Senescence involves increased expression of proteases, which may participate in nitrogen recycling or cellular signalling. 2D zymograms detected two protein species with increased proteolytic activity in senescing leaves of Arabidopsis thaliana. A proteomic analysis revealed that both protein species correspond to a subtilisin protease encoded by At3g14067, termed Senescence-Associated Subtilisin Protease (SASP). SASP mRNA levels and enzyme activity increase during leaf senescence in leaves senescing during both the vegetative or the reproductive phase of the plant life cycle, but this increase is more pronounced in reproductive plants. SASP is expressed in all above-ground organs, but not in roots. Putative AtSASP orthologues were identified in dicot and monocot crop species. A phylogenetic analysis shows AtSASP and its putative orthologues clustering in one discrete group of subtilisin proteases in which no other Arabidospsis subtilisin protease is present. Phenotypic analysis of two knockout lines for SASP showed that mutant plants develop more inflorescence branches during reproductive development. Both AtSASP and its putative rice orthologue (OsSASP) were constitutively expressed in sasp-1 to complement the mutant phenotype. At maturity, sasp-1 plants produced 25% more inflorescence branches and siliques than either the wild-type or the rescued lines. These differences were mostly due to an increased number of second and third order branches. The increased number of siliques was compensated for by a small decrease (5.0%) in seed size. SASP downregulates branching and silique production during monocarpic senescence, and its function is at least partially conserved between Arabidopsis and rice.
The apoplast, i.e. the cellular compartment external to the plasma membrane, undergoes important changes during senescence. Apoplastic fluid volume increases quite significantly in senescing leaves, thereby diluting its contents. Its pH elevates by about 0.8 units, similar to the apoplast alkalization in response to abiotic stresses. The levels of 159 proteins decrease, whereas 24 proteins increase in relative abundance in the apoplast of senescing leaves. Around half of the apoplastic proteins of non-senescent leaves contain a N-terminal signal peptide for secretion, while all the identified senescence-associated apoplastic proteins contain the signal peptide. Several of the apoplastic proteins that accumulate during senescence also accumulate in stress responses, suggesting that the apoplast may constitute a compartment where developmental and stress-related programs overlap. Other senescence-related apoplastic proteins are involved in cell wall modifications, proteolysis, carbohydrate, ROS and amino acid metabolism, signaling, lipid transport, etc. The most abundant senescence-associated apoplastic proteins, PR2 and PR5 (e.g. pathogenesis related proteins PR2 and PR5) are related to leaf aging rather than to the chloroplast degradation program, as their levels increase only in leaves undergoing developmental senescence, but not in dark-induced senescent leaves. Changes in the apoplastic space may be relevant for signaling and molecular trafficking underlying senescence.
The family Sapindaceae is one of the most important groups of neotropical climbers and it has several cambial variants, some of which are exclusive. In this contribution, we described the wood development of Serjania meridionalis, the southernmost distributed species of the genus, studied by conventional anatomical techniques and based on material from Martín García island (Buenos Aires, Argentina). The primary vascular system is eustelic. Secondary growth begins with a typical cambium; however, secondary thickening supernumerary meristems dedifferentiate later. These meristems produce new vascular cylinders (secondary xylem and phloem) in the periphery of the original vascular cylinder. The presence of corded vascular cylinders in older stems of S. meridionalis is new for the species and raises the need to review the presence-absence of cambial variants dichotomy in many identification keys.
Extracellular RNA (exRNA) has long been considered as cellular waste that plants can degrade and utilize to recycle nutrients. However, recent findings highlight the need to reconsider the biological significance of RNAs found outside of plant cells. A handful of studies suggest that the exRNA repertoire, which turns out to be an extremely heterogenous group of non-coding RNAs, comprises species as small as a dozen nucleotides to hundreds of nucleotides long. They are found mostly in free form or associated with RNA-binding proteins, while very few are found inside extracellular vesicles (EVs). Despite their low abundance, small RNAs associated with EVs have been a focus of exRNA research due to their putative role in mediating transkingdom RNA interference. Therefore, non-vesicular exRNAs have remained completely under the radar until very recently. Here we summarize our current knowledge of the RNA species that constitute the extracellular RNAome and discuss mechanisms that could explain the diversity of exRNAs, focusing not only on the potential mechanisms involved in RNA secretion but also on post-release processing of exRNAs. We will also share our thoughts on the putative roles of vesicular and extravesicular exRNAs in plant-pathogen interactions, intercellular communication, and other physiological processes in plants.
