Plant defense mechanism and current understanding of salicylic acid and NPRs in activating SAR

Plants, in contrast to vertebrates, lack specific mobile resistance cells and a circulatory system, but still, the plant cells are capable of launching an operative immune response. SA is the most important plant hormones, which plays a vital role in mediating the SAR (systemic acquired resistance) in plants; however, its receptor has not yet been completely unraveled. It is believed that NPR1, NPR3, and NPR4 are the receptors of SA, where NPR3 and NPR4 work as adaptors that finally break down NPR1 in an SA-regulated manner. Moreover, a new investigation has revealed that the NPR1 localization occurs in two steps: SA-activated monomerization and SNF1-RELATED PROTEIN KINASE 2.8 (SnRK2.8)-mediated phosphorylation, which further allows plants to trigger SAR and escape the harmful effects of high levels of SA in proximal tissues. This fascinating research gives directions to the designation of specific ways triggering SnRK2.8, possibly containing nitric oxide, and also further elucidation of the role of additional mobile immune signals in triggering NPR1.