Root enhancement in cytokinin-deficient oilseed rape causes leaf mineral enrichment, increases the chlorophyll concentration under nutrient limitation and enhances the phytoremediation capacity

Background Cytokinin is a negative regulator of root growth, and a reduction of the cytokinin content or signalling causes the formation a larger root system in model plants, improves their growth under drought and nutrient limitation and causes increased accumulation of elements in the shoot. Roots are an important but understudied target of plant breeding. Here we have therefore explored whether root enhancement by lowering the cytokinin content can also be achieved in oilseed rape (Brassica napus L.) plants. Results Transgenic plants overexpressing the CKX2 gene of Arabidopsis thaliana encoding a cytokinin-degrading cytokinin oxidase/dehydrogenase showed higher CKX activity and a strongly reduced cytokinin content. Cytokinin deficiency led to the formation of a larger root system under different growth conditions, which was mainly due to an increased number of lateral and adventitious roots. In contrast, shoot growth was comparable to wild type, which caused an enhanced root-to-shoot ratio. Transgenic plants accumulated in their leaves higher concentrations of macro- and microelements including P, Ca, Mg, S, Zn, Cu, Mo and Mn. They formed more chlorophyll under Mg- and S-deficiency and accumulated a larger amount of Cd and Zn from contaminated medium and soil. Conclusions These findings demonstrate the usefulness of ectopic CKX gene expression to achieve root enhancement in oilseed rape and underpin the functional relevance of a larger root system. Furthermore, the lack of major developmental consequences on shoot growth in cytokinin-deficient oilseed rape indicates species-specific differences of CKX gene and/or cytokinin action

Cytokinin Determines Thiol-Mediated Arsenic Tolerance and Accumulation

The presence of arsenic in soil and water is a constant threat to plant growth in many regions of the world. Phytohormones act in the integration of growth control and stress response, but their role in plant responses to arsenic remains to be elucidated. Here, we show that arsenate [As(V)], the most prevalent arsenic chemical species in nature, causes severe depletion of endogenous cytokinins (CKs) in the model plant Arabidopsis (Arabidopsis thaliana). We found that CK signaling mutants and transgenic plants with reduced endogenous CK levels showed an As(V)-tolerant phenotype. Our data indicate that in CK-depleted plants exposed to As(V), transcript levels of As(V)/phosphate-transporters were similar or even higher than in wild-type plants. In contrast, CK depletion provoked the coordinated activation of As(V) tolerance mechanisms, leading to the accumulation of thiol compounds such as phytochelatins and glutathione, which are essential for arsenic sequestration. Transgenic CK-deficient Arabidopsis and tobacco lines show a marked increase in arsenic accumulation. Our findings indicate that CK is an important regulatory factor in plant adaptation to arsenic stress

Funktionelle Charakterisierung der B-Typ Response Regulatoren von Arabidopsis thaliana

