The calcium channel in plants helps close the door to intruders

Disease-causing microorganisms can invade plants through pores in the leaves called stomata, which rapidly close in a calcium-dependent manner upon detection of such a hazard. Finally, the calcium channels involved have been identified.

In plants, calcium ions (Ca 2+ ) function as a central signal for various stimuli, ranging from internal developmental signals to physical or biological stresses such as infections. However, the transient nature of Ca signals 2+ and the enigmatic identities of Ca channels 2+ of the plant have made it difficult to study the role of these ions. Furthermore, the connection between the Ca channels 2+ and the specific responses of plants are often unclear. Writing in Nature ,Thor et al. 1 now I clarify one of those connections and they report their finding of a type of Ca channel 2+ that is activated during a specific response against infection.

Two specialized moon-shaped cells, called guard cells, form a leaf pore called a stoma (Fig. 1a). Stomata allow gas exchange, including entry of carbon dioxide for the energy-generating process of photosynthesis. Therefore, they are essential for the survival of plants. However, disease-causing microorganisms (pathogens) can use the stomata as a gateway for invasion. To limit infection, plants close the stomata upon recognition of such an attack, in a defense response called stomatal immunity 2 . The surfaces of plant and animal cells have receptor proteins that contain regions called kinase domains, and these proteins can recognize evolutionarily conserved microbial molecular motifs called pathogen-associated molecular patterns (PAMPs) and initiate signaling pathways necessary for defense.

Figure 1 | Calcium channel that regulates stomatal closure. a, Plants, such as the model species Arabidopsis thaliana, have leaf pores called stomata. If the plant detects a disease-causing agent (called a pathogen), the protective cells in the stomata rapidly close in response. b, Thor et al. 1 describe the identification of the calcium ion channel (Ca 2+) in the pathway that leads to stomatal closure. Pathogens are detected by the FLS2 receptor protein, which forms a complex with the BAK1 protein. When this complex detects a bacterial protein fragment, called flg22, it adds a phosphate group (P) to the BIK1 protein, activating it. BIK1 then phosphorylates the Ca 2+ channel. Thor et al. report that two proteins of the OSCA family, OSCA1.3 and OSCA1.7, can function as Ca 2+ channels during this response (it is unknown whether one or both of them together fulfill this function). It is not clear how an influx of Ca 2+ through the canal causes the stomata to close. One possibility is that enzymes called calcium-dependent protein kinases (CDPKs) activate S-type anion channels (SLACs). SLACs allow anions (negatively charged ions) to leave the cell, leading to water loss that prompts stomatal closure.

 

On the model floor Arabidopsis thaliana , a receptor protein called FLS2, which has a kinase domain, binds to the bacterial flagellin protein, recognizing a region of this PAMP called flg22. This recognition event causes FLS2 to form an active receptor complex with another cell surface receptor kinase called BAK1. The complex adds a phosphate group to a cytoplasmic kinase called BIK1. This phosphorylation of BIK1 activates immune responses 3 , such as the production of reactive oxygen species by the RBOHD protein. BIK1 is necessary for stomatal immunity 4 , and if the guard cells contain a mutant version of the gene encoding this kinase, the plant cannot respond to flg22. However, the link between the recognition of PAMP and Ca 2+Stomatal closure mediated and regulated by BIK1 has not been clear.

To connect the dots, Thor et al. speculated that, through direct phosphorylation, BIK1 controls Ca channels 2+ necessary for stomatal immunity. The authors focused on an ion channel called OSCA1.3, which is phosphorylated by detecting flg22. Thor and colleagues report that OSCA1.3 is permeable to Ca 2+ , and that phosphorylation of OSCA1.3 by BIK1 at amino acid residue of serine 54 (in the same type of motif as that phosphorylated by BIK1 in RBOHD) activates this channel in recognition of pathogens (Fig. 1b). Furthermore, our observation that the gene encoding OSCA1.3 is specifically expressed in stomata is consistent with the role of the channel in stomatal immunity.

