Isolation and characterization of Arabidopsis mutants defective in the induction of ethylene biosynthesis by cytokinin

Ethylene Biosynthesis and Signaling: An Overview

Hormones are key regulators of plant growth and development. Genetic and biochemical studies have identified major factors that mediate ethylene biosynthesis and signal transduction. Substantial progress in the elucidation of the ethylene signal transduction pathway has been made, mainly by research on Arabidopsis thaliana. Research on ethylene biosynthesis and its regulation provided new insights, particularly on the posttranslational regulation of ethylene synthesis and the feedback from ethylene signal transduction. The identification of new components in the ethylene-response pathway and the elucidation of their mode of action provide a framework for understanding not only how plants sense and respond to this hormone but also how the signal is integrated with other inputs, ultimately determining the plant phenotype.

N-Glucosylation of Cytokinins by Glycosyltransferases of Arabidopsis thaliana

Cytokinins are plant hormones that can be glucosylated to form O-glucosides and N-glucosides. The glycoconjugates are inactive and are thought to play a role in homeostasis of the hormones. Although O-glucosyltransferases have been identified that recognize cytokinins, the enzymes involved in N-glucosylation have not been identified even though the process has been recognized for many years. This study utilizes a screening strategy in which 105 recombinant glycosyltransferases (UGTs) of Arabidopsis have been analyzed for catalytic activity toward the classical cytokinins: trans-zeatin, dihydrozeatin, N(6)-benzyladenine, N(6)-isopentenyladenine, and kinetin. Five UGTs were identified in the screen. UGT76C1 and UGT76C2 recognized all cytokinins and glucosylated the hormones at the N(7) and N(9) positions. UGT85A1, UGT73C5, and UGT73C1 recognized trans-zeatin and dihydrozeatin, which have an available hydroxyl group for glucosylation and formed the O-glucosides. The biochemical characteristics of the N-glucosyltransferases were analyzed, and highly effective inhibitors of their activities were identified. Constitutive overexpression of UGT76C1 in transgenic Arabidopsis confirmed that the recombinant enzyme functioned in vivo to glucosylate cytokinin applied to the plant. The role of the N-glucosyltransferases in cytokinin metabolism is discussed.

Modulation of sensitivity and selectivity in plant signaling by proteasomal destabilization

The ubiquitin (Ub) system of intracellular protein degradation regulates the abundance of numerous proteins that control plant growth and development. Recent advances have begun to illustrate how environmental and endogenous signals affect plant responses through Ub-related proteolysis, the importance of combinatorial control in regulated protein destruction and how multiprotein complexes confer sensitivity and selectivity to ubiquitination. Further insight into the cell biology of Ub-chain assembly and proteasomal degradation, as well as into the relationship between proteolysis and other regulatory modifications, will be essential for understanding the mechanistic basis of the integration of diverse plant signals.

Mutations at CRE1 impair cytokinin-induced repression of phosphate starvation responses in Arabidopsis

Plants display a number of responses to low phosphate availability, involving biochemical and developmental changes. Recently we have shown that many of these responses can be repressed in roots by exogenous addition of cytokinins. In order to understand the genetic basis to this effect of cytokinins, and its relation with the better known roles of cytokinins in the control of cell-cycle and differentiation, we have undertaken mutant screening and characterization using a transgenic line of Arabidopsis thaliana harbouring a reporter gene specifically responsive to Pi starvation (AtIPS1::GUS). One type of mutant identified displayed reduced sensitivity of AtIPS1::GUS to cytokinin repression. Several other Pi starvation response genes showed reduced cytokinin sensitivity in these lines. These mutants also showed reduced cytokinin repression of the anthocyanin accumulation induced by Pi starvation in the aerial part of the plants. Mapping and molecular characterization of these mutants showed that they were allelic of CRE1/WOL, a locus known to encode a cytokinin receptor. CRE1 is downregulated by Pi starvation and induced by cytokinins, both in the wild-type and in the cre1 mutants, in which cre1 mRNA levels are higher. These results reveal the existence of a positive feed-back loop, in addition to the already established negative feedback loop, in cytokinin signalling and indicate that the negative regulation of Pi starvation responses by cytokinins involves a two-component signalling circuitry, as it is the case of other types of cytokinin response.

