Diphenylurea-derived cytokinin oxidase/dehydrogenase inhibitors for biotechnology and agriculture

H2O2 promotes trimming-induced tillering by regulating energy supply and redox status in bermudagrass

Tillering/branching pattern plays a significant role in determining the structure and diversity of grass, and trimming has been found to induce tillering in turfgrass. Recently, it has been reported that hydrogen peroxide (H2O2) regulates axillary bud development. However, the role of H2O2 in trimming-induced tillering in bermudagrass, a kind of turfgrass, remains unclear. Our study unveils the significant impact of trimming on promoting the sprouting and growth of tiller buds in stolon nodes, along with an increase in the number of tillers in the main stem. This effect is accompanied by spatial-temporal changes in cytokinin and sucrose content, as well as relevant gene expression in axillary buds. In addition, the partial trimming of new-born tillers results in an increase in sucrose and starch reserves in their leaves, which can be attributed to the enhanced photosynthesis capacity. Importantly, trimming promotes a rapid H2O2 burst in the leaves of new-born tillers and axillary stolon buds. Furthermore, exogenous application of H2O2 significantly increases the number of tillers after trimming by affecting the expression of cytokinin-related genes, bolstering photosynthesis potential, energy reserves and antioxidant enzyme activity. Taken together, these results indicate that both endogenous production and exogenous addition of H2O2 enhance the inductive effects of trimming on the tillering process in bermudagrass, thus helping boost energy supply and maintain the redox state in newly formed tillers.

Inspired by Nature: Isostere Concepts in Plant Hormone Chemistry

Chemical concepts such as isosteres and scaffold hopping have proven to be powerful tools in agrochemical innovation processes. They offer opportunities to modify known molecular lead structures with the aim to improve a range of parameters, including biological efficacy and spectrum, physicochemical properties, stability, and toxicity. While recent biochemical insights into plant-specific receptors and signaling pathways trigger the discovery of the first lead structures, the disclosure of such a new chemical structure sparks a broad range of synthesis activities giving rise to diverse chemical innovation and often a considerable boost in biological activity. Herein, recent examples of isostere concepts in plant-hormone chemistry will be discussed, outlining how synthetic creativity can broaden the scope of natural product chemistry and giving rise to new opportunities in research fields such as abiotic stress tolerance and growth promotion.

Antimicrobial and anthelmintic activities of aryl urea agents

OBJECTIVES

This study aimed to characterise compounds with activity against carbapenemase-expressing Gram-negative bacteria and nematodes and evaluate their cytotoxicity to non-cancerous human cells.

METHODS

The antimicrobial activity and toxicity of a series of phenyl-substituted urea derivatives were evaluated using broth microdilution, chitinase, and resazurin reduction assays.

RESULTS

The effects of different substitutions present on the nitrogen atoms of the urea backbone were investigated. Several compounds were active against Staphylococcus aureus and Escherichia coli control strains. Specifically, derivatives 7b, 11b, and 67d exhibited antimicrobial activity against Klebsiella pneumoniae 16, a carbapenemase-producing Enterobacteriaceae species, with minimum inhibitory concentration (MIC) values of 100, 50, and 72 µM (32, 64, and 32 mg/L), respectively. In addition, the MICs obtained against a multidrug-resistant E. coli strain were 100, 50, and 36 µM (32, 16, and 16 mg/L) for the same compounds, respectively. Furthermore, the urea derivatives 18b, 29b, 50c, 51c, 52c, 55c-59c, and 62c were very active towards the nematode Caenorhabditis elegans.

CONCLUSIONS

Testing on non-cancerous human cell lines suggested that some of the compounds have the potential to affect bacteria, especially helminths, with limited cytotoxicity to humans. Given the simplicity of synthesis for this class of compounds and their potency against Gram-negative, carbapenemase-expressing K. pneumoniae, aryl ureas possessing the 3,5-dichloro-phenyl group certainly warrant further investigation to exploit their selectivity.

