Systems Biology at Washington University

FIBR


High Light Response in Synechocystis 6803, Arabidopsis and Moss:

Light is a key substrate for the photosynthetic organisms that support life on this planet. Light can become deleterious when energy can not be completely utilized towards photosynthetic process. High light was used to understand how photosynthetic organisms cope with the excessive excitation. Global changes in the mRNA level in Synechocystis 6803, Arabidopsis and Moss were measured using high density DNA microarray.

Key Findings: Synechocystis 6803, Arabidopsis and Moss show similar physiological response. Differential expression of ~20% of genes ( table ) Majority are down-regulated, necessary to maintain internal homeostasis Comprehensive integrated homeostatic response between energy production and consumption. Key genes involved in protection against ROS and maintenance of redox homeostasis Presence of excess amount of key nutrients does not modify the overall response to high light (Figure) Future Goals: Validating the role of key regulatory genes by constructing gene specific mutant strains Profiling changes globally in metabolites and proteins during the high light treatment

Microarray Experiments : Design and Data
KEGG Metabolic Pathways for Differentially Regulated Genes



Physiological Response

By Abhay SIngh and Abha Khandelwal

Time course of changes in Fv/Fm ratio following high light treatment. Synechocystis 6803 cells grown at 30 µE.m-2.s-1 at 30°C were exposed to 300 µE.m-2.s-1 (HL) at 30°C, samples were withdrawn at indicated time period and Fv/Fm was measured in FL100. The Fv/Fm ratio is interpreted as a measure of the maximal quantum efficiency of PS II photochemistry. As can be seen, the Fv/Fm ratio declined rapidly in response to 15 min of HL treatment and remained lower up to the 3 h. By 6 h of HL treatment, the Fv/Fm ratio began to increase and significant increase in the Fv/Fm ratio was observed during further HL treatment.
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Integrated homeostatic response between energy production and energy consumption

By Abhay Singh

Schematic presentation of interaction between energy generation (photosynthesis) and major energy consuming pathways (carbon fixation and nitrate assimilation). Energy and reducing power generated by photosynthetic process is utilized towards fixation of CO₂ that can either be stored in the form of glycogen or utilized for synthesis of 2-oxoglutarate which is a final acceptor used to assimilate nitrogen, a pathway that also requires the energy and reducing powers generated by photosynthetic process. The C and N are eventually assembled in glutamate which is utilized for biosynthesis of pigments, amino acids, nucleic acids etc. Our results show that exposure of Synechocystis 6803 to high light in the presence of ambient CO₂ resulted in decreased energy production by photosynthetic process as evident by Fv/Fm ratio and expression of genes coding for proteins involved in photosynthetic process (Table). This decrease in energy production resulted in decrease CO₂ fixation evident by down-regulation of genes involved in CCM and CO₂ fixation (table) and ultimately cells were limited in carbon availability as evident by up-regulation of genes involved in carbon-transport (table). The limited carbon availability resulted in lesser synthesis of 2-oxoglutarate which resulted in lesser assimilation of nitrogen. This was manifested in the coordinated down-regulation of genes coding for (1) ABC-type transporters involved in import of nitrogen-related compunds; (2) GS/GOGAT pathway and (3) PII and NtcA. In addition, two genes coding for proteins IF7 and IF17 were up-regulated. These proteins inactivate GS activity by directly associating with it. The overall effect of lesser assimilation of carbon and nitrogen in the chain of glutamate affected widespread effect such as synthesis of pigments, amino acids, nucleic acids etc and gene expression data shows the coordinated response on regulation of genes coding for proteins involved in these pathways.
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Protection against ROS and maintenance of redox homeostasis

By Abhay Singh

A schematic diagram depicting the generation of ROS by photosynthetic complexes and its role as signaling molecule either directly or indirectly. PS I generates superoxide and hydrogen peroxide by reducing oxygen (Mehler reaction) whereas PS II generates singlet oxygen following generation of triplet chlorophyll. Generation of these ROS by PS II and PS I directly correlates with light intensity. Our data shows that cells employ multiple mechanism to minimize the excessive production of ROS and maintain redox homeostasis. This included up-regulation of genes coding for MvrA, flavoprotein containing monooxygenase, various redoxins etc (Table).
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Microarray Experiments : Design and Data

KEGG Metabolic Pathways for Differentially Regulated Genes