Chloroplasts are a key player in plant immunity

Chloroplasts are a key player in plant immunity

by UC Davis

Experimental seedlings in the laboratory. UC Davis plant biologists have discovered how chloroplasts, responsible for photosynthesis in green plants, also play a key role in plant immunity to infections. Credit: Sasha Bakhter, UC Davis College of Biological Sciences

Scientists have long known that chloroplasts help plants turn the sun’s energy into food, but a new study, led by plant biologists at the University of California, Davis, shows that they are also essential for plant immunity to viral and bacterial pathogens.

Chloroplasts are generally spherical, but a small percentage of them change their shape and send out tube-like projections called “stromules.” First observed over a century ago, the biological function of stromules has remained enigmatic.

Previous studies have shown that chloroplasts produce more stromules when a plant detects an infection. Stromules aid in clustering chloroplasts around the nucleus and function as conduits to transport pro-defense signals from chloroplasts to the nucleus. Despite these findings, researchers have not been able to determine the role of stromules in immunity, as no genes involved with the formation of stromules have been identified.

In the new study, Savithramma Dinesh-Kumar, professor and chair in the Department of Plant Biology, graduate student Nathan Meier and colleagues have identified a key protein involved in stromule biogenesis during immunity. Their findings were published Oct. 25 in Science Advances.

A hidden player in immune defense

In order to test the stromules’ role in immunity, researchers need to switch them off and then observe how stromule-less plant cells fare when faced with a pathogen. However, without knowing which genes are involved with the creation of stromules, researchers have had no way to know which genes to switch off.

To overcome this roadblock, Dinesh-Kumar and his colleagues turned to kinesins, proteins that function as tiny motors that allow molecules and organelles to move around a cell. This intracellular movement usually involves the cell’s cytoskeleton, which is made up of two different types of fiber: large microtubules and smaller actin filaments.

The researchers wanted to investigate a type of kinesin that is unique to plants and capable of binding both microtubules and actin filaments. The researchers found that overexpression of one of these kinesins, KIS1, induced stromule formation in the absence of pathogen infection.

When the researchers manipulated tobacco and Arabidopsis plants so that they could not produce the KIS1 kinesin, they found that neither plant was able to form stromules, and their chloroplasts did not migrate toward the nucleus. This left the plants unable to defend themselves from introduced pathogens.

Secrets of chloroplast movement

To disentangle the roles of microtubules and actin, the researchers engineered one set of KIS1 variants that could only bind to microtubules, and another that could only bind to actin. Expression of these variants in tobacco showed that KIS1 needs to bind to microtubules in order for chloroplasts to form stromules, but in order for chloroplasts to move toward the nucleus, it must also bind to actin.

The team also wanted to know how stromules fit into the bigger picture of plant immunity. By using genetic manipulation to switch different immune signals off, they found that stromule formation is triggered by molecular signaling and that an intact immune signaling system is needed in order for stromules to form.

“If we remove any of the known immune signaling genes, the chloroplasts lose the ability to make stromules, which suggests that these structures are an integral part of the immune signaling pathways that activate defense,” said Dinesh-Kumar.

New light on plant immunity

This study is the first evidence of a plant kinesin directly involved in plant immunity. It’s also the first time that scientists have identified a gene—KIS1—involved in chloroplast stromule biogenesis, which opens the door to understanding the role of chloroplast stromules and why chloroplasts cluster around the nucleus during plant immune defense.

“If we can better understand at the cellular level how organelles like chloroplasts help cells to defend themselves, we could help to engineer resistance to the pathogen,” Dinesh-Kumar said.

