Guillermo E. Santa-María

The HAK1 gene of barley is a member of a large gene family and encodes a high-affinity potassium transporter.

The high-affinity K+ uptake system of plants plays a crucial role in nutrition and has been the subject of extensive kinetic studies. However, major components of this system remain to be identified. We isolated a cDNA from barley roots, HvHAK1, whose translated sequence shows homology to the Escherichia coli Kup and Schwanniomyces occidentalis HAK1 K+ transporters. HvHAK1 conferred high-affinity K+ uptake to a K(+)-uptake-deficient yeast mutant exhibiting the hallmark characteristics of the high-affinity K+ uptake described for barley roots. HvHAK1 also mediated low-affinity Na+ uptake. Another cDNA (HvHAK2) encoding a polypeptide 42% identical to HvHAK1 was also isolated. Analysis of several genomes of Triticeae indicates that HvHAK1 belongs to a multigene family. Translated sequences from bacterial DNAs and Arabidopsis, rice, and possibly human cDNAs show homology to the Kup-HAK1-HvHAK1 family of K+ transporters.

Thylakoid-Bound Ascorbate Peroxidase Mutant Exhibits Impaired Electron Transport and Photosynthetic Activity

In chloroplasts, stromal and thylakoid-bound ascorbate peroxidases (tAPX) play a major role in the removal of H2O2 produced during photosynthesis. Here, we report that hexaploid wheat (Triticum aestivum) expresses three homeologous tAPX genes (TaAPX-6A, TaAPX-6B, and TaAPX-6D) mapping on group-6 chromosomes. The tAPX activity of a mutant line lacking TaAPX-6B was 40% lower than that of the wild type. When grown at high-light intensity photosystem II electron transfer, photosynthetic activity and biomass accumulation were significantly reduced in this mutant, suggesting that tAPX activity is essential for photosynthesis. Despite the reduced tAPX activity, mutant plants did not exhibit oxidative damage probably due to the reduced photochemical activity. This might be the result of a compensating mechanism to prevent oxidative damage having as a consequence a decrease in growth of the tAPX mutant plants.

Nitric oxide as a key component in hormone-regulated processes.

Nitric oxide (NO) is a small gaseous molecule, with a free radical nature that allows it to participate in a wide spectrum of biologically important reactions. NO is an endogenous product in plants, where different biosynthetic pathways have been proposed. First known in animals as a signaling molecule in cardiovascular and nervous systems, it has turned up to be an essential component for a wide variety of hormone-regulated processes in plants. Adaptation of plants to a changing environment involves a panoply of processes, which include the control of CO2 fixation and water loss through stomatal closure, rearrangements of root architecture as well as growth restriction. The regulation of these processes requires the concerted action of several phytohormones, as well as the participation of the ubiquitous molecule NO. This review analyzes the role of NO in relation to the signaling pathways involved in stomatal movement, plant growth and senescence, in the frame of its interaction with abscisic acid, auxins, gibberellins, and ethylene.

The Ionic Environment Controls the Contribution of the Barley HvHAK1 Transporter to Potassium Acquisition

The control of potassium (K+) acquisition is a critical requirement for plant growth. Although HAK1 (high affinity K+ 1) transporters provide a pathway for K+ acquisition, the effect exerted by the ionic environment on their contribution to K+ capture remains essentially unknown. Here, the influence of the ionic environment on the accumulation of transcripts coding for the barley (Hordeum vulgare) HvHAK1 transporter as well as on HvHAK1-mediated K+ capture has been examined. In situ mRNA hybridization studies show that HvHAK1 expression occurs in most root cells, being augmented at the outermost cell layers. Accumulation of HvHAK1 transcripts is enhanced by K+ deprivation and transiently by exposure to high salt concentrations. In addition, studies on the accumulation of transcripts coding for HvHAK1 and its close homolog HvHAK1b revealed the presence of two K+-responsive pathways, one repressed and the other insensitive to ammonium. Experiments with Arabidopsis (Arabidopsis thaliana) HvHAK1-expressing transgenic plants showed that K+ deprivation enhances the capture of K+ mediated by HvHAK1. A detailed study with HvHAK1-expressing Saccharomyces cerevisiae cells also revealed an increase of K+ uptake after K+ starvation. This increase did not occur in cells grown at high Na+ concentrations but took place for cells grown in the presence of NH4+. 3,3′-Dihexyloxacarbocyanine iodide accumulation measurements indicate that the increased capture of K+ in HvHAK1-expressing yeast cells cannot be explained only by changes in the membrane potential. It is shown that the yeast protein phosphatase PPZ1 as well as the halotolerance HAL4/HAL5 kinases negatively regulate the HvHAK1-mediated K+ transport.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant’s ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant’s priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Point mutations in the barley HvHAK1 potassium transporter lead to improved K+-nutrition and enhanced resistance to salt stress.

Members of group I KT‐HAK‐KUP transporters play an important role in K+ acquisition by plant roots, a process that is strongly affected by salt stress. A PCR‐based random mutagenesis approach on HvHAK1 allowed identification of V366I and R591C substitutions, which confer enhanced K+‐capture, and improved NaCl, LiCl and NH4Cl tolerance, to yeast cells. Improved K+‐capture was linked to an enhanced V max. Results reveal an intrinsic protective effect of K+, and assign an important role to the 8th transmembrane domain, as well as the C‐terminus, in determining the maximum capacity for the transport of K+ in KT‐HAK‐KUP transporters.

