Nitrogen (N) is one of the most important elements that has a central impact on plant growth and yield. N is also widely involved in plant stress responses, but its roles in host-pathogen interactions are complex as each affects the other. In this review, we summarize the relationship between N nutrition and plant disease and stress its importance for both host and pathogen. From the perspective of the pathogen, we describe how N can affect the pathogen’s infection strategy, whether necrotrophic or biotrophic. N can influence the deployment of virulence factors such as type III secretion systems in bacterial pathogen or contribute nutrients such as gamma-aminobutyric acid to the invader. Considering the host, the association between N nutrition and plant defence is considered in terms of physical, biochemical and genetic mechanisms. Generally, N has negative effects on physical defences and the production of anti-microbial phytoalexins but positive effects on defence-related enzymes and proteins to affect local defence as well as systemic resistance. N nutrition can also influence defence via amino acid metabolism and hormone production to affect downstream defence-related gene expression via transcriptional regulation and nitric oxide (NO) production, which represents a direct link with N. Although the critical role of N nutrition in plant defences is stressed in this review, further work is urgently needed to provide a comprehensive understanding of how opposing virulence and defence mechanisms are influenced by interacting networks.

Plant secondary metabolites (SMs) play crucial roles in plant‐environment interactions and contribute greatly to human health. Global climate changes are expected to dramatically affect plant secondary metabolism, yet a systematic understanding of such influences is still lacking. Here, we employed medicinal and aromatic plants (MAAPs) as model plant taxa and performed a meta‐analysis from 360 publications using 1828 paired observations to assess the responses of different SMs levels and the accompanying plant traits to elevated carbon dioxide (eCO₂), elevated temperature (eT), elevated nitrogen deposition (eN) and decreased precipitation (dP). The overall results showed that phenolic and terpenoid levels generally respond positively to eCO₂ but negatively to eN, while the total alkaloid concentration was increased remarkably by eN. By contrast, dP promotes the levels of all SMs, while eT exclusively exerts a positive influence on the levels of phenolic compounds. Further analysis highlighted the dependence of SM responses on different moderators such as plant functional types, climate change levels or exposure durations, mean annual temperature and mean annual precipitation. Moreover, plant phenolic and terpenoid responses to climate changes could be attributed to the variations of C/N ratio and total soluble sugar levels, while the trade‐off supposition contributed to SM responses to climate changes other than eCO₂. Taken together, our results predicted the distinctive SM responses to diverse climate changes in MAAPs and allowed us to define potential moderators responsible for these variations. Further, linking SM responses to C‐N metabolism and growth‐defence balance provided biological understandings in terms of plant secondary metabolic regulation.

Potassium (K) availability in plant cells is critical for maintaining plant productivity across many terrestrial ecosystems. Yet, there is no comprehensive assessment of the mechanisms by which plants respond to potassium application in such conditions, despite the global challenge of escalating osmotic stress. Herein, we conducted a meta-analysis using data from 2381 paired observations to investigate plant responses to potassium application across various morphological, physiological, and biochemical parameters under both osmotic and non-osmotic stress. Globally, our results showed the significant effectiveness of potassium application in promoting plant productivity (e.g., +12~30% in total dry weight), elevating photosynthesis (+12~30%), and alleviating osmotic damage (e.g., -19~26% in malonaldehyde), particularly under osmotic stress. Moreover, we found evidence of interactive effects between osmotic stress and potassium on plant traits, which were more pronounced under drought than salt stress, and more evident in C₃ than C₄ plants. Our synthesis verifies a global potassium control over osmotic stress, and further offers valuable insights into its management and utilization in agriculture and restoration efforts.