CC BY-NC-SA 2.0)

In plants, the first line of defence against microbes involves the recognition of molecules that are produced by a wide variety of microbes. For example, many plants can detect the protein EF-Tu — which is essential for DNA replication in bacteria — and a molecule called chitin, which is found in the cell walls of fungi. Detection of these molecules activates defence responses that are thought to be able to repel most microbes.

However, some microbes are able to avoid these defences and so plants need to be able to employ other defence strategies that are targeted at those particular microbes. These defences involve genes known as resistance (or R) genes that detect specific molecules produced by the microbe. These genes have been widely used in the breeding of wheat and other crops to improve disease resistance. However, this form of resistance can quickly become ineffective as the microbes mutate or lose the gene(s) that make the molecules detected by the R genes. So, the search is on for more durable forms of resistance.

Mind the GAP: understanding how rice plants defend against disease

Image by Mathieu Schoutteten via Flickr (CC BY-NC-ND 2.0)

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.

Sugar helps plants to tell the time

Clocks within a clock. The cacti in this sundial all have their own internal circadian clocks. Image by the author.

Plants can harness light energy to produce their own sugars from carbon dioxide and water in a process known as photosynthesis. Much of the sugar produced during the day is stored as starch to be used as an energy source overnight when photosynthesis is not possible. To enable the plant to maximise photosynthesis during the day and regulate the use of its starch energy stores at night, plants need to be able to “tell the time”. Plants have an internal “circadian” clock, which maintains 24-hour rhythms that modulate many plant processes, including photosynthesis.

At the core of the plant circadian clock are three proteins called CCA1, LHY and TOC1. These proteins (known as transcription factors) can regulate the production of other proteins. In the morning, the CCA1 and LHY proteins repress the production of TOC1 protein (see figure below). Over the course of the day, the levels of CCA1 and LHY proteins decline as their production is repressed by other transcription factors, including PRR7. With fewer CCA1 and LHY proteins to prevent its production, TOC1 levels rise at dusk and this further represses CCA1/LHY production. During the night TOC1 production falls and CCA1/LHY levels start to rise again.

Sabotage of plant cell communication by invading bacteria

Leaf speck on a tomato leaf caused by Pseudomonas syringae infection. Image by Alan Collmer via Wikimedia Commons (CC0).

To protect themselves from infection by disease-causing microbes, plants have systems that detect potentially harmful microbes and activate defence responses. Disease-causing microbes can overcome these defences by producing proteins called effectors that can enter host plant cells and disrupt them. Understanding what these effectors do in host plants could be useful for the development of more disease-resistant crop plants. Unfortunately, the roles of many effector proteins are not yet understood.

One of the ways effector proteins can interfere with plant defence responses is to prevent the relay of danger messages from the site of microbe detection at the plasma membrane to other locations in the cell. For the signal relays to function, the various protein components need to be located in the right places in the cell (plasma membrane, cytoplasm, nucleus, vacuole etc.). The cytoskeleton, consisting of filaments of the protein actin, is required for this organisation and moves proteins contained within (or on) small membrane-bound structures called vesicles.

Making waves at the Plant Calcium Signalling meeting

Last week I went to the Plant Calcium Signalling Meeting in Münster, Germany. I really enjoyed the meeting and it was a great opportunity to get an update on the most recent research in the area.

I have a guest blog on Annals of Botany blog about my personal highlights of the conference click the link to read it.

If you haven’t seen it already, read the article I posted earlier this week about a new drought-tolerant barley variety that has been developed by some of my colleagues at the John Innes Centre in collaboration with researchers at the University of Jordan.

And don’t forget that the Organism of the Month here at Plant Scientist is the poppy. There are still loads in flower in the UK at the moment to take a look if you can. If you want to know more read Kirsty Jackson’s article.

CCaMK: a protein switch in plant-microbe symbioses

Most land plants can form symbioses with soil-living microbes. The microbes provide nutrients to the plants in return for carbon, in the form of sugars. The plants and microbes need to be able to communicate to enable the microbe to infect into the plant roots. Chemical signals from the microbe activate a signal relay (pathway) in plant cells (Figure 1). Central to this pathway is the activation of nuclear calcium oscillations (repeated increases and decreases in the concentration of calcium ions) and the subsequent activation of a protein called CCaMK.

Figure 1: Chemical signal from microbe is detected by receptors on the plant cell membrane. This leads to the activation of repeated increases and decreases (oscillations) in calcium, the activation of CCaMK and the switching on of symbiosis genes. Diagram by S. Shailes

CCaMK is a very unusual protein because it can bind to calcium ions in two different ways. Firstly, Calcium ions can bind directly to EF-hand domains on the protein. The second way is indirect via the binding of a calcium-binding protein called calmodulin (CaM) to another part of the protein (CaM-binding domain). CCaMK has similarities with the animal CaMKII protein. CaMKII is also activated by calcium oscillations but unlike CCaMK it can only bind calcium via CaM, not directly (it doesn’t have any EF-hands). So why does CCaMK need two ways to bind calcium?

Guest Post. Barring the gates: How plants defend against invading bacteria

This week I have swapped blogs with S.E. Gould (@labratting) from Scientific American. Visit her blog to see my post on communication between legume plants and bacteria during symbiosis.

Barring the gates: How plants defend against invading bacteria

Bacteria will exploit any opportunity to invade a new living space, in particular taking advantage of any easily-colonisable entrances into other living organisms. In plants one of these entrances is a doorway between the interior of the leaf and the outside air in order to exchange gases. Plants require carbon dioxide in order to carry out photosynthesis, and this has to be brought into the interior of the leaves. Similarly the excess oxygen produced must leave the cells before it builds up to toxic levels. In order to achieve this, plants have small holes in the leaves formed by two curved cells, known as guard cells, with a gap between them. These gaps are called stomata (singular stoma).

Pollen tubes: getting the sperm to the egg

Scanning electon micrograph of pollen grains. Image by Carsten Pietzsch distributed under a CC SA-BY 3.0 licence

Pollen. Enemy of hayfever sufferers everywhere, including myself. However, as a plant scientist I can (almost) forgive pollen for making me feel unwell because I find the role it plays in plant sexual reproduction fascinating.

A typical pollen grain from a flowering plant (angiosperm) contains a vegetative cell and a reproductive cell containing two nuclei. One of the nuclei from the reproductive cell splits to form two sperm cells. The group of cells is surrounded by a cellulose rich cell wall and a chemically resistant outer cell wall made mostly of sporopollenin. Within the pollen grain the sperm cells are protected from the environment (e.g. UV radiation or drying out) during the transfer from the anther (where pollen is produced) to the stigma, the entry point to the female part of the flower.