Micaela Carvajal

Diurnal variations in hydraulic conductivity and root pressure can be correlated with the expression of putative aquaporins in the roots of Lotus japonicus

Abstract. The hydraulic conductivity of excised roots (Lpr) of the legume Lotus japonicus (Regel) K. Larsen grown in mist (aeroponic) and sand cultures, was found to vary over a 5-fold range during a day/night cycle. This behaviour was seen when Lpr was measured in roots exuding, either under root pressure (osmotic driving force), or under an applied hydrostatic pressure of 0.4 MPa which produced a rate of water flow similar to that in a transpiring plant. A similar daily pattern of variation was seen in plants grown in natural daylight or in controlled-environment rooms, in plants transpiring at ambient rates or at greatly reduced rates, and in plants grown in either aeroponic or sand culture. When detached root systems were connected to a root pressure probe, a marked diurnal variation was seen in the root pressure generated. After excision, this circadian rhythm continued for some days. The hydraulic conductivity of the plasma membrane of individual root cells was measured during the diurnal cycle using a cell pressure probe. Measurements were made on the first four cell layers of the cortex, but no evidence of any diurnal fluctuation could be found. It was concluded that the conductance of membranes of endodermal and stelar cells may be responsible for the observed diurnal rhythm in root Lpr. When mRNAs from roots were probed with cDNA from the Arabidopsis aquaporin AthPIP1a gene, an abundant transcript was found to vary in abundance diurnally under high-stringency conditions. The pattern of fluctuations resembled closely the diurnal pattern of variation in root Lpr. The plasma membranes of root cells were found to contain an abundant hydrophobic protein with a molecular weight of about 31 kDa which cross-reacted strongly to an antibody raised against the evolutionarily conserved N-terminal amino acid sequence of AthPIP1a.

Gas exchange, chlorophyll and nutrient contents in relation to Na+ and Cl- accumulation in 'Sunburst' mandarin grafted on different rootstocks

Abstract We investigated effects of salinity on the growth and net gas exchange of CO2 and water vapour in leaves of 2-year-old ‘Sunburst’ mandarin [(Citrus reticulata Blanco)×(Citrus paradisi Macf.×C. reticulata)] trees grafted on either Cleopatra mandarin (C. reticulata) or Carrizo citrange (Citrus sinensis L. Osb.×Poncirus trifoliata L.) rootstocks. Trees were grown in a greenhouse and watered with a complete nutrient solution containing 0, 30, 60 or 90 mM NaCl. After 6 weeks of treatment, tree growth, net gas exchange of leaves, leaf chlorophyll and mineral nutrient concentration were measured. Salinity decreased growth and net gas exchange of leaves on all trees. Cleopatra roots accumulated higher concentrations of Cl− and Na+ than Carrizo roots but ‘Sunburst’ leaves on Cleopatra accumulated less Cl− and Na+ and had higher CO2 exchange rates than those on Carrizo. This corresponded to higher concentrations of leaf N and chlorophyll in leaves of trees on Cleopatra than on Carrizo. Salinity increased N and decreased K+ contents in roots of both rootstocks. Salinity increased N and decreased K+ concentrations in leaves of trees on Cleopatra but not in trees on Carrizo. Salinity also increased Ca2+ concentration and reduced Mg2+ in leaves on Cleopatra. The lower Cl− and Na+ concentration in leaves of ‘Sunburst’ grafted on Cleopatra than on Carrizo, suggests that the salinity tolerance of Cleopatra is associated with ion sequestration in roots with less transport to leaves.

Changes in plasma membrane lipids, aquaporins and proton pump of broccoli roots, as an adaptation mechanism to salinity

Salinity stress is known to modify the plasma membrane lipid and protein composition of plant cells. In this work, we determined the effects of salt stress on the lipid composition of broccoli root plasma membrane vesicles and investigated how these changes could affect water transport via aquaporins. Brassica oleracea L. var. Italica plants treated with different levels of NaCl (0, 40 or 80mM) showed significant differences in sterol and fatty acid levels. Salinity increased linoleic (18:2) and linolenic (18:3) acids and stigmasterol, but decreased palmitoleic (16:1) and oleic (18:1) acids and sitosterol. Also, the unsaturation index increased with salinity. Salinity increased the expression of aquaporins of the PIP1 and PIP2 subfamilies and the activity of the plasma membrane H(+)-ATPase. However, there was no effect of NaCl on water permeability (P(f)) values of root plasma membrane vesicles, as determined by stopped-flow light scattering. The counteracting changes in lipid composition and aquaporin expression observed in NaCl-treated plants could allow to maintain the membrane permeability to water and a higher H(+)-ATPase activity, thereby helping to reduce partially the Na(+) concentration in the cytoplasm of the cell while maintaining water uptake via cell-to-cell pathways. We propose that the modification of lipid composition could affect membrane stability and the abundance or activity of plasma membrane proteins such as aquaporins or H(+)-ATPase. This would provide a mechanism for controlling water permeability and for acclimation to salinity stress.

