There are pores on the underside of a typical plant leaf that allow carbon dioxide in while letting oxygen and water vapour out to support photosynthesis and the movement of water up the plant. As such these pores support oxygen and water movements on the planet, enabling life. These pores are called stomata, which open and close in response to a plant’s needs during the day, night, rain, drought and other environmental conditions.
It is also necessary for a plant to close their stomata to prevent pathogenic microbes from entering the interior of the leaf. It’s a bit like the fire doors of a building closing automatically when a smoke detector is activated. However, not all of the links between the detector and the closure mechanism are known in the case of stomata.
Professor Cyril Zipfel (at The Sainsbury Laboratory and the University of Zurich) and colleagues from different international laboratories have now identified one of the key missing links in the chain.
Stomata are composed of two guard cells that form two sides of a ring-like shape. These cells can expand or contract, leading to closure or opening, respectively. One of the early events in the signalling mechanism is the rapid influx of calcium ions into the guard cells. However, the identity of the channel through which the ions move in response to the presence of a pathogen was unknown.
As described in their Nature paper, the international team, including groups in UK, Switzerland, USA, Germany and Finland, has shown that OSCA1.3 is the long sought-after channel. They also showed that the opening of this channel is a specific response to the presence of pathogens, rather than environmental changes such as drought. They proved the channel is activated by a core component of the plant immune system, a protein called BIK1. Thus, a direct link has been made between microbial detection and the closure mechanisms. This finding also reveals the first plant calcium channel identified with a role in stomatal closure in response to any stimulus.
The OSCA1.3 channel is a member of a widespread family of channels known to exist in many organisms, including humans. Notably, OSCA1.3 seems specifically activated upon detection of pathogens. It will now be interesting to see if other members of the family are involved in responses to other stimuli known to trigger stomatal closure.
The identification of this channel has implications for the development of crops that are more resistant to disease. Just like with a smoke alarm, it is important to ensure that closure occurs when there is a real and significant threat. Equally as important is the need to prevent false alarms which cause unnecessary losses in photosynthetic efficiency. Achieving an optimum balance would achieve significant agricultural benefits.
Professor Cyril Zipfel said “despite the physiological and ecological relevance of stomatal closure, the identity of some of the key components mediating this closure were still unknown. The identification of OSCA1.3 now fills one of these important gaps. In the context of plant immunity this work is particularly apt in 2020, the UN International Year of Plant Health.”.
Dr Kathrin Thor, one of the lead researchers, said “identifying OSCA1.3 as a key component in the signalling pathway leading to stomatal closure upon pathogen attack is an important contribution to our understanding of plants’ responses to biotic stress, such as attack by pathogens, which can cause severe yield losses in crops. Along with members of the OSCA family several other potential calcium channels exist, for some of them functions have been described, others are still unknown. It will therefore be fascinating to see how future research can combine our finding with those on other proteins, plant organs and stresses, as well as their interactions, to paint an overall picture of how plants are able to respond to changes in their environment”.
Professor José Feijó at the University of Maryland, who collaborated on this study, said “this is a perfect example on how a collaborative effort between labs with different expertise can bring about important conclusions that would be difficult on solo efforts. It was very exciting for us to be able to characterize the novel properties of this channel.”.