New cytokinin metabolites in IPT transgenic Arabidopsis thaliana plants

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.

Effects of aluminum (Al) stress on the isoprenoid metabolism of two Citrus species differing in Al-tolerance

Isoprenoid metabolism and its derivatives took part in photosynthesis, growth regulation, signal transduction, and plant defense to biotic and abiotic stresses. However, how aluminum (Al) stress affects the isoprenoid metabolism and whether isoprenoid metabolism plays a vital role in the Citrus plants in coping with Al stress remain unclear. In this study, we reported that Al-treatment-induced alternation in the volatilization rate of monoterpenes (α-pinene, β-pinene, limonene, α-terpinene, γ-terpinene and 3-carene) and isoprene were different between Citrus sinensis (Al-tolerant) and C. grandis (Al-sensitive) leaves. The Al-induced decrease of CO2 assimilation, maximum quantum yield of primary PSII photochemistry (Fv/Fm), the lower contents of glucose and starch, and the lowered activities of enzymes involved in the mevalonic acid (MVA) pathway and 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway might account for the different volatilization rate of isoprenoids. Furthermore, the altered transcript levels of genes related to isoprenoid precursors and/or derivatives metabolism, such as geranyl diphosphate (GPP) synthase (GPPS) in GPP biosynthesis, geranylgeranyl diphosphate synthase (GGPPS), chlorophyll synthase (CHS) and GGPP reductase (GGPPR) in chlorophyll biosynthesis, limonene synthase (LS) and α-pinene synthase (APS) in limonene and α-pinene synthesis, respectively, might be responsible for the different contents of corresponding products in C. grandis and C. sinensis. Our data suggested that isoprenoid metabolism was involved in Al tolerance response in Citrus, and the alternation of some branches of isoprenoid metabolism could confer different Al-tolerance to Citrus species.

Comprehensive Phytohormone Profiling of Kohlrabi during In Vitro Growth and Regeneration: The Interplay with Cytokinin and Sucrose

The establishment of an efficient protocol for in vitro growth and regeneration of kohlrabi (Brassica oleracea var. gongylodes) allowed us to closely examine the phytohormone profiles of kohlrabi seedlings at four growth stages (T1-T4), additionally including the effects of cytokinins (CKs)-trans-zeatin (transZ) and thidiazuron (TDZ)-and high sucrose concentrations (6% and 9%). Resulting phytohormone profiles showed complex time-course patterns. At the T2 stage of control kohlrabi plantlets (with two emerged true leaves), levels of endogenous CK free bases and gibberellin GA₂₀ increased, while increases in jasmonic acid (JA), JA-isoleucine (JA-Ile), indole-3-acetic acid (IAA) and indole-3-acetamide (IAM) peaked later, at T3. At the same time, the content of most of the analyzed IAA metabolites decreased. Supplementing growth media with CK induced de novo formation of shoots, while both CK and sucrose treatments caused important changes in most of the phytohormone groups at each developmental stage, compared to control. Principal component analysis (PCA) showed that sucrose treatment, especially at 9%, had a stronger effect on the content of endogenous hormones than CK treatments. Correlation analysis showed that the dynamic balance between the levels of certain bioactive phytohormone forms and some of their metabolites could be lost or reversed at particular growth stages and under certain CK or sucrose treatments, with correlation values changing between strongly positive and strongly negative. Our results indicate that the kohlrabi phytohormonome is a highly dynamic system that changes greatly along the developmental time scale and also during de novo shoot formation, depending on exogenous factors such as the presence of growth regulators and different sucrose concentrations in the growth media, and that it interacts intensively with these factors to facilitate certain responses.

Transcriptome mining of genes in Zanthoxylum armatum revealed ZaMYB86 as a negative regulator of prickly development

