Cytokinin Biosynthesis Research Articles

A wheat phytohormone atlas spanning major tissues across the entire life cycle provides novel insights into cytokinin and jasmonic acid interplay.

Although numerous studies have focused on the specific organs or tissues at different development stages or under various abiotic and biotic stress, our understanding of the distribution and relative abundance of phytohormones throughout the entire life cycle of plant organs and tissues remains insufficient. Here, we present a phytohormone atlas resource covering the quantitative analysis of eight major classes of phytohormones, comprising a total of 40 hormone-related compounds, throughout the complete life cycle of wheat. In combination with transcriptome analysis, we established a Wheat Phytohormone Metabolic Regulatory Network (WPMRN). Using our WPMRN dataset and GO enrichment analysis, we swiftly characterized the function of TaLOG5-B1 in cytokinin biosynthesis. Furthermore, a detailed investigation of the WPMRN dataset uncovered transcription factor-mediated co-regulation mechanisms among different classes of phytohormones. We focused specifically on the metabolic regulatory involving cytokinin and jasmonic acid. To achieve this, we characterized genes TaLOG3-D1 and TaAOS-D1 involved in the biosynthesis of these phytohormones, along with their regulatory transcription factors TaDOF3A and TaDOF5.6B. The functions of these genes were validated in transgenic plants, revealing their ability to co-regulate radicle length. These findings serve as a case study that highlights the utility of this resource for studying phytohormone metabolic regulatory networks in cereal crops and for gaining insights into the roles of phytohormones in enhancing agronomic traits.

Expression of Agrobacterium Isopentenyl transferase (IPT) gene in wheat improves drought tolerance.

Drought, as an abiotic stressor, globally limits cereal productivity, leading to early aging of leaves and lower yields. The expression of the isopentenyl transferase (IPT) gene, which is involved in cytokinin (CK) biosynthesis, can delay drought-induced leaf senescence. In this study, the Agrobacterium Isopentenyl transferase (IPT) gene was introduced into two local hexaploid wheat cultivars, NR-421 and FSD-2008. The expression cassette was developed containingthe IPT gene under transcriptional regulation of the stress-inducible promoter 'Dehydrin,' sourced from Hordeum vulgare. The gene expression cassette was assembled in pSB219M, a modified transformation vector for monocots, equipped with both an antibiotic (spectinomycin) and an herbicide selection marker (BASTA). Initial screening of transgenic plants involved BASTA selection (2 and 3mg/L) and was subsequently confirmed through PCR analysis. The transformation efficiencies of NR-421 and FSD-2008 were 0.4% and 0.3%, respectively. The qRT-PCR analysis under stress conditions showed a 13.5-fold higher expression of the IPT gene in T2 transgenic plants of NR-421 and a 5.8-fold higher expression in those of FSD-2008 than in non-transgenic controls. Under stress conditions, the wheat transgenic plants exhibited increased chlorophyll and relative water content. Additionally, for total soluble proteins, two transgenic lines from the NR-421 variety showed a significant increase, whereas no notable change was observed in the FSD-2008 transgenics. Moreover, the transgenic lines displayed increased plant height, higher fresh and dry biomass, and increased seed weight compared to the non-transgenic controls. These findings highlight that stress-inducible expression of the IPT gene in wheat leads to enhanced grain yield and subsequently improved drought tolerance.

Ectopic expression of LONELY GUY7 in epidermis of internodal segments for de novo shoot regeneration without phytohormone treatment in ipecac.

