Cloning, expression, purification and preliminary X-ray analysis of the protein kinase domain of constitutive triple response 1 (CTR1) from Arabidopsis thaliana

Identification of mebendazole as an ethylene signaling activator reveals a role of ethylene signaling in the regulation of lateral root angles

The lateral root angle or gravitropic set-point angle (GSA) is an important trait for root system architecture (RSA) that determines the radial expansion of the root system. The GSA therefore plays a crucial role for the ability of plants to access nutrients and water in the soil. Only a few regulatory pathways and mechanisms that determine GSA are known. These mostly relate to auxin and cytokinin pathways. Here, we report the identification of a small molecule, mebendazole (MBZ), that modulates GSA in Arabidopsis thaliana roots and acts via the activation of ethylene signaling. MBZ directly acts on the serine/threonine protein kinase CTR1, which is a negative regulator of ethylene signaling. Our study not only shows that the ethylene signaling pathway is essential for GSA regulation but also identifies a small molecular modulator of RSA that acts downstream of ethylene receptors and that directly activates ethylene signaling.

Expression profiling of CTR1-like and EIN2-like genes in buds and leaves of Populus tremula, and in vitro study of the interaction between their polypeptides

In Arabidopsis, the serine/threonine protein kinase Constitutive Triple Response 1 (CTR1) and Ethylene Insensitive 2 polypeptide (EIN2) functions are key negative and positive components, respectively, in the ethylene signalling route. Here, we report on an in silico study of members of the CTR1-like and EIN2-like polypeptide families from poplars. The expression of CTR1-like and EIN2-like genes such as Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a was studied in Populus tremula buds and leaves in response to dehydration, various light conditions and under senescence. In buds under dehydration, the maximal fold-change of the Ptre-CTR1, Ptre-CTR3 and Ptre-EIN2a expression level recorded almost identical values. This suggests that maintenance of a constant ratio between the transcript levels of genes encoding positive and negative ethylene signalling components is required under stress. The expression of the studied genes was 1.4-to 3-fold higher in response to darkness, but 4.5- to 51.2-fold and 21.6- to 51.2-fold higher under the early and moderate leaf senescence, respectively. It is worth noting that the senescence-related Ptre-EIN2a and Ptre-CTR3a expression profiles were very similar. Using in vitro assays, we demonstrated the ability of the catalytic domain of Ptre-CTR1 to phosphorylate the Ptre-EIN2a-like polypeptide, which is similar to that in Arabidopsis. The target substrate, the Ptre-CEND2a polypeptide (C-terminal part of Ptre-EIN2a), was only phosphorylated by the protein kinase Ptre-CTR1 and not by Ptre-CTR3. Moreover, the addition of Ptre-CTR3 polypeptides (-CTR3a or -CTR3b forms) to the reaction mixture had an inhibitory effect on Ptre-CTR1 auto- and trans-phosphorylation. In contrast to Ptre-CTR1, Ptre-CTR3 may act as a positive regulator in ethylene signalling in poplar; however, this hypothesis requires in vivo confirmation. Thus, the ethylene signalling route in poplar seems to be under the control of certain additional mechanisms which have not been reported in Arabidopsis.

Dominant and recessive mutations in the Raf-like kinase HT1 gene completely disrupt stomatal responses to CO2 in Arabidopsis

HT1 (HIGH LEAF TEMPERATURE 1) is the first component associated with changes in stomatal aperture in response to CO2 to be isolated by forward genetic screening. The HT1 gene encodes a protein kinase expressed mainly in guard cells. The loss-of-function ht1-1 and ht1-2 mutants in Arabidopsis thaliana have CO2-hypersensitive stomatal closure with concomitant reductions in their kinase activities in vitro In addition to these mutants, in this study we isolate or obtaine five new ht1 alleles (ht1-3, ht1-4, ht1-5, ht1-6, and ht1-7). Among the mutants, only ht1-3 has a dominant mutant phenotype and has widely opened stomata due to CO2 insensitivity. The ht1-3 mutant has a missense mutation affecting a non-conserved residue (R102K), whereas the other six recessive mutants have mutations in highly conserved residues in the catalytic domains required for kinase activity. We found that the dominant mutation does not affect the expression of HT1 or the ability to phosphorylate casein, a universal kinase substrate, but it does affect autophosphorylation activity in vitro A 3D structural model of HT1 also shows that the R102 residue protrudes from the surface of the kinase, implying a role for the formation of oligomers and/or interaction with its targets. We demonstrate that both the loss-of-function and gain-of-function ht1 mutants have completely disrupted CO2 responses, although they have normal responses to ABA. Furthermore, light-induced stomatal opening is smaller in ht1-3 and much smaller in ht1-2 Taken together, these results indicate that HT1 is a critical regulator for CO2 signaling and is partially involved in the light-induced stomatal opening pathway.

Expression profiles of a MhCTR1 gene in relation to banana fruit ripening

The banana (Musa spp.) is a typical climacteric fruit of high economic importance. The development of bananas from maturing to ripening is characterized by increased ethylene production accompanied by a respiration burst. To elucidate the signal transduction pathway involved in the ethylene regulation of banana ripening, a gene homologous to Arabidopsis CTR1 (constitutive triple response 1) was isolated from Musa spp. (Hsien Jin Chiao, AAA group) and designated as MhCTR1. MhCTR1 spans 11.5 kilobases and consists of 15 exons and 14 introns with consensus GT-AG nucleotides situated at their boundaries. MhCTR1 encodes a polypeptide of 805 amino acid residues with a calculated molecular weight of 88.6 kDa. The deduced amino acid sequence of MhCTR1 demonstrates 55%, 56% and 55% homology to AtCTR1, RhCTR1, and LeCTR1, respectively. MhCTR1 is expressed mostly in the mature green pulp and root organs. During fruit development MhCTR1 expression increases just before ethylene production rises. Moreover, MhCTR1 expression was detected mainly in the pulps at ripening stage 3, and correlated with the onset of peel yellowing, while MhCTR1 was constitutively expressed in the peels. MhCTR1 expression could be induced by ethylene treatment (0.01 μL L(-1)), and MhCTR1 expression decreased in both peel and pulp 24 h after treatment. Overall, changes observed in MhCTR1 expression in the pulp closely related to the regulation of the banana ripening process.

Protein kinase domain of CTR1 from Arabidopsis thaliana promotes ethylene receptor cross talk

Ethylene controls many aspects of plant growth and development. Signaling by the gaseous phytohormone is initiated by disulfide-linked membrane-bound receptors, and the formation of heteromeric receptor clusters contributes to the broad range of ethylene responsiveness. In Arabidopsis thaliana, the TCS-like ethylene receptors interact with the cytosolic serine/threonine kinase constitutive triple response 1 (CTR1), a proposed mitogen-activated protein kinase kinase kinase. In the absence of the hormone, the receptor and therefore CTR1 are active. Hence, ethylene acts as an inverse agonist of its signaling pathway. The three-dimensional structures of the active, triphosphorylated and the unphosphorylated, inactive kinase domain of CTR1 in complex with staurosporine illustrate the conformational rearrangements that form the basis of activity regulation. Additionally, in analytical ultracentrifugation experiments, active kinase domains form back-to-back dimers, while inactive and activation loop variants are monomers. Together with a front-to-front activation interface, the active protein kinase dimers thereby engage in interactions that promote CTR1-mediated cross talk between ethylene receptor clusters. This model provides a structural foundation for the observed high sensitivity of plants to ethylene.