Zanthoxylum armatum DC. is an important economic tree species. Prickle is a type of trichome with special morphology, and there are a lot of prickles on the leaves of Z. armatum, which seriously restricts the development of Z. armatum industry. In this study, the leaves of Z. armatum cv. Zhuye (ZY) and its budding variety 'Rongchangwuci' (WC) (A less prickly mutant variety) at different developmental stages were used as materials, and the transcriptome sequencing data were analyzed. A total of 96,931 differentially expressed genes (DEGs) were identified among the samples, among which 1560 were candidate DEGs that might be involved in hormone metabolism. The contents of JA, auxin and CK phytohormones in ZY leaves were significantly higher than those in WC leaves. Combined with weighted gene co-expression network analysis, eight genes (MYC, IAA, ARF, CRE/AHK, PP2C, ARR-A, AOS and LOX) were identified, including 25 transcripts, which might affect the metabolism of the three hormones and indirectly participate in the formation of prickles. Combining with the proteins successfully reported in other plants to regulate trichome formation, ZaMYB86, a transcription factor of R2R3 MYB family, was identified through local Blast and phylogenetic tree analysis, which might regulate prickle formation of Z. armatum. Overexpression of ZaMYB86 in mutant A. thaliana resulted in the reduction of trichomes in A. thaliana leaves, which further verified that ZaMYB86 was involved in the formation of pickles. Yeast two-hybrid results showed that ZaMYB86 interacted with ZaMYB5. Furthermore, ZaMYB5 was highly homologous to AtMYB5, a transcription factor that regulated trichomes development, in MYB DNA binding domain. Taken together, these results indicated that ZaMYB86 and ZaMYB5 act together to regulate the formation of prickles in Z. armatum. Our findings provided a new perspective for revealing the molecular mechanism of prickly formation.

Cytokinin glucosyl transferases, key regulators of cytokinin homeostasis, have potential value for wheat improvement

The cytokinins, which are N⁶ -substituted adenine derivatives, control key aspects of crop productivity. Cytokinin levels are controlled via biosynthesis by isopentenyl transferase (IPT), destruction by cytokinin oxidase/dehydrogenase (CKX), and inactivation via glucosylation by cytokinin glucosyl transferases (CGTs). While both yield components and tolerance to drought and related abiotic stressors have been positively addressed via manipulation of IPT and/or CKX expression, much less attention has been paid to the CGTs. As naming of the CGTs has been unclear, we suggest COGT, CNGT, CONGT and CNOGT to describe the O-, N- and dual function CGTs. As specific CGT mutants of both rice and arabidopsis showed impacts on yield components, we interrogated the wheat genome database, IWGSC RefSeq v1.0 & v2.0, to investigate wheat CGTs. Besides providing unambiguous names for the 53 wheat CGTs, we show their expression patterns in 70 developmental tissues and their response characteristics to various stress conditions by reviewing more than 1000 RNA-seq data sets. These revealed various patterns of responses and showed expression generally being more limited in reproductive tissues than in vegetative tissues. Multiple cis-regulatory elements are present in the 3 kb upstream of the start codons of the 53 CGTs. Elements associated with abscisic acid, light and methyl jasmonate are particularly over-represented, indicative of the responsiveness of CGTs to the environment. These data sets indicate that CGTs have potential value for wheat improvement and that these could be targeted in TILLING or gene editing wheat breeding programmes.

Putrescine Depletion Affects Arabidopsis Root Meristem Size by Modulating Auxin and Cytokinin Signaling and ROS Accumulation

Polyamines (PAs) dramatically affect root architecture and development, mainly by unknown mechanisms; however, accumulating evidence points to hormone signaling and reactive oxygen species (ROS) as candidate mechanisms. To test this hypothesis, PA levels were modified by progressively reducing ADC1/2 activity and Put levels, and then changes in root meristematic zone (MZ) size, ROS, and auxin and cytokinin (CK) signaling were investigated. Decreasing putrescine resulted in an interesting inverted-U-trend in primary root growth and a similar trend in MZ size, and differential changes in putrescine (Put), spermidine (Spd), and combined spermine (Spm) plus thermospermine (Tspm) levels. At low Put concentrations, ROS accumulation increased coincidently with decreasing MZ size, and treatment with ROS scavenger KI partially rescued this phenotype. Analysis of double AtrbohD/F loss-of-function mutants indicated that NADPH oxidases were not involved in H2O2 accumulation and that elevated ROS levels were due to changes in PA back-conversion, terminal catabolism, PA ROS scavenging, or another pathway. Decreasing Put resulted in a non-linear trend in auxin signaling, whereas CK signaling decreased, re-balancing auxin and CK signaling. Different levels of Put modulated the expression of PIN1 and PIN2 auxin transporters, indicating changes to auxin distribution. These data strongly suggest that PAs modulate MZ size through both hormone signaling and ROS accumulation in Arabidopsis.

