Changes in Cell Ultrastructure and Zeatin Riboside Concentrations in Hedera helix, Pelargonium zonale, Prunus avium, and Rubus ulmifolius Leaves Infected by Fungi

Cytokinin Regulation of Source-Sink Relationships in Plant-Pathogen Interactions

Cytokinins are plant hormones known for their role in mediating plant growth. First discovered for their ability to promote cell division, this class of hormones is now associated with many other cellular and physiological functions. One of these functions is the regulation of source-sink relationships, a tightly controlled process that is essential for proper plant growth and development. As discovered more recently, cytokinins are also important for the interaction of plants with pathogens, beneficial microbes and insects. Here, we review the importance of cytokinins in source-sink relationships in plants, with relation to both carbohydrates and amino acids, and highlight a possible function for this regulation in the context of plant biotic interactions.

Engineered nanomaterials inhibit Podosphaera pannosa infection on rose leaves by regulating phytohormones

In the present study, we investigated the antifungal effects of engineered nanomaterials (ENMs) against Podosphaera pannosa (P. pannosa), a fungal pathogen that causes powdery mildew on plants in the rose family. Four commercial ENMs, including multi-wall carbon nanotubes (MWCNTs), reduced graphene oxide (rGO), copper oxide (CuO) nanoparticles (NPs) and titanium dioxide (TiO₂) NPs, were used to prepare 50 or 200 mg/L NP suspensions in deionized water. Rose leaves in water-agar plates were sprayed by different ENM suspensions mixed with P. pannosa conidia. After a 19-day standard infection test, the growth of P. pannosa on rose leaves was evaluated. All four ENMs inhibited infection by P. pannosa at the concentration 200 mg/L, whereas only CuO NPs decreased fungal growth at 50 mg/L. The phytohormone content of the leaves was measured across all treatments to investigate potential ENMs antifungal mechanisms. The results suggest that ENMs increased plant resistance to fungal infection by altering the content of endogenous hormones, particularly zeatin riboside (ZR). Our study demonstrates that ENMs exhibited distinctly antifungal effects against P. pannosa on roses, and could be utilized as a novel plant protection strategy after a comprehensive assessment of potential environmental risk.

Yeasts producing zeatin

The present paper describes the first screening study of the ability of natural yeast strains to synthesize in culture the plant-related cytokine hormone zeatin, which was carried out using HPLC-MS/MS. A collection of 76 wild strains of 36 yeast species (23 genera) isolated from a variety of natural substrates was tested for the production of zeatin using HPLC-MS/MS. Zeatin was detected in more than a half (55%) of studied strains and was more frequently observed among basidiomycetous than ascomycetous species. The amount of zeatin accumulated during the experiment varied among species and strains. Highest zeatin values were recorded for basidiomycete Sporobolomyces roseus and ascomycete Taphrina sp. that produced up to 8,850.0 ng and 5,166.4 ng of zeatin per g of dry biomass, respectively. On average, the ability to produce zeatin was more pronounced among species isolated from the arctic-alpine zone than among strains from tropical and temperate climates. Our study also demonstrated that epiphytic strains and pigmented yeast species, typically for phyllosphere, are able to more often produce a plant hormone zeatin than other yeasts.

Should I fight or should I grow now? The role of cytokinins in plant growth and immunity and in the growth-defence trade-off

BACKGROUND

Perception and activation of plant immunity require a remarkable level of signalling plasticity and control. In Arabidopsis and other plant species, constitutive defence activation leads to resistance to a broad spectrum of biotrophic pathogens, but also frequently to stunted growth and reduced seed set. Plant hormones are important integrators of the physiological responses that influence the outcome of plant-pathogen interactions.

SCOPE

We review the mechanisms by which the plant hormone cytokinin regulates both plant growth and response to pathogens, and how cytokinins may connect these two processes, ultimately affecting the growth trade-offs observed in plant immunity.

