Shanti S. Sharma

The relationship between metal toxicity and cellular redox imbalance

The relationship between cellular redox imbalances leading to oxidative stress and metal toxicity in plants has been studied intensely over the past decades. This interdependency was often considered to reflect a rather indirect metal effect of cellular disregulation and progressive secondary damage development. By contrast, recent experiments revealed a clear relationship between metal stress and redox homeostasis and antioxidant capacity. Analysis of plants expressing targeted modifications of components of the antioxidant system, the comparison of closely related plant species with different degrees of toxic metal sensitivity and effector studies with, for instance, salicylic acid have established a link between the degree of plant tolerance to metals and the level of antioxidants.

Proline accumulation in wheat seedlings exposed to zinc and copper

Abstract The changes in proline level of wheat seedlings in response to Zn and Cu (10 ppm) treatment have been monitored. The proline content in shoots increased progressively with an increase in the length of time of exposure to metals (time course) as well as in a concentration-dependent manner (dose-response). Roots exhibited a fluctuating pattern of proline accumulation. Detached leaves also showed a rise in proline level, though of a lower magnitude than in intact seedlings, in response to applied metals. Copper proved to be a stronger inducer of proline accumulation than Zn and it was more toxic than Zn in terms of chlorophyll inhibition. The possibility of proline involvement in metal tolerance is suggested.

Vacuolar compartmentalization as indispensable component of heavy metal detoxification in plants

Plant cells orchestrate an array of molecular mechanisms for maintaining plasmatic concentrations of essential heavy metal (HM) ions, for example, iron, zinc and copper, within the optimal functional range. In parallel, concentrations of non-essential HMs and metalloids, for example, cadmium, mercury and arsenic, should be kept below their toxicity threshold levels. Vacuolar compartmentalization is central to HM homeostasis. It depends on two vacuolar pumps (V-ATPase and V-PPase) and a set of tonoplast transporters, which are directly driven by proton motive force, and primary ATP-dependent pumps. While HM non-hyperaccumulator plants largely sequester toxic HMs in root vacuoles, HM hyperaccumulators usually sequester them in leaf cell vacuoles following efficient long-distance translocation. The distinct strategies evolved as a consequence of organ-specific differences particularly in vacuolar transporters and in addition to distinct features in long-distance transport. Recent molecular and functional characterization of tonoplast HM transporters has advanced our understanding of their contribution to HM homeostasis, tolerance and hyperaccumulation. Another important part of the dynamic vacuolar sequestration syndrome involves enhanced vacuolation. It involves vesicular trafficking in HM detoxification. The present review provides an updated account of molecular aspects that contribute to the vacuolar compartmentalization of HMs.

Subcellular distribution of the V-ATPase complex in plant cells, and in vivo localisation of the 100 kDa subunit VHA-a within the complex

BackgroundVacuolar H+-ATPases are large protein complexes of more than 700 kDa that acidify endomembrane compartments and are part of the secretory system of eukaryotic cells. They are built from 14 different (VHA)-subunits. The paper addresses the question of sub-cellular localisation and subunit composition of plant V-ATPase in vivo and in vitro mainly by using colocalization and fluorescence resonance energy transfer techniques (FRET). Focus is placed on the examination and function of the 95 kDa membrane spanning subunit VHA-a. Showing similarities to the already described Vph1 and Stv1 vacuolar ATPase subunits from yeast, VHA-a revealed a bipartite structure with (i) a less conserved cytoplasmically orientated N-terminus and (ii) a membrane-spanning C-terminus with a higher extent of conservation including all amino acids shown to be essential for proton translocation in the yeast. On the basis of sequence data VHA-a appears to be an essential structural and functional element of V-ATPase, although previously a sole function in assembly has been proposed.ResultsTo elucidate the presence and function of VHA-a in the plant complex, three approaches were undertaken: (i) co-immunoprecipitation with antibodies directed to epitopes in the N- and C-terminal part of VHA-a, respectively, (ii) immunocytochemistry approach including co-localisation studies with known plant endomembrane markers, and (iii) in vivo-FRET between subunits fused to variants of green fluorescence protein (CFP, YFP) in transfected cells.ConclusionsAll three sets of results show that V-ATPase contains VHA-a protein that interacts in a specific manner with other subunits. The genomes of plants encode three genes of the 95 kDa subunit (VHA-a) of the vacuolar type H+-ATPase. Immuno-localisation of VHA-a shows that the recognized subunit is exclusively located on the endoplasmic reticulum. This result is in agreement with the hypothesis that the different isoforms of VHA-a may localize on distinct endomembrane compartments, as it was shown for its yeast counterpart Vph1.

Responses of wild type and abscisic acid mutants ofArabidopsis thaliana to cadmium

Summary Heavy metal-induced inhibitory effects are reported to be concomitant with an increase in endogenous abscisic acid (ABA) levels in plant tissues indicating the possibility of this phytohormone mediating a part of the metal-imposed phytotoxicity. We examined this possibility by comparing the seed germination and seedling growth responses of ABA-deficient (aba-1, aba-3 and aba-4) and ABA-insensitive (abi2-1 and abi3-1) mutants of Arabidopsis thaliana to Cd with those of the wild type (Landsberg erecta, Ler). Assuming that Cd imposed a part of its toxic influence via affecting a rise in endogenous ABA level, all ABA mutants studied could be predicted to exhibit reduced responsiveness to Cd exposure in comparison to the wild type. However, the data obtained both in germination and growth assays were not consistent with this prediction. In germination assays, all ABA mutants proved consistently more sensitive than the wild type to Cd. In case of growth (root length and seedling fresh weight), the magnitude of Cd-induced inhibition in ABA mutants (aba-1, abi2-1 and abi3-1) was generally comparable to that in the wild type. Based on these observations a direct mediatory role of ABA in Cd-imposed phytotoxic effects on early growth could be excluded. The possible significance of heavy metal-dependent increase in endogenous ABA levels in plant tissues is discussed.

