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Phosphate (Pi) is an essential nutrient for all organisms. Roots are underground organs, but the majority of the root biology studies have been done on root systems growing in the presence of light. Root illumination alters the Pi starvation response (PSR) at different intensities. Thus, we have analyzed morphological, transcriptional and physiological responses to Pi starvation in dark-grown roots. We have identified new genes and pathways regulated by Pi starvation that were not described previously. We also show that Pi-starved plants increase the cis-zeatin (cZ) : trans-zeatin (tZ) ratio. Transcriptomic analyses show that tZ preferentially represses cell cycle and PSR genes, whereas cZ induces genes involved in cell and root hair elongation and differentiation. In fact, cZ-treated seedlings show longer root system as well as longer root hairs compared with tZ-treated seedlings, increasing the total absorbing surface. Mutants with low cZ concentrations do not allocate free Pi in roots during Pi starvation. We propose that Pi-starved plants increase the cZ : tZ ratio to maintain basal cytokinin responses and allocate Pi in the root system to sustain its growth. Therefore, cZ acts as a PSR hormone that stimulates root and root hair elongation to enlarge the root absorbing surface and to increase Pi concentrations in roots.

Cytokinins (CKs) are plant hormones affecting numerous developmental processes. Zeatin and its derivatives are the most important group of isoprenoid CKs. Zeatin occurs as two isomers: while frans-zeatin (transZ) was found to be a bioactive substance, cis-zeatin (cisZ) was reported to have a weak biological impact. Even though cisZ derivatives are abundant in various plant materials their biological role is still unknown. The comprehensive screen of land plants presented here suggests that cisZ-type CKs occur ubiquitously in the plant kingdom but their abundance might correlate with a strategy of life rather than with evolutionary complexity. Changing levels of transZ and cisZ during Arabidopsis ontogenesis show that levels of the two zeatin isomers can differ significantly during the life span of the plant, with cisZ-type CKs prevalent in the developmental stages associated with limited growth. A survey of the bioassays employed illustrates mild activity of cisZ and its derivatives. No cis ↔ trans isomerization, which would account for the effects of cisZ, was observed in tobacco cells and oat leaves. Differences in uptake between the two isomers resulting in distinct bioactivity have not been detected. In contrast, cisZ and transZ have a different metabolic fate in oat and tobacco. Analysis of a CK-degrading enzyme, cytokinin oxidase/dehydrogenase (CKX), reveals that Arabidopsis possesses two isoforms, AtCKX1 expressed in stages of active growth, and AtCKX7, both of which have the highest affinity for the cisZ isomer. Based on the present results, the conceivable function of cisZ-type CKs as delicate regulators of CK responses in plants under growth-limiting conditions is hypothesized.

Organ-to-organ signal transmission is essential for higher organisms to ensure coordinated biological reactions during metabolism and morphogenesis. Similar to organs in animals, plant organs communicate by various signalling molecules. Among them, cytokinins, a class of phytohormones, play a key role as root-to-shoot long-distance signals, regulating various growth and developmental processes in shoots 1 , 2 . Previous studies have proposed that trans -zeatin-riboside, a type of cytokinin precursor, is a major long-distance signalling form in xylem vessels and its action depends on metabolic conversion via the LONELY GUY enzyme in proximity to the site of action 3 – 5 . Here we report an additional long-distance signalling form of cytokinin: trans -zeatin, an active form. Grafting between various cytokinin biosynthetic and transportation mutants revealed that root-to-shoot translocation of trans -zeatin, a minor component of xylem cytokinin, controls leaf size but not meristem activity-related traits, whereas that of trans -zeatin riboside is sufficient for regulating both traits. Considering the ratio of trans -zeatin to trans -zeatin-riboside in xylem and their delivery rate change in response to environmental conditions, this dual long-distance cytokinin signalling system allows plants to fine-tune the manner of shoot growth to adapt to fluctuating environments. After trans -zeatin riboside, the active cytokinin trans -zeatin is identified as the second major xylem cytokinin translocated from root to shoot. Interestingly, these two forms of root-derived cytokinins play different roles in regulating shoot development.