Naturally Occurring and Artificial N9-Cytokinin Conjugates: From Synthesis to Biological Activity and Back

Cytokinins and their sugar or non-sugar conjugates are very active growth-promoting factors in plants, although they occur at very low concentrations. These compounds have been identified in numerous plant species. This review predominantly focuses on 9-substituted adenine-based cytokinin conjugates, both artificial and endogenous, sugar and non-sugar, and their roles in plants. Acquired information about their biological activities, interconversions, and metabolism improves understanding of their mechanisms of action and functions in planta. Although a number of 9-substituted cytokinins occur endogenously, many have also been prepared in laboratories to facilitate the clarification of their physiological roles and the determination of their biological properties. Here, we chart advances in knowledge of 9-substituted cytokinin conjugates from their discovery to current understanding and reciprocal interactions between biological properties and associated structural motifs. Current organic chemistry enables preparation of derivatives with better biological properties, such as improved anti-senescence, strong cell division stimulation, shoot forming, or more persistent stress tolerance compared to endogenous or canonical cytokinins. Many artificial cytokinin conjugates stimulate higher mass production than naturally occurring cytokinins, improve rooting, or simply have high stability or bioavailability. Thus, knowledge of the biosynthesis, metabolism, and activity of 9-substituted cytokinins in various plant species extends the scope for exploiting both natural and artificially prepared cytokinins in plant biotechnology, tissue culture, and agriculture.

Phytochrome and Phytohormones: Working in Tandem for Plant Growth and Development

Being sessile organisms, plants need to continually adapt and modulate their rate of growth and development in accordance with the changing environmental conditions, a phenomenon referred to as plasticity. Plasticity in plants is a highly complex process that involves a well-coordinated interaction between different signaling pathways, the spatiotemporal involvement of phytohormones and cues from the environment. Though research studies are being carried out over the years to understand how plants perceive the signals from changing environmental conditions and activate plasticity, such remain a mystery to be resolved. Among all environmental cues, the light seems to be the stand out factor influencing plant growth and development. During the course of evolution, plants have developed well-equipped signaling system that enables regulation of both quantitative and qualitative differences in the amount of perceived light. Light influences essential developmental switches in plants ranging from germination or transition to flowering, photomorphogenesis, as well as switches in response to shade avoidances and architectural changes occurring during phototropism. Abscisic acid (ABA) is controlling seed germination and is regulated by light. Furthermore, circadian clock adds another level of regulation to plant growth by integrating light signals with different hormonal pathways. MYB96 has been identified as a regulator of circadian gating of ABA-mediated responses in plants by binding to the TIMING OF CAB EXPRESSION 1(TOC1) promoter. This review will present a representative regulatory model, highlight the successes achieved in employing novel strategies to dissect the levels of interaction and provide perspective for future research on phytochrome-phytohormones relationships toward facilitating plant growth, development, and function under abiotic-biotic stresses.

The novel transcription factor TRP interacts with ZFP5, a trichome initiation-related transcription factor, and negatively regulates trichome initiation through gibberellic acid signaling

The trichome-related protein (TRP) is a novel transcription factor (TF) that negatively regulates trichome initiation-related TFs through gibberellin (GA) signaling. Trichomes, which are outgrowths of leaf epidermal cells, provide the plant with a first line of defense against damage from herbivores and reduce transpiration. The initiation and development of trichomes are regulated by a network of positively or negatively regulating transcription factors (TFs). However, little information is currently available on transcriptional regulation related to trichome formation. Here, we report a novel TF Trichome-Related Protein (TRP) that was observed to negatively regulate the trichome initiation-related TFs through gibberellic acid (GA) signaling. ProTRP:GUS revealed that TRP was only expressed in the trichome. The TRP loss-of-function mutant (trp) had an increased number of trichomes on the flower, cauline leaves, and main inflorescence stems compared to the wild-type. In contrast, TRP overexpression lines (TRP-Ox) exhibited a decreased number of trichomes on cauline leaves and main inflorescence stem following treatment with exogenous GA. Moreover, the expressions of trichome initiation regulators (GIS, GIS2, ZFP8, GL1, and GL3) increased in trp plants but decreased in TRP-Ox lines after GA treatment. TRP was observed to physically interact with ZFP5, a C2H2 TF that controls trichome cell development through GA signaling, both in vivo and in vitro. Based on these results, we suggest that TRP functions upstream of the trichome initiation regulators and represses the binding of ZFP5 to the ZFP8 promoter.

Flowering and trichome development share hormonal and transcription factor regulation

Gibberellins (GAs) and cytokinins (CKs) are plant hormones that act either synergistically or antagonistically during the regulation of different developmental processes. In Arabidopsis thaliana, GAs and CKs overlap in the positive regulation of processes such as the transition from the vegetative to the reproductive phase and the development of epidermal adaxial trichomes. Despite the fact that both developmental processes originate in the rosette leaves, they occur separately in time and space. Here we review how, as genetic and molecular mechanisms are being unraveled, both processes might be closely related. Additionally, this shared genetic network is not only dependent on GA and CK hormone signaling but is also strictly controlled by specific clades of transcription factor families. Some key flowering genes also control other rosette leaf developmental processes such as adaxial trichome formation. Conversely, most of the trichome activator genes, which belong to the MYB, bHLH and C2H2 families, were found to positively control the floral transition. Furthermore, three MADS floral organ identity genes, which are able to convert leaves into floral structures, are also able to induce trichome proliferation in the flower. These data lead us to propose that the spatio-temporal regulation and integration of diverse signals control different developmental processes, such as floral induction and trichome formation, which are intimately connected through similar genetic pathways.

