The wonderful world of intrinsic and intricate immunity responses in plants against pathogens
Cunchun Yang, Aria Dolatabadian, W. G. Dilantha Fernando
Abstract
Plants, unlike animals, lack specialized mobile immune cells, so they do not have an adaptive immune system. Instead, plants can launch specific, self-tolerant immune responses and establish immune memory. Plants possess defense mechanisms that efficiently detect and ward off potentially dangerous microorganisms. These defense mechanisms start with multiple signalling processes responsible for sensation, recognition, signal collection and conveying information between cells. Recognition occurs when microbial or pathogen-associated molecular patterns (MAMPs or PAMPs) are detected, leading to MAMP- or PAMP-triggered immunity (MTI or PTI). Plant cells also recognize pathogens through effector-triggered immunity (ETI), which relies on the function of the pathogen’s avirulence (Avr) gene-coded effector proteins and host’s resistance (R) gene-coded R proteins, resulting in the activation of apoptosis-like cell death, known as the hypersensitive response. In addition, reactive oxygen species (ROS) act as a double-edged sword; they are either toxic or versatile signalling molecules in plants. ROS generation is an integral part of hormone regulation and function in plant defense mechanisms. Plant hormones are also implicated in plant defense signalling pathways; salicylic acid, jasmonic acid, and ethylene have been increasingly studied in plant responses to pathogens. These innate immune system components interact with each other and provide protection against invading pathogens. We review advances in understanding the molecular aspects of plant defense mechanisms and describe the role of ROS, mitogen-activated protein kinase (MAPK) cascades, and hormones in modulating defense responses. We also provide an overview of how these plant defense components interact for a balanced and appropriate defense response.