Background Symbiotic Methylobacterium strains comprise a significant part of plant microbiomes. Their presence enhances plant productivity and stress resistance, prompting classification of these strains as plant growth-promoting bacteria (PGPB). Methylobacteria can synthesize unusually high levels of plant hormones, called cytokinins (CKs), including the most active form, trans-Zeatin (tZ). Results This study provides a comprehensive inventory of 46 representatives of Methylobacterium genus with respect to phytohormone production in vitro, including 16 CK forms, abscisic acid (ABA) and indole-3-acetic acid (IAA). High performance-liquid chromatography—tandem mass spectrometry (HPLC–MS/MS) analyses revealed varying abilities of Methylobacterium strains to secrete phytohormones that ranged from 5.09 to 191.47 pmol mL −1 for total CKs, and 0.46 to 82.16 pmol mL −1 for tZ. Results indicate that reduced methanol availability, the sole carbon source for bacteria in the medium, stimulates CK secretion by Methylobacterium . Additionally, select strains were able to transform L-tryptophan into IAA while no ABA production was detected. Conclusions To better understand features of CKs in plants, this study uncovers CK profiles of Methylobacterium that are instrumental in microbe selection for effective biofertilizer formulations.

This study determined if the variation in grain filling parameters between two different spikelet types of rice (Oryza sativa L.) is regulated by the hormonal levels in the grains. Two rice mutants, namely, a large-grain mutant (AZU-M) and a small-grain mutant (ZF802-M), and their respective wild types (AZU-WT and ZF802-WT) were grown in the field. The endosperm cell division rate, filling rate, and hormonal levels: zeatin + zeatin riboside (Z+ZR), indo-3-acetic acid (IAA), polyamines (PAs), and abscisic acid (ABA) were determined. The results showed that there was no significant difference between the filling and endosperm cell division rates. These rates were synchronous between the superior and inferior spikelets for both mutants. However, the abovementioned parameters were significantly different between the two spikelet types for the two wild types. The superior spikelets filled faster and their filling rate was higher compared to the inferior ones. Changes in the concentrations of plant hormones were consistent with the observed endosperm cell division rate and the filling rate for both types of spikelets of mutant and wild type plants. Regression analysis showed a significant positive correlation between cell division and filling rates with the concentrations of the investigated hormones. Exogenous chemical application verified the role of ABA, IAA, and PAs in grain filling. The results indicate that poor filling of inferior spikelets in rice occurs primarily due to the reduced hormone concentrations therein, leading to lower division rate of endosperm cells, fewer endosperm cells, slower filling rate, and smaller grain weight.

Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as ‘novel’ stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

Cytokinin (CK) N -glucosides are the most abundant group of CK metabolites in many species; however, their physiological role in planta was for a long time perceived as irreversible storage CK forms only. Recently, a comprehensive screen showed that only vascular plants form CK N -glucosides in contrast to mosses, algae, and fungi. The formation of CK N -glucosides as biologically inactive CK conjugates thus represents an evolutionarily young mechanism for deactivation of CK bases. Even though CK N -glucosides are not biologically active themselves due to their inability to activate the CK perception system, new data on CK N -glucoside metabolism show that trans -zeatin (tZ) N7- and N9-glucosides are metabolized in vivo , efficiently releasing free CK bases that are most probably responsible for the biological activities observed in a number of bioassays. Moreover, CK N -glucosides’ subcellular localization as well as their abundance in xylem both point to their possible plasma membrane transport and indicate a role also as CK transport forms. Identification of the enzyme(s) responsible for the hydrolysis of tZ N7- and N9-glucosides, as well as the discovery of putative CK N -glucoside plasma membrane transporter, would unveil important parts of the overall picture of CK metabolic interconversions and their physiological importance.