Arabidopsis COP10 is a ubiquitin-conjugating enzyme variant that acts together with COP1 and the COP9 signalosome in repressing photomorphogenesis

A group of evolutionarily conserved pleiotropic COP/DET/FUS proteins was initially defined by their ability to repress photomorphogenesis in Arabidopsis. It was proposed that this regulation be mediated by targeting degradation of key cellular regulators that promote photomorphogenesis. Among them, COP1 and the COP9 signalosome have been hypothesized to fulfill the roles as an ubiquitin ligase (E3) and an essential E3 modulator. Here we report that COP10 encodes a protein similar to ubiquitin-conjugating enzyme (E2) variant proteins (UEV). COP10 is part of a nuclear protein complex and capable of directly interacting with both COP1 and the COP9 signalosome. Our data indicates that COP10 defines a possible E2 activity, thus validating the working hypothesis that the pleiotropic COP/DET/FUS group of proteins defined a protein ubiquitination pathway.

Tumorous shoot development (TSD) genes are required for co-ordinated plant shoot development

This report describes the identification of novel plant genes that are required to ensure co-ordinated post-embryonic development. After germination the tumorous shoot development mutants of Arabidopsis thaliana develop disorganized tumorous tissue instead of organized leaves and stems. This results in green callus-like structures, which are capable of unlimited growth in vitro on hormone-free medium. The tsd mutants are recessive and belong to three complementation groups (tsd1, tsd2, tsd3). The genes were mapped to the bottom of chromosomes 5 and 1, and the top of chromosome 3, respectively. Histological analyses showed that the tsd mutants have different developmental defects. The shoot apical meristem of tsd1 formed only rudimentary leaves and was characterized by a degenerating L1 cell layer. tsd2 mutants had reduced cell adhesion and altered cell division planes in the L2 and L3 cell layers. The tumorous tissue of tsd3 mutants originated from the base of the leaf. Cytokinin levels that are inhibitory to the growth of wild-type seedlings bring about an enhanced growth response in all the tsd mutants. The steady state transcript levels of the histidine kinase CKI1 gene and the KNAT1 and STM homeobox genes were increased in tsd mutants, while mRNA levels of cell cycle genes were not altered. We hypothesize that the TSD gene products negatively regulate cytokinin-dependent meristematic activity during vegetative development of Arabidopsis.

The cytokinin 2-isopentenyladenine causes partial reversion to skotomorphogenesis and induces formation of prolamellar bodies and protochlorophyllide657 in the lip1 mutant of pea

When grown in darkness the photomorphogenic lip1 mutant of pea (Pisum sativum L.) has a slender stem, expanded leaves, prolamellar body (PLB) lacking plastids with the size of chloroplasts and a low level of phytochrome A. The lack of PLBs in a dark-grown material (lip1) created a possibility to further study the regulation of their formation in relation to plant development. Inclusion of a cytokinin, 2-isopentenyladenine (2iP), in a medium supporting growth of the pea seedlings in darkness was found to reduce epicotyl length in the wild type. In lip1 the formation of a slender stem was inhibited and a short epicotyl developed. Furthermore, leaf expansion was inhibited, the plastid size reduced and the formation of PLBs induced. The PLB formation in lip1 was not accompanied by an increase in the amount of protochlorophyllide (Pchlide) or Pchilde oxidoreductase (POR). In the presence of 2iP the level of phytochrome A protein was increased in lip1 and the POR mRNA levels decreased in both lip1 and wild-type plants. The chloroplast characteristic trans-3-hexadecenoate acyl group of phosphatidylglycerol, present in the plastids of dark-grown lip1, was not influenced by 2iP. Thus, not all photomorphogenic processes reacted similarly in the lip1 mutant, but leaf expansion and plastid differentiation, including PLB formation, seemed to be regulated by the same signal transduction chain. Exogenously applied brassinolide could rescue neither dark- nor light-grown defects of the lip1 mutant. Thus, cytokinins but not brassinolides seem to be involved in the regulation of certain characteristic traits of skotomorphogenesis in pea, including plastid development and PLB formation.

