Mukesh Kumar Raval

Senescence-induced loss in photosynthesis enhances cell wall β-glucosidase activity.

A link between senescence-induced decline in photosynthesis and activity of beta-glucosidase is examined in the leaves of Arabidopsis. The enzyme is purified and characterized. The molecular weight of the enzyme is 58 kDa. It shows maximum activity at pH 5.5 and at temperature of 50 degrees C. Photosynthetic measurements and activity of the enzyme are conducted at different developmental stages including senescence of leaves. Senescence causes a significant loss in total chlorophyll, stomatal conductance, rate of evaporation and in the ability of the leaves for carbon dioxide fixation. The process also brings about a decline in oxygen evolution, quantum yield of photosystem II (PS II) and quantum efficiency of PS II photochemistry of thylakoid membrane. The loss in photosynthesis is accompanied by a significant increase in the activity of the cell wall-bound beta-glucosidase that breaks down polysaccharides to soluble sugars. The loss in photosynthesis as a signal for the enhancement in the activity of the enzyme is confirmed from the observation that incubation of excised mature leaves in continuous dark or in light with a photosynthesis inhibitor 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) that leads to sugar starvation enhances the activity of the enzyme. The work suggests that in the background of photosynthetic decline, the polysaccharides bound to cell wall that remains intact even during late phase of senescence may be the last target of senescing leaves for a possible source of sugar for remobilization and completion of the energy-dependent senescence program.

Topology and photoprotective role of carotenoids in photosystem II of chloroplast: a hypothesis.

On the basis of existing evidence, a model is proposed for the topology of carotenoids in photosystem II (PS II) of chloroplasts. suggesting their possible roles in the photoprotection and stability of PS II complex. The presence of one cis and one trans beta-carotene at reaction centre II (RC II), with different photoprotective functions, is suggested. The core antennae (CP43, CP47) are presumed to contain beta-carotene molecules in clusters. The possible molecular mechanism for formation of a quenching complex in the minor LHC II, involving zeaxanthin, chlorophyll a and the glutamic acid side chain of the light harvesting protein, is worked out. This complex is proposed to be an efficient triplet quencher, in addition to its role as a quencher of singlet chlorophyll energy. The migration of triplet energy from RC II to the quenching complex is surmised. It is suggested that the carotenoids in RC II and in different LHCs form an integrated photoprotective unit.

Response of senescing leaves of wheat seedlings to UVA radiation: inhibition of PSII activity in light and darkness

Abstract Ultraviolet A (UVA) induced inhibition in photosystem II (PSII) photochemistry of primary leaves of wheat seedlings senescing in light or darkness has been examined. O 2 evolution and 2,6-dichlorophenol indophenol (DCPIP) photoreduction diminished in the presence of light. Results of thermoluminescence (TL) studies suggest that UVA inhibits the activity of PSII by accelerating the process of senescence-induced modifications of Q B protein conformation and/or S-state transitions.

Leaf Senescence and Transformation of Chloroplasts to Gerontoplasts

Senescence of green leaves brings about several structural and functional alterations in the cells. The process causes modifications in mitochondrial cristae, condensation of the nucleus, shrinkage of chloroplasts and extensive alteration of thylakoid structure. Senescence-induced changes in chloroplasts are extensive. These changes during senescence result in transdifferentiation of a chloroplast into a gerontoplast, a plastid form with unique structural features and physiology. During leaf senescence, the cells lose essential macromolecules including proteins, lipids and nucleic acids. The stroma proteins and lipids of the thylakoid membrane are the major targets for degradation. In addition to macromolecular degradation, the process causes loss in the photosynthetic pigments, namely chlorophylls and carotenoids. The enzymes that participate in chlorophyll degradation and their regulation are now known. However, the mechanism of degradation of carotenoids still remains a mystery.

Response of Photosynthetic Organelles to Abiotic Stress: Modulation by Sulfur Metabolism

Sulfur metabolism mediated modulation of plant response to various abiotic stress factors is the focus of this review. Since chloroplast is extremely sensitive to abiotic stress factors and at the same time a major location of sulfur assimilation, the organelle plays a major role in the modulation of stress response. The photosynthetic organelle coordinates carbon, nitrogen, and sulfur metabolic pathways and provides the essential precursors for synthesis of sulfur compounds. The abiotic stress factors like high light, low light, temperature extremes, drought, and UV radiations which the organelle experiences lead to creation of an oxidative environment and production of reactive oxygen species (ROS). Sulfur metabolites containing thiol residues with reversible oxidation-reduction potential effectively scavenge ROS in a series of biochemical reactions. Abiotic stress factors cause upand downregulations of several stress-related genes. The stress signals, their transmission, and downstream signaling network regulating gene expression are complex. The stress-induced redox signals generated in chloroplast play a major role in different signal transduction systems and expression of stress-responsive genes in green plants.

