Vicente Martínez

Hormonal changes in relation to biomass partitioning and shoot growth impairment in salinized tomato (Solanum lycopersicum L.) plants

Following exposure to salinity, the root/shoot ratio is increased (an important adaptive response) due to the rapid inhibition of shoot growth (which limits plant productivity) while root growth is maintained. Both processes may be regulated by changes in plant hormone concentrations. Tomato plants (Solanum lycopersicum L. cv Moneymaker) were cultivated hydroponically for 3 weeks under high salinity (100 mM NaCl) and five major plant hormones (abscisic acid, ABA; the cytokinins zeatin, Z, and zeatin-riboside, ZR; the auxin indole-3-acetic acid, IAA; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, ACC) were determined weekly in roots, xylem sap, and leaves. Salinity reduced shoot biomass by 50–60% and photosynthetic area by 20–25% both by decreasing leaf expansion and delaying leaf appearance, while root growth was less affected, thus increasing the root/shoot ratio. ABA and ACC concentrations strongly increased in roots, xylem sap, and leaves after 1 d (ABA) and 15 d (ACC) of salinization. By contrast, cytokinins and IAA were differentially affected in roots and shoots. Salinity dramatically decreased the Z+ZR content of the plant, and induced the conversion of ZR into Z, especially in the roots, which accounted for the relative increase of cytokinins in the roots compared to the leaf. IAA concentration was also strongly decreased in the leaves while it accumulated in the roots. Decreased cytokinin content and its transport from the root to the shoot were probably induced by the basipetal transport of auxin from the shoot to the root. The auxin/cytokinin ratio in the leaves and roots may explain both the salinity-induced decrease in shoot vigour (leaf growth and leaf number) and the shift in biomass allocation to the roots, in agreement with changes in the activity of the sink-related enzyme cell wall invertase.

Salinity induced potassium deficiency in maize plants

Summary Maize (Zea mays L.) plants were grown in ½ Hoagland nutrient solutions to study the effect of two K+ levels (0.1 and 1 mmol/L) combined with low (0.1mmol/L) and high NaCl salinity (100mmol/L) on K+ uptake and translocation. Net uptake rates of K+ were determined by disappearance in the medium and by plant content. Salinity significantly decreased shoot growth when the level of K+ in the solution was 0.1 mmol/L, probably due to a salinity-induced K+ deficiency. NaCl reduced K+ net uptake rates and to a greater extent K+ translocation from root to shoot, which resulted in a lower K+ shoot content and a higher K+ root content. The inhibitory effect of salinity on K+ translocation was stronger with low K+ concentration in the nutrient solution. Net uptake of K+ was dependent on K+ concentration in the root medium and on K+ status of the root.

Root K+ Acquisition in Plants: The Arabidopsis thaliana Model

K(+) is an essential macronutrient required by plants to complete their life cycle. It fulfills important functions and it is widely used as a fertilizer to increase crop production. Thus, the identification of the systems involved in K(+) acquisition by plants has always been a research goal as it may eventually produce molecular tools to enhance crop productivity further. This review is focused on the recent findings on the systems involved in K(+) acquisition. From Epsteins pioneering work >40 years ago, K(+) uptake was considered to consist of a high- and a low-affinity component. The subsequent molecular approaches identified genes encoding K(+) transport systems which could be involved in the first step of K(+) uptake at the plant root. Insights into the regulation of these genes and the proteins that they encode have also been gained in recent studies. A demonstration of the role of the two main K(+) uptake systems at the root, AtHKA5 and AKT1, has been possible with the study of Arabidopsis thaliana T-DNA insertion lines that knock out these genes. AtHAK5 was revealed as the only uptake system at external concentrations <10 μM. Between 10 and 200 μM both AtHAK5 and AKT1 contribute to K(+) acquisition. At external concentrations >500 μM, AtHAK5 is not relevant and AKT1s contribution to K(+) uptake becomes more important. At 10 mM K(+), unidentified systems may provide sufficient K(+) uptake for plant growth.

Relative contribution of AtHAK5 and AtAKT1 to K+ uptake in the high-affinity range of concentrations.

The relative contribution of the high-affinity K(+) transporter AtHAK5 and the inward rectifier K(+) channel AtAKT1 to K(+) uptake in the high-affinity range of concentrations was studied in Arabidopsis thaliana ecotype Columbia (Col-0). The results obtained with wild-type lines, with T-DNA insertion in both genes and specific uptake inhibitors, show that AtHAK5 and AtAKT1 mediate the NH4+-sensitive and the Ba(2+)-sensitive components of uptake, respectively, and that they are the two major contributors to uptake in the high-affinity range of Rb(+) concentrations. Using Rb(+) as a K(+) analogue, it was shown that AtHAK5 mediates absorption at lower Rb(+) concentrations than AtAKT1 and depletes external Rb(+) to values around 1 muM. Factors such as the presence of K(+) or NH4+ during plant growth determine the relative contribution of each system. The presence of NH4+ in the growth solution inhibits the induction of AtHAK5 by K(+) starvation. In K(+)-starved plants grown without NH4+, both systems are operative, but when NH4+ is present in the growth solution, AtAKT1 is probably the only system mediating Rb(+) absorption, and the capacity of the roots to deplete Rb(+) is reduced.

