AcERF2, an ethylene-responsive factor of Atriplex canescens, positively modulates osmotic and disease resistance in Arabidopsis thaliana.

Ethylene-responsive factors (ERFs) comprise a large family of transcription factors in plants and play important roles in developmental processes and stress responses. Here, we characterized a novel AP2/ERF transcription factor, AcERF2, from the halophyte Atriplex canescens (four-wing saltbush, Chenopodiaceae). AcERF2 was proved to be a transcriptional activator in yeast and localized to the nucleus upon transient expression in Nicotiana benthamiana, indicating its potential role as a transcription factor. Overexpression of AcERF2 driven by a CaMV35S promoter led to decreased accumulation of reactive oxygen species (ROS) and malondialdehyde (MDA), and increased antioxidant enzymatic activities, as well as rapid stomatal closure under osmotic treatment in Arabidopsis. Arabidopsis plants overexpressing AcERF2 were hypersensitive to abscisic acid (ABA) during germination, seedling establishment, and primary root elongation, and exhibited significant tolerance to osmotic stress. Furthermore, overexpression of AcERF2 induced transcript accumulation of plant defense-related genes (PR1, PR2, PR5, ERF1 and ERF3) and increased Arabidopsis resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 and the necrotrophic fungal pathogen Botrytis cinerea. These results suggest that AcERF2 may play a positive modulation role in response to osmotic stress and pathogen infection in plants.

AcEBP1, an ErbB3-Binding Protein (EBP1) from halophyte Atriplex canescens, negatively regulates cell growth and stress responses in Arabidopsis.

An ErbB-3-binding protein gene AcEBP1, also known as proliferation-associated 2G4 gene (PA2G4s) belonging to the M24 superfamily, was obtained from the saltbush Atriplex canescens. Subcellular localization imaging showed the fusion protein AcEBP1-eGFP was located in the nucleus of epidermal cells in Nicotiana benthamiana. The AcEBP1 gene expression levels were up-regulated under salt, osmotic stress, and hormones treatment as revealed by qRT-PCR. Overexpression of AcEBP1 in Arabidopsis demonstrated that AcEBP1 was involved in root cell growth and stress responses (NaCl, osmotic stress, ABA, low temperature, and drought). These phenotypic data were correlated with the expression patterns of stress responsive genes and PR genes. The AcEBP1 transgenic Arabidopsis plants also displayed increased sensitivity under low temperature and evaluated resistance to drought stress. Together, these results demonstrate that AcEBP1 negatively affects cell growth and is a regulator under stress conditions.

Rapid regulation of the plasma membrane H⁺-ATPase activity is essential to salinity tolerance in two halophyte species, Atriplex lentiformis and Chenopodium quinoa.

BACKGROUND AND AIMS: The activity of H(+)-ATPase is essential for energizing the plasma membrane. It provides the driving force for potassium retention and uptake through voltage-gated channels and for Na(+) exclusion via Na(+)/H(+) exchangers. Both of these traits are central to plant salinity tolerance; however, whether the increased activity of H(+)-ATPase is a constitutive trait in halophyte species and whether this activity is upregulated at either the transcriptional or post-translation level remain disputed. METHODS: The kinetics of salt-induced net H(+), Na(+) and K(+) fluxes, membrane potential and AHA1/2/3 expression changes in the roots of two halophyte species, Atriplex lentiformis (saltbush) and Chenopodium quinoa (quinoa), were compared with data obtained from Arabidopsis thaliana roots. KEY RESULTS: Intrinsic (steady-state) membrane potential values were more negative in A. lentiformis and C. quinoa compared with arabidopsis (-144 ± 3·3, -138 ± 5·4 and -128 ± 3·3 mV, respectively). Treatment with 100 mm NaCl depolarized the root plasma membrane, an effect that was much stronger in arabidopsis. The extent of plasma membrane depolarization positively correlated with NaCl-induced stimulation of vanadate-sensitive H(+) efflux, Na(+) efflux and K(+) retention in roots (quinoa > saltbush > arabidopsis). NaCl-induced stimulation of H(+) efflux was most pronounced in the root elongation zone. In contrast, H(+)-ATPase AHA transcript levels were much higher in arabidopsis compared with quinoa plants, and 100 mm NaCl treatment led to a further 3-fold increase in AHA1 and AHA2 transcripts in arabidopsis but not in quinoa. CONCLUSIONS: Enhanced salinity tolerance in the halophyte species studied here is not related to the constitutively higher AHA transcript levels in the root epidermis, but to the plant's ability to rapidly upregulate plasma membrane H(+)-ATPase upon salinity treatment. This is necessary for assisting plants to maintain highly negative membrane potential values and to exclude Na(+), or enable better K(+) retention in the cytosol under saline conditions.