Damage of photosynthetic apparatus in the senescing basal leaf of Arabidopsis thaliana: a plausible mechanism of inactivation of reaction center II.

Significant decline in oxygen evolution and DCPIP photoreduction and a marginal restoration of the later with DPC as an electron donor suggest the inactivation of reaction center of photosystem II. The declines in the height of thermoluminescence bands support the view and the damage of reaction center II could be central to the senescence process in Arabidopsis leaves. The enhancement in the number of reduced quinones, signifying a loss in redox homeostasis in the electron transport chain between photosystem II and I leads to the creation of an energy imbalance. The view is supported by the decline in actual quantum yield of photosystem II in the light adapted state and maximum quantum yield of primary photochemistry in the dark adapted state of chlorophyll fluorescence. An increase in chlorophyll a fluorescence polarization and decline in carotenoid to chlorophyll energy transfer efficiency suggest the perturbation in thylakoid structure. A plausible mechanism illustrating the senescence mediated inactivation of oxygen evolving complex has been proposed.

The Role of Combined OSR and SDF Method for Pre-Processing of Microarray Data That Accounts for Effective Denoising and Quantification

Microarray data is inherently noisy due to the noise contaminated from various sources during the preparation of microarray slide and thus it greatly affects the accuracy of the gene expression. How to eliminate the effect of the noise constitutes a challenging problem in microarray analysis. Efficient denoising is often a necessary and the first step to be taken before the image data is analyzed to compensate for data corruption and for effective utilization for these data. Hence preprocessing of microarray image is an essential to eliminate the background noise in order to enhance the image quality and effective quantification. Existing denoising techniques based on transformed domain have been utilized for microarray noise reduction with their own limitations. The objective of this paper is to introduce novel preprocessing techniques such as optimized spatial resolution (OSR) and spatial domain filtering (SDF) for reduction of noise from microarray data and reduction of error during quantification process for estimating the microarray spots accurately to determine expression level of genes. Besides combined optimized spatial resolution and spatial filtering is proposed and found improved denoising of microarray data with effective quantification of spots. The proposed method has been validated in microarray images of gene expression profiles of Myeloid Leukemia using Stanford Microarray Database with various quality measures such as signal to noise ratio, peak signal to noise ratio, image fidelity, structural content, absolute average difference and correlation quality. It was observed by quantitative analysis that the proposed technique is more efficient for denoising the microarray image which enables to make it suitable for effective quantification.

Changes in Chlorophyll Fluorescence during Aging of Cell Free Chloroplasts

Summary Aging of cell-free chloroplasts of fern leaves exhibits almost complete loss of 2–6 dichlorophenol indophenol photoreduct ion without any significant loss of pigments, which would suggest a change in thylakoid structure independent of chlorophyll carotenoid loss.The changes in thylakoid organization as probed by Chla fluorescence are relatively slow in dark.When aging is slow (dark treatment), an increase in Chla fluorescence indicates uncoupling of light absorption photochemical reactions.However, a rapid membrane disorganization in light causes a significant quenching of fluorescence emission.An increase in fluorescence polarization is correlated to aging induced conversion of thylakoid membrane to a gel phase.

Proplastid to Chloroplast

The development of chloroplast has normally been examined with either proplastid or etioplast as the precursor.

New encoded single-indicator sequences based on physico-chemical parameters for efficient exon identification

The first step in gene identification problem based on genomic signal processing is to convert character strings into numerical sequences. These numerical sequences are then analysed spectrally or using digital filtering techniques for the period-3 peaks, which are present in exons (coding areas) and absent in introns (non-coding areas). In this paper, we have shown that single-indicator sequences can be generated by encoding schemes based on physico-chemical properties. Two new methods are proposed for generating single-indicator sequences based on hydration energy and dipole moments. The proposed methods produce high peak at exon locations and effectively suppress false exons (intron regions having greater peak than exon regions) resulting in high discriminating factor, sensitivity and specificity.