The combined effect of salinity and heat reveals a specific physiological, biochemical and molecular response in tomato plants

Many studies have described the response mechanisms of plants to salinity and heat applied individually; however, under field conditions some abiotic stresses often occur simultaneously. Recent studies revealed that the response of plants to a combination of two different stresses is specific and cannot be deduced from the stresses applied individually. Here, we report on the response of tomato plants to a combination of heat and salt stress. Interestingly, and in contrast to the expected negative effect of the stress combination on plant growth, our results show that the combination of heat and salinity provides a significant level of protection to tomato plants from the effects of salinity. We observed a specific response of plants to the stress combination that included accumulation of glycine betaine and trehalose. The accumulation of these compounds under the stress combination was linked to the maintenance of a high K(+) concentration and thus a lower Na(+) /K(+) ratio, with a better performance of the cell water status and photosynthesis as compared with salinity alone. Our findings unravel new and unexpected aspects of the response of plants to stress combination and provide a proposed list of enzymatic targets for improving crop tolerance to the abiotic field environment.

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.

A putative role for the plasma membrane potential in the control of the expression of the gene encoding the tomato high-affinity potassium transporter HAK5.

A chimeric CaHAK1–LeHAK5 transporter with only 15 amino acids of CaHAK1 in the N-terminus mediates high-affinity K+ uptake in yeast cells. Kinetic and expression analyses strongly suggest that LeHAK5 mediates a significant proportion of the high-affinity K+ uptake shown by K+-starved tomato (Solanum lycopersicum) plants. The development of high-affinity K+ uptake, putatively mediated by LeHAK5, was correlated with increased LeHAK5 mRNA levels and a more negative electrical potential difference across the plasma membrane of root epidermal and cortical cells. However, this increase in high-affinity K+ uptake was not correlated with the root K+ content. Thus, (i) growth conditions that result in a hyperpolarized root plasma membrane potential, such as K+ starvation or growth in the presence of NH4+, but which do not decrease the K+ content, lead to increased LeHAK5 expression; (ii) the presence of NaCl in the growth solution, which prevents the hyperpolarization induced by K+ starvation, also prevents LeHAK5 expression. Moreover, once the gene is induced, depolarization of the plasma membrane potential then produces a decrease in the LeHAK5 mRNA. On the basis of these results, we propose that the plant membrane electrical potential plays a role in the regulation of the expression of this gene encoding a high-affinity K+ transporter.

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.

The Arabidopsis thaliana HAK5 K+ Transporter Is Required for Plant Growth and K+ Acquisition from Low K+ Solutions under Saline Conditions

K(+) uptake in the high-affinity range of concentrations and its components have been widely studied. In Arabidposis thaliana, the AtHAK5 transporter and the AtAKT1 channel have been shown to be the main transport proteins involved in this process. Here, we study the role of these two systems under two important stress conditions: low K(+) supply or the presence of salinity. T-DNA insertion lines disrupting AtHAK5 and AtAKT1 are employed for long-term experiments that allow physiological characterization of the mutant lines. We found that AtHAK5 is required for K(+) absorption necessary to sustain plant growth at low K(+) in the absence as well as in the presence of salinity. Salinity greatly reduced AtHAK5 transcript levels and promoted AtAKT1-mediated K(+) efflux, resulting in an important impairment of K(+) nutrition. Although having a limited capacity, AtHAK5 plays a major role for K(+) acquisition from low K(+) concentrations in the presence of salinity.

K+ uptake in plant roots. The systems involved, their regulation and parallels in other organisms ☆

Potassium (K(+)) is an essential macronutrient for plants. It is taken into the plant by the transport systems present in the plasma membranes of root epidermal and cortical cells. The identity of these systems and their regulation is beginning to be understood and the systems of K(+) transport in the model species Arabidopsis thaliana remain far better characterized than in any other plant species. Roots can activate different K(+) uptake systems to adapt to their environment, important to a sessile organism that needs to cope with a highly variable environment. The mechanisms of K(+) acquisition in the model species A. thaliana are the best characterized at the molecular level so far. According to the current model, non-selective channels are probably the main pathways for K(+) uptake at high concentrations (>10mM), while at intermediate concentrations (1mM), the inward rectifying channel AKT1 dominates K(+) uptake. Under lower concentrations of external K(+) (100μM), AKT1 channels, together with the high-affinity K(+) uptake system HAK5 contribute to K(+) acquisition, and at extremely low concentrations (<10μM) the only system capable of taking up K(+) is HAK5. Depending on the species the high-affinity system has been named HAK5 or HAK1, but in all cases it fulfills the same functions. The activation of these systems as a function of the K(+) availability is achieved by different mechanisms that include phosphorylation of AKT1 or induction of HAK5 transcription. Some of the characteristics of the systems for root K(+) uptake are shared by other organisms, whilst others are specific to plants. This indicates that some crucial properties of the ancestral of K(+) transport systems have been conserved through evolution while others have diverged among different kingdoms.