In many plant species, the application of exogenous phytohormones is crucial for initiating de novo shoot regeneration. However, ipecac [Carapichea ipecacuanha (Brot) L. Andersson] has a unique ability to develop adventitious shoots on the epidermis of internodal segments without phytohormone treatment. This characteristic allows us to evaluate the effects of endogenous phytohormones in this species. Here, we showed that the presence of the pith, including vascular bundles in the internodal segment, is required to activate both endogenous cytokinin (CK) biosynthesis and adventitious shoot formation. Adventitious shoots were mainly formed in the apical region of internodal segments, where the CK biosynthesis genes ISOPENTENYL TRANSFERASE 3 (CiIPT3) and LONELY GUY 7 (CiLOG7) were spontaneously upregulated in the early culture stage on phytohormone-free medium. In addition, CiIPT3 and CiLOG7 were respectively expressed in the pith and the epidermis of the internodal segments. The expression of CiLOG7 was localized as several spots on the epidermis. These findings suggest that CK precursors are generated in the pith, transferred to the epidermis, and then converted into active CKs, facilitating adventitious shoot formation on the epidermis. Conversely, auxin levels rapidly decreased during culture and remained low in the region of shoot formation. Auxin is transferred to the basal region of internodal segments, and strongly suppressed the CiLOG7 expression and decreased the CK levels. Thus, we conclude that the ectopic expression of CiLOG7 in the epidermis of internodal segments contributes to de novo shoot regeneration in ipecac.

The cytokinin efflux transporter ABCC4 participates in Arabidopsis root system development

Abstract The directional and sequential flow of cytokinin in plants is organized by a complex network of transporters. Genes involved in several aspects of cytokinin transport have been characterized; however, much of the elaborate system remains elusive. In this study, we used a transient expression system in tobacco (Nicotiana benthamiana) leaves to screen Arabidopsis (Arabidopsis thaliana) transporter genes and isolated ATP-BINDING CASSETTE TRANSPORTER C4 (ABCC4). Validation through drug-induced expression in Arabidopsis and heterologous expression in budding yeast revealed that ABCC4 effluxes the active form of cytokinins. During the seedling stage, ABCC4 was highly expressed in roots, and its expression was upregulated in response to cytokinin application. Loss-of-function mutants of ABCC4 displayed enhanced primary root elongation, similar to mutants impaired in cytokinin biosynthesis or signaling, that was suppressed by exogenous trans-zeatin treatment. In contrast, overexpression of the gene led to suppression of root elongation. These results suggest that ABCC4 plays a role in the efflux of active cytokinin, thereby contributing to root growth regulation. Additionally, cytokinin-dependent enlargement of stomatal aperture was impaired in the loss-of-function and overexpression lines. Our findings contribute to unraveling the many complexities of cytokinin flow and enhance our understanding of the regulatory mechanisms underlying root system development and stomatal opening in plants.

GmbZIP4a/b Positively Regulate Nodule Number by Affecting Cytokinin Biosynthesis in Glycine max.

Legumes have the capability to form nodules that facilitate symbiotic nitrogen fixation (SNF) with rhizobia. Given the substantial energy consumption during the process of SNF, legumes need to optimize nodule number in response to everchanging environmental scenarios. The TGACG BINDING FACTOR1/4 (TGA1/4) are key players in the basal immune response of plants. In this study, both β-glucuronidase staining and quantitative reverse transcription PCR (qRT-PCR) demonstrated that both GmbZIP4a and GmbZIP4b are inducible upon rhizobial inoculation. To investigate their roles further, we constructed gmbzip4a/b double mutants using CRISPR/Cas9 system. Nodulation assessments revealed that these double mutants displayed a reduction in the number of infection threads, which subsequently resulted in a decreased nodule number. However, the processes associated with nodule development including nodule fresh weight, structural characteristics, and nitrogenase activity, remained unaffected in the double mutants. Subsequent transcriptome analyses revealed that zeatin biosynthesis was downregulated in gmbzip4a/b mutants post rhizobial inoculation. Supporting these findings, genes associated with cytokinin (CTK) signaling pathway were upregulated in Williams 82 (Wm82), but this upregulation was not observed in the double mutants after rhizobial treatment. These results suggest that GmbZIP4a/b positively influences nodule formation by promoting the activation of the CTK signaling pathway during the early stages of nodule formation.

Gibberellin and cytokinin signaling antagonistically control female-germline cell specification in Arabidopsis.