Anatomía de los órganos vegetativos de dos especies de Atriplex (Chenopodiaceae) de Venezuela / Anatomy of the vegetative organs of two species of Atriplex (Chenopodiaceae) from Venezuela

In Venezuela, Atriplex is represented by A. cristata and A. oestophora, the latter being endemic; they inhabit coastal areas with high temperatures, high solar radiation and sandy soils with high salt content. This work aimed to provide information to facilitate and clarify these species taxonomic delimitation, throughout the study of the anatomy of their vegetative organs; this may also clarify our understanding of their adaptability to soil and climatic conditions prevailing in areas they inhabit. The plant material was collected from at least three individu- als of each species in Punta Taima Taima and Capatárida, Falcon. Segments of roots, located near the neck and towards the apex, apical, middle and basal internodes of stems, were taken; and of leaves, located in the middle portion of plants. This material was fixed in FAA (formaldehyde, acetic acid, 70% ethanol) until processing. Semipermanent and permanent microscope slides were prepared with transverse or longitudinal sections, made using a razor (free-hand) or a rotation microtome, in this latter case, after paraffin embedding; besides, additional plates were mounted with portions of leaf epidermis, obtained by the maceration technique. The sections were stained with aqueous toluidine blue (1%) or safranin-fast-green, and mounted in water-glycerin or in Canada balsam. In order to calculate the vulnerability index, the vessel diameter in the vascular rings of roots, as well as their density, were quantified. Our results revealed structural features in the different organs, that resulted of taxonomic value and allowed the distinction of the species: in the leaf, the presence of aquifer tissue, the number of vascular bundles and their organization in the midrib, and the collenchyma differentiation in this part of the leaf; in the roots, the xylem and phloem arrangement in the growth rings, the nature of conjunctive tissue, and the presence of included phloem in one species. In addition, the species showed typical anatomical features of halophytes and xerophytes, such as: high density of trichomes on leaves and young stems which act as salt secreting glands, abundant sclerenchyma in stems and roots, water storage tissue and Kranz anatomy in leaves, narrow cortical region in young roots, presence of cambial variants in stems and roots, as well as short and narrow xylem vessels. Vulnerability index calculations indicated that both species tend to assure conduction but not the efficiency of the system. Atriplex species have anatomical characters which facilitate their adaptation to the special conditions prevailing in their habitats and that may be used for taxonomic delimitation.

En Venezuela, Atriplex está representado por A. cristata y A. oestophora, siendo esta última endémica, las mismas habitan zonas costeras con altas temperaturas, alta radiación solar y suelos arenosos con alto contenido de sales. Se caracterizaron anatómicamente sus órganos vegetativos con el fin de aportar rasgos para delimitarlas taxonómicamente y precisar caracteres que contribuyan a su adaptabilidad a las condiciones edafoclimáticas imperantes en su hábitat. El material vegetal fue recolectado en tres individuos de cada especie en Punta Taima Taima y Capatárida (Falcón). Se recolectaron segmentos de raíces próximos al cuello y al ápice; entrenudos basales, medios y apicales, y hojas ubicadas en la porción media de las plan- tas. Este material fue fijado en FAA (formaldehido, ácido acético y etanol 70%) hasta su procesamiento. Se prepara- ron láminas semipermanentes y permanentes con secciones transversales y longitudinales hechas a mano alzada o con micrótomo, en este último caso posterior a la inclusión en parafina. Adicionalmente, se realizaron macerados con el fin de obtener las epidermis foliares. Las secciones fueron teñidas con azul de toluidina acuosa (1%) o con safranina- fastgreen, montándolas en agua/glicerina o en bálsamo de Canadá. Se cuantificó el número de vasos y su densidad en los anillos vasculares de las raíces, para calcular el índice de vulnerabilidad. Se encontraron rasgos estructurales de valor taxonómico: la presencia de tejido acuífero en la lámina foliar, el número de haces vasculares y su organización en el nervio medio, así como la diferenciación de colénquima en el mismo; además, el arreglo del xilema/ floema en los anillos de crecimiento, la naturaleza del tejido conjuntivo, así como la presencia de floema incluso, en las raíces. Se detectaron caracteres anatómicos típicos de halófitas y xerófitas, como son: alta densidad de tricomas en hojas y tallos jóvenes, que actúan como glándulas secretoras de sal, abundancia de esclerénquima en tallos y raíces, tejidos reservantes de agua y anatomía Kranz en hojas, región cortical estrecha en raíces jóvenes, presencia de variantes cambiales en tallos y raíces, así como vasos cortos y estrechos en el xilema. El índice de vulnerabilidad indica que ambas especies tienden a asegurar la conducción, pero no la eficiencia del sistema. Las especies de Atriplex que crecen en Venezuela tienen caracteres que permiten su delimitación taxonómica y que facilitan su adaptación a las condiciones particulares de su hábitat.

