Salt stress alleviation in citrus plants by plant growth-promoting rhizobacteria Pseudomonas putida and Novosphingobium sp.

Key message This work reveals the protective role of two rhizobacteria, Pseudomonas putida and Novosphingobium sp., on citrus plants subjected to salt stress conditions. Abstract Detrimental salt stress effects on crops are likely to increase due to climate change reducing the quality of irrigation water. Plant growth-promoting rhizobacteria (PGPRs) can mitigate stress-induced damage in plants cultivated under high salinity conditions. In this work, Citrus macrophylla (alemow) plants inoculated with the rhizobacteria Pseudomonas putida KT2440 or Novosphingobium sp. HR1a were subjected to salt stress for 30 days. Results showed that in absence of salt stress, Novosphingobium sp. HR1a induced a decrease of transpiration (E) and stomatal conductance (gs). Both rhizobacteria reduced salt stress-induced damage. Levels of abscisic acid (ABA) and salicylic acid (SA) were lower in inoculated plants under salt stress conditions. Similarly, under stress conditions maximum efficiency of photosystem II (Fv/Fm) in inoculated plants decreased to a lower extent than in non-inoculated ones. In stressed plants, Novosphingobium sp. HR1a also induced leaf accumulation of 3-indole acetic acid (IAA) and a delay in the decrease of quantum yield (ΦPSII). P. putida KT2440 inhibited root chloride and proline accumulation in response to salt stress. Although both bacterial species had beneficial effects on salt-stressed citrus plants, Novosphingobium sp. HR1a induced a better plant performance. Therefore, both strains could be candidates to be used as PGPRs in programs of inoculation for citrus protection against salt stress

La metabolómica como herramienta para la evaluación fisiológica y nutricional en citricultura

La metabolómica tiene como objetivo el análisis de todos los metabolitos de bajo peso molecular presentes en un organismo, tejido o tipo celular concreto en un estadio de desarrollo dado y bajo unas condiciones ambientales particulares, proporcionando así una descripción detallada del fenotipo bioquímico. En este trabajo se analizaron, mediante cromatografía líquida en fase reversa acoplada a espectrometría de masas (RPLC/ESI-QTOF-MS) los perfiles de metabolitos secundarios de tres patrones de cítricos: citrange Carrizo, citrumelo CPB4475 y mandarino Cleopatra cultivados en condiciones óptimas en tres localizaciones distintas. Los metabolitos se emplearon como marcadores para clasificar las muestras atendiendo al genotipo y a la localización mediante análisis clúster jerárquico (HCA) seguido de análisis discriminante basado en mínimos cuadrados parciales (PLS-DA), que permite la identificación de las variables importantes en dicha clasificación. Este análisis reveló una influencia importante del genotipo incluso por encima del ambiente, aunque éste también influyó en la composición metabólica de forma significativa. En conclusión, esta tecnología puede ser empleada para investigar variaciones sutiles en el metabolismo debidas a alteraciones genéticas o cambios en el ambiente, a modo de técnica de evaluación fisiológica.Este estudio fue financiado por el Ministerio de Educación y Ciencia y la Universitat Jaume 1a través de los proyectos AGL201 0-22195-C03-01, P11B2012-06 Y P11B2013-2

Effects of cadmium on gas exchange and phytohormone contents in citrus

The effect of increased Cd2+ concentrations in the watering solution on citrus physiology was studied by using two citrus genotypes, Cleopatra mandarin and Carrizo citrange. Cadmium content in roots and leaves was tested together with measurements of leaf damage, gas exchange parameters, and hormonal contents. Citrus roots efficiently retained Cd2+ avoiding its translocation to the shoots and Cleopatra mandarin translocated less Cd2+ than Carrizo. With increasing Cd2+ concentration all gas exchange parameters were decreased more in Carrizo than in Cleopatra mandarin. Cd-induced increases in abscisic acid and salicylic acid contents were observed in leaves but not in roots of both genotype

NaCl protects against Cd and Cu-induced toxicity in the halophyte Atriplex halimus

The objective of the present work was to evaluate the extent of Cd- and Cu-induced oxidative stress and the antioxidant response triggered in the halophyte species Atriplex halimus after metallic trace elements exposure. Plants were treated for one month with Cd2+ or Cu2+ (400 μM) in the absence or presence of 200 mM NaCl in the irrigation solution. The interaction between salinity and heavy metal stress was analyzed in relation to plant growth, tissue ion contents (Na+, K+ and Mg2+), oxidative damage and antioxidative metabolism. Data indicate that shoot and root weight significantly decreased as a consequence of Cd2+- or Cu2+-induced stress. Metallic stress leads to unbalanced nutrient uptake by reducing the translocation of K+ and Mg2+ from the root to the shoot. The levels of malondialdehyde increased in root tissue when Cd, and especially Cu, were added to the irrigation solution, indicating that oxidative damage occurred. Results showed that NaCl gave a partial protection against Cd and Cu induced toxicity, although these contaminants had distinct influence on plant physiology. It can be concluded that salinity drastically modified heavy metal absorption and improved plant growth. Salinity also decreased oxidative damage, but differently in plants exposed to Cd or Cu stress.Ministerio de Economía y Competitividad, Spain (GL2013-42038-R; AGL2016-76574-R); Universitat Jaume I (P1- 1B2012-06)