En esta contribución se presentan los resultados de un estudio sobre el leño caulinar de la liana Ephedra tweediana Fisch. & C.A. Mey. emend. J.H. Hunz. (Ephedraceae), mediante técnicas anatómicas convencionales. Se da a conocer por primera vez para el género la presencia de la variante cambial “tallos lobulados”; y por primera vez para la especie la presencia de leño de reacción (expresado por la médula excéntrica), parénquima radial diagonal y radios agregados.
La senescencia constituye la etapa final del desarrollo ontogénico de las hojas, que se caracteriza por el desmantelamiento altamente organizado de componentes celulares, y la degradación masiva de sus macromoléculas (proteínas, ARN, ADN, lípidos). Este síndrome tiene un impacto positivo para la planta, dado que posibilita la exportación de nutrientes, principalmente N, de las hojas senescentes y su re-utilización en otros órganos. A nivel celular, los cloroplastos albergan la mayor parte de las proteínas foliares, y su desensamblado es central y diagnóstico del progreso de la senescencia. Es por esto que la mayor parte de los estudios en el tema se han enfocado en los cloroplastos, y los eventos extra-plastidiales que acompañan estos cambios son menos conocidos. Menos aún se conoce qué ocurre fuera de la membrana plasmática en las células senescentes. El apoplasto representa un compartimiento “extracelular”, externo, limitado por la membrana plasmática, que incluye las paredes celulares, los espacios intercelulares y el fluido apoplástico (FA). La composición (iones, macromoléculas, etc) de este espacio varía en forma dinámica contribuyendo al desarrollo de la planta y la adaptación a las condiciones ambientales. En el apoplasto tiene lugar la percepción y las primeras respuestas a varios estreses, que han sido estudiadas y caracterizadas en detalle tanto para estreses bióticos y abióticos. Dado que el apoplasto es escenario de procesos fisiológicos como la absorción y el transporte de azúcares y aminoácidos, la comunicación intercelular, la señalización sistémica, etc, es altamente probable que en este compartimiento tengan lugar cambios funcionales vinculados con la senescencia. En este trabajo se examinaron cambios en parámetros fisiológicos del apoplasto como compartimiento, y variaciones en la composición del fluido apoplástico a lo largo de la senescencia en Arabidopsis thaliana, a fin de profundizar el conocimiento sobre la dinámica de este espacio, y su papel en la degradación y removilización de nitrógeno durante la senescencia. En el capítulo 3 se describen las técnicas utilizadas para la obtención y análisis de fluido apoplástico foliar (FA). Este capítulo se enfoca en la optimización de las técnicas de infiltración-centrifugación para la extracción de FA de hojas senescentes de Arabidopsis, y del Índigo Carmín para calcular el volumen original de FA y el factor de dilución del FA aislado, ambos parámetros necesarios para estimar la concentración fisiológica de los componentes del FA. Luego, en el capítulo 4 se caracteriza la senescencia foliar como etapa ontogénica y los FAs obtenidos de hojas senescentes y no senescentes. En este capítulo se describe la selección de hojas representativas de distintas etapas de desarrollo para realizar un análisis sobre los cambios asociados a la senescencia en el fluido apoplástico foliar. En el capítulo 5 se presentan cambios fisiológicos en el compartimiento extracelular asociados a la senescencia, y en el capítulo 6 se analizan variaciones en las proteínas y proteoma del fluido apoplástico. En el capítulo 7 se muestran resultados de experimentos manipulativos de la senescencia foliar, y las respuestas a nivel de variaciones en el contenido estable de aminoácidos, péptidos y proteínas. Se analizó el rol del apoplasto en la degradación de proteínas foliares y la removilización de nitrógeno durante la senescencia, natural y acelerada por oscuridad y el papel de la autofagia en la exportación de aminoácidos al apoplasto.
En esta contribución se describe una estrategia de ascenso novedosa para el conjunto de las plantas trepadoras. Se ha denominado “entramado caulinar” y ha sido observado en Ephedra tweediana Fisch. & C.A. Mey. emend. J.H. Hunz. (Ephedraceae), a partir de diversos estudios realizados en individuos que crecen en la Isla Martín García (Buenos Aires, Argentina). Asimismo, se clarifica el tipo biológico básico de la especie, definido como “liana” (planta escandente leñosa).
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