Improving photosynthetic efficiency of plants - a genetic approach

Seminar

Title: Improving photosynthetic efficiency of plants- a genetic approach

Speaker: Baishnab Tripathy

Affiliation:School of Life Sciences, Jawaharlal Nehru University, New Delhi, India

Host: Nelson Saibo

Abstract:

Chlorophyll (Chl) b is synthesized by oxidation of a methyl group on the B ring of the porphyrin molecule to a formyl group by chlorophyllide a oxygenase (CAO).  The overexpression of Arabidopsis thaliana full length CAO (AtCAO) in tobacco (Nicotiana tabacum) resulted in an increased Chl synthesis and a decreased Chl a/b ratio in low-light-grown (LL) as well as in high-light-grown (HL) tobacco plants, where the effect was more pronounced. In HL-plants, Chl biosynthesis potential and Protochlorophyllide Oxido Reductase (POR) activity increased that compensated for the HL-induced loss of Chl. Increased availability of the substrate chlorophyllide and the enzyme CAO in CAO over-expressing (CAOx) plants resulted in augmented Chl b synthesis and decreased Chl a/b ratio; this, in turn showed an increased abundance of light-harvesting chlorophyll-proteins (LHCPs) and other proteins of electron transport chain that led to efficient capture of solar energy and enhanced (40 - 80%) electron transport rates of Photosystem I and Photosystem II at both limiting and saturating light intensities. However, this was not accompanied by equivalent enhanced whole chain electron transport; where the percent increase was lower (20-50%).  Further, the rate of respiration increased in CAOx-HL plants resulting in increased light compensation point.  Consequently, net CO2 fixation in attached leaves increased partially and quantum yield remained unchanged in CAOx plants.  Although controlled up-regulation of Chl b biosynthesis co-modulates the expression of chloroplastic proteins that increases the antenna size and electron transport rates, it may not lead to significant up-regulation of net photosynthesis.

 Carbon, nitrogen and sulphur metabolisms are highly dependent upon one another. Carbon metabolism is usually mediated by photosynthesis and respiration. Nitrogen metabolism is regulated via nitrate uptake by the root system and its reduction to NH4+. The S and N assimilation is mediated by a prosthetic group siroheme, synthesized from uroporphyrinogen III, an intermediate of biosynthetic pathway of Chl, an essential pigment of carbon assimilation in oxygenic prokaryotic and eukaryotic autotrophs. The metabolism of chlorophyll, nitrogen and sulphur was genetically manipulated by over-expression of a chloroplastic enzyme sirohydrochlorin ferrochelatase (SirB) required for nitrogen and sulphur assimilation. It is demonstrated that N and S assimilation, protein contents and photosynthetic rates increased due to enhanced AtSirB expression. AtSirBx plants were bigger in size and greener in colour as compared to that of WT. The fresh weight and dry weight of AtSirBx were more than that of WT whereas in antisense plants were less than that of WT.  This could be due to increased nitrogen, sulphur and carbon assimilation in sense plants.  The Chl and carotenoids contents of AtSirBx plants were higher than that of WT.  Similar to Chl, the total protein and N contents of mature (3-week-old) AtSirBx plants were higher than that of WT. This was due to increased NR and NiR activities of AtSirBx plants. In a N deficient medium the phenotype of WT plants looked pale-green and in extreme N starvation (0.1N of control), they almost blanched.  This was due to reduced Chl accumulation. Under identical growth conditions the AtSirBx plants looked greener than WT and had higher amounts of Chl than that of WT.  Sulphate is taken up from soil, reduced to S in chloroplasts by sulphite reductase and then assimilated to cysteine. After 7 days of growth in S-deficient medium (0.1S) the phenotype of WT plants were deficient in Chl and looked pale-green whereas the AtSirBx plants accumulated higher amounts of Chl and looked greener than that of WT.