More information: Nathan Meier et al, Calponin-homology domain containing kinesin, KIS1, regulates chloroplast stromule formation and immunity, Science Advances (2023). DOI: 10.1126/

Journal information: Science Advances 

Provided by UC Davis 

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New insight into plants’ self-defense




Monday, 20 November 2023 12:05:05

Grahame Jackson posted a new submission ‘TOMATO BROWN RUGOSE FRUIT VIRUS – MOROCCO: FIRST REPORT’




Source: European Plant Protection Organisation (EPPO) Reporting Service 10/2023/235 [summ. Mod.DHA, edited]
The NPPO [National Plant Protection Organisation] of Morocco recently informed EPPO of the occurrence of _Tomato brown rugose fruit virus_ (_Tobamovirus_, ToBRFV – EPPO A2 List) on its territory. During the production season 2022-2023, about 10 outbreaks have been confirmed on tomato (_Solanum lycopersicum_) grown in glasshouses for fruit production. The sources of the outbreaks are infected imported seed.

ToBRFV has been a priority quarantine pest in Morocco since 2018, and official measures are taken in case of findings. They include the destruction of infected plants, restriction on cultivation of host plants and hygiene measures. In 2023, yield losses were observed, as well as increased management costs.

Communicated by:
[_Tomato brown rugose fruit virus_ (ToBRFV) was identified as a new member of the genus _Tobamovirus_ (type member _Tobacco mosaic virus_, TMV) in Jordan and soon after in Israel (see links below). Since then, it has also been reported from Europe and the Mediterranean region, where it continues to spread (see links below), as well as from China and North America, but so far not from South America. The virus was shown to affect also capsicum and has been detected in both plants and seeds of both crops. ToBRFV symptoms on tomato vary depending on host cultivar but may include chlorosis, mottling, mosaic, crinkling (rugosis) on leaves; necrotic spots on petioles and calyces; yellowish mottling, brown spots and rugosis on fruit to make them unmarketable. On capsicum, leaf symptoms are similar; fruits may be deformed with yellow mottling or green stripes. Almost 100% incidence was reported for some outbreaks in tomato, but not every fruit on an infected plant may show symptoms.

ToBRFV (like many tobamoviruses) is seed transmitted and can also be spread by mechanical means, contaminated equipment, as well as with plant or other materials. It is very stable and can remain infectious for months outside a host. Bumblebees, which are used widely as commercial pollinators in glasshouse tomato production, have been shown to be effective vectors of ToBRFV (see link below). Volunteer crop plants and solanaceous weed species are likely pathogen reservoirs. The Tm-22 resistance gene used in some tomato cultivars to protect from other tobamoviruses (such as _Tomato mosaic virus_) does not appear to be effective against ToBRFV. Disease management relies mainly on exclusion but may include phytosanitation (disinfecting tools, removing crop debris) and control of virus reservoirs. Use of certified clean seeds or crop transplants is crucial. Research on possible seed treatments to eliminate the virus is being carried out (see link below). Tomato seeds are traded widely and are known to pose a risk of spreading viruses and other pathogens internationally (e.g., ProMED post 20140122.2222560).

Coinfection of ToBRFV with _Pepino mosaic virus_ (genus _Potexvirus_) and _Tomato spotted wilt virus_ (TSWV; genus _Orthotospovirus_) has been found in tomato (ProMED posts 20191029.6751082, 20200507.7307615), as well as with TSWV in capsicum (see link below). It is thought that the respective symptoms may have been due to either virus or synergism. Further research is needed to clarify a potential role of ToBRFV in coinfections and to determine whether its presence in coinfections may have led to earlier cases of misdiagnosis and delayed identification of this new virus.
ToBRFV on tomato:×0/4137.jpg and×0/4138.jpg
ToBRFV symptoms on capsicum:

Information and characterisation of ToBRFV: (with distribution and host list), (Jordan), (Israel), (TSWV co-infection, capsicum) and via
ToBRFV spread: (new reservoir hosts) and (by pollinators)
Tomato resistance breeding:, and
ToBRFV seed treatment:
Recent ToBRFV updates. Europe:, in-sardinia,, (1st report Slovakia, ex Austria) and (first at seed production and breeding site)
International spread of tobamoviruses by seeds (review):
Virus taxonomy via:
EPPO A2 quarantine list:
– Mod.DHA