Root Involvement in Plant Responses to Adverse Environmental Conditions

limate change is altering the environment in which plants grow and survive. An increase in worldwide Earth surface temperatures has been already observed, together with an increase in the intensity of other abiotic stress conditions such as water deficit, high salinity, heavy metal intoxication, etc., generating harmful conditions that destabilize agricultural systems. Stress conditions deeply affect physiological, metabolic and morphological traits of plant roots, essential organs for plant survival as they provide physical anchorage to the soil, water and nutrient uptake, mechanisms for stress avoidance, specific signals to the aerial part and to the biome in the soil, etc. However, most of the work performed until now has been mainly focused on aerial organs and tissues. In this review, we summarize the current knowledge about the effects of different abiotic stress conditions on root molecular and physiological responses. First, we revise the methods used to study these responses (omics and phenotyping techniques). Then, we will outline how environmental stress conditions trigger various signals in roots for allowing plant cells to sense and activate the adaptative responses. Later, we discuss on some of the main regulatory mechanisms controlling root adaptation to stress conditions, the interplay between hormonal regulatory pathways and the global changes on gene expression and protein homeostasis. We will present recent advances on how the root system integrates all these signals to generate different physiological responses, including changes in morphology, long distance signaling and root exudation. Finally, we will discuss the new prospects and challenges in this field

La actividad fotosintética como indicador de la tolerancia de los cítricos al estrés provocado por la inundación del sustrato

Soil waterlogging is a seasonal phenomenon associated to the incidence of heavy rains in the mediterranean basin. The main effect of substrate flooding is the reduction in the availability of O2 in the root environment which is directly related to a decrease in the plant ability for water absorption and the reduction in gas exchange parameters. In general, citrus are considered sensitive to root waterlogging although significant differences have been described to this respect among genotypes. In this sense, Cleopatra mandarin is considered highly sensitive whereas other genotypes such as Swingle citrumelo or Carrizo citrange are classified as more tolerant. In the present work, the relationship of different photosynthetic parameters such as gas exchange and chlorophyll fluorescence with tolerance to continuous flooding was assessed in the three above-mentioned citrus genotypes. Cleopatra mandarin revealed as very sensitive, citrumelo as moderately tolerant and Carrizo citrange as very tolerant. This gradation was correlated with the ability to keep elevated photosynthetic activity and maintaining it stable for longer time. These results suggest that photosynthetic activity could be used as indicator of tolerance to root waterlogging in citrusEn la cuenca mediterránea, el encharcamiento del suelo es un fenómeno estacional asociado a la incidencia de lluvias torrenciales. El principal efecto de la inundación del sustrato es la reducción en la disponibilidad de O2 en el entorno radicular, lo cual se traduce en una reducción de la capacidad de absorción de agua de las plantas y en un descenso del intercambio gaseoso. Los cítricos se consideran, en general, sensibles a la asfixia radicular aunque se han descrito notables diferencias entre genotipos. En este sentido, se considera al mandarino Cleopatra altamente sensible mientras que otros patrones como citrumelo Swingle o citrange Carrizo están clasificados como más tolerantes. En el presente trabajo, se evaluó la relación de distintos parámetros fotosintéticos, tales como el intercambio gaseoso y la fluorescencia de clorofilas, con la tolerancia al encharcamiento en los tres genotipos mencionados. Se observó una gradación en la tolerancia al encharcamiento continuado, siendo mandarino Cleopatra muy sensible, citrumelo moderadamente tolerante y citrange Carrizo muy tolerante. Esta gradación se correlacionó con la capacidad de mantener una elevada actividad fotosintética estable durante más tiempo. Estos resultados sugieren que la actividad fotosintética podría ser utilizada como indicador de la tolerancia al encharcamiento radicular en cítrico

Root exudates from citrus plants subjected to abiotic stress conditions have a positive effect on rhizobacteria

