BUSCADOR RECOLECTA

Drought stress is a major factor limiting symbiotic nitrogen fixation (NF) in soybean crop production. However, the regulatory mechanisms involved in this inhibition are still controversial. Soybean plants were symbiotically grown in a split-root system (SRS), which allowed for half of the root system to be irrigated at field capacity while the other half remained water deprived. NF declined in the water-deprived root system while nitrogenase activity was maintained at control values in the well-watered half. Concomitantly, amino acids and ureides accumulated in the water-deprived belowground organs regardless of transpiration rates. Ureide accumulation was found to be related to the decline in
their degradation activities rather than increased biosynthesis. Finally, proteomic analysis suggests that plant carbon metabolism, protein synthesis, amino acid metabolism, and cell growth are among the processes most altered in soybean nodules under drought stress. Results presented here support the hypothesis of a local regulation of NF taking place in soybean and downplay the role of ureides in the inhibition of NF, This work was financed by the Spanish Ministry of Economy and Competitiveness (AGL 2011–23738 and AGL 2011-30386-C02-01). EG-Q and AS are holders of PhD fellowships from the Public University of Navarre (735/2008 and 134/2012). EL is a recipient of a Marie Curie International Outgoing Fellowship for Career Development (PIOF-GA-2009–253141).

Drought stress is a major factor limiting nitrogen fixation (NF) in crop production. However, the regulatory mechanism involved and the origin of the inhibition, whether local or systemic, is still controversial and so far scarcely studied in temperate forage legumes. Medicago truncatula plants were symbiotically grown with a split-root system and exposed to gradual water deprivation. Physiological parameters, NF activity, and amino acid content were measured. The partial drought treatment inhibited NF in the nodules directly exposed to drought stress. Concomitantly, in the droughted below-ground organs, amino acids accumulated prior to any drop in evapotranspiration (ET). It is concluded that drought exerts a local inhibition of NF and drives an overall accumulation of amino acids in diverse plant organs which is independent of the decrease in ET. The general increase in the majority of single amino acids in the
whole plant questions the commonly accepted concept of a single amino acid acting as an N-feedback signal., This work was financed by the Spanish Ministry of Economy and Competitiveness (AGL 2011–23738 and AGL 2011-30386-C02-01). EG-Q is a holder of a PhD fellowship from the Public University of Navarre (735/2008). EL is a recipient of a Marie Curie International Outgoing Fellowship for Career Development (PIOF-GA-2009–253141).

The symbiotic association between Medicago truncatula and Sinorhizobium meliloti is a well-established model system in the legume–Rhizobium community. Despite its wide use, the symbiotic efficiency of this model has been recently questioned and an alternative microsymbiont, S. medicae, has been proposed. However, little is known about the physiological mechanisms behind the higher symbiotic efficiency of S. medicae WSM419. In the present study, we inoculated M. truncatula Jemalong A17 with either S. medicae WSM419 or S. meliloti 2011 and compared plant growth, photosynthesis, N2-fixation rates, and plant nodule carbon and nitrogen metabolic activities in the two systems. M. truncatula plants in symbiosis with S. medicae showed increased biomass and photosynthesis rates per plant. Plants grown in symbiosis with S. medicae WSM419 also showed higher N2-fixation rates, which were correlated with a larger nodule biomass, while nodule number was similar in both systems. In terms of plant nodule metabolism, M. truncatula–S. medicae WSM419 nodules showed increased sucrose-catabolic activity, mostly associated with sucrose synthase, accompanied by a reduced starch content, whereas nitrogen-assimilation activities were comparable to those measured in nodules infected with S. meliloti 2011. Taken together, these results suggest that S. medicae WSM419 is able to enhance plant carbon catabolism in M. truncatula nodules, which allows for the maintaining of high symbiotic N2-fixation rates, better growth and improved general plant performance., This
work has been partially funded by the Spanish National Research
and Development Programmes (AGL2011-23738 and AGL2011-
30386-C02-01). Estíbaliz Larrainzar and Erena Gil-Quintana are
funded by the European FP7-PEOPLE program (253141). Amaia
Seminario is funded by a predoctoral fellowship from the Public
University of Navarre.