Cytokinins are essential for the regulation of many developmental processes in plants. In Arabidopsis the signal transduction of cytokinins is mediated by a multi-step His-to-Asp phospho-relay system. The B-type response regulators are one component of this phospho-relay system. The B-type response regulators are transcription factors that at least partially mediate the response to cytokinin. In planta functional analysis of this protein family is hampered by the high level of functional redundancy of its eleven members. In order to explore the functions of the B-type response regulators and to overcome their functional redundancy, the chimeric repressor silencing technology (CRES-T) was employed by generating a dominant-negative version of the Arabidopsis response regulator ARR1 (ARR1-SRDX). The 35S:ARR1-SRDX transgenic Arabidopsis plants showed phenotypic changes reminiscent of plants with a reduced cytokinin status, such as a strongly reduced leaf size, an enhanced root system and larger seeds. Several bioassays showed that 35S:ARR1-SRDX plants have an increased resistance towards cytokinin. Molecular analysis indicated attenuation of the early transcriptional response to cytokinin. In addition, a role for B-type ARRs in mediating crosstalk with other pathways was supported by the resistance of 35S:ARR1-SRDX seeds to phyB-mediated inhibition of germination by far-red light. Components downstream of the B-type ARRs were identified by performing expression profiling using CATMA arrays. The rapid induction of a large part of cytokinin response genes was dampened. The transcript levels of more than 500 genes were >2.5-fold reduced in 35S:ARR1-SRDX transgenic seedlings suggesting a broad function of the B-type ARRs. A total of 106 genes were identified as putative target genes of the B-type ARRs, and altered expression profiles of some of these genes were confirmed by qRT-PCR. In order to identify specific target genes of ARR1, arr1 and 35S:ARR1 transgenic plants were characterised, and their transcription profiles were analysed. A set of 24 genes was identified as putative specific target genes of ARR1. After finding these potential target genes by microarray analysis, the promoter of one target gene (ARR6) was analysed to identify cis- acting elements by promoter deletion analysis. The result of this analysis confirmed for the first time the in planta function of a known ARR1 binding motif. Also, a new promoter region important for the activation of the ARR6 gene was identified. The newly found 27 bp promoter region will be useful for further studies to pinpoint the binding motif of ARR1 and/or other B-type ARRs. In addition, attempts were made to obtain potentially relevant information for the regulation of ARR1 activity by studying proteins that were recognised as ARR1 interactors. In order to investigate whether the target genes of the B-type ARRs are conserved across species and to study the efficiency of the CRES-T in other plant species, 35S:ARR1-SRDX transgenic tomato plants were generated and characterized. These transgenic tomato plants also showed the cytokinin deficiency syndrome similar to Arabidopsis, suggesting the conservation of the target genes and target gene sequences of these transcription factors across species. Further, these transgenic tomato plants produced seedless tomato fruits thereby indicating a role for cytokinin in parthenocarpic fruit development. In conclusion, the suppression of pleiotropic cytokinin activities by a dominant-negative version of a B-type ARR indicates that this protein family is involved in mediating most, if not all, of the cytokinin activities in Arabidopsis. The 35S:ARR1-SRDX Arabidopsis and tomato transgenic plants and the microarray data sets comprising the putative target genes of B-type ARRs and ARR1 are valuable tools for investigating these functions.Cytokinine sind entscheidend an der Regulation diverser pflanzlicher Entwicklungsprozesse beteiligt. In Arabodopsis thaliana wird die Signaltranduktion dieses Hormons über eine mehrstufige Phosphatkaskade vermittelt. Die so genannten Arabidopsis Response Regulatoren (ARRs) des B-Typs sind als Transkriptionsfaktorproteine Bestandteil dieses Signalsystems und vermitteln als Transaktivatoren die Cytokininantwort auf molekularer Ebene. Eine hohe funktionelle Redundanz unter den 11 Mitgliedern dieser Genfamilie erschwert jedoch funktionelle Analysen in planta. Um die Funktion der B-Typ ARRs zu ergründen und die Redundanz innerhalb dieser Genfamilie zu überbrücken, wurde die Chimeric Repressor Silencing Technologie (kurz CRES-T) angewandt, durch die eine dominant-negative Repressorvariante von ARR1 generiert wurde (ARR1-SRDX). Die erzeugten transgenen 35S:ARR1-SRDX Arabidopsis-Pflanzen wiesen phänotypische Veränderungen auf, die denen von Pflanzen mit reduziertem Cytokiningehalt ähnelten. Dazu gehörten eine reduzierte Blattgröße, ein verstärktes Wurzelsystem und größere Samen. Diverse Cytokininsensitivitätstests zeigten auf eine verstärkte Cytokininresistenz der 35S:ARR1-SRDX transgenen Pflanzen. Molekulare Analysen konnten zudem eine Abschwächung der frühen Antwort auf Cytokinin auf transkriptioneller Ebene zeigen. Darüber hinaus wiesen 35S:ARR1-SRDX transgene Samen eine Resistenz gegenüber der phyB-vermittelten Inhibierung der Keimung durch dunkelrotes Licht auf, was auf eine B-Typ ARR-vermittelte Interaktion zwischen der Cytokininsignaltransduktion und anderen Signalwegen hindeutet. Des weiteren wurden durch Erstellung von Expressionsprofilen mittels CATMA-Arrays den B-Typ ARRs nachgeschaltete Komponenten im Signalweg identifiziert. Die schnelle Induktion einer Vielzahl an Cytokininantwortgenen war hierbei abgeschwächt. Insgesamt war in 35S:ARR1-SRDX transgenen Keimlingen die relative Transkriptabundanz von mehr als 500 Genen um mehr als 2,5- fach reduziert. Dies deutet auf ein weites Funktionsspektrum der B-Typ ARRs hin. 106 Gene wurden als mögliche Zielgene der B-Typ ARRs identifiziert und für einige von ihnen wurde die veränderte Expression mittels quantitativer PCR verifiziert. Um ARR1-spezifische Zielgene zu identifizieren, wurden neben einer molekularen und phänotypischen Charakterisierung der Mutante arr1 und von 35S:ARR1 transgenen Pflanzen die Transkriptionsprofile beider Genotypen mittels Microarrays analysiert. Dabei wurden insgesamt 24 Gene als potentielle spezifische Zielgene von ARR1, u. a. ARR6, identifiziert. Der Promotor von ARR6 wurde für Promotordeletionsanalysen verwendet, um cis-aktive Elemente zu identifizieren. Durch diese Analysen konnte erstmals die Funktion eines zuvor bereits bekannten ARR1 Bindemotivs in planta gezeigt werden. Des weiteren wurde eine neue für die Aktivierung von ARR6 notwendige Region im Promotor ermittelt. Diese 27 bp lange Region wird für weitergehende Analysen zur Identifizierung des Bindemotivs von ARR1 und/oder weiterer B-Typ ARRs benutzt werden. Darüber hinaus wurden mit ARR1 interagierende Proteine untersucht, um möglicherweise relevante Informationen zur Regulation der ARR1 Aktivität zu erhalten. Um zu überprüfen, ob die ARR1-Zielgene über die Speziesgrenze konserviert sind und um die Effizienz der CRES-Technologie auch in anderen Pflanzenspezies zu zeigen, wurden 35S:ARR1-SRDX transgene Tomatenpflanzen generiert und charakterisiert. Diese Pflanzen zeigten das Cytokinindefizienzsyndrom ähnlich dem in Arabidopsis, was auf die Konservierung der Zielgene und Zielsequenzen dieser Transkriptionsfaktoren über die Speziesgrenze hinaus hinweist. Das Fehlen von Samen in diesen transgenen Pflanzen weist darüber hinaus auf eine Rolle von Cytokinin bei der parthenocarpischen Fruchtentwicklung hin. Zusammenfassend zeigen die erzielten Resultate, dass die pleiotropen Cytokininaktivitäten, die in der vorliegenden Arbeit durch Verwendung einer dominanten Repressorvariante eines B-Typ ARR unterdrückt worden waren, im wesentlichen, wenn nicht sogar vollständig, auf die Proteinfamilie der B-Typ ARRs zurückzuführen sind. Die 35S:ARR1-SRDX transgenen Arabidopsis-Pflanzen, die transgenen Tomatenpflanzen und die erzeugten Microarray-Daten, die die möglichen Zielgene von ARR1 und anderer B-Typ ARRs umfassen, sind wertvolle Hilfsmittel, um diese Funktionen weiter zu untersuchen