Auxin-independent effects of apical dominance induce changes in phytohormones correlated with bud outgrowth

The inhibition of shoot branching by the growing shoot tip of plants, termed apical dominance, was originally thought to be mediated by auxin. Recently, the importance of the shoot tip sink strength during apical dominance has re-emerged with recent studies highlighting roles for sugars in promoting branching. This raises many unanswered questions on the relative roles of auxin and sugars in apical dominance. Here we show that auxin depletion after decapitation is not always the initial trigger of rapid cytokinin (CK) increases in buds that are instead correlated with enhanced sugars. Auxin may also act through strigolactones (SLs) which have been shown to suppress branching after decapitation, but here we show that SLs do not have a significant effect on initial bud outgrowth after decapitation. We report here that when sucrose or CK is abundant, SLs are less inhibitory during the bud release stage compared to during later stages and that SL treatment rapidly inhibits CK accumulation in pea (Pisum sativum) axillary buds of intact plants. After initial bud release, we find an important role of gibberellin (GA) in promoting sustained bud growth downstream of auxin. We are, therefore, able to suggest a model of apical dominance that integrates auxin, sucrose, SLs, CKs, and GAs and describes differences in signalling across stages of bud release to sustained growth.

GWAS across multiple environments and WGCNA suggest the involvement of ZmARF23 in embryonic callus induction from immature maize embryos

Combined GWAS, WGCNA, and gene-based association studies identified the co-expression network and hub genes for maize EC induction. ZmARF23 bound to ZmSAUR15 promoter and regulated its expression, affecting EC induction. Embryonic callus (EC) induction in immature maize embryos shows high genotype dependence, which limits the application of genetic transformation in transgenic breeding and gene function elucidation in maize. Herein, we conducted a genome-wide association mapping (GWAS) for four EC induction-related traits, namely rate of embryonic callus induction (REC), increased callus diameter (ICD), ratio of shoot formation (RSF), and length of shoot (LS) across different environments. A total of 77 SNPs were significantly associated these traits under three environments and using the averages (across environments). Among these significant SNPs, five were simultaneously detected under multiple environments and 11 had respective phenotypic variation explained > 10%. A total of 257 genes were located in the linkage disequilibrium decay of these REC- and ICD-associated SNPs, of which 178 were responsive to EC induction. According to the expression values of the 178 genes, we performed a weighted gene co-expression network analysis (WGCNA) and revealed an EC induction-associated module and five hub genes. Hub gene-based association studies uncovered that the intragenic variations in GRMZM2G105473 and ZmARF23 influenced EC induction efficiency among different maize lines. Dual-luciferase reporter assay indicated that ZmARF23 bound to the promoter of a known causal gene (ZmSAUR15) for EC induction and positively regulated its expression on the transcription level. Our study will deepen the understanding of genetic and molecular mechanisms underlying EC induction and contribute to the use of genetic transformation in maize.

Lactate anion catalyzes aminolysis of polyesters with anilines

Chemical transformation of spent polyesters into value-added chemicals is substantial for sustainable development but still challenging. Here, we report a simple, metal-free, and efficient aminolysis strategy to upcycle polylactic acid by anilines over lactate-based ionic liquids (e.g., tetrabutylammonium lactate), accessing a series of N-aryl lactamides under mild conditions. This strategy is also effective for degradation of poly(bisphenol A carbonate), affording bisphenol A and corresponding diphenylurea derivatives. It is found that, with the assistance of water, lactate anion as hydrogen-bond donor can efficiently activate carbonyl C atom of polyesters via hydrogen bonding with carbonyl O atom; meanwhile, as hydrogen-bond acceptor, it can enhance nucleophilicity of the N atom of anilines via hydrogen bonding with amino H atom. The nucleophilic attack of N atom of anilines on carbonyl C atom of polyesters results in cleavage of C─O bond of polymers and formation of the target products.

