Muhammad Naseem

Cytokinins Mediate Resistance against Pseudomonas syringae in Tobacco through Increased Antimicrobial Phytoalexin Synthesis Independent of Salicylic Acid Signaling

Cytokinins are phytohormones that are involved in various regulatory processes throughout plant development, but they are also produced by pathogens and known to modulate plant immunity. A novel transgenic approach enabling autoregulated cytokinin synthesis in response to pathogen infection showed that cytokinins mediate enhanced resistance against the virulent hemibiotrophic pathogen Pseudomonas syringae pv tabaci. This was confirmed by two additional independent transgenic approaches to increase endogenous cytokinin production and by exogenous supply of adenine- and phenylurea-derived cytokinins. The cytokinin-mediated resistance strongly correlated with an increased level of bactericidal activities and up-regulated synthesis of the two major antimicrobial phytoalexins in tobacco (Nicotiana tabacum), scopoletin and capsidiol. The key role of these phytoalexins in the underlying mechanism was functionally proven by the finding that scopoletin and capsidiol substitute in planta for the cytokinin signal: phytoalexin pretreatment increased resistance against P. syringae. In contrast to a cytokinin defense mechanism in Arabidopsis (Arabidopsis thaliana) based on salicylic acid-dependent transcriptional control, the cytokinin-mediated resistance in tobacco is essentially independent from salicylic acid and differs in pathogen specificity. It is also independent of jasmonate levels, reactive oxygen species, and high sugar resistance. The novel function of cytokinins in the primary defense response of solanaceous plant species is rather mediated through a high phytoalexin-pathogen ratio in the early phase of infection, which efficiently restricts pathogen growth. The implications of this mechanism for the coevolution of host plants and cytokinin-producing pathogens and the practical application in agriculture are discussed.

Integrated Systems View on Networking by Hormones in Arabidopsis Immunity Reveals Multiple Crosstalk for Cytokinin

Predicting the system response to the invading pathogen, dynamic modeling of hormone disease networks explores the effect of cytokinin on immune defense. Together with validation experiments on disease phenotypes and monitoring hormonal responses and gene expression data, this work identified synergistic crosstalk for cytokinin and antagonism between auxin and cytokinin regarding immunity. Phytohormones signal and combine to maintain the physiological equilibrium in the plant. Pathogens enhance host susceptibility by modulating the hormonal balance of the plant cell. Unlike other plant hormones, the detailed role of cytokinin in plant immunity remains to be fully elucidated. Here, extensive data mining, including of pathogenicity factors, host regulatory proteins, enzymes of hormone biosynthesis, and signaling components, established an integrated signaling network of 105 nodes and 163 edges. Dynamic modeling and system analysis identified multiple cytokinin-mediated regulatory interactions in plant disease networks. This includes specific synergism between cytokinin and salicylic acid pathways and previously undiscovered aspects of antagonism between cytokinin and auxin in plant immunity. Predicted interactions and hormonal effects on plant immunity are confirmed in subsequent experiments with Pseudomonas syringae pv tomato DC3000 and Arabidopsis thaliana. Our dynamic simulation is instrumental in predicting system effects of individual components in complex hormone disease networks and synergism or antagonism between pathways.

The Role of Auxin-Cytokinin Antagonism in Plant-Pathogen Interactions

It has been several decades since Skoog and Miller described the contrasting behavior of auxin and cytokinin in promoting the growth of root and shoot, respectively [1]. In recent years, a lot of progress has been made in understanding the regulation of stem cell niche and cell fate in both shoot and root apical meristems. Developmental processes such as the maintenance of root meristems [2], lateral root formation [3], leaf position determination [4], and de novo auxin-induced organogenesis [5] are finetuned by the mutual interactions between auxin and cytokinin. Auxin exerts its inhibition on cytokinin on several levels; mechanisms range from its biosynthesis to the suppression of its signaling [6]. Reciprocally, cytokinin antagonistically impacts the flux, distribution, and signaling of auxin [7]. Antagonism between auxin and cytokinin is not the only type of interaction that governs developmental outputs in plants. Rather, synergistic interaction between auxin and cytokinins also exists in processes such as nodule organogenesis [8], light-mediated leaf initiation, and organ positioning [9]. With the majority of previous studies focusing on these hormones in development, auxin-cytokinin interplay has not been extensively analyzed in the context of plant immunity. Mutual interactions between stress-specific hormones such as salicylic acid and jasmonic acid/ethylene (SA-JA/ET) are regarded as the central backbone of the immunity [10]. However, growth-promoting hormones (auxin, cytokinins, gibberellic acid, and abscisic acid) either inhibit or potentiate this balance in mediating the protection or susceptibility of the plant against the invading pathogen [10,11]. For a comprehensive understanding of hormonal crosstalk in disease, a systems-biological perspective is critical, as plant hormones act in concert [11]. We focus on recent progress regarding the individual effects of auxin and cytokinins and their combined effect on immune dynamics in plant-pathogen systems.

The nexus between growth and defence signalling: auxin and cytokinin modulate plant immune response pathways

Plants deploy a finely tuned balance between growth and defence responses for better fitness. Crosstalk between defence signalling hormones such as salicylic acid (SA) and jasmonates (JAs) as well as growth regulators plays a significant role in mediating the trade-off between growth and defence in plants. Here, we specifically discuss how the mutual antagonism between the signalling of auxin and SA impacts on plant growth and defence. Furthermore, the synergism between auxin and JA benefits a class of plant pathogens. JA signalling also poses growth cuts through auxin. We discuss how the effect of cytokinins (CKs) is multifaceted and is effective against a broad range of pathogens in mediating immunity. The synergism between CKs and SA promotes defence against biotrophs. Reciprocally, SA inhibits CK-mediated growth responses. Recent reports show that CKs promote JA responses; however, in a feedback loop, JA suppresses CK responses. We also highlight crosstalk between auxin and CKs and discuss their antagonistic effects on plant immunity. Efforts to minimize the negative effects of auxin on immunity and a reduction in SA- and JA-mediated growth losses should lead to better sustainable plant protection strategies.

