Essentiality of mitochondrial oxidative metabolism for photosynthesis: optimization of carbon assimilation and protection against photoinhibition.

The review emphasizes the essentiality of mitochondrial oxidative metabolism for photosynthetic carbon assimilation. Photosynthetic activity in chloroplasts and oxidative metabolism in mitochondria interact with each other and stimulate their activities. During light, the partially modified TCA cycle supplies oxoglutarate to cytosol and chloroplasts. The marked stimulation of O2 uptake after few minutes of photosynthetic activity, termed as light enhanced dark respiration (LEDR), is now a well-known phenomenon. Both the cytochrome and alternative pathways of mitochondrial electron transport are important in such interactions. The function of chloroplast is optimized by the complementary nature of mitochondrial metabolism in multiple ways: facilitation of export of excess reduced equivalents from chloroplasts, shortening of photosynthetic induction, maintenance of photorespiratory activity, and supply of ATP for sucrose biosynthesis as well as other cytosolic needs. Further, the mitochondrial oxidative electron transport and phosphorylation also protects chloroplasts against photoinhibition. Besides mitochondrial respiration, reducing equivalents (and ATP) are used for other metabolic phenomena, such as sulfur or nitrogen metabolism and photorespiration. These reactions often involve peroxisomes and cytosol. The beneficial interaction between chloroplasts and mitochondria therefore extends invariably to also peroxisomes and cytosol. While the interorganelle exchange of metabolites is the known basis of such interaction, further experiments are warranted to identify other biochemical signals between them. The uses of techniques such as on-line mass spectrometric measurement, novel mutants/transgenics, and variability in metabolism by growth conditions hold a high promise to help the plant biologist to understand this

Importance of the cytochrome pathway of mitochondrial electron transport over the alternative pathway during the Kok effect in leaf discs of pea (Pisum sativum).

The Kok effect refers to the progressive light-induced inhibition of dark respiration at low light intensities, which saturates around the light compensation point. This appears as a sudden break around the light compensation point in the plot of photosynthesis versus light intensity. The magnitude of the break can be considered as a measure of the Kok effect. In the present work, the importance of different components of dark respiration during the Kok effect was investigated by using low concentrations of mitochondrial inhibitors in leaf discs of pea (Pisum sativum L. cv. Azad P1). The effects of glucose (stimulates respiration) and 0.8 M sorbitol (imposes osmotic stress and inhibits photosynthesis) were also studied for comparison. The magnitude of the break decreased significantly in the presence of antimycin A or oligomycin (inhibitors of cytochrome pathway of mitochondrial electron transport and ATP synthase, respectively). In contrast, there was no significant change with salicylhydroxamic acid (SHAM; an inhibitor of alternative pathway of mitochondrial electron transport). The magnitude of the break increased significantly with glucose, and decreased on exposure to osmotic stress. Our results suggest that the Kok effect (inhibition of dark respiration in light) is modulated by inhibitors of cytochrome pathway and ATP synthesis, but not that of the alternative pathway.

Ocular Lesions in Leprosy.

Out of 100 cases of leprosy studied or ocular complications, 46 were suffering from lepromatous leprosy and -54 from nonlepromatous leprosy; 72 were males and 28 females. The patients were in the age group of 13-70 years. with the mum incidence (45%) in fifth -decade of life. The mean duration of leprosy was 11.8 years, with a range between 4 and 36 years. Single or multiple ocular lesions were found in 84% of patients. Impaired or absent corneal sensation (70%) was the most frequently seen ocular lesion, in this study, - followed by madarosis of eye brows and eye lids (56%), anterior uveitis (21%) lagopthalmos (19%), ectropion of lower lid (8%) and scleral lesions (4%) etc. Uniocular blindness was observed in 8% of patients. Periodical check up of all leprosy patients for any ocular involvement is essential un order to detect and treat the eye complications in early stages.