Auxin and cytokinin act during gynoecial patterning and the development of ovules from the meristematic medial domain

The gynoecium is the female reproductive structure of flowering plants, and is the site of ovule and seed development. The gynoecium is critical for reproductive competence and for agricultural productivity in many crop plants. In this review we focus on molecular aspects of the development of the Arabidopsis thaliana gynoecium. We briefly introduce gynoecium structure and development and then focus on important research advances published within the last year. We highlight what has been learned recently with respect to: (1) the role of auxin in the differential development of the medial and lateral domains of the Arabidopsis gynoecium; (2) the interaction between cytokinin and auxin during gynoecial development; (3) the role of auxin in the termination of the floral meristem and in the transition of floral meristem to gynoecium; and (4) recent studies that suggest a degree of evolutionary conservation of auxin mechanisms during gynoecial development in other eudicots.

Role of plant hormones and their interplay in development and ripening of fleshy fruits

Plant hormones have been extensively studied for their roles in the regulation of various aspects of plant development. However, in the last decade important new insights have been made into their action during development and ripening, in both dry and fleshy fruits. Emerging evidence suggests that relative functions of plant hormones are not restricted to a particular stage, and a complex network of more than one plant hormone is involved in controlling various aspects of fruit development. Though some areas are extensively covered, considerable gaps in our knowledge and understanding still exist in the control of hormonal networks and crosstalk between different hormones during fruit expansion, maturation, and various other aspects of ripening. Here, we evaluate the new knowledge on their relative roles during tomato fruit development with a view to understand their mechanism of action in fleshy fruits. For a better understanding, pertinent evidences available on hormonal crosstalk during fruit development in other species are also discussed. We envisage that such detailed knowledge will help design new strategies for effective manipulation of fruit ripening.

An integrative model of the control of ovule primordia formation

Upon hormonal signaling, ovules develop as lateral organs from the placenta. Ovule numbers ultimately determine the number of seeds that develop, and thereby contribute to the final seed yield in crop plants. We demonstrate here that CUP-SHAPED COTYLEDON 1 (CUC1), CUC2 and AINTEGUMENTA (ANT) have additive effects on ovule primordia formation. We show that expression of the CUC1 and CUC2 genes is required to redundantly regulate expression of PINFORMED1 (PIN1), which in turn is required for ovule primordia formation. Furthermore, our results suggest that the auxin response factor MONOPTEROS (MP/ARF5) may directly bind ANT, CUC1 and CUC2 and promote their transcription. Based on our findings, we propose an integrative model to describe the molecular mechanisms of the early stages of ovule development.

Endogenous cytokinin and auxin profiles during in vitro organogenesis from vegetative buds of Pinus radiata adult trees

In Pinus radiata D. Don, the transition from the juvenile to the mature phase is characterized by a reduction in the tree's organogenic potential, which is usually reverted in breeding programs by reinvigoration procedures to enable vegetative propagation. In this work, we have determined the best culture conditions for in vitro reinvigoration of radiata pine buds, tested different cytokinin (CK) types [N⁶-benzyladenine (BA), meta-topolin (mT) and trans-zeatin] and concentrations (25 and 50 µM), and studied the effect of culture conditions on endogenous CK and indole-3-acetic acid (IAA) levels at different stages of the organogenic process. To this end, the levels of 43 CKs and IAA were determined in P. radiata buds before and during the reinvigoration process. When BA or mT was applied to the induction medium, we did not observe any significant increase or decrease in endogenous isoprenoid CK content. We also report for the first time the presence of O-glucosides in non-treated P. radiata explants from the field and remark the importance of O-glucosides as storage forms.

Endosperm: food for humankind and fodder for scientific discoveries

The endosperm is an essential constituent of seeds in flowering plants. It originates from a fertilization event parallel to the fertilization that gives rise to the embryo. The endosperm nurtures embryo development and, in some species including cereals, stores the seed reserves and represents a major source of food for humankind. Endosperm biology is characterized by specific features, including idiosyncratic cellular controls of cell division and epigenetic controls associated with parental genomic imprinting. This review attempts a comprehensive summary of our current knowledge of endosperm development and highlights recent advances in this field.

