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Publikation

Serra, P.; Carbonell, A.; Navarro, B.; Gago-Zachert, S.; Li, S.; Di Serio, F.; Flores, R.; Symptomatic plant viroid infections in phytopathogenic fungi: A request for a critical reassessment Proc. Natl. Acad. Sci. U.S.A. 117, 10126-10128, (2020) DOI: 10.1073/pnas.1922249117

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Bücher und Buchkapitel

Flores, R.; Gago-Zachert, S.; Serra, P.; De la Peña, M.; Navarro, B.; Chrysanthemum Chlorotic Mottle Viroid (Hadidi, A., et al., eds.). 331-338, (2017) DOI: 10.1016/B978-0-12-801498-1.00031-0

Chrysanthemum chlorotic mottle viroid (CChMVd) (398–401 nt) belongs to genus Pelamoviroid, family Avsunviroidae and, like other members of this family, replicates in plastids through a rolling-circle mechanism involving hammerhead ribozymes. CChMVd RNA adopts a branched conformation stabilized by a kissing-loop interaction, resembling peach latent mosaic viroid in this respect. Chrysanthemum is the only natural and experimental host for CChMVd, which in the most sensitive varieties induces leaf mottling and chlorosis, delay in flowering, and dwarfing. The viroid has been found in major chrysanthemum growing areas including Europe and Asia. There are natural variants in which the change (UUUC→GAAA) mapping at a tetraloop in the CChMVd branched conformation is sufficient to change the symptomatic phenotype into a nonsymptomatic one without altering the viroid titer. Preinfection with nonsymptomatic variants prevents challenge inoculation with symptomatic ones. Moreover, experimental coinoculation with symptomatic and nonsymptomatic CChMVd variants results in symptomless phenotypes only when the latter is in vast excess, thus indicating its lower fitness.
Publikation

Flores, R.; Gago-Zachert, S.; Serra, P.; Sanjuán, R.; Elena, S. F.; Viroids: Survivors from the RNA World? Annu. Rev. Microbiol. 68, 395-414, (2014) DOI: 10.1146/annurev-micro-091313-103416

Because RNA can be a carrier of genetic information and a biocatalyst, there is a consensus that it emerged before DNA and proteins, which eventually assumed these roles and relegated RNA to intermediate functions. If such a scenario—the so-called RNA world—existed, we might hope to find its relics in our present world. The properties of viroids that make them candidates for being survivors of the RNA world include those expected for primitive RNA replicons: (a) small size imposed by error-prone replication, (b) high G + C content to increase replication fidelity, (c) circular structure for assuring complete replication without genomic tags, (d) structural periodicity for modular assembly into enlarged genomes, (e) lack of protein-coding ability consistent with a ribosome-free habitat, and (f) replication mediated in some by ribozymes, the fingerprint of the RNA world. With the advent of DNA and proteins, those protoviroids lost some abilities and became the plant parasites we now know.
Publikation

Abel, S.; Theologis, A.; Odyssey of Auxin Cold Spring Harb. Perspect. Biol. 2, a004572, (2010) DOI: 10.1101/cshperspect.a004572

The history of plant biology is inexorably intertwined with the conception and discovery of auxin, followed by the many decades of research to comprehend its action during growth and development. Growth responses to auxin are complex and require the coordination of auxin production, transport, and perception. In this overview of past auxin research, we limit our discourse to the mechanism of auxin action. We attempt to trace the almost epic voyage from the birth of the hormonal concept in plants to the recent crystallographic studies that resolved the TIR1-auxin receptor complex, the first structural model of a plant hormone receptor. The century-long endeavor is a beautiful illustration of the power of scientific reasoning and human intuition, but it also brings to light the fact that decisive progress is made when new technologies emerge and disciplines unite.
Publikation

Serra, P.; BANI HASHEMIAN, S. M.; PENSABENE-BELLAVIA, G.; Gago, S.; DURAN-VILA, N.; An artificial chimeric derivative of Citrus viroid V involves the terminal left domain in pathogenicity Mol. Plant Pathol. 10, 515-522, (2009) DOI: 10.1111/j.1364-3703.2009.00553.x

The recently described Citrus viroid V (CVd‐V) induces, in Etrog citron, mild stunting and very small necrotic lesions and cracks, sometimes filled with gum. As Etrog citron plants co‐infected with Citrus dwarfing viroid (CDVd) and CVd‐V show synergistic interactions, these host–viroid combinations provide a convenient model to identify the pathogenicity determinant(s). The biological effects of replacing limited portions of the rod‐like structure of CVd‐V with the corresponding portions of CDVd are reported. Chimeric constructs were synthesized using a novel polymerase chain reaction‐based approach, much more flexible than those based on restriction enzymes used in previous studies. Of the seven chimeras (Ch) tested, only one (Ch5) proved to be infectious. Plants infected with Ch5 showed no symptoms and, although this novel chimera was able to replicate to relatively high titres in singly infected plants, it was rapidly displaced by either CVd‐V or CDVd in doubly infected plants. The results demonstrate that direct interaction(s) between structural elements in the viroid RNA (in this case, the terminal left domain) and as yet unidentified host factors play an important role in modulating viroid pathogenicity. This is the first pathogenic determinant mapped in species of the genus Apscaviroid.
Publikation

Serra, P.; Gago, S.; Duran-Vila, N.; A single nucleotide change in Hop stunt viroid modulates citrus cachexia symptoms Virus Res. 138, 130-134, (2008) DOI: 10.1016/j.virusres.2008.08.003