How do growth hormones interact to specify female-germline cell types in flowering plants and control production of the first female-germline cell? Here, we find that gibberellin (GA) biosynthesis and signaling are restricted in ovule primordia, with overexpression of receptors and biosynthetic enzymes resulting in multiple and enlarged megaspore mother cells (MMCs) in Arabidopsis. GA signaling machinery interacts with and promotes the degradation of cytokinin (CK) type-B Arabidopsis response regulators (ARR1/10/12), which also directly interact with DELLA proteins. CK biosynthesis and signaling components are expressed in both MMCs and sporophytic cells, with signaling negatively controlled by GA in ovule primordia, and perturbations leading to the induction of multiple, enlarged MMC-like cells. The vacuolar sorting protein SHRUBBY (SHBY) interacts with GA and CK signaling components to block GA-induced degradation. CK signaling restricts multiple sub-epidermal cells in distal ovule primordia from acquiring MMC identity. By balancing degradation activity, GA and CK signaling antagonistically control female-germline cell specification.

Methylocystis borbori sp.nov., a novel methanotrophic bacterium from the sludge of a freshwater lake and its metabolic properties.

A novel methanotrophic strain 9NT was isolated from the sludge of a freshwater lake. Cells were aerobic, Gram-stain-negative, non-motile pleomorphic rods with intracytoplasmic membrane systems that appropriate type-II methanotrophs and hemispherical and spherical exocellular formations on the perimeter of the cell wall surface. The novel isolate grows only on methane or methanol as the sole carbon and energy source, at 10-37°C (optimum 28-30°C), pH 4.5-9.0 (optimum 7.0-7.5), up to 1% NaCl (optimum 0.3-0.5%). Methanol supported the growth of the strain 9NT in a wide range of concentrations from 0.05 to 5.0% (v/v) with an optimum of 0.5% (v/v). The major fatty acids were C18:1ω8c and C18:1ω7c. Based on 16S rRNA gene sequence phylogenetic analysis, strain 9NT was closely related to representatives of the genus Methylocystis (96.5-98.3%). The genome of strain 9NT was 3.34Mbp in size with 63.5% of G + C content. The average nucleotide identity and digital DNA-DNA hybridization values between strain 9NT and closely related type strains of genus Methylocystis were 78.0-82.4% and 20.9-24.3%, respectively. The careful genome annotation of the novel strain shows it possessed genes for the detoxification of arsenate and cyanides as well as genes potentially involved in plant growth promotion (such as biosynthesis of indoles, cytokinins, polyhydroxybutyrate, siderophore production, and nitrogen fixation). Based on the phylogenetic, phenotypic, chemotaxonomic and genomic data, we propose Methylocystis borbori sp. nov. as novel species of the genus Methylocystis. The type strain is 9NT (= VKM B-3616T = KCTC 92566T).

KNUCKLES regulates floral meristem termination by controlling auxin distribution and cytokinin activity.

The termination of floral meristem (FM) activity is essential for the normal development of reproductive floral organs. During this process, KNUCKLES (KNU), a C2H2-type zinc finger protein, crucially regulates FM termination by directly repressing the expression of both the stem cell identity gene WUSCHEL (WUS) and the stem cell marker gene CLAVATA3 (CLV3) to abolish the WUS-CLV3 feedback loop required for FM maintenance. In addition, phytohormones auxin and cytokinin are involved in FM regulation. However, whether KNU modulates auxin and cytokinin activities for FM determinacy control remains unclear. Here, we show that the auxin distribution and the cytokinin activity mediated by KNU in Arabidopsis (Arabidopsis thaliana) promote the termination of FM during stage 6 of flower development. Mutation of KNU leads to altered distribution of auxin and cytokinin in the FM of a stage 6 floral bud. Moreover, KNU directly represses the auxin transporter gene PIN-FORMED1 (PIN1) and the cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE7 (IPT7) via mediating H3K27me3 deposition on these 2 loci to regulate auxin and cytokinin activities. Our study presents a molecular regulatory network that elucidates how the transcriptional repressor KNU integrates and modulates the activities of auxin and cytokinin, thus securing the timed FM termination.

Reduction of Plasmodiophora brassicae infection on Brassica rapa through host-induced gene silencing of two secreted genes.