Medicago truncatula es una planta forrajera anual con gran interés agronómico y científico, siendo de hecho empleada como planta modelo en el estudio de la biología de las leguminosas. Teniendo en cuenta el contexto actual del cambio climático, es de vital importancia mantener o aumentar el rendimiento de los cultivos para así poder responder a los requerimientos derivados del constante aumento de la población mundial. Para ello, es imprescindible entender las res-puestas adaptativas de las plantas al estrés hídrico, siendo muy útil el empleo de esta planta modelo para su estudio. En este trabajo hemos estudiado el comportamiento de varios órganos vegetales con especial énfasis en el sistema radical, permitiéndonos un conocimiento más inte-grado de los mecanismos de respuesta al estrés hídrico a nivel de planta entera.
El sistema radical de M. truncatula fue estudiado en el Capítulo 1, en el cual hicimos una distinción entre la raíz primaria, más gruesa, y las raíces laterales o “raíz fibrosa”, mucho más finas. Se estudió entonces el comportamiento de ambos tipos de raíz en condiciones control, remarcando la gran diferencia metabólica entre ambas, teniendo la raíz primaria un papel más activo que como mero almacén de reservas. Además, se observó una mayor resiliencia de la raíz primaria al estrés hídrico, pudiendo tener la modulación de la degradación de la sacarosa y del metabolismo de la prolina un papel esencial en la adaptación del sistema radical al estrés hídrico.
En el Capítulo 2 hemos abordado diversos tipos de estrés hídrico, empleando para ello con-diciones iso-osmóticas de salinidad (NaCl y KCl), falta de riego y un agente osmótico (PEG-6000). Esta comparativa nos permite identificar las semejanzas y diferencias en los mecanismos de respuesta a cada estrés a nivel de planta entera. Por un lado, los resultados obtenidos nos llevaron a descartar el uso de PEG como un compuesto apto para semejar condiciones de sequía, mientras que la exposición de M. truncatula a NaCl y KCl provocó respuestas similares, con un ligero mayor efecto negativo en el metabolismo por parte de este último. Al comparar la res-puesta a la falta de riego se observó un mayor énfasis en la protección del sistema radical, mien-tras que la exposición a NaCl conllevó una mayor respuesta a nivel de parte aérea. El estudio del floema nos permitió una mejor comprensión de las respuestas sistémicas de la planta a los di-versos tipos de estrés hídrico.
En resumen, este estudio proporciona un mayor conocimiento de la respuesta de M. trunca-tula a condiciones de estrés hídrico a nivel de planta entera y desde un punto de vista bioquí-mico, metabólico y fisiológico., Medicago truncatula is a forage legume with agricultural but also scientifical interest, being used as a model plant for the study of legumes’ biology. Within a climate change context, it is of great importance to maintain/increase plant yield in stressful growth conditions to meet the requirements of the increasing world population. In order to achieve this, it is mandatory to further understand the adaptive response of plants to water-deficit stress, for which the use of this model plant results of great utility. In the present study, the simultaneous study of various plant organs with particular focus on the root system allows us a more integrative understanding of water-deficit response mechanisms from a whole-plant perspective.
The root tissue was studied in Chapter 1, distinguishing between the thick taproot and the much thinner fibrous root. The different behaviour of both root types under well-watered as well as under water-deficit conditions was studied from a physiological and metabolic perspec-tive. This study highlighted the active role of the taproot rather than being considered a mere nutrient storage organ. The taproot showed a more resilient nature towards water-deficit stress than the fibrous root, while sucrose cleavage modulation, together with proline metabolism sug-gested a crucial role of these pathways in the root adaptation to water-deficit stress.
In Chapter 2 we aimed to address different water-deficit conditions that can affect plant water status, using iso-osmotical conditions of salinity (NaCl and KCl), lack of irrigation and an osmoticum (PEG). This approach allows us to identify the similarities and differences in the mechanisms involved in the response to each stress at the whole-plant level. While PEG was dismissed as a reliable drought-stress mimicker, NaCl and KCl led to similar responses, with a slightly higher negative effect of KCl on plant metabolism. On the other hand, an emphasis on the shoot and root protection was observed for NaCl and no-irrigation stress, respectively. The study of the phloem sap allowed us to better understand the responses to the different water-deficit conditions at a whole-plant level.
In summary, this study provides further insight into the response at the whole-plant level of M. truncatula to water-deficit conditions from a biochemical, metabolic and physiological point of view., This work was funded by the Spanish Ministry of Economy and Competitiveness (AGL 2011-23738) and the Public University of Navarra/Caja Navarra Foundation (7442-1941/2016). Verónica Castañeda has been a holder of a phD fellowship from the Basque Country Government (BFI-2012-97) and she has received a mobility grant from the Public University of Navarra as well as from the University of Hamburg (13/10/2014 and 20/05/2014, respectively)., Programa de Doctorado en Agrobiología Ambiental (RD 99/2011), Ingurumen Agrobiologiako Doktoretza Programa (ED 99/2011)

Incluye 3 ficheros de datos, Symbiotic nitrogen fixation is one of the first physiological
processes inhibited in legume plants under water-deficit conditions.
Despite the progress made in the last decades, the
molecular mechanisms behind this regulation are not fully
understood yet. Recent proteomic work carried out in the
model legume Medicago truncatula provided the first indications
of a possible involvement of nodule methionine (Met)
biosynthesis and related pathways in response to waterdeficit
conditions. To better understand this involvement, the
drought-induced changes in expression and content of
enzymes involved in the biosynthesis of Met, S-adenosyl-Lmethionine
(SAM) and ethylene in M. truncatula root and
nodules were analyzed using targeted approaches. Nitrogenfixing
plants were subjected to a progressive water deficit and
a subsequent recovery period. Besides the physiological characterization
of the plants,the content of total sulphur,sulphate
and main S-containing metabolites was measured. Results
presented here show that S availability is not a limiting factor
in the drought-induced decline of nitrogen fixation rates in
M. truncatula plants and provide evidences for a downregulation
of the Met and ethylene biosynthesis pathways in
roots and nodules in response to water-deficit conditions., This work was supported by the Spanish Ministry of Economy and Competitiveness (AGL 2011–23738 and AGL 2011–30386-C02-01).E.L. is a recipient of a Marie Curie International Outgoing Fellowship for Career Development (PIOF-GA-2009–253141). E.G-Q. received a PhD fellowship from the Public University of Navarre (735/2008).