Near-isogenic wheat lines carrying altered function alleles of the Rht-1 genes exhibit differential responses to potassium deprivation.

Most of the elements involved in the integration of signals of low external K(+)-supply into a physiological response pathway remain essentially unknown. The aim of this work was to study the influence exerted by DELLA proteins, which are known to be key components for the control of growth, on plant responses during K(+) deprivation in wheat (Triticum aestivum) by using two sets of near-isogenic lines (NILs) in the Maringa and April Bearded cultivars. After K(+) shortage, the NILs of both cultivars containing the Rht-B1b,Rht-D1b alleles, which encode altered function DELLA proteins, displayed either a slight or no decrease in chlorophyll content, in contrast to the sharp decrease observed in the NILs having the wild type alleles (Rht-B1a,Rht-D1a). That difference was accompanied by a lower relative decrease of biomass accumulation only in the Maringa cultivar. In both cultivars, high chlorophyll retention was coupled with K(+) starvation-induced differences in superoxide dismutase and ascorbate peroxidase activities, which were enhanced in K(+)-starved Rht-B1b,Rht-D1b NILs. In addition, Rht-B1b,Rht-D1b and Rht-B1a,Rht-D1a NILs markedly differed in the accumulation of the major cations Ca(2+), Na(+) and K(+). These results suggest a major role of the Rht-1 genes in the control of physiological responses during K(+) deprivation.

The regulation of zinc uptake in wheat plants

Abstract The control of net zinc-uptake rate can be a critical factor for plant survival and growth in heavy metal polluted environments. Here we report the results of an study on the regulation of net Zn-uptake in wheat plants (Triticum aestivum L.) grown in solution culture at supra optimum levels of Zn supply. As external Zn-concentration was increased, net Zn-uptake rate increased. The higher the external Zn-concentration the higher both Zn-influx and efflux, with Zn-efflux increasing more. However the relative increase in the outward flux of Zn was not enough to prevent the potential increase of Zn-concentration within plant tissues up to toxic levels. On the other hand, when the external Zn-concentration was changed to a higher or a lower level, the unidirectional Zn-influx changed almost instantaneously, while the net uptake rate of Zn changed slowly, towards the level exhibited by plants kept in the previous solution. These findings are consistent with the concept that regulation of Zn fluxes, at supra optimum levels of Zn supply, appear to be controlled primarily by Zn-efflux and not by short or long term regulation of Zn-influx.

An exogenous source of nitric oxide modulates zinc nutritional status in wheat plants.

The effect of addition of the nitric oxide donor S-nitrosoglutathione (GSNO) on the Zn nutritional status was evaluated in hydroponically-cultured wheat plants (Triticum aestivum cv. Chinese Spring). Addition of GSNO in Zn-deprived plants did not modify biomass accumulation but accelerated leaf senescence in a mode concomitant with accelerated decrease of Zn allocation to shoots. In well-supplied plants, Zn concentration in both roots and shoots declined due to long term exposure to GSNO. A further evaluation of net Zn uptake rate (ZnNUR) during the recovery of long-term Zn-deprivation unveiled that enhanced Zn-accumulation was partially blocked when GSNO was present in the uptake medium. This effect on uptake was mainly associated with a change of Zn translocation to shoots. Our results suggest a role for GSNO in the modulation of Zn uptake and in root-to-shoot translocation during the transition from deficient to sufficient levels of Zn-supply.

DELLAs Contribute to Set the Growth and Mineral Composition of Arabidopsis thaliana Plants Grown Under Conditions of Potassium Deprivation

DELLAs proteins play a major role in the modulation of plant responses to fluctuations in environmental conditions. In this work, we examined to what extent Arabidopsis thaliana plants lacking DELLAs activity (5xdella mutant) or carrying an altered function allele of one of the DELLAs coding genes (gai-1 mutant) display differential responses, in terms of growth and shoot elemental composition, relative to WT plants when deprived of potassium (K). Studies with plants grown in hydroponic media unveiled that the shoot mineral composition of gai-1 constitutively differs from that of WT and 5xdella plants. Tolerance to K-deprivation, as estimated by the relative decline of biomass accumulation, followed the order gai-1 > WT > 5xdella. In turn, the degree of responsiveness of the shoot composition to the stress condition followed the order 5xdella > WT > gai-1, suggesting a correspondence between the degree of injury and changes in the elemental composition. Internal efficiency of K-utilization was maximized in WT relative to 5xdella plants. Interestingly, the acquisition of K was severely impaired in gai-1 plants well supplied, or deprived of, K. Complementary studies indicated that influx and root-to-shoot transport of Rubidium, a K-analogue, were reduced in those plants. Furthermore, evidence obtained supports the view that the effect of altered DELLAs derives, at least partially, from controlling the accumulation of transcripts coding for the AtHAK5 transporter. These results, together with the observation that K-deprivation promotes the accumulation of a DELLA protein (RGA) fused to GFP in root cells, suggest a pivotal role of DELLAs in key plant responses to K-deprivation.