Clubroot disease caused by the biotrophic pathogen Plasmodiophora brassicae, is one of the most serious threats to cruciferous crops production worldwide. P. brassicae is known for rapid adaptive evolution to overcome resistance in varieties. It is urgent to establish alternative management to control P. brassicae. In this study, we identified two P. brassicae secretory proteins that were up-regulated during infection and effected plant defense. We established a method for transient expression in the roots of seedlings and demonstrated that P. brassicae could take up substances from the environment of root cells. Using a RNA interference (RNAi)-based host-induced gene silencing (HIGS) by expression of hairpin RNAi constructs with sequence homology to P. brassicae effector Pb48 or Pb52 in susceptible Brassica rapa plants enhanced host disease resistance. After silencing these two effectors, the transcription levels of cytokinin biosynthesis gene IPT1 and the regulation gene of auxin homeostasis GH3.5 were down-regulated. These results suggested that RNAi-based HIGS of effectors has a great practical application of improving crop resistance against P. brassicae and can contribute to environmentally sustainable agriculture.

Evolutionary Analysis and Catalytic Function of LOG Proteins in Plants.

The plant hormone cytokinin is a conserved regulator of plant development. LONELY GUY (LOG) proteins are pivotal in cytokinin biosynthesis. However, their origin, evolutionary history, and enzymatic characteristics remain largely uncharacterized. To elucidate LOG family evolution history and protein motif composition, we conducted phylogenetic and motif analyses encompassing representative species across the whole green plant lineage. Catalytic activity and structure analysis were conducted to thoroughly characterize the LOG proteins. Our phylogeny showed that LOG proteins could be divided into five groups and revealed three major duplication events giving rise to four distinct groups of vascular LOG proteins. LOG proteins share a conserved structure characterized by a canonical motif arrangement comprising motifs 1, 2, 3, 4, 5, 6, and 7. Two significant changes in LOG motif composition occurred during the transition to land plants: the emergence of motif 3 in charophyte LOG sequences and the subsequent acquisition of motif 8 at the C-terminus of LOG proteins. Enzymatic assays demonstrated that LOG proteins can be classified into two groups based on their enzyme activity. One group act as cytokinin riboside 5'-monophosphate phosphoribohydrolase and primarily convert iPRMP to iP, while the other group act as 5'-ribonucleotide phosphohydrolase, and preferentially produce iPR from the same substrates. TaLOG5-4A1, TaLOG5-4A2, TaLOG5-5B2, and TaLOG5-D1 shared conserved residues in the critical motif and were predicted to have similar protein structures, but displayed distinct enzymatic activities. Our findings provide a comprehensive overview of LOG protein phylogeny and lay a foundation for further investigations into their functional diversification.

The role of the cytokinin biosynthesis pathways in the rate of tobacco leaf senescence

The regulation of leaf senescence depends on endogenous and exogenous factors, among them phytohormones like cytokinins (CKs). CKs are key players regulating the senescing process, as their endogenous concentration is linked to the onset and rate of senescence progression. Thus, this study aimed to identify the relationship between the activity of endogenous CKs biosynthesis pathways - the cytosolic mevalonate (MVA) and the plastid methyl-erythritol phosphate (MEP) and the rate of leaf senescence. To this end, three distinct tobacco (Nicotiana tabacum L.) cultivars – Xanthi, Golden Virginia and Monte Calme Yellow were analysed. The study involved treatment with exogenous CK – benzyladenine – and two different CK synthesis inhibitors: lovastatin and clomazone. The progression of senescence was induced by light deprivation and monitored with chlorophyll level (SPAD), photosynthetic activity (PAM) and changes in the Rubisco protein profile (SDS-PAGE). Analyses showed that the Xanthi cultivar was characterized by delayed onset of senescence and stay-green phenotype, while Golden Virginia, and particularly Monte Calme Yellow showed rapid leaf senescence. The studies provided valuable information regarding the role of MEP and MVA pathway of CK synthesis in the regulation of tobacco leaf senescence.