Drought provokes a number of physiological changes in plants including oxidative damage. Ascorbic acid (AsA), also known as vitamin C, is one of the most abundant water-soluble antioxidant compound present in plant tissues. However, little is known on the regulation of AsA biosynthesis under drought stress conditions. In the current work we analyze the effects of water deficit on the biosynthesis of AsA by measuring its content, in vivo biosynthesis and the expression level of genes in the Smirnoff-Wheeler pathway in one of the major legume crop, soybean (Glycine max L. Merr). Since the pathway has not been described in legumes, we first searched for the putative orthologous genes in the soybean genome. We observed a significant genetic redundancy, with multiple genes encoding each step in the pathway. Based on RNA-seq analysis, expression of the complete pathway was detected not only in leaves but also in root tissue. Putative paralogous genes presented differential expression patterns in response to drought, suggesting the existence of functional specialization mechanisms. We found a correlation between the levels of AsA and GalLDH biosynthetic rates in leaves of drought-stressed soybean plants. However, the levels of GalLDH transcripts did not show significant differences under water deficit conditions. Among the other known regulators of the pathway, only the expression of VTC1 genes correlated with the observed decline in AsA in leaves., This work was supported by the Spanish Ministry of Economy and Competitiveness (AGL 2011–23738 to EG and a “Juan de la Cierva” JCI-2012-13175 post-doctoral contract to EL) the Fundación Caja Navarra project FCN2016-7442 to EG and the project 2016/PI013 LEGUMINOSAS from the Government of Navarra to EL. AS received a Ph.D. fellowship from the Public University of Navarra (1287/2011).

Root performance represents a target factor conditioning plant development under drought conditions. Moreover, recent root phenotyping studies remark relevant differences on functionality of the different root types. However, despite its relevance, the performance of different types of roots such as primary/taproot (tapR) and lateral/fibrous roots (fibR) under water stress conditions is largely unknown. In the current study, the impact of water stress on target C and N metabolism (namely sucrose and proline) processes were characterized in tapR and fibR of Medicago truncatula plants exposed to different water stress severity regimes (moderate versus severe). While both root types exhibit some common responses to face water stress, the study highlighted important physiological and metabolic differences between them. The tapR proved to have an essential role on carbon and nitrogen partitioning rather than just on storage. Moreover, this root type showed a higher resilience towards water deficit stress. Sucrose metabolization at sucrose synthase level was early blocked in this tissue together with a selective accumulation of some amino acids such as proline and branched chain amino adds, which may act as alternative carbon sources under water deficit stress conditions. The decline in respiration, despite the over-accumulation of carbon compounds, suggests a modulation at sucrose cleavage level by sucrose synthase and invertase. These data not only provide new information on the carbon and nitrogen metabolism modulation upon water deficit stress but also on the different role, physiology, and metabolism of the taproot and fibrous roots. In addition, obtained results highlight the fact that both root types show distinct performance under water deficit stress; this factor can be of great relevance to improve breeding programs for increasing root efficiency under adverse conditions., VC was funded by the Basque Country Government (BFI-2012-97). This work was financed by the Spanish Ministry of Economy and Competitiveness (AGL 2011-23738) and the Public University of Navarra/Caja Navarra Foundation (7442-1941/2016).

Drought is considered the more harmful abiotic stress resulting in crops yield loss. Legumes in symbiosis with rhizobia are able to fix atmospheric nitrogen. Biological nitrogen fixation (SNF) is a very sensitive process to drought and limits legumes agricultural productivity. Several factors are known to regulate SNF including oxygen availability to bacteroids, carbon and nitrogen metabolisms; but the signalling pathways leading to SNF inhibition are largely unknown. In this work, we have performed a proteomic approach of pea plants grown in split-root-system where one half of the root was well-irrigated and the other was subjected to drought. Water stress locally provoked nodule water potential decrease that led to SNF local inhibition. The proteomic approach revealed 11 and 7 nodule proteins regulated by drought encoded by P. sativum and R. leguminosarum genomes respectively. Among these 18 proteins, three proteins related to flavonoid metabolism, two to sulphur metabolism and three RNA-binding proteins were identified. These proteins could be molecular targets for future studies focused on the improvement of legumes tolerance to drought. Moreover, this work also provides new hints for the deciphering of SNF regulation machinery in nodules., This work has been partially fundedby the Spanish National Research and Development Programme (AGL2011-30386-CO2-1 and AGL2011-23738).