Abstract
Higher plants and some algae convert the absorbed light into chemical energy through one of the most important organelles, chloroplast, for photosynthesis and store it in the form of organic compounds to supply their life activities. However, more and more studies have shown that the role of chloroplasts is more than a factory for photosynthesis. In the process of light conversion to chemical energy, any damage to the components of chloroplast may affect the photosynthesis efficiency and promote the production of by-products, reactive oxygen species, that are mainly produced in the chloroplasts. Substantial evidence show that chloroplasts are also involved in the battle of plants and microbes. Chloroplasts are important in integrating a variety of external environmental stimuli and regulate plant immune responses by transmitting signals to the nucleus and other cell compartments through retrograde signaling pathways. Besides, chloroplasts can also regulate the biosynthesis and signal transduction of phytohormones, including salicylic acid and jasmonic acid, to affect the interaction between the plants and microbes. Since chloroplasts play such an important role in plant immunity, correspondingly, chloroplasts have become the target of pathogens. Different microbial pathogens target the chloroplast and affect its functions to promote their colonization in the host plants.
Highlights
Chloroplasts are considered to be organelles produced by endosymbiotic bacteria in plants that undergo photoautotrophy (Ding et al, 2019)
Most of the chloroplast-targeted proteins encoded in host nucleus contain a chloroplast transit peptide at their N-terminal, which subsequently enters chloroplasts by interacting with the translocators located in the inner membrane (TIC) and outer membrane (TOC) of the chloroplast and could be cleaved off by a stromal processing peptidase (SPP)
Once PAMP-triggered immunity (PTI) or effector-triggered immunity (ETI) signal is relayed to the chloroplast, Ca2+ from the thylakoid lumen, which contains a high concentration of Ca2+, are transported to the stroma by Calcium-sensing receptor (CAS), resulting in a continuous high concentration of Ca2+ in the stroma, and the signal is transduced from the chloroplast to the nucleus through the 1O2-mediated retrograde signaling pathway (Kim and Apel, 2013), which regulates the defense responses through transcriptional reprogramming of defenserelated genes (Nomura et al, 2012)
Summary
Chloroplasts are considered to be organelles produced by endosymbiotic bacteria in plants that undergo photoautotrophy (Ding et al, 2019). Once PTI or ETI signal is relayed to the chloroplast, Ca2+ from the thylakoid lumen, which contains a high concentration of Ca2+, are transported to the stroma by CAS, resulting in a continuous high concentration of Ca2+ in the stroma, and the signal is transduced from the chloroplast to the nucleus through the 1O2-mediated retrograde signaling pathway (Kim and Apel, 2013), which regulates the defense responses through transcriptional reprogramming of defenserelated genes (Nomura et al, 2012) Another outstanding example is that the calcium protein kinase 16 (CPK16), localized on the PM, undergoes N-myristoylation in normal condition and relocalizes from the PM to chloroplasts upon flg or immune elicitors treatment to promote chloroplast-dependent defenses (Medina-Puche et al, 2020). Chloroplast-targeted protein 1 (CTP1) secreted by Melampsora larici-populina (Petre et al, 2016) and the secreted protein PvRXLR8620 of Plasmopara viticola
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