Heritable gene editing in tomato through viral delivery of isopentenyl transferase and single-guide RNAs to latent axillary meristematic cells

Realizing the full potential of genome editing for crop improvement has been slow due to inefficient methods for reagent delivery and the reliance on tissue culture for creating gene-edited plants. RNA viral vectors offer an alternative approach for delivering gene engineering reagents and bypassing the tissue culture requirement. Viruses, however, are often excluded from the shoot apical meristem, making virus-mediated gene editing inefficient in some species. Here, we developed effective approaches for generating gene-edited shoots in Cas9-expressing transgenic tomato plants using RNA virus-mediated delivery of single-guide RNAs (sgRNAs). RNA viral vectors expressing sgRNAs were either delivered to leaves or sites near axillary meristems. Trimming of the apical and axillary meristems induced new shoots to form from edited somatic cells. To further encourage the induction of shoots, we used RNA viral vectors to deliver sgRNAs along with the cytokinin biosynthesis gene, isopentenyl transferase. Abundant, phenotypically normal, gene-edited shoots were induced per infected plant with single and multiplexed gene edits fixed in the germline. The use of viruses to deliver both gene editing reagents and developmental regulators overcomes the bottleneck in applying virus-induced gene editing to dicotyledonous crops such as tomato and reduces the dependency on tissue culture.

Genome-Wide Identification, Expression, and Protein Analysis of CKX and IPT Gene Families in Radish (Raphanus sativus L.) Reveal Their Involvement in Clubroot Resistance.

Cytokinins (CKs) are a group of phytohormones that are involved in plant growth, development, and disease resistance. The isopentenyl transferase (IPT) and cytokinin oxidase/dehydrogenase (CKX) families comprise key enzymes controlling CK biosynthesis and degradation. However, an integrated analysis of these two gene families in radish has not yet been explored. In this study, 13 RsIPT and 12 RsCKX genes were identified and characterized, most of which had four copies in Brassica napus and two copies in radish and other diploid Brassica species. Promoter analysis indicated that the genes contained at least one phytohormone or defense and stress responsiveness cis-acting element. RsIPTs and RsCKXs were expanded through segmental duplication. Moreover, strong purifying selection drove the evolution of the two gene families. The expression of the RsIPT and RsCKX genes distinctly showed diversity in different tissues and developmental stages of the root. Expression profiling showed that RsCKX1-1/1-2/1-3 was significantly upregulated in club-resistant materials during primary infection, suggesting their vital function in clubroot resistance. The interaction network of CKX proteins with similar 3D structures also reflected the important role of RsCKX genes in disease resistance. This study provides a foundation for further functional study on the IPT and CKX genes for clubroot resistance improvement in Raphanus.

Five unaddressed questions about cytokinin biosynthesis.

Cytokinins, a class of phytohormones, play crucial roles in regulating plant growth and stress responses through finely tuned feedback loops involving metabolic and signaling cascades. Cytokinin metabolism modulates the abundance of these biologically active molecules. Over the past 25 years, studies have identified key genes involved in cytokinin biosynthesis and inactivation pathways. Nevertheless, several gaps remain in our understanding, particularly regarding the movement of intermediate metabolites between subcellular compartments and the discrepancy between the product of adenosine phosphate-isopentenyltransferase (IPT) and the substrate preferences of subsequent reactions. In addition, recent gene discoveries related to lonely guy (LOG)-independent pathways suggest a spatial extension of cytokinin biosynthesis into the apoplast. Other intriguing issues remain to be addressed, i.e., elucidating the synthetic pathway for cis-zeatin and unraveling the molecular mechanisms governing selective substrate use by the cytokinin biosynthetic enzyme tumor morphology root (Tmr) derived from the phytopathogen Agrobacterium tumefaciens during crown gall formation. Further studies are needed to reveal a fully comprehensive picture of cytokinin metabolism.

The combination of a microbial and a non-microbial biostimulant increases yield in lettuce (Lactuca sativa) under salt stress conditions by up-regulating cytokinin biosynthesis.