Influence of saline stress on root hydraulic conductance and PIP expression inArabidopsis

Measurements of the root hydraulic conductance (L0) of roots of Arabidopsis thaliana were carried out and the results were compared with the expression of aquaporins present in the plasma membrane of A. thaliana. L0 of plants treated with different NaCl concentrations was progressively reduced as NaCl concentration was increased compared to control plants. Also, L0 of plants treated with 60 mmol/L NaCl for different lengths of time was measured. Variations during the light period were seen, but only for the controls. A good correlation between mRNA expression and L0 was observed in both experiments. Control plants and plants treated with 60 mmol/L NaCl were incubated with Hg and then with DTT. For these plants, L0 and cell-to-cell pathway contributions to root water transport were determined. These results revealed that in control plants most water movement occurs via the cell-to-cell pathway, thus implying aquaporin involvement. But, in NaCl-stressed plants, the Hg-sensitive cell-to-cell pathway could be inhibited already by the effect of NaCl on water channels. Therefore, short periods of NaCl application to Arabidopsis plants are characterised by decreases in the L0 of roots, and are related to down-regulation of the expression of the PIP aquaporins. This finding indicates that the well known effect of salinity on L0 could involve regulation of aquaporin expression.

Arbuscular mycorrhizal symbiosis increases relative apoplastic water flow in roots of the host plant under both well-watered and drought stress conditions.

BACKGROUND AND AIMS The movement of water through mycorrhizal fungal tissues and between the fungus and roots is little understood. It has been demonstrated that arbuscular mycorrhizal (AM) symbiosis regulates root hydraulic properties, including root hydraulic conductivity. However, it is not clear whether this effect is due to a regulation of root aquaporins (cell-to-cell pathway) or to enhanced apoplastic water flow. Here we measured the relative contributions of the apoplastic versus the cell-to-cell pathway for water movement in roots of AM and non-AM plants. METHODS We used a combination of two experiments using the apoplastic tracer dye light green SF yellowish and sodium azide as an inhibitor of aquaporin activity. Plant water and physiological status, root hydraulic conductivity and apoplastic water flow were measured. KEY RESULTS Roots of AM plants enhanced significantly relative apoplastic water flow as compared with non-AM plants and this increase was evident under both well-watered and drought stress conditions. The presence of the AM fungus in the roots of the host plants was able to modulate the switching between apoplastic and cell-to-cell water transport pathways. CONCLUSIONS The ability of AM plants to switch between water transport pathways could allow a higher flexibility in the response of these plants to water shortage according to the demand from the shoot.

The Physiological Importance of Glucosinolates on Plant Response to Abiotic Stress in Brassica

Glucosinolates, a class of secondary metabolites, mainly found in Brassicaceae, are affected by the changing environment. This review is focusing on the physiological significance of glucosinolates and their hydrolysis products in the plant response to different abiotic stresses. Special attention is paid to the crosstalk between some of the physiological processes involved in stress response and glucosinolate metabolism, with the resulting connection between both pathways in which signaling mechanisms glucosinolate may act as signals themselves. The function of glucosinolates, further than in defense switching, is discussed in terms of alleviating pathogen attack under abiotic stress. The fact that the exogenous addition of glucosinolate hydrolysis products may alleviate certain stress conditions through its effect on specific proteins is described in light of the recent reports, but the molecular mechanisms involved in this response merit further research. Finally, the transient allocation and re-distribution of glucosinolates as a response to environmental changes is summarized.

Ammonium, bicarbonate and calcium effects on tomato plants grown under saline conditions.