CYTOKININ METABOLISM AND ACTION

Cytokinins are structurally diverse and biologically versatile. The chemistry and physiology of cytokinin have been studied extensively, but the regulation of cytokinin biosynthesis, metabolism, and signal transduction is still largely undefined. Recent advances in cloning metabolic genes and identifying putative receptors portend more rapid progress based on molecular techniques. This review centers on cytokinin metabolism with connecting discussions on biosynthesis and signal transduction. Important findings are summarized with emphasis on metabolic enzymes and genes. Based on the information generated to date, implications and future research directions are presented.

The ethylene pathway: a paradigm for plant hormone signaling and interaction

To dissect the web of signals that control plant growth, it is important to understand how the individual components of the pathway are modulated. Ethylene is a plant hormone involved in a large number of developmental processes. Biochemical and genetic approaches have provided a detailed view of the biosynthetic and signal transduction pathways of this hormone in the reference plant Arabidopsis thaliana. The effects of several hormones and of developmental changes on the regulation of the key enzymes of ethylene biosynthesis, ACC synthase and ACC oxidase, serve as a clear example of interaction between signals in the generation of complex responses. We now have a picture of how ethylene is sensed by the ethylene receptors and how the signal is further transduced to the nucleus. Although some of the ethylene receptors show a tissue-specific pattern of expression, little is known about the regulation of the components of the ethylene transduction cascade by other hormones or developmental factors. Once the ethylene signal reaches the nucleus, it activates a transcriptional cascade that results in changes in the expression of a number of genes. We describe some of the results that suggest an interaction at the transcriptional level between ethylene, other hormones, and stress signals.

Influence of cytokinins on the expression of phosphate starvation responsive genes in Arabidopsis

The increase in the ratio of root growth to shoot growth that occurs in response to phosphate (Pi) deprivation is paralleled by a decrease in cytokinin levels under the same conditions. However, the role of cytokinin in the rescue system for Pi starvation remains largely unknown. We have isolated a gene from Arabidopsis thaliana (AtIPS1) that is induced by Pi starvation, and studied the effect of cytokinin on its expression in response to Pi deprivation. AtIPS1 belongs to the TPSI1/Mt4 family, the members of which are specifically induced by Pi starvation, and the RNAs of which contain only short, non-conserved open reading frames. Pi deprivation induces AtIPS1 expression in all cells of wild-type plants, whereas in the pho1 mutant grown on Pi-rich soils, AtIPS1 expression in the root was delimited by the endodermis. This supports the view that pho1 is impaired in xylem loading of Pi, and that long-distance signals controlling the Pi starvation responses act via negative control. Exogenous cytokinins repress the expression of AtIPS1 and other Pi starvation-responsive genes in response to Pi deprivation. However, cytokinins did not repress the increase in root-hair number and length induced by Pi starvation, a response dependent on local Pi concentration rather than on whole-plant Pi status. Our results raise the possibility that cytokinins may be involved in the negative modulation of long-distance, systemically controlled Pi starvation responses, which are dependent on whole-plant Pi status.

A gain-of-function phenotype conferred by over-expression of functional subunits of the COP9 signalosome in Arabidopsis

The COP9 signalosome is a conserved cellular regulator present in diverse organisms. To understand the structural and functional relationship of the COP9 signalosome with its subunits, we expressed in wild-type and mutant Arabidopsis backgrounds two orthologues of subunit 1, rice FUS6 (rFUS6) and human GPS1, and Arabidopsis subunit 8 (COP9). In Arabidopsis, rFUS6 can functionally replace Arabidopsis endogenous FUS6 to form the COP9 signalosome complex and rescue the null fus6-1 mutant phenotype. Moreover, light-grown rFUS6 over-expression seedlings displayed longer hypocotyls and reduced anthocyanin accumulation in comparison to wild-type seedlings, which is opposite to the fus6/cop11 mutant phenotype. The long-hypocotyl phenotype was also observed in transgenic seedlings over-expressing Arabidopsis COP9. This finding indicates that over-expression of a functional subunit 1 or subunit 8 of the COP9 signalosome confers a gain-of-function phenotype relative to the complex. Human GPS1, when expressed in the fus6-1 null mutant of Arabidopsis, can assemble into a chimeric COP9 signalosome at low efficiency, demonstrating the structural conservation of the complexes between human and Arabidopsis. This low-abundancy chimeric complex is insufficient to fully rescue the mutant but is able to attenuate the mutant severity.