The multipotent thidiazuron: A mechanistic overview of its roles in callogenesis and other plant cultures in vitro

Thidiazuron (TDZ) is an active substituted phenyl urea compound that has found a significant role as a plant growth regulator. The most exciting aspect of its function is that it can mimic auxins and cytokinin but is chemically different from these two. Many theories have been put forward, and experiments performed to understand the mode of action of TDZ in callogenesis. One suggested mechanism presents that it works by inhibiting the cytokinin degrading enzymes that compete with cytokinin for an active site on the enzyme. An example is the TDZ-induced suppressed expression of gibberellic acid (GA) biosynthesis genes encoding GA3 and GA20 oxidases. This is entailed with a slightly increased expression of GA catabolism genes encoding GA20 oxidase. Similarly, one of the recommendations is that TDZ induces the expression of specific genes and transcription regulatory sequences that are either responsible directly for callus formation or in turn induce other auxins or cytokinin for callogenesis. There is no concise review available that discusses the details of TDZ-induced callus, specifically and other in vitro cultures in general. This review is an attempt to explore all these pathways and mechanisms involved in callogenesis in plants stimulated by TDZ.

Genomic Variation Underlying the Breeding Selection of Quinoa Varieties Longli-4 and CA3-1 in China

Quinoa (Chenopodium quinoa) is a well-known climate-resilient crop and has been introduced into multiple marginal lands across the world, including China, to improve food security and/or balanced nutrient supplies. Conventional breeding has been widely applied in the selection and breeding of quinoa varieties in China since 1980s; however, few studies have been implemented on the genetic variances among different varieties developed by diversity breeding objectives. In this study, the phenotypic and genetic differences between two varieties (Longli-4 and CA3-1) from China were systematically analyzed. A total of 407,651 and 2,731,411 single nucleotide polymorphisms (SNPs) and 212,724 and 587,935 small insertion and deletion (INDELs) were detected for Longli-4 and CA3-1, respectively, when compared with the reference genome of PI614886. The SNPs/INDELs were unevenly distributed across each chromosome for both varieties. There were 143,996 SNPs and 83,410 INDELs shared between Longli-4 and CA3-1, accounting for 4% of the total variances. The variation was then screened based on the SNP effects. There were 818 and 73 genes with the variety-specific non-synonymous and stop-gain variation in Longli-4, whereas there were 13,701 and 733 genes in CA3-1. Specifically, 3501 genes with the non-synonymous variation and 74 genes with the stop-gain variation were found in both Longli-4 and CA3-1. These results suggest that convergent selection occurred during the different breeding processes. A set of candidate genes related to agronomic traits and domestication were further selected to detect the genetic divergence in detail in the two varieties. Only one domestication gene was identified having Longli-4-specific stop-gain variation. Twelve candidate genes related to betalain (1), flowering (4), seed size (2), domestication (1), and saponin (4) were identified having CA3-1-specific stop-gain variation. Interestingly, one seed size gene homologous of CKX1 (cytokinin oxidase/dehydrogenase 1) had the stop-gain variation in both varieties. This research will therefore provide guidance for the molecular-assisted breeding in quinoa.

Cytokinin oxidase/dehydrogenase inhibitors: outlook for selectivity and high efficiency

Inhibitors of cytokinin oxidase/dehydrogenase (CKX) reduce the degradation of cytokinins in plants, and this effect can be exploited in agriculture and in plant tissue culture. In this study, we examine the structure-activity relationship of two series of CKX inhibitors based on diphenylurea. The compounds of Series I were derived from the recently published CKX inhibitors 3TFM-2HM and 3TFM-2HE, and we identified key substituents with increased selectivity for maize ZmCKX1 and ZmCKX4a over AtCKX2 from Arabidopsis. Series II contained compounds that further exceled in CKX inhibitory activity as well as in the ease of their synthesis. The best inhibitors exhibited half-maximal inhibitory concentration (IC50) values in low nanomolar ranges with ZmCKX1 and especially with ZmCKX4a, which is generally more resistant to inhibition. The activity of the key compounds was verified in tobacco and lobelia leaf-disk assays, where N6-isopentenyladenine was protected from degradation and promoted shoot regeneration. All the prepared compounds were further tested for toxicity against Caenorhabditis elegans, and the assays revealed clear differences in toxicity between compounds with and without a hydroxyalkyl group. In a broader perspective, this work increases our understanding of CKX inhibition and provides a more extensive portfolio of compounds suitable for agricultural and biotechnological research.