Tomato plants (70 days old) were grown in hydroponic culture into a greenhouse, where supply of inorganic carbon, ammonium and calcium to saline nutrient solution, was investigated in order to reduce the negative effect of salinity. After 70 days, an ameliorating effect upon the decrease in growth observed under salinity was only observed with the treatments NaCl+Ca(2+) and NaCl+HCO(3)(-)+NH(4)(+)+Ca(2+). A large reduction of hydraulic conductance (L(0)) and stomatal conductance (G(s)) was observed with all treatments, compared with the control. However, the reductions were less when NaCl and Ca(2+) were added together. Organic acids (mainly malic acid) in the xylem were decreased with all treatments except with NaCl+NH(4)(+) and with all single treatments added together (NaCl+HCO(3)(-)+NH(4)(+)+Ca(2+)). Amino acid concentrations in the xylem (mainly asparagine and glutamine) decreased when plants were treated with NaCl and NaCl+Ca(2+), but there was a large increase in the plants treated with NaCl+NH(4)(+) or with all treatments together. As HCO(3)(-) is an important source of carbon for NH(4)(+) assimilation, the increase in the concentration of amino acids and organic acids caused by the treatments that contained NH(4)(+), support the idea that fixation of dissolved inorganic carbon was occurring and that the products were transported via the xylem to the shoot. The ameliorating effect of Ca(2+) on root hydraulic conductivity plus the increase of NH(4)(+) incorporation into the amino acid synthesis pathway possibly due to dissolved inorganic carbon fixation, could reduce the negative effect of salinity on tomato plants.

Fruit quality of grafted tomato plants grown under saline conditions

Summary Tomato seedlings (cvs. ‘Fanny’ and ‘Goldmar’) were grafted onto the tomato rootstock hybrid AR-9704, using the cleft grafting method, and grown under greenhouse conditions. Tomato fruit yield and quality from plants exposed to 0, 30 or 60 mM NaCl, comparing grafted with ungrafted plants, was studied. Fruit yield was determined, and the chemical quality of fruit was analysed by measurements of sugars, lycopene, -carotene, ascorbic acid and mineral composition, and by determination of titratable acidity and soluble solids. Fruit yield was higher in grafted than in ungrafted plants for both varieties, for 0, 30 and 60 mM NaCl. Salinity increased soluble solids, glucose and fructose, mainly at 60 mM NaCl, while ascorbic acid increased significantly only for grafted plants at 0 mM NaCl. The concentrations of -carotene and lycopene were not affected by salinity, but a large increase due to grafting was observed for all treatments. Sodium, chloride and nitrate ion concentrations were higher in ungrafted than in grafted plants, for the 60 mM NaCl treatment. Thus, grafting could be a useful tool to increase tomato fruit quality.

Growing Hardier Crops for Better Health: Salinity Tolerance and the Nutritional Value of Broccoli

To evaluate the variations in the nutritional components of a broccoli cultivar under saline stress, two different NaCl concentrations (40 and 80 mM) were assayed. Glucosinolates, phenolic compounds, and ascorbic and dehydroascorbic acids (vitamin C) were analyzed by HPLC, and mineral composition was determined by ICP spectrophotometry. Qualitative differences were observed for several bioactive compounds depending on the plant organ and the intensity of the salt stress. Glucosinolate content showed the most significant increase in the florets; phenolic compounds also increased in the florets, whereas no variation in the vitamin C content was observed as a result of the saline treatments. The mineral composition of the edible parts of the inflorescences remained within the range of the recommended values for human consumption. Overall, the nutritional quality of the edible florets of broccoli was improved under moderate saline stress.

SALINITY AND AMMONIUM/NITRATE INTERACTIONS ON TOMATO PLANT DEVELOPMENT, NUTRITION, AND METABOLITES

The interactions between NaCl and different NO3 −NH4 + ratios were investigated. Tomato plants (Lycopersicon esculentum Mill.) were grown in a greenhouse, in 120L capacity containers filled with continuously aerated Hoagland nutrient solution. Treatments were added to observe the combined effect of two NaCl levels (30 and 60 mM) and three millimolar ratios of nitrate: ammonium (14:0, 12:2, 10:4) on growth, nutrition, and contents of chlorophyll and sugars. Saline treatments decreased growth, which was partly restored by NH4 + treatment. The leaf mineral composition showed a marked effect of nitrogen (N) source, while salinity only affected NO3 − concentration. Changing the NO3 −:NH4 + ratio from 14:0 to 12:2 and 10:4 produced progressive increases in the concentrations of iron (Fe), chlorophyll, and reducing sugars in leaves. Therefore, the deleterious effect of salinity on biomass production can be minimized by the use of nutrient solutions containing higher NH4 + concentrations, since this seemed to be correlated with increases in nitrogen assimilation and the levels of Fe and chlorophyll.