Root engineering in maize by increasing cytokinin degradation causes enhanced root growth and leaf mineral enrichment

Key message Root-specific expression of a cytokinin-degrading CKX gene in maize roots causes formation of a larger root system leading to higher element content in shoot organs. The size and architecture of the root system is functionally relevant for the access to water and soil nutrients. A great number of mostly unknown genes are involved in regulating root architecture complicating targeted breeding of plants with a larger root system. Here, we have explored whether root-specific degradation of the hormone cytokinin, which is a negative regulator of root growth, can be used to genetically engineer maize (Zea mays L.) plants with a larger root system. Root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene of Arabidopsis caused the formation of up to 46% more root dry weight while shoot growth of these transgenic lines was similar as in non-transgenic control plants. The concentration of several elements, in particular of those with low soil mobility (K, P, Mo, Zn), was increased in leaves of transgenic lines. In kernels, the changes in concentration of most elements were less pronounced, but the concentrations of Cu, Mn and Zn were significantly increased in at least one of the three independent lines. Our data illustrate the potential of an increased root system as part of efforts towards achieving biofortification. Taken together, this work has shown that root-specific expression of a CKX gene can be used to engineer the root system of maize and alter shoot element composition

Root engineering in maize by increasing cytokinin degradation causes enhanced root growth and leaf mineral enrichment

The size and architecture of the root system is functionally relevant for the access to water and soil nutrients. A great number of mostly unknown genes are involved in regulating root architecture complicating targeted breeding of plants with a larger root system. Here, we have explored whether root-specific degradation of the hormone cytokinin, which is a negative regulator of root growth, can be used to genetically engineer maize (Zea mays L.) plants with a larger root system. Root-specific expression of a CYTOKININ OXIDASE/DEHYDROGENASE (CKX) gene of Arabidopsis caused the formation of up to 46% more root dry weight while shoot growth of these transgenic lines was similar as in non-transgenic control plants. The concentration of several elements, in particular of those with low soil mobility (K, P, Mo, Zn), was increased in leaves of transgenic lines. In kernels, the changes in concentration of most elements were less pronounced, but the concentrations of Cu, Mn and Zn were significantly increased in at least one of the three independent lines. Our data illustrate the potential of an increased root system as part of efforts towards achieving biofortification. Taken together, this work has shown that root-specific expression of a CKX gene can be used to engineer the root system of maize and alter shoot element composition

The jacalin-related lectin HvHorcH is involved in the physiological response of barley roots to salt stress

Salt stress tolerance of crop plants is a trait with increasing value for future food production. In an attempt to identify proteins that participate in the salt stress response of barley, we have used a cDNA library from salt-stressed seedling roots of the relatively salt-stress-tolerant cv. Morex for the transfection of a salt-stress-sensitive yeast strain (Saccharomyces cerevisiae YSH818 Deltahog1 mutant). From the retrieved cDNA sequences conferring salt tolerance to the yeast mutant, eleven contained the coding sequence of a jacalin-related lectin (JRL) that shows homology to the previously identified JRL horcolin from barley coleoptiles that we therefore named the gene HvHorcH. The detection of HvHorcH protein in root extracellular fluid suggests a secretion under stress conditions. Furthermore, HvHorcH exhibited specificity towards mannose. Protein abundance of HvHorcH in roots of salt-sensitive or salt-tolerant barley cultivars were not trait-specific to salinity treatment, but protein levels increased in response to the treatment, particularly in the root tip. Expression of HvHorcH in Arabidopsis thaliana root tips increased salt tolerance. Hence, we conclude that this protein is involved in the adaptation of plants to salinity