Killing two birds with a single stone-genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

The development of high-yielding, bio-fortified, stress-tolerant crop cultivars is the need of the hour in the wake of increasing global food insecurity, abrupt climate change, and continuous shrinking of resources and landmass suitable for agriculture. The cytokinin group of phytohormones positively regulates seed yield by simultaneous regulation of source capacity (leaf senescence) and sink strength (grain number and size). Cytokinins also regulate root-shoot architecture by promoting shoot growth and inhibiting root growth. Cytokinin oxidase/dehydrogenase (CKX) are the only enzymes that catalyze the irreversible degradation of active cytokinins and thus negatively regulate the endogenous cytokinin levels. Genetic manipulation of CKX genes is the key to improve seed yield and root-shoot architecture through direct manipulation of endogenous cytokinin levels. Downregulation of CKX genes expressed in sink tissues such as inflorescence meristem and developing seeds, through reverse genetics approaches such as RNAi and CRISPR/Cas9 resulted in increased yield marked by increased number and size of grains. On the other hand, root-specific expression of CKX genes resulted in decreased endogenous cytokinin levels in roots which in turn resulted in increased root growth indicated by increased root branching, root biomass, and root-shoot biomass ratio. Enhanced root growth provided enhanced tolerance to drought stress and improved micronutrient uptake efficiency. In this review, we have emphasized the role of CKX as a genetic factor determining yield, micronutrient uptake efficiency, and response to drought stress. We have summarised the efforts made to increase crop productivity and drought stress tolerance in different crop species through genetic manipulation of CKX family genes.

Priming with Small Molecule-Based Biostimulants to Improve Abiotic Stress Tolerance in Arabidopsis thaliana

Biostimulants became a hotspot in the fight to alleviate the consequences of abiotic stresses in crops. Due to their complex nature, it is challenging to obtain stable and reproducible final products and more challenging to define their mechanism of action. As an alternative, small molecule-based biostimulants, such as polyamines have promoted plant growth and improved stress tolerance. However, profound research about their mechanisms of action is still missing. To go further, we tested the effect of putrescine (Put) and its precursor ornithine (Orn) and degradation product 1,3-diaminopropane (DAP) at two different concentrations (0.1 and 1 mM) as a seed priming on in vitro Arabidopsis seedlings grown under optimal growth conditions, osmotic or salt stress. None of the primings affected the growth of the seedlings in optimal conditions but altered the metabolism of the plants. Under stress conditions, almost all primed plants grew better and improved their greenness. Only Orn-primed plants showed different plant responses. Interestingly, the metabolic analysis revealed the implication of the N- acetylornithine and Orn and polyamine conjugation as the leading player regulating growth and development under control and stress conditions. We corroborated polyamines as very powerful small molecule-based biostimulants to alleviate the adverse abiotic stress effects.

Cytokinin Oxygenase/Dehydrogenase Inhibitors: An Emerging Tool in Stress Biotechnology Employed for Crop Improvement

In order to meet the global challenges of food security, one of the foremost solutions lies in enhancing the crop productivity. This can be attained by considering key plant hormones such as cytokinins as agrochemicals as cytokinins in particular are known to control the essential processes of the plants. Even though, it has already been established since 1980s that the enzyme, cytokinin oxidase/dehydrogenase (CKO/CKX) deactivates cytokinins; the potential applications of manipulating these enzymes have mostly been speculated to have a high potential in the biotechnology industry and spreads to agriculture, horticulture and agroforestry. The enzyme is critical in maintaining a balanced level of cytokinins in plants. However, it is yet to be fully established that inhibiting this enzyme can be the constant source of improvement in the productivity of plants, even though success has been obtained in some economically important plant species. Furthermore, the impact efficiency of this enzyme may vary from plant to plant, which needs to be evaluated employing tissue culture and other extrinsic applications. This review intends to cover the relevant studies addressing any biological activity of this enzyme in the current context and any associated biotechnological applications specific to enhanced grain yield, abiotic stress tolerance, delayed senescence and in vitro organogenesis among various plants and not only cereals. Moreover, our study will identify the present gaps in research with respect to many important food crops, which will be useful for researchers who are actively involved in providing a foundation for a variety of genetically improved plants achieved through this manner. In addition to this, other ways of engineering the amount of cytokinin levels appropriate for signaling also needs to be analyzed in order to extend the benefits of cytokinin biology to other crops too. The application of these inhibitors can be considered among the best alternates as well as addition to genetically modified plants for overcoming the gaps in crop demand.