Trichoderma is an important genus of symbiotic fungi, commonly used around the world as biocontrol agents and as biofertilizer. Although their beneficial effects are well known and are successfully exploited in sustainable agriculture practices, the biochemical mechanisms of plant growth-promoting actions of Trichoderma and their anti-pathogen characteristics are not well understood. This study biochemically surveyed 22 strains of Trichoderma and shows that Trichoderma produces cytokinins (CKs), which has not been reported to date. The phytohormone profiles ranged from 5.34 to 379.99 pmol CKs released to 10 mL of the growth medium and comprised riboside and nucleotide derivatives of cis-zeatin (cZ) and isopentenyladenine (iP), suggesting that fungal CKs originate from a tRNA degradation pathway. We reveal a connection between the levels of free base cZ produced by Trichoderma and the inhibition rate against the pathogen Fusarium graminearum among the tested strains. Furthermore, we analyzed CK profiles of Arabidopsis plants cultured in vitro in the presence of Trichoderma strains. The inoculated plants showed increased levels of cZ-type (cZR, cZROG) and iP-type (iP, iPR) CKs—the forms which dominated CK profiles of all the fungal in vitro cultures tested in this study. The increase in the levels of cZ derivatives was accompanied by a significant reduction in plant trans-zeatin (tZ)-type CKs (tZR, tZNT, tZOG, tZ7G, tZ9G) in Arabidopsis when co-cultured with the fungus. Our work suggests that CKs produced by plant symbiotic Trichoderma strains can be used for plant growth stimulation, may impact the colonization strategy of symbiotic fungi, and include alterations to the host plant phytohormones for enhanced plant resistance against pathogens.

Plants growing at high densities can detect competitors through changes in the composition of light reflected by neighbours. In response to this far-red-enriched light, plants elicit adaptive shade avoidance responses for light capture, but these need to be balanced against other input signals, such as nutrient availability. Here, we investigated how Arabidopsis integrates shade and nitrate signalling. We unveiled that nitrate modulates shade avoidance via a previously unknown shade response pathway that involves root-derived trans -zeatin (tZ) signal and the BEE1 transcription factor as an integrator of light and cytokinin signalling. Under nitrate-sufficient conditions, tZ promotes hypocotyl elongation specifically in the presence of supplemental far-red light. This occurs via PIF transcription factors-dependent inhibition of type-A ARRs cytokinin response inhibitors. Our data thus reveal how plants co-regulate responses to shade cues with root-derived information about nutrient availability, and how they restrict responses to this information to specific light conditions in the shoot. The authors here investigate information integration in plants using light and nutrient stimuli and identify a novel regulation pathway towards photoreceptor-regulated stem growth.

Summary Cold stress severely limits tea plant (Camellia sinensis) productivity, yet the molecular mechanisms underlying cold adaptation remain elusive. Here, we identified a cold‐inducible glycosyltransferase, CsUGT71A60, through integrative genome‐wide association studies (GWAS) and proteomic profiling. Natural variation in CsUGT71A60 was strongly associated with cold tolerance, as evidenced by linkage disequilibrium analysis of flanking SNPs. Functional characterization revealed that CsUGT71A60 specifically catalyses the glycosylation of cis‐zeatin to form cis‐zeatin 9‐O‐glucoside in vitro and in vivo. Overexpression of CsUGT71A60 in Arabidopsis enhanced cold tolerance and agronomic traits, including germination rate, tiller number and seed weight, while delaying flowering. Transient silencing of CsUGT71A60 in tea plants disrupted cis‐zeatin homoeostasis, impairing antioxidant defences and osmotic regulation under cold stress. Mechanistically, the transcription factor ARR (TEA021099) directly binds to CRM elements in the CsUGT71A60 promoter, activating its expression to fine‐tune cytokinin signalling. This study unveils a dual‐function glycosyltransferase that orchestrates stress tolerance and developmental plasticity, offering a strategic target for breeding climate‐tolerance crops without yield penalties.