Polyploidy enhances the occupation of heterogeneous environments through hydraulic related trade-offs in Atriplex canescens (Chenopodiaceae).

Plant hydraulic characteristics were studied in diploid, tetraploid and hexaploid cytotypes of Atriplex canescens (Chenopodiaceae) to investigate the potential physiological basis underlying the intraspecific habitat differentiation among plants of different ploidy levels. Populations of A. canescens from different habitats of the Chihuahuan Desert (New Mexico, USA) were analyzed using flow cytometry to determine ploidy levels. Traits related to xylem water transport efficiency and safety against drought-induced hydraulic failure were measured in both stems and leaves. At the stem level, cytotypes of higher ploidy showed consistently lower leaf-specific hydraulic conductivity but greater resistance to drought-induced loss of hydraulic conductivity. At the leaf level, comparisons in hydraulics between cytotypes did not show a consistent pattern, but exhibited high plasticity to proximal environmental conditions related to soil water availability. The results suggest that a trade-off between stem hydraulic efficiency and safety across ploidy levels underlies niche differentiation among different cytotypes of A. canescens. Polyploidization may have been facilitated by environmental heterogeneity related to water availability, and variation in water-related physiology found in the present study suggests an important functional basis for the niche differentiation and coexistence of A. canescens cytotypes in desert environments.

Cadmium has contrasting effects on polyethylene glycol-sensitive and resistant cell lines in the Mediterranean halophyte species Atriplex halimus L.

Beside a direct toxicity, cadmium impact on plants involves both a secondary-induced water stress and an oxidative stress. Proliferating cell lines of Atriplex halimus were selected for their sensitivity or resistance to polyethylene glycol (PEG 10,000, 20%) and then exposed to 100 microM CdCl2 in the simultaneous presence or absence of PEG 20% or 150 mM NaCl. The PEG resistant cell line exhibited a higher growth in the presence of Cd than the sensitive line, although Cd acccumulation was higher in the former than in the latter. Exogenous PEG induced an increase in Cd concentration in the sensitive but not in the resistant cell line while NaCl induced a decrease in Cd accumulation in both cell lines. In the presence of Cd alone, the water content (WC) was higher and the osmotic potential was lower in PEG-sensitive than in PEG resistant line. The presence of PEG in the Cd-containing medium increased the WC and decreased the osmotic potential in PEG-resistant line comparatively to Cd stress alone, while an inverse trend was observed for the sensitive line. The PEG-resistant cell line displayed a higher ability to cope with oxidative stress in relation to an increase of endogenous antioxidants (glutathione and ascorbic acid), a high constitutive superoxide dismutase (EC 1.15.1.1) activity and an efficient Cd-induced increase in glutathione reductase (GR) (EC 1.6.4.1) and ascorbate peroxidase (APX) (EC 1.11.1.11). Cadmium tolerance in PEG-resistant line is thus not related to any strategy of Cd exclusion or osmotic adjustment but to tolerance mechanisms allowing the tissue to restrict the deleterious impact of accumulated Cd.

Abscisic acid has contrasting effects on salt excretion and polyamine concentrations of an inland and a coastal population of the Mediterranean xero-halophyte species Atriplex halimus.

BACKGROUND AND AIMS: Different populations of the Mediterranean xerohalophyte species Atriplex halimus exhibit different levels of resistance to salt and osmotic stress depending on the nature of the osmocompatible solute they accumulate. There is, however, no conclusive description of the involvement of abscisic acid (ABA) in the plant response to NaCl or osmotic stress in this species. METHODS: Seedlings issued from an inland water-stress-resistant population (Sbikha) and from a coastal salt-resistant one (Monastir) were exposed in nutrient solutions to NaCl (40 or 160 mm) or to 15 % PEG for 1 d and 10 d in the presence or absence of 50 microm ABA. KEY RESULTS: Plants from Sbikha accumulated higher amounts of ABA in response to osmotic stress than those of Monastir, while an opposite trend was recorded for NaCl exposure. Exogenous ABA improved osmotic stress resistance in Monastir through an improvement in the efficiency of stomatal conductance regulation. It also improved NaCl resistance in Sbikha through an increase in sodium excretion through the external bladders. It is suggested that polyamines (spermidine and spermine) are involved in the salt excretion process and that ABA contributes to polyamine synthesis as well as to the conversion from the bound and conjugated to the free soluble forms of polyamine. Proline accumulated in response to osmotic stress and slightly increased in response to ABA treatment while glycinebetaine accumulated in response to salinity and was not influenced by ABA. CONCLUSIONS: It is concluded that ABA is involved in both salt and osmotic stress resistance in the xerohalophyte species Atriplex halimus but that it acts on different physiological cues in response to those distinct environmental constraints.