The Role of Cytokinin During Infection of Arabidopsis thaliana by the Cyst Nematode Heterodera schachtii

Plant-parasitic cyst nematodes induce the formation of hypermetabolic feeding sites, termed syncytia, as their sole source of nutrients. The formation of the syncytium is orchestrated by the nematode, in part, by modulation of phytohormone responses, including cytokinin. In response to infection by the nematode Heterodera schachtii, cytokinin signaling is transiently induced at the site of infection and in the developing syncytium. Arabidopsis lines with reduced cytokinin sensitivity show reduced susceptibility to nematode infection, indicating that cytokinin signaling is required for optimal nematode development. Furthermore, lines with increased cytokinin sensitivity also exhibit reduced nematode susceptibility. To ascertain why cytokinin hypersensitivity reduces nematode parasitism, we examined the transcriptomes in wild type and a cytokinin-hypersensitive type-A arr Arabidopsis mutant in response to H. schachtii infection. Genes involved in the response to biotic stress and defense response were elevated in the type-A arr mutant in the absence of nematodes and were hyperinduced following H. schachtii infection, which suggests that the Arabidopsis type-A arr mutants impede nematode development because they are primed to respond to pathogen infection. These results suggest that cytokinin signaling is required for optimal H. schachtii parasitism of Arabidopsis but that elevated cytokinin signaling triggers a heightened immune response to nematode infection.

Cytokinin concentrations and CHASE-DOMAIN CONTAINING HIS KINASE 2 (NaCHK2)- and NaCHK3-mediated perception modulate herbivory-induced defense signaling and defenses in Nicotiana attenuata

Herbivore attack elicits changes in cytokinins (CKs), but how these changes influence defense signaling remains poorly described. We investigated the influence of the CK pathway on the well-described inducible defense pathways of Nicotiana attenuata in response to wounding with and without elicitors from the specialist herbivore Manduca sexta. CK pathway manipulation often suffers from substantial side effects on plant growth and development. We therefore used multiple manipulation tools including spray application of CKs, chemically-inducible expression of the CK biosynthesis enzyme isopentenyltransferase, and transient and constitutive RNAi-mediated gene silencing of CK receptors to resolve the function of CKs in plant defense. The results demonstrated that CK concentrations in leaves and perception through CHASE-DOMAIN CONTAINING HIS KINASE 2 (NaCHK2) and NaCHK3 were important for the accumulation of jasmonic acid (JA) and phenolamides and proteinase inhibitor activity. By contrast, the CK pathway did not promote the accumulation of the active JA-isoleucine conjugate and negatively regulated the release of specific green leaf volatile esters. Interestingly, CK signaling also promotes the systemic phenolamide accumulation. We conclude that the CK pathway is an important regulator of herbivory-inducible defense signaling and chemistry, which expands its reported participation in adjusting a plant's physiology to abiotic and biotic stress responses.

Are green islands red herrings? Significance of green islands in plant interactions with pathogens and pests

The term green island was first used to describe an area of living, green tissue surrounding a site of infection by an obligately biotrophic fungal pathogen, differentiated from neighbouring yellowing, senescent tissue. However, it has now been used to describe symptoms formed in response to necrotrophic fungal pathogens, virus infection and infestation by certain insects. In leaves infected by obligate biotrophs such as rust and powdery mildew pathogens, green islands are areas where senescence is retarded, photosynthetic activity is maintained and polyamines accumulate. We propose such areas, in which both host and pathogen cells are alive, be termed green bionissia. By contrast, we propose that green areas associated with leaf damage caused by toxins produced by necrotrophic fungal pathogens be termed green necronissia. A range of biotrophic/hemibiotrophic fungi and leaf-mining insects produce cytokinins and it has been suggested that this cytokinin secretion may be responsible for the green island formation. Indeed, localised cytokinin accumulation may be a common mechanism responsible for green island formation in interactions of plants with biotrophic fungi, viruses and insects. Models have been developed to study if green island formation is pathogen-mediated or host-mediated. They suggest that green bionissia on leaves infected by biotrophic fungal pathogens represent zones of host tissue, altered physiologically to allow the pathogen maximum access to nutrients early in the interaction, thus supporting early sporulation and increasing pathogen fitness. They lead to the suggestion that green islands are 'red herrings', representing no more than the consequence of the infection process and discrete changes in leaf senescence.

Plants and biotrophs: a pivotal role for cytokinins?

Plants infected with biotrophic fungal pathogens exhibit reduced photosynthetic rates, nutrient mobilization towards infection sites and, in interactions where discrete pustules are formed, green islands are induced. The ability of cytokinins to mobilize nutrients towards sites of application and to delay senescence led researchers to speculate that cytokinins might be involved in nutrient mobilization and green island formation in plants infected with biotrophic fungi. There is evidence that the reduction in photosynthesis in infected leaves results from early increases in invertase activity, leading to carbohydrate accumulation and the downregulation of photosynthetic metabolism. In this Opinion article, we propose that these seemingly disparate changes in host physiology in infected plants are the result of cytokinin-induced increases in invertase activity occurring early on in the host-pathogen interaction.