[Genetic screening and analysis of suppressors of asa1-1 (soa) defective in jasmonate-mediated lateral root formation in Arabidopsis]

It has been shown that jasmonate modulates the lateral root development through crosstalk with auxin in Arabidopsis thaliana. Exogenous application of jasmonate stimulates lateral root formation in wild type but inhibits lateral root formation in asa1-1. Our previous work has demonstrated that the lateral root formation defect of asa1-1 is co-related with jasmonte effect on PIN2 protein levels. To further elucidate the molecular mechanisms underlying jasmonate-mediated reduction of plasma membrane (PM)-resident PIN2 abundance, we have conducted a genetic screen to identify suppressors of asa1-1 (soa), which showed lateral root formation in the presence of jasmonate. Here, we described the basic characterization of soa563 and soa856. We showed that both soa563 and soa856 displayed restored lateral root formation in response to exogenous jasmonate. In addition, jasmonate-induced PIN2:GFP reduction was blocked in these two mutants. Our on-going effort to identify genes defined by these mutants promise to shed new light on the understanding of the molecular mechanisms controlling jasmonate-mediated regulation of PIN2 protein trafficking and turnover.

Cytokinins in the bryophyte Physcomitrella patens: analyses of activity, distribution, and cytokinin oxidase/dehydrogenase overexpression reveal the role of extracellular cytokinins

Ultra-performance liquid chromatography-tandem mass spectrometry was used to establish the cytokinin profile of the bryophyte Physcomitrella patens (Hedw.) B.S.G.; of 40 analyzed cytokinins, 20 were detected. cis-Zeatin-riboside-O-glucoside, N(6)-(Delta(2)-isopentenyl)adenosine-5'-monophosphate (iPRMP), and trans-zeatin-riboside-O-glucoside were the most abundant intracellular cytokinins. In addition, the aromatic cytokinins N(6)-benzyladenosine (BAR), N(6)-benzyladenine, meta-, and ortho-topolin were detected. Unexpectedly, the most abundant extracellular cytokinin was the nucleotide iPRMP, and its identity was confirmed by quadrupole time-of-flight mass spectrometry. The effects of overexpressing a heterologous cytokinin oxidase/dehydrogenase (CKX; EC 1.4.3.18/1.5.99.12) gene (AtCKX2 from Arabidopsis [Arabidopsis thaliana]) on the intracellular and extracellular distribution of cytokinins was assessed. In cultures of CKX-transformed plants, ultra-performance liquid chromatography-tandem mass spectrometry measurements showed that there were pronounced reductions in the extracellular concentrations of N(6)-(Delta(2)-isopentenyl)adenine (iP) and N(6)-(Delta(2)-isopentenyl)adenosine (iPR), but their intracellular cytokinin concentrations were only slightly affected. In vitro and in vivo measured CKX activity was shown to be strongly increased in the transformants. Major phenotypic changes observed in the CKX-overexpressing plants included reduced and retarded budding, absence of sexual reproduction, and abnormal protonema cells. In bud-induction bioassays with wild-type Physcomitrella, the nucleotides iPRMP, trans-zeatin-riboside-5'-monophosphate, BAR monophosphate, and the cis-zeatin forms cZ and cZR had no detectable effects, while the activities displayed by other selected cytokinins were in the following order: iP > tZ > N(6)-benzyladenine > BAR > iPR > tZR > meta-topolin > dihydrozeatin > ortho-topolin. The results on wild type and CKX transgenics suggest that extracellular iP and iPR are the main cytokinins responsible for inducing buds in the bryophyte Physcomitrella. Cytokinin profile is discussed regarding the evolution of cytokinin biosynthetic pathways.

Three-dimensional reconstruction of anomalous chloroplasts in transgenic ipt tobacco

Anomalies in the ultrastructure of chloroplasts, from transgenic ipt tobacco, overproducing endogenous cytokinins (CKs) were studied. Detailed analyses of CKs and their metabolites showed that Pssu-ipt tobacco contained enhanced contents of CKs both in leaves and in isolated chloroplasts. The role of CKs in the formation of anomalous structures is suggested. Pssu-ipt chloroplasts frequently formed the distinct peripheral reticulum with a system of caverns that often involved mitochondria and/or peroxisomes. Large crystalloids, which were found in chloroplasts of Pssu-ipt, occupied up to 16% of chloroplast volume. We suggested that the crystalloids were formed by LHC II aggregates. This was supported by analysis of the fluorescence emission spectra at 77 degrees K, chlorophyll a/b ratio, immunogold staining of the structures, and crystallographic unit size analysis.