Abstract
Chloroplasts play a central role in plant immunity through the synthesis of secondary metabolites and defense compounds, as well as phytohormones, such as jasmonic acid and salicylic acid. Additionally, chloroplast metabolism results in the production of reactive oxygen species and nitric oxide as defense molecules. The impact of viral and bacterial infections on plastids and chloroplasts has been well documented. In particular, bacterial pathogens are known to introduce effectors specifically into chloroplasts, and many viral proteins interact with chloroplast proteins to influence viral replication and movement, and plant defense. By contrast, clear examples are just now emerging for chloroplast-targeted effectors from fungal and oomycete pathogens. In this review, we first present a brief overview of chloroplast contributions to plant defense and then discuss examples of connections between fungal interactions with plants and chloroplast function. We then briefly consider well-characterized bacterial effectors that target chloroplasts as a prelude to discussing the evidence for fungal effectors that impact chloroplast activities.
Highlights
Plastids are dynamic plant organelles that differentiate during development into photosynthetic chloroplasts as well as leucoplasts, chromoplasts, and additional subtypes [1]
In the context of plant immunity, most studies focus on chloroplasts, which are responsible for the formation of energy equivalents, such as ATP and NADPH during photosynthesis
HopNI is another well-studied P. syringae effector protein that is targeted to the chloroplast using a non-cleavable transit peptide [74]
Summary
Plastids are dynamic plant organelles that differentiate during development into photosynthetic chloroplasts as well as leucoplasts, chromoplasts, and additional subtypes [1]. In the context of plant immunity, most studies focus on chloroplasts, which are responsible for the formation of energy equivalents, such as ATP and NADPH during photosynthesis. These energy equivalents can be used during and after carbon fixation to produce primary carbon-containing metabolites, such as sugars, starch, nucleotides, amino acids and fatty acids/lipids. Chloroplasts, make important contributions to plant defense against pathogens, including participation in the pathogen-associated molecular pattern (PAMP), triggered immunity (PTI) and effector-triggered immunity (ETI) [2,3,4,5,6,7,8]. Information on bacterial interactions is included to reinforce findings of key chloroplast functions that mediate defense or serve as targets of pathogen attack. We discuss future directions, including the prediction that fungal effectors targeting the chloroplast will be prominent contributors to disease
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