Plants are constantly releasing root exudates to the rhizosphere. These compounds are responsible for different (positive or negative) interactions with other organisms, including plants, fungi or bacteria. In this work, the effect of root exudates obtained from in vitro cultured citrus plants on two rhizobacteria (Pseudomonas putida KT2440 and Novosphingobium sp. HR1a) was evaluated. Root exudates were obtained from two citrus genotypes differing in their sensitivity to salt and heat stress and differentially affected the growth of both rhizobacteria. Root exudates from salt-stressed plants of C. macrophylla (salt tolerant) induced an increase in bacterial growth higher than that obtained from Carrizo citrange exudates (salt sensitive). Root exudates from heat-stressed plants also had a positive effect on bacterial growth, which was more evident in the heat-sensitive C. macrophylla. These results reveal that the growth of these rhizobacteria can be modulated through citrus root exudates and can change depending on both the stress conditions as well as the genotype. Biosensors P. putida KT2442 (pMIS5) and Novosphingobium sp. HR1a (pPAH) were used to test the presence of proline and salicylates in root exudates by measuring β-galactosidase activity. This activity increased in the presence of root exudates obtained from stressed plants to a higher extent in the case of exudates obtained from the genotype resistant to each particular stress, indicating that those root exudates contain larger quantities of proline and salicylates, as it has been described previously. Our data reveals that both P. putida KT2442 (pMIS5) and Novosphingobium sp. HR1a (pPAH), could be used as biosensors of plant stress

Identification and expression of the Cucurbita WRKY transcription factors in response to water deficit and salt stress

WRKY transcription factors (TFs) have been reported to play important roles in plant responses to various stress conditions. Although several studies on the genomic organization of the WRKY gene family in various species have been reported, the information related to the genus Cucurbita is scarce, and null in the case of Cucurbita pepo. The present study aimed to examine the response of Cucurbita pepo to water deficit and salt stress. Additionally, WRKY gene family has been identified and characterized in this species. Shoot growth was negatively affected by both adverse situations. Similarly, both salt and water stress conditions reduced transpiration and stomatal conductance in C. pepo plants. However, the quantum efficiency of PSII decreased only in those plants exposed to salt stress. The increase in proline concentration recorded in C. pepo plants subjected to salt or drought stress point out the important role of this amino acid for plant tolerance to both stress conditions. Based on the genome sequence, 95 CmWRKY genes were found and classified into three main groups according to their orthologues in Arabidopsis. Among these, 24 and 14 CmWRKY genes were responsive to water and salt stresses, respectively. Three water stress-responsive genes were upregulated under the adverse condition. The expression of six CmWRKY genes was induced by NaCl treatment. Therefore, a total of nine up-regulated genes related to both stresses were identified, suggesting their putative involvement in the plant response to water deficit and salt stress

Photosynthetic and antioxidant responses of Mexican lime (Citrus aurantifolia) plants to Citrus tristeza virus infection

The effect of Citrus tristeza virus (CTV) infection on photosynthetic activity and antioxidant metabolism was analysedin plants of the highly susceptible citrus genotype Mexican lime (Citrus aurantifolia). Two virus isolates differing intheir virulence (the severe T318 and the mild T385) were used in the experiments. CTV infection caused a reduction inphotosynthetic capacity in infected plants. This limitation was mainly due to a reduction in the carboxylative efficiencywhereas the limitation of CO2diffusion through the stoma had lower impact. The virus did not damage the antennaeand did not reduce the efficiency of light harvesting complexes. Oxidative damage occurred in infected plants, as evi-denced by the increase in malondialdehyde levels. Indeed, CTV infection caused an increase in ascorbate peroxidaseactivity in new shoots developed in infected plants during the 2 years of the experiment. Data suggest that the H2O2removal machinery was not damaged as a result of stress but the defence mechanism was overwhelmed with time dueto the continuing pressure of biotic stressUniversitat Jaume I (SPAIN) P1IB2012-06 P1IB2013-23 Spanish Ministerio de Economia y Competitividad (MINECO) AGL2010-22195-C03-0

Effect of cadmium and calcium treatments on phytochelatin and glutathione levels in citrus plants

Industry residues, phosphate fertilisers and wastewater as a source of irrigation have considerably increased levels of heavy metals in the soil, mainly cadmium (Cd2+). To test the effects of a calcium (Ca2+) treatment on Cd2+ accumulation and plant tolerance to this heavy metal, plants of two citrus genotypes, Cleopatra mandarin (CM) and Carrizo citrange (CC), were watered with increasing concentrations of Cd2+, and phytochelatin (PC) and glutathione (GSH) content were measured. Both genotypes were able to synthesise PCs in response to heavy metal intoxication, although CM seems to be a better Cd2+ excluder than CC. However, data indicate that CC plants had a higher capacity for regenerating GSH than CM plants. In this context, the effects of Ca2+ treatment on Cd2+ accumulation, plant survival and PC, GSH and oxidised glutathione (GSSG) content were assessed. Data indicate that treatment with Ca2+ had two positive effects on citrus physiology: it reduced Cd+2 uptake into roots and also increased GSH content (even in the absence of Cd2+). Overall, the data indicate that although Cd2+ exclusion is a powerful mechanism to avoid heavy metal build-up into photosynthetic organs, the capacity to maintain optimum GSH levels to feed PC biosynthesis could also be an important factor in stress tolerance