Influence of Abscisic Acid and Trans Cinnamic Acid on Senescence of Detached Tropaeolum majus Leaves in Relation to Stomatal Movements

Summary The influence of abscisic acid (ABA) and t cinnamic acid, individually and in combination, on senescence of excised nasturtium ( Tropaeolum majus ) leaves as related to stomatal movements has been studied. ABA (10 ∔5 M) induced an acceleration of chlorophyll loss in dark that was accompanied by an increase in stomatal diffusive resistance (SDR) of leaves. Trans-cinnamic acid (tCA) prevented the ABA-induced loss of chlorophyll and there was a proportionate decrease in SDR. When applied alone tCA, in general, delayed the chlorophyll loss SDR being slightly lower at 10 ∔5 M than in control. tCA treatment resulted in a massive release of potassium ions from senescing leaves. Significance of the findings is discussed.

Characterization of seed germination, seedling growth, and associated metabolic responses of Brassica juncea L. cultivars to elevated nickel concentrations

Seed germination and seedling growth responses of three Brassica juncea L. cultivars, namely Varuna, Kranti, and Pusa Jai Kisan, to nickel have been characterized. Ni suppressed the seed germination differentially, suppression being greater in cv. Kranti than in others. On the basis of seedling growth performance, cv. Varuna proved most tolerant to Ni. The Ni accumulation in seedlings differed strongly among the three cultivars. The lowest and highest Ni contents were observed in the seedlings of cvs. Kranti and Pusa Jai Kisan, respectively. Despite substantial Ni accumulation, cv. Varuna was most tolerant to Ni. Ni accumulation in seedlings was accompanied by differentially altered Fe and K contents and increased levels of non-protein thiols and free proline. The O2ˉ and H2O2 contents and their respective scavenging enzymes in the seedlings responded differentially to the Ni treatment suggesting the involvement of redox imbalance in the development of Ni toxicity. Interestingly, the greater Ni tolerance of cv. Varuna coincided with the elevated constitutive activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX). The data have implications for seedling establishment under Ni-rich conditions and in turn for phytoremediation.

Tomatine, its effect and interaction with abscisic acid on stomatal opening in Commelina communis

Abstract Tomatine, a glycoalkaloid, induces stomatal closure in epidermal peels of Commelina communis . It is more potent than abscisic acid (ABA) and reverses ABA-induced stomatal closure.

Storage-Dependent Changes in Dormancy and Germination of Himalayan Mayapple (Podophyllum hexandrum Royle) Seeds and their Response to Gibberellic Acid

The storage-dependent changes in viability and dormancy/germination status of seeds of a Western population of Himalayan Mayapple (Podophyllum hexandrum), an endangered plant species of high medicinal value, have been monitored with particular reference to the changes in seed responsiveness to GA3. Shortly after harvest (3 months), seeds exhibited dormancy that was marginally overcome after storage for 30 months. GA3 treatment was effective in overcoming the dormancy, particularly when concentrated H2SO4 was used. The responsiveness of the seeds to GA3, however, decreased with the storage period. A subset of seeds that failed to germinate under a GA3-H2SO4-GA3 treatment eventually deteriorated. GA3-induced enhancement of the activities of α-amylase and dehydrogenases (2,3,5 triphenyltetrazolium chloride (TTC) reduction) corresponded to germination improvement in seeds stored for a short duration, but not in seeds stored for long period of time. Lipid peroxidation in seeds did not change owing to storage or GA3 treatment, whereas catalase activity tended to decline marginally. Seed phenolic contents were not involved in the seed germination behavior. Possible mechanisms of heterogeneity in responsiveness of seeds to GA3 are discussed.

Interference between arsenic-induced toxicity and hypoxia: Interference between hypoxia and arsenic toxicity

Plants often face combinatorial stresses in their natural environment. Here, arsenic (As) toxicity was combined with hypoxia (Hpx) in the roots of Arabidopsis thaliana as it often occurs in nature. Arsenic inhibited growth of both roots and leaves, whereas root growth almost entirely ceased in Hpx. Growth efficiently resumed, and Hpx marker transcripts decreased upon reaeration. Compromised recovery from HpxAs treatment following reaeration indicated some persistent effects of combined stresses despite lower As accumulation. Root glutathione redox potential turned more oxidized in Hpx and most strongly in HpxAs. The more oxidizing root cell redox potential and the lowered glutathione amounts may be conducive to the growth arrest of plants exposed to HpxAs. The stresses elicited changes in elemental and transcriptomic composition. Thus, calcium, magnesium, and phosphorous amounts decreased in rosettes, but the strongest decline was seen for potassium. The reorganized potassium-related transcriptome supports the conclusion that disturbed potassium homeostasis contributes to the growth phenotype. In a converse manner, photosynthesis-related parameters were hardly affected, whereas accumulated carbohydrates under all stresses and anthocyanins under Hpx exclude carbohydrate limitation. The study demonstrates the existence of both synergistic since mutually aggravating effects and antagonistic effects of single and combined stresses.