The Cytokinin-hypersensitive genes of Arabidopsis negatively regulate the cytokinin-signaling pathway for cell division and chloroplast development

We isolated Arabidopsis thaliana mutants that respond more sensitively than the wild type to cytokinins. The calli produced from the mutants exhibit typical cytokinin responses, including rapid proliferation and chloroplast development in response to lower levels of cytokinins than in the wild type. The mutations are recessive and belong to two complementation groups designated ckh1 and ckh2 for cytokinin-hypersensitive. CKH1 and CKH2 were mapped to the top of chromosome I and the middle of chromosome II, respectively. The cytokinin levels in these mutants were not increased. We speculate that the CKH1 and CKH2 gene products negatively regulate the signaling pathway leading from cytokinin perception to cell proliferation and chloroplast development.

Role of hormones in the induction of iron deficiency responses in Arabidopsis roots

In "strategy I" plants, several alterations in root physiology and morphology are induced by Fe deficiency, although the mechanisms by which low Fe levels are translated into reactions aimed at alleviating Fe shortage are largely unknown. To prove whether changes in hormone concentration or sensitivity are involved in the adaptation to suboptimal Fe availability, we tested 45 mutants of Arabidopsis defective in hormone metabolism and/or root hair formation for their ability to increase Fe(III) chelate reductase activity and to initiate the formation and enlargement of root hairs. Activity staining for ferric chelate reductase revealed that all mutants were responsive to Fe deficiency, suggesting that hormones are not necessary for the induction. Treatment of wild-type plants with the ethylene precursor 1-aminocyclopropane-1-carboxylic acid caused the development of root hairs in locations normally occupied by non-hair cells, but did not stimulate ferric reductase activity. Ectopic root hairs were also formed in -Fe roots, suggesting a role for ethylene in the morphological responses to Fe deficiency. Ultrastructural analysis of rhizodermal cells indicated that neither Fe deficiency nor 1-aminocyclopropane-1-carboxylic acid treatment caused transfer-cell-like alterations in Arabidopsis roots. Our data indicate that the morphological and physiological components of the Fe stress syndrome are regulated separately.

A strong loss-of-function mutation in RAN1 results in constitutive activation of the ethylene response pathway as well as a rosette-lethal phenotype

A recessive mutation was identified that constitutively activated the ethylene response pathway in Arabidopsis and resulted in a rosette-lethal phenotype. Positional cloning of the gene corresponding to this mutation revealed that it was allelic to responsive to antagonist1 (ran1), a mutation that causes seedlings to respond in a positive manner to what is normally a competitive inhibitor of ethylene binding. In contrast to the previously identified ran1-1 and ran1-2 alleles that are morphologically indistinguishable from wild-type plants, this ran1-3 allele results in a rosette-lethal phenotype. The predicted protein encoded by the RAN1 gene is similar to the Wilson and Menkes disease proteins and yeast Ccc2 protein, which are integral membrane cation-transporting P-type ATPases involved in copper trafficking. Genetic epistasis analysis indicated that RAN1 acts upstream of mutations in the ethylene receptor gene family. However, the rosette-lethal phenotype of ran1-3 was not suppressed by ethylene-insensitive mutants, suggesting that this mutation also affects a non-ethylene-dependent pathway regulating cell expansion. The phenotype of ran1-3 mutants is similar to loss-of-function ethylene receptor mutants, suggesting that RAN1 may be required to form functional ethylene receptors. Furthermore, these results suggest that copper is required not only for ethylene binding but also for the signaling function of the ethylene receptors.

