ICAR AIEEA PG 2024 - Plant Biotechnology

The Light Compensation Point (LCP) is the irradiance level at which the rate of photosynthetic carbon fixation exactly balances the rate of respiratory carbon release ($P_n = 0$). Shade-adapted plants (sciophytes) and shade-tolerant leafy crops (such as Betelvine, grown in shaded 'bareja' structures, and Lettuce) exhibit very low dark respiration rates and high light-harvesting antenna efficiency, resulting in a distinctly low Light Compensation Point. In contrast, heliophytes and $C_4$ grasses like Fingermillet (Eleusine coracana) have high respiration rates and require much higher light intensities to reach compensation.

Classification of free-living diazotrophs based on oxygen relationship:
Clostridium (e.g., Clostridium pasteurianum): Strictly anaerobic, fermentative, chemoheterotrophic free-living nitrogen fixer.
Rhodospirillum (e.g., Rhodospirillum rubrum): Purple non-sulfur phototrophic bacterium that fixes atmospheric $N_2$ under anaerobic conditions.
Azotobacter (e.g., Azotobacter chroococcum): Strictly aerobic chemoheterotroph with high respiratory protection for nitrogenase.
Gloeotheca: Aerobic unicellular cyanobacterium capable of fixing nitrogen in the light.

Biochemical matching of electron donors:

• Nitrite reductase: Localized in chloroplasts, reduces nitrite to ammonium ($NO_2^- \to NH_4^+$) utilizing 6 electrons supplied directly by reduced ferredoxin ($Fd_{red}$) -> (IV)
• APS sulfotransferase (adenylyl-sulfate reductase): Essential in plant sulfate assimilation, transfers sulfate from APS using reduced glutathione (GSH) as the physiological electron donor -> (III)
• De novo fatty acid synthesis: Operates in plant plastids where repetitive reductive steps catalyzed by $\beta$-ketoacyl-ACP reductase and enoyl-ACP reductase specifically utilize NADPH -> (II)
• Nitrate reductase: Soluble cytosolic enzyme reducing nitrate to nitrite ($NO_3^- \to NO_2^-$) utilizing NADH (or sometimes NADPH) as the physiological electron donor -> (I)

Subcellular localization of plant metabolic enzymes:

• Triacylglycerol synthesis: Assembly of TAG from glycerol-3-phosphate and acyl-CoAs (the Kennedy pathway) is catalyzed by membrane-bound acyltransferases located in the Endoplasmic Reticulum -> (III)
• Glycolate oxidase: Peroxisomal flavoprotein that oxidizes glycolate to glyoxylate in the $C_2$ photorespiratory pathway, generating $H_2O_2$ -> (IV)
• Hexose Monophosphate Shunt (Pentose Phosphate Pathway): Operates parallelly in the soluble stroma of Plastids (chloroplasts/leucoplasts) and the cytosol, providing NADPH for biosynthetic reactions -> (II)
• Serine synthesis in photorespiration: Occurs in Mitochondria where the glycine decarboxylase multienzyme complex and serine hydroxymethyltransferase condense two glycine molecules into one serine molecule -> (I)