Salinization poses a significant challenge in agriculture, exacerbated by anthropogenic global warming. Biostimulants, derived from living microorganisms or natural extracts, have emerged as valuable tools for conventional and organic agriculture. However, our understanding of the molecular mechanisms underlying the effects of biostimulants is very limited, especially in crops under real cultivation conditions. In this study, we adopted an integrative approach to investigate the effectiveness of the combined application of plant growth-promoting bacterium (Bacillus megaterium strain BM08) and a non-microbial biostimulant under control conditions (normal watering) and salt stress. After confirming the yield increase under both conditions, we investigated the molecular mechanisms underlying the observed effect by measuring a number of physiological parameters (i.e., lipid peroxidation, antioxidants, chlorophylls, total phenolics and phytohormone content), as well as RNA sequencing and primary metabolite analyses. Our findings reveal that the combined effect of the microbial and non-microbial biostimulants led to a decrease in the antioxidant response and an up-regulation of genes involved in cytokinin biosynthesis under salt stress conditions. This, in turn, resulted in a higher concentration of the bioactive cytokinin, isopentenyladenosine, in roots and leaves and an increase in γ-aminobutyric acid, a non-proteic amino acid related to abiotic stress responses. In addition, we observed a decrease in malic acid, along with an abscisic acid (ABA)-independent up-regulation of SR-kinases, a family of protein kinases associated with abiotic stress responses. Furthermore, we observed that the single application of the non-microbial biostimulant triggers an ABA-dependent response under salt stress; however, when combined with the microbial biostimulant, it potentiated the mechanisms triggered by the BM08 bacterial strain. This comprehensive investigation shows that the combination of two biostimulants is able to elicit a cytokinin-dependent response that may explain the observed yield increase under salt stress conditions.

A sharp discovery.

Plant prickles are controlled by genes involved in the final step of cytokinin biosynthesis.

Seed yield as a function of cytokinin-regulated gene expression in wild Kentucky bluegrass (Poa pratensis)

BackgroundKentucky bluegrass (Poa pratensis L.) panicle development is a coordinated process of cell proliferation and differentiation with distinctive phases and architectural changes that are pivotal to determine seed yield. Cytokinin (CK) is a key factor in determining seed yield that might underpin the second “Green Revolution”. However, whether there is a difference between endogenous CK content and seed yields of Kentucky bluegrass, and how CK-related genes are expressed to affect enzyme regulation and downstream seed yield in Kentucky bluegrass remains enigmatic.ResultsIn order to establish a potential link between CK regulation and seed yield, we dissected and characterized the Kentucky bluegrass young panicle, and determined the changes in nutrients, 6 types of endogenous CKs, and 16 genes involved in biosynthesis, activation, inactivation, re-activation and degradation of CKs during young panicle differentiation of Kentucky bluegrass. We found that high seed yield material had more meristems compared to low seed yield material. Additionally, it was found that seed-setting rate (SSR) and lipase activity at the stage of spikelet and floret primordium differentiation (S3), as well as 1000-grain weight (TGW) and zeatin-riboside (ZR) content at the stages of first bract primordium differentiation (S1) and branch primordium differentiation (S2) showed a significantly positive correlation in the two materials. And zeatin, ZR, dihydrozeatin riboside, isopentenyl adenosine and isopentenyl adenosine riboside contents were higher in seed high yield material than those in seed low yield material at S3 stage. Furthermore, the expressions of PpITP3, PpITP5, PpITP8 and PpLOG1 were positively correlated with seed yield, while the expressions of PpCKX2, PpCKX5 and PpCKX7 were negatively correlated with seed yield in Kentucky bluegrass.ConclusionsOverall, our study established a relationship between CK and seed yield in Kentucky bluegrass. Perhaps we can increase SSR and TGW by increasing lipase activity and ZR content. Of course, using modern gene editing techniques to manipulate CK related genes such as PpITP3/5/8, PpLOG1 and PpCKX2/5/7, will be a more direct and effective method in Kentucky bluegrass, which requires further trial validation.

VlMYB4 and VlCDF3 co-targeted the VlLOG11 promoter to regulate fruit setting in grape (Vitis vinifera L).

