Saptaparna Dasgupta, Bennett University
Chloroplasts turn off photosynthesis during immunological activation, generate antimicrobial chemicals, and connect to a nucleus through tubes known as stromules. Although the alteration of chloroplast in a light reaction is well documented, the dynamics of the migration of chloroplasts in response to pathogen attack are less characterized. It is depicted in the article regarding the accumulation of chloroplast at the pathogen interface at the time of infection by Phytophthora infestans. Considering that the immune recognition mechanism of similar effectors stimulates the stromules via various pathways. It thus is important to implicate the response of the chloroplasts upon pathogen invasion.
An insight into the pathogen Phytophthora infestans
Phytophthora infestans is known to cause potato late blight. It is an oomycete, a fungus-like micro-organism, identified as a devastating pathogen, infecting the economically important crops. This pathogen invades host cells by haustoria, a structure extending from its invasive intercellular hyphae. Haustoria is circled by an extrahaustorial membrane (EHM) produced by the plants, which transfers effectors released by the pathogen within the host cell. This interface is crucial for the success or failure of the infection and is aimed at the focused immune response of the plant. This involves the deposition of the callose, autophagy redirection, and migration of the nucleus to the site of infection. However, the biological makeup of the EHM and plasma membrane varies markedly (in Fig 1).
Mechanism of action of the pathogen:
These pathogen species are associated with a specialized membrane that could engulf the pathogen haustorium. Chemical inhibition of actin polymerization decreases the chloroplast accumulation in pathogenic haustoria. This suggests that the procedure is partially dependent on the actin cytoskeleton. However, the accumulation of chloroplasts in haustoria does not always depend on the movements of the nucleus to this interface. It also states that the light conditions are not affected.
Stromules are usually induced to form dynamic organelle clusters during infection, enveloping the haustoria and facilitate interactions amongst chloroplasts. It was observed that the development of inflectionally induced stromule formation to rely on Brassinosteroid Insensitive 1-Associated Kinase 1 (BAK1) mediated surface immune signalling. Following the defence-related activation of stromules, effector-mediated inhibition of BAK1 reduces the development of stromule formation during infection.
The response generated against the pathogen:
It has been studied that the EHM does not have surface localized pattern recognition receptors (PRRs). Thus, it activates the downstream pathway of the immune response utilizing the recognition of pathogen-associated molecular patterns (PAMPs). Upon triggering of the PAMPs, the co-receptor, BAK1 induces the basal immune responses. This makes up the first line of defence in the host against the attacking pathogen (in Fig. 2). Pathogens usually use host-translocated effectors to sabotage immunity to this impact.
Study models against Phytophthora infestans:
Nicotiana benthamiana proves to be an excellent model for the study of basal plant immunity functional principles against Phytophthora infestans. This is based on a host translocated RXLR effector study to suppress the BAK1 mediated surface immune response via the AVR3a suppressor in Phytophthora infestans. But, despite the suppression, the basal responses of the plant were still able to respond and provide immunity against the pathogen. This indicated that the immune response generated through the effectors is partial. Further downregulation of BAK1 gene expression in Nicotiana benthamiana,contributes to enhanced pathogen growth. Therefore, it is observed to provide surface-mediated immunity against the pathogen.
Chloroplast as the target for pathogens:
The activation of immunity on the surface of the cell encourages chloroplasts to stop photosynthesis. This allows the production of defence hormone precursors and the creation of reactive oxygen species (ROS). Pathogens with effector proteins are known to target chloroplasts, which further emphasizes their immunological relevance. Interestingly, several genes have been identified to encode chloroplast-localized proteins, linked with oomycete resistance pathogens. In response to several elicitors including phytohormones, ROS and bacterial PAMP, flg22, etc. chloroplasts generate stroma-filled tubules (stromules). Although the specific functions of stromules remain uncertain, the immunity, mobility of chloroplast, and linkage to plant cell nucleus have been involved. PAMPs and ROS also stimulate the interaction of chloroplasts with the nucleus with possible defensive functions in a redistribution of chloroplast during the time of infection.
Results:
From the study, it can be inferred that the cytoskeleton, especially the actin molecules are involved in the movement and accumulation of chloroplasts and nucleus. In conjunction with it, it is also responsible for the stromules interactions and polarization of cells at the site of pathogen invasion. With the help of optical tweezers, a proportion (17%) of the chloroplasts were exported to the non-haustorial cells. GFP was used for the quantification of the number of haustoria associated with chloroplasts. This also helped in the determination of the position of the chloroplasts around the haustoria, in correlation with the nucleus. The chloroplasts could form links by the stromules, or the chloroplasts could follow the trail of themselves when connected by stromules. Thus, the results successfully depict the role of chloroplast during pathogen invasion.
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Reference:
- Savage, Z., Duggan, C., Toufexi, A., Pandey, P., Liang, Y., Eugenia Segretin, M., Him Yuen, L., Gaboriau, D. C. A., Leary, A. Y., Tumtas, Y., Khandare, V., Ward, A. D., Botchway, S. W., Bateman, B. C., Pan, I., Schattat, M., Sparkes, I., & Bozkurt, T. O. (2021). Chloroplasts alter their morphology and accumulate at the pathogen interface during infection by Phytophthora infestans. The Plant Journal, tpj.15416. https://doi.org/10.1111/tpj.15416
Author info:
Saptaparna Dasgupta, currently a B. Tech 3rd-year student, pursuing Biotechnology, is a diligent student and determined in terms of her career goals. Being a budding biotechnologist, she is open to all research fields of her course and passionate about knowledge. She is focused and constantly tries to improve her writing skills, also a project enthusiast and is fond of gaining hands-on experience in laboratories. She believes that all hard work and efforts pays off eventually and follows this as the motto of her life. Check out some of her works on:
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