Molecular mechanisms of cytokinin action

Cytokinins have been implicated in many aspects of plant development, including a crucial role in regulating cell proliferation. Recent studies indicate that cytokinins may elevate cell division rates by induction of expression of CycD3, which encodes a D-type cyclin thought to play a role in the G1-->M transition of the cell cycle. Progress has also been made in our understanding of cytokinin perception as homologs of two-component phosphorelay systems have emerged as likely signaling elements.

GENETIC ANALYSIS OF HORMONE SIGNALING

Phytohormones influence many diverse developmental processes ranging from seed germination to root, shoot, and flower formation. Recently, mutational analysis using the model plant Arabidopsis thaliana has been instrumental in determining the individual components of specific hormone signal transduction pathways. Moreover, epistasis and suppressor studies are beginning to explain how these genes and their products relate to one another. While no hormone transduction pathway is completely understood, the genes identified to date suggest that simple molecular rules can be established to explain how plant hormone signals are transduced. This review describes some of the shared characteristics of plant hormone signal transduction pathways and the properties for informational transfer common to many of the genes that specify the transduction of the signal.

Two Arabidopsis mutants that overproduce ethylene are affected in the posttranscriptional regulation of 1-aminocyclopropane-1-carboxylic acid synthase

The Arabidopsis mutants eto1 (ethylene overproducer) and eto3 produce elevated levels of ethylene as etiolated seedlings. Ethylene production in these seedlings peaks at 60 to 96 h, and then declines back to almost wild-type levels. Ethylene overproduction in eto1 and eto3 is limited mainly to etiolated seedlings; light-grown seedlings and various adult tissues produce close to wild-type amounts of ethylene. Several compounds that induce ethylene biosynthesis in wild-type, etiolated seedlings through distinct 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACS) isoforms were found to act synergistically with eto1 and eto3, as did the ethylene-insensitive mutation etr1 (ethylene resistant), which blocks feedback inhibition of biosynthesis. ACS activity, the rate-limiting step of ethylene biosynthesis, was highly elevated in both eto1 and eto3 mutant seedlings, even though RNA gel-blot analysis demonstrated that the steady-state level of ACS mRNA was not increased, including that of a novel Arabidopsis ACS gene that was identified. Measurements of the conversion of ACC to ethylene by intact seedlings indicated that the mutations did not affect conjugation of ACC or the activity of ACC oxidase, the final step of ethylene biosynthesis. Taken together, these data suggest that the eto1 and eto3 mutations elevate ethylene biosynthesis by affecting the posttranscriptional regulation of ACS.

Cytokinin signaling

Although cytokinin plays a central role in plant development, our knowledge of the biosynthesis, distribution, perception and signal transduction of cytokinin is limited. Recent molecular-genetic studies have, however, implicated involvement of a two-component system in cytokinin signal transduction. Furthermore, new mutants with altered cytokinin responses and genes involved in cytokinin signaling have been identified.

Cytokinin action: two receptors better than one?

Cytokinins are ubiquitous plant hormones that have dramatic effects on growth and development, but almost nothing is known of their molecular mode of action. Recently, evidence has emerged that cytokinin action may involve a G-protein-coupled receptor and/or a two-component signaling pathway.

Two genes with similarity to bacterial response regulators are rapidly and specifically induced by cytokinin in Arabidopsis

Cytokinins are central regulators of plant growth and development, but little is known about their mode of action. By using differential display, we identified a gene, IBC6 (for induced by cytokinin), from etiolated Arabidopsis seedlings, that is induced rapidly by cytokinin. The steady state level of IBC6 mRNA was elevated within 10 min by the exogenous application of cytokinin, and this induction did not require de novo protein synthesis. IBC6 was not induced by other plant hormones or by light. A second Arabidopsis gene with a sequence highly similar to IBC6 was identified. This IBC7 gene also was induced by cytokinin, although with somewhat slower kinetics and to a lesser extent. The pattern of expression of the two genes was similar, with higher expression in leaves, rachises, and flowers and lower transcript levels in roots and siliques. Sequence analysis revealed that IBC6 and IBC7 are similar to the receiver domain of bacterial two-component response regulators. This homology, coupled with previously published work on the CKI1 histidine kinase homolog, suggests that these proteins may play a role in early cytokinin signaling.