The VlLOG11 mediates the cytokinin signaling pathway to regulate grape fruit setting. Fruit set, as an accepted agronomic trait, is inextricably linked with fruit quality and yield. Previous studies have demonstrated that exogenous treatment with the synthetic cytokinin analog, forchlorfenuron (CPPU), significantly enhances fruit set. In this study, a significant reduction in endogenous cytokinins was found by measuring the content of cytokinins in young grape berries after CPPU treatment. LONELY GUYs (VlLOGs), a key cytokinin-activating enzyme working in the biosynthesis pathway of cytokinins, exhibited differential expression. Some differentially expressed VlLOGs genes were presented by RNA seq data and their functions and regulation patterns were further investigated. The results showed that VlLOG11 was differentially expressed in young grape berries after CPPU treatment. Overexpression of VlLOG11 in tomato increases the amount of fruit set, and upregulated the expression of genes associated with cytokinin signaling including SlHK4, SlHK5, SlHP3, SlHP4, SlPHP1, SlPHP2. VlMYB4 and VlCDF3 could regulate the expression of VlLOG11 by directly binding to its promoter in young grape berries during fruit set. These results strongly demonstrated that VlMYB4/VlCDF3-VlLOG11 regulatory module plays a key role in the process of fruit setting in grape. This provided a basis for the molecular mechanism of VlLOG11-mediated cytokinin biosynthesis in young grape fruit set.

Spider mite infestation triggers coordinated hormonal trade-offs enabling plant survival with a fitness cost.

Tetranychus urticae is an important pest that causes severe damage to a wide variety of plants and crops, leading to a substantial productivity loss. Previous research has been focused on plant defence response to T. urticae to improve plant resistance. However, plant growth, development and reproduction throughout the infestation process have not been previously studied. Through physiological, biochemical, transcriptomic and hormonomic evaluation, we uncover the molecular mechanisms directing the defence-growth trade-off established in Arabidopsis upon T. urticae infestation. Upon mite attack, plants suffer an adaptation process characterized by a temporal separation between the defence and growth responses. Jasmonic and salicylic acids regulate the main defence responses in combination with auxin and abscisic acid. However, while the reduction of both auxin signalling and gibberellin, cytokinin and brassinosteroid biosynthesis lead to initial growth arrest, increasing levels of growth hormones at later stages enables growth restart. These alterations lead to a plant developmental delay that impacts both seed production and longevity. We demonstrate that coordinated trade-offs determine plant adaptation and survival, revealing mite infestation has a long-lasting effect negatively impacting seed viability. This study provides additional tools to design pest management strategies that improve resistance without penalty in plant fitness.

Systemic strategies for cytokinin biosynthesis and catabolism in Arabidopsis roots and leaves under prolonged ammonium nutrition

Cytokinins are growth-regulating plant hormones that are considered to adjust plant development under environmental stresses. During sole ammonium nutrition, a condition known to induce growth retardation of plants, altered cytokinin content can contribute to the characteristic ammonium toxicity syndrome. To understand the metabolic changes in cytokinin pools, cytokinin biosynthesis and degradation were analyzed in the leaves and roots of mature Arabidopsis plants. We found that in leaves of ammonium-grown plants, despite induction of biosynthesis on the expression level, there was no active cytokinin build-up because they were effectively routed toward their downstream catabolites. In roots, cytokinin conjugation was also induced, together with low expression of major synthetic enzymes, resulting in a decreased content of the trans-zeatin form under ammonium conditions. Based on these results, we hypothesized that in leaves and roots, cytokinin turnover is the major regulator of the cytokinin pool and does not allow active cytokinins to accumulate. A potent negative-regulator of root development is trans-zeatin, therefore its low level in mature root tissues of ammonium-grown plants may be responsible for occurrence of a wide root system. Additionally, specific cytokinin enhancement in apical root tips may evoke a short root phenotype in plants under ammonium conditions. The ability to flexibly regulate cytokinin metabolism and distribution in root and shoot tissues can contribute to adjusting plant development in response to ammonium stress.