Thor and cabbage . did not observe a clear effect on the immune response to flg22 in a mutant plant in which the gene OSCA1.3 it was off. However, in a plant also designed to have a mutant version of another member of clade 1, the gene OSCA1.7 , the stomatal closure when perceiving flg22 was affected and the susceptibility to bacterial infection increased, compared to the response in the wild-type plant. OSCA1.7 has a protein motif similar to that phosphorylated in OSCA1.3 by BIK1, and is activated by phosphorylation by BIK1 to generate an influx of Ca 2+ in cells. Therefore, it appears that OSCA1.3 and OSCA1.7 are Ca 2+channels that regulate stomatal immunity and probably function redundantly, so that if OSCA1.3 is absent, OSCA1.7 can fulfill its role. It is unknown whether only one or both of these proteins together form Ca channels. 2+ that act on stomatal immunity.

In addition to identifying these Ca channels 2+ ,Thor et al. explored the role of the plant hormone abscisic acid (ABA), which regulates the closure of stomata when the plant detects a water deficit. This hormone also controls the defenses of the stomata, because the stomata of ABA-deficient plants do not close effectively when perceiving pathogens 2 . However, the authors found that a plant with mutations in the genes encoding OSCA1.3 and OSCA1.7 fully responds to ABA, indicating that these channels are not involved in ABA-mediated stomatal closure. This observation corroborates the previous evidence 4 of that the regulation of stomatal immunity by BIK1 does not require ABA. Also, Thor et al . report that the total Ca Activation signal 2+ by flg22 in leaves that had mutations in the genes encoding OSCA1.3 and OSCA1.7 was not affected; guard cells make up only a small fraction of leaf cells. This result strongly supports the specific role of these channels in stomatal immunity, rather than general immunity, although BIK1 is required for both types of response.

How changes in Ca concentration 2+ generate specific cellular responses to stimuli is a central question in this area of ​​research. One proposed idea is that specific temporal patterns of stimulation of cytoplasmic levels of Ca 2+ could provide a key clue, and that these 'signatures of Ca 2+ 'could be generated and decoded by specific Ca binding components 2+ , such as calmodulin proteins or calcium-dependent protein kinases 8 . The results of Thor and his colleagues suggest, instead, that the Ca channels themselves 2+ could determine specificity, at least for stomatal immunity.

Although OSCA proteins allow stomata to close regardless of ABA involvement, closure mediated by ABA or in response to infection likely involves the same mechanism, which ultimately closes stomata by moving water away from guard cells . Therefore, both ways should converge at some point. Activation of channels that allow negatively charged ions (anions) to exit the cell, such as S-type anion channels, called SLACs, is a crucial step in stomatal movement. 9 . The OPEN STOMATA1 protein kinase, which is a component of an ABA-mediated signaling pathway, activates SLACs and has been proposed 2 as a point of convergence for defense responses and ABA signaling. However, some calcium-dependent protein kinases also activate SLAC 10 , and such Ca The decoders of 2+ signals, or perhaps even the anion channels themselves, could be the point of convergence.

An emerging theme in studies of Ca channels 2+ of plants is its regulation by phosphorylation. Previous studies 11 , 12 reported that BIK1 and BAK1 phosphorylate members of another group of Ca channels 2+ of plants, ion channels activated by cyclic nucleotides, to regulate their function or stability. The phosphorylation of OSCA1.3 and OSCA1.7 by BIK1 underscores the connection between receptor kinases and Ca channels 2+ , presumably to generate Ca signals 2+ specific stimulus. It will be interesting to determine whether proteins in the OSCA family interact with other components on the surface of cells to form a structure called a canalosome, a group of signaling molecules that surround an ion channel.13 .

Until now, OSCA proteins have been primarily related to the detection of osmotic stress. They are categorized as a type of mechanical sensing channel, one that converts physical forces into biochemical signals. 14 , 15 . Are OSCA1.3 and OSCA1.7 activated by osmotic stress or mechanical stimulation, in addition to their activation by BIK1? Did the two proteins develop a specific defense role or do they also have other functions in stomata? Understand the biological function of each OSCA and Ca signals 2+ that they generate will shed light on stomatal biology. Such insights could be crucial for plant bioengineering to meet future challenges in crop production.

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