Mechanisms and balanced regulation of plant immunity.

Plants have evolved sophisticated immune systems to defend against a wide array of pathogens. These defence mechanisms are primarily mediated by cell surface pattern recognition receptors (PRRs) and intracellular nucleotide-binding leucine-rich repeat receptors (NLRs). Recent studies indicate that immune signalling involving PRRs and NLRs converges on cytosolic calcium (Ca2+) and other signals, which act synergistically to mount robust plant defences. However, uncontrolled activation of these defence mechanisms often negatively affects plant growth. In this Review, we discuss recent findings on the signalling mechanisms of PRRs and NLRs, including microbial recognition, receptor activation and signal transduction, as well as mechanisms that mitigate the negative impact on plant fitness associated with growth-defence trade-offs. We also discuss the interplay between the immune system and environmental stresses and explore strategies for engineering disease-resistant plants with optimized fitness.