Cachexia disease of citrus is caused by Hop stunt viroid (HSVd). In citrus, pathogenic and non-pathogenic strains differ by a “cachexia expression motif” of five to six nucleotides located in the variable domain of the proposed rod-like secondary structure. Here, site-directed mutants were generated to investigate if all these nucleotides were required for infectivity and/or symptom expression. Specifically an artificial cachexia inducing mutant M0 was generated by introducing the six nucleotides changes of the “cachexia expression motif” into a non-pathogenic sequence variant and M0 was used as a template to systematically restore some of the introduced changes. The resulting mutants in which specific changes introduced to generate M0, were restored presented a variety of responses: (i) M1, obtained by introducing two insertions forming a base-pair, was infectious but non-pathogenic; (ii) M2, obtained by introducing an insertion and restoring a substitution, presented low infectivity and the resulting progeny reverted to M0; (iii) M3, obtained by restoring a single substitution in the lower strand of the viroid secondary structure, was infectious but induced only mild cachexia symptoms; (iv) M4, obtained by restoring a single susbtitution in the upper strand of the viroid secondary structure, was non-infectious. These results confirm that the “cachexia expression motif” plays a major role in inciting cachexia symptoms, and that subtle changes within this motif affect symptom severity and may even suppress symptom expression.
Publikation

Morgan, K. E.; Zarembinski, T. I.; Theologis, A.; Abel, S.; Biochemical characterization of recombinant polypeptides corresponding to the predicted βαα fold in Aux/IAA proteins FEBS Lett. 454, 283-287, (1999) DOI: 10.1016/S0014-5793(99)00819-4

The plant hormone indoleacetic acid (IAA or auxin) transcriptionally activates a select set of early genes. The Auxl IAA class of early auxin-responsive genes encodes a large family of short-lived, nuclear proteins. Aux/IAA polypeptides homo-and heterodimerize, and interact with auxin-response transcription factors (ARFs) via C-terminal regions conserved in both protein families. This shared region contains a predicted βαα motif similar to the prokaryotic β-Ribbon DNA binding domain, which mediates both protein dimerization and DNA recognition. Here, we show by circular dichroism spectroscopy and by chemical cross-linking experiments that recombinant peptides corresponding to the predicted βαα region of three Aux/IAA proteins from Arabidopsis thaliana contain substantial α-helical secondary structure and undergo homo- and heterotypic interactions in vitro. Our results indicate a similar biochemical function of the plant βαα domain and suggest that the βαα fold plays an important role in mediating combinatorial interactions of Aux/IAA and ARF proteins to specifically regulate secondary gene expression in response to auxin.
Publikation

Abel, S.; Theologis, A.; Early Genes and Auxin Action Plant Physiol. 111, 9-17, (1996) DOI: 10.1104/pp.111.1.9

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Publikation

Abel, S.; Ballas, N.; Wong, L.-M.; Theologis, A.; DNA elements responsive to auxin BioEssays 18, 647-654, (1996) DOI: 10.1002/bies.950180808

Genes induced by the plant hormone auxin are probably involved in the execution of vital cellular functions and developmental processes. Experimental approaches designed to elucidate the molecular mechanisms of auxin action have focused on auxin perception, genetic dissection of the signaling apparatus and specific gene activation. Auxin‐responsive promoter elements of early genes provide molecular tools for probing auxin signaling in reverse. Functional analysis of several auxin‐specific promoters of unrelated early genes suggests combinatorial utilization of both conserved and variable elements. These elements are arranged into autonomous domains and the combination of such modules generates uniquely composed promoters. Modular promoters allow for auxin‐mediated transcriptional responses to be revealed in a tissue‐ and development‐specific manner.
Publikation

Wong, L. M.; Abel, S.; Shen, N.; de la Foata, M.; Mall, Y.; Theologis, A.; Differential activation of the primary auxin response genes, PS-IAA4/5 and PS-IAA6, during early plant development Plant J. 9, 587-599, (1996) DOI: 10.1046/j.1365-313X.1996.9050587.x

The plant growth hormone auxin typified by indoleacetic acid (IAA) transcriptionally activates early genes in pea, PS‐IAA4/5 and PS‐IAA6 , that are members of a multigene family encoding short‐lived nuclear proteins. To gain first insight into the biological role of PS‐IAA4/5 and PSIAA6 , promoter‐β‐glucuronidase (GUS) gene fusions were constructed and their expression during early development of transgenic tobacco seedlings was examined. The comparative analysis reveals spatial and temporal expression patterns of both genes that correlate with cells, tissues, and developmental processes known to be affected by auxin. GUS activity in seedlings of both transgenic lines is located in the root meristem, sites of lateral root initiation and in hypocotyls undergoing rapid elongation. In addition, mutually exclusive cell‐specific expression is evident. For instance, PS‐IAA4/5—GUS but not PS‐IAA6—GUS is expressed in root vascular tissue and in guard cells, whereas only PS‐IAA6—GUS activity is detectable in glandular trichomes and redistributes to the elongating side of the hypocotyl upon gravitropic stimulation. Expression of PS‐IAA4/5 and PS‐IAA6 in elongating, dividing, and differentiating cell types indicates multiple functions during development. The common and yet distinct activity patterns of both genes suggest a combinatorial code of spatio‐temporal co‐expression of the various PS‐IAA4/ 5‐like gene family members in plant development that may mediate cell‐specific responses to auxin.
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