Plant Growth Regulators INCYDE and TD-K Underperform in Cereal Field Trials

Using plant growth regulators to alter cytokinin homeostasis with the aim of enhancing endogenous cytokinin levels has been proposed as a strategy to increase yields in wheat and barley. The plant growth regulators INCYDE and CPPU inhibit the cytokinin degrading enzyme cytokinin oxidase/dehydrogenase (CKX), while TD-K inhibits the process of senescence. We report that the application of these plant growth regulators in wheat and barley field trials failed to enhance yields, or change the components of yields. Analyses of the endogenous cytokinin content showed a high concentration of trans-zeatin (tZ) in both wheat and barley grains at four days after anthesis, and statistically significant, but probably biologically insignificant, increases in cisZ-O-glucoside, along with small decreases in cZ riboside (cZR), dihydro Z (DHZ), and DHZR and DHZOG cytokinins, following INCYDE application to barley at anthesis. We discuss possible reasons for the lack of efficacy of the three plant growth regulators under field conditions and comment on future approaches to manipulating yield in the light of the strong homeostatic mechanisms controlling endogenous cytokinin levels.

Novel cytokinin oxidase/dehydrogenase inhibitors for enhancing grain yield in crop plants and potential applications in the biotechnology industry

This article comments on:

Nisler J, Kopečný D, Pěkná Z, Končitíková R, Koprna R, Murvanidze N, Werbrouck SPO, Havlíček L, De Diego N, Wimmer Z, Briozzo P, Moréra S, Zalabák D, Spíchal L, Strnad M. 2021. Diphenylurea-derived cytokinin oxidase/dehydrogenase inhibitors for biotechnology and agriculture. Journal of Experimental Botany 72, 355–370.

Targeting Cytokinin Homeostasis in Rapid Cycling Brassica rapa with Plant Growth Regulators INCYDE and TD-K

Modifying the cytokinin content in plants is a means of improving plant productivity. Here, we report the development and biological activity of compound TD-K (1-(furan-2-ylmethyl)-3-(1,2,3-thiadiazol-5-yl)urea)which is related to thidiazuron. TD-K—which exhibited extremely high antisenescence activity in the wheat leaf bioassay—and INCYDE (2-chloro-6-(3-methoxyphenyl)aminopurine)—a plant growth regulator reported to inhibit cytokinin oxidase/dehydrogenase (CKX), an enzyme involved in the degradation of the plant hormone cytokinin—were selected for investigation of their effects on the model plant Rapid Cycling Brassica rapa (RCBr). We monitored the expression of BrCKX and isopentenyl transferase (BrIPT), which codes for the key cytokinin biosynthesis enzyme, in developing leaves following INCYDE and TD-K application. Growth room experiments revealed that INCYDE increased RCBr seed yield per plant, but only when applied multiple times and when grown in 5 mM KNO₃. Expression in control leaves showed transient, high levels of expression of BrCKX and BrIPT at true leaf appearance. Following INCYDE application, there was a rapid and strong upregulation of BrCKX3, and a transient downregulation of BrIPT1 and BrIPT3. Interestingly, the upregulation of BrCKX3 persisted in a milder form throughout the course of the experiment (16 days). TD-K also upregulated BrCKX3. However, in contrast to INCYDE, this effect disappeared after two days. These results suggest that both compounds (CKX inhibitor and cytokinin TD-K) influenced cytokinin homeostasis in RCBr leaves, but with different mechanisms.