Cytokinins for immunity beyond growth, galls and green islands.

Cytokinins are essential plant hormones that control almost every aspect of plant growth and development. Their function in mediating plant susceptibility to fungal biotrophs and gall-causing pathogens is well known. Here we highlight the interaction between cytokinins and salicylic acid pathways. Furthermore, we discuss ways in which cytokinin signaling could crosstalk with plant immune networks. Some of these networks are modulated by pathogens to propagate disease, whereas others help the host to mitigate an infection.

How Many Salicylic Acid Receptors Does a Plant Cell Need

Paralogous proteins NPR1, NPR3, and NPR4 are receptors for the plant immunity mediator salicylic acid. The small-molecule hormone salicylic acid (SA) is a plant immune signal for which the receptors have only recently been identified. Two recent studies reported that the transcriptional coactivator nonexpresser of pathogenesis-related genes 1 (NPR1) and its paralogues NPR3 and NPR4 are bona fide SA immune signal receptors in plants. Fu et al. demonstrated that because of their binding affinity for SA, NPR3 and NPR4 are SA receptors for immune responses in Arabidopsis thaliana. Both NPR3 and NPR4 function as adaptors in proteasomal degradation of NPR1 in an SA-dependent manner. By applying nonequilibrium methods, they showed very low binding affinity of NPR1 for SA, suggesting that it may not qualify as an SA immune signal transduction receptor. However, using a method of equilibrium dialysis, Wu et al. found that SA binds to NPR1 and induces a conformational change in NPR1 or introduces steric hindrance that relieves repression of the transcriptional activation domain of NPR1 by an autoinhibitory N-terminal domain. This derepression leads to the expression of SA-dependent defense genes. Here, we discuss the importance of emerging SA perception models.

Probing the unknowns in cytokinin-mediated immune defense in Arabidopsis with systems biology approaches.

Plant hormones involving salicylic acid (SA), jasmonic acid (JA), ethylene (Et), and auxin, gibberellins, and abscisic acid (ABA) are known to regulate host immune responses. However, plant hormone cytokinin has the potential to modulate defense signaling including SA and JA. It promotes plant pathogen and herbivore resistance; underlying mechanisms are still unknown. Using systems biology approaches, we unravel hub points of immune interaction mediated by cytokinin signaling in Arabidopsis. High-confidence Arabidopsis protein—protein interactions (PPI) are coupled to changes in cytokinin-mediated gene expression. Nodes of the cellular interactome that are enriched in immune functions also reconstitute sub-networks. Topological analyses and their specific immunological relevance lead to the identification of functional hubs in cellular interactome. We discuss our identified immune hubs in light of an emerging model of cytokinin-mediated immune defense against pathogen infection in plants.

The impact of cytokinin on jasmonate-salicylate antagonism in Arabidopsis immunity against infection with Pst DC3000

Cytokinin has long been shown to be an essential modulator of growth and development in plants. However, its implications in plant immunity have only recently been realized. The interaction between jasmonate and salicylate pathways is regarded as a central backbone of plant immune defense. However, the effect of cytokinin on the jasmonate and salicylate mediated balance in plant immunity is still not known. Here, we analyze the impact of cytokinin on the jasmonate-salicylate antagonism in Arabidopsis immunity regarding infection with hemibiotrophic pathogen Pseudomonas syringae pv tomato DC3000 (Pst DC3000). Systems biology analysis of a refined hormone immune pathway model provides insights into the impact of cytokinin on the balance between jasmonate and salicylic acid pathways in Arabidopsis. Targeted experiments validate model simulations monitoring bacterial growth in wild type plants as well as in jasmonate pathway mutants. An integrated analysis shows that CK promotes the SA pathway of plant immunity and does not promote JA-mediated Arabidopsis susceptibility against infection with Pst DC3000. Finally, we discuss these results in the context of an emerging model of auxin-cytokinin antagonism in plant immunity.

LONELY-GUY Knocks Every Door: Crosskingdom Microbial Pathogenesis

Many plant microbial pathogens utilize cytokinins to establish inter-actions with their host. However, the production of cytokinins by an animal pathogen has just been reported for the first time. Here we discuss the impact of microbial secreted cytokinins on the infection dynamics in plant and animal cells.

Integration of boolean models on hormonal interactions and prospects of cytokinin-auxin crosstalk in plant immunity

Crosstalk between auxin and cytokinin in plant growth and development has already been established. However, their dynamics in plant immunity is still not well understood and requires systems biological approaches for analysis. Omics based public databases are exploited for the reconstruction, integration and analysis of Boolean models for hormonal interactions in plants. We established a meta-network by combining the plant immune regulatory network and the pathogen virulence specific network and used it as substrate for dynamic simulations on hormonal aspects of plant immunity. Our integrated analysis of this meta-network reveals antagonistic crosstalk between auxin and cytokinin in the Pst DC3000 and Arabidopsis interaction. Moreover, we discuss here the importance of Boolean models in unfolding inferences relevant to plant pathogen interactions.