Chl Breakdown Research Articles

Enzymatic Degradation of Chlorophyll in <italic>Chenopodium album</italic>

We describe here the breakdown of Chl in crude extracts of Chenopodium album in complete darkness. The results show that the degradation of Chl was the result of enzymatic reactions and suggest the possible existence of a Mg-releasing enzyme, «Mg-dechelatase». The possibility of the inhibitory accumulation of pheophorbide species is discussed

Senescence and Stomatal Aperture as Affected by Antibiotics in Darkness and Light

In leaves of barley (Hordeum vulgare), as previously found with oats (Avena sativa), a group of six antibiotics that interfere in different ways with the sequence DNA --> mRNA --> protein all delay senescence in the dark, acting to conserve chlorophyll (Chl) and protein and also to open the stomata. Among the active compounds is chloramphenicol, which had previously been reported to act only on procaryotes. It is now shown that all these compounds with senescence-delaying action in darkness have the opposite effect in light, accelerating Chl destruction and partially or completely closing the stomata. Leaves of the dicot Tropaeolum majus show most of the same responses, though the changes in protein and amino acids are more variable. The data as a whole support the previous conclusion that the synthesis of one or more proteins controls both the opening and the closing of the stomata. An additional compound, kanamycin, acts in the same way as the other six compounds on oats and barley, though its action on proteolysis is unclear. On Tropaeolum, however, it opens the stomata in both light and darkness. Anisomycin and ethidium bromide have comparably atypical effects. Thus, although changes in stomatal opening or closing in the majority of cases are closely linked to the breakdown or preservation of Chl, the occasional exception shows that the biochemical phenomena of senescence cannot be under the direct control of changes in stomatal aperture.

Effects of Exogenous 1,3-Diaminopropane and Spermidine on Senescence of Oat Leaves

Excision and dark incubation of oat (Avena sativa L., var. Victory) leaves cause a sharp increase in protease activity, which precedes Chl loss. Both these senescence processes are inhibited by exogenously applied 1,3-diaminopropane (Dap), which occurs naturally in leaf segments. The inhibition of protease activity is much greater in vivo than in vitro, suggesting inhibition of protease synthesis as well as protease action by Dap. Chl breakdown in leaves of radish and broccoli, which also senesce rapidly in the dark, is only slightly inhibited by DaP. These differences between cereal and dicotyledonous plants are correlated with the natural occurrence of Dap in cereals. In the light, Dap promotes, rather than retards, the loss of Chl in oat leaves. This resembles previously described effects of other polyamines. Addition of Mg(2+) to the medium does not antagonize this effect. In the dark, the accumulated Dap also inhibits ethylene production and decreases titer of other polyamines. Addition of Ca(2+) to the incubation medium containing Dap competitively reduces the effects of Dap. Thus, Dap, like other polyamines, seems to require an initial attachment to a membrane site shared with Ca(2+) before exerting its antisenescence action.

Investigation of the Mechanism of Action of a Chlorosis-Inducing Toxin Produced by Pseudomonas phaseolicola

A toxin that induced chlorotic haloes (typifying haloblight disease) on primary leaves of Phaseolus vulgaris L. (var. Canadian Wonder) was partially purified from culture filtrates of the causative agent Pseudomonas phaseolicola (Burkh.) Dowson. This material was used to investigate chlorosis induction. Haloes could only be induced in those bean leaves that were expanding and synthesizing chlorophyll (Chl); the toxin, therefore, does not promote Chl breakdown. Chl, carotene, and xanthophyll synthesis were inhibited in sections of greening barley (Hordeum vulgare L.) leaves, irrespective of the irradiance level. In parallel experiments, the toxin decreased the level of 5-aminolevulinic acid by amounts sufficient to account for toxin-inhibition of Chl synthesis. Electron microscopy revealed no difference between the transformation of etioplasts into chloroplasts in toxin-treated and control tissue, despite a 60% reduction in Chl in the former. The incorporation of [(14)C]acetate into lipid by greening barley leaf sections and by isolated Pisum sativum chloroplasts in the light and the dark was inhibited about 60% by the toxin. The distribution of radioactivity among the spectra of acyl residues was the same in the control and toxin-treated material. It is suggested that the toxin interferes with an early process common to the synthesis of different lipids, including Chl.