Studio degli effetti dell’inquinamento da Cadmio sulle piante di Spinacia Oleracea L. mediante analisi proteomica
Author(s)
Fagioni, Marco
Date Issued
February 20, 2009
Type
Doctoral Thesis
Abstract
The changes induced in the photosynthetic apparatus of spinach (Spinacia oleracea L.) leaves upon addition of cadmium to hydroponics solution were characterized. Two proteomic approaches coupled with chlorophyll, xanthophylls and phytochelatins analysis and in vivo measurements of photosynthesis were used. Cadmium only accumulated in basal leaves, that were therefore used for assessment of Cd-induced changes. Cadmium strongly reduced chlorophyll concentration in leaves, especially chlorophyll a. Lutein, neoxanthin and violaxanthin increased during the treatment but no zeaxanthin and anteraxanthin were produced, indicating that the violaxanthin-zeaxanthin de-epoxidation cycle was impaired by Cd. Cadmium reduced significantly the amount of antenna proteins of PSI, while PSII antennae were affected to a minor extent, with exception of the isomeric Lhcb1.1 which decreased significantly already at the onset of the treatment. Cytochrome b6/f and the ATP-synthase complex did not change following the Cd treatment. No new protein was formed and no specific protein disappeared in the photosynthetic apparatus of Cd-treated leaves, while an increasing amount of phytochelatins was recorded over time. Fluorescence analysis revealed that Cd damage to photosystems only influences photosynthesis when light intensity drives high rates of electron transport, which in turn require large rates of RuBP regeneration. Upon removal of Cd, a rapid re-synthesis of both chlorophylls was detected, lutein decreased again and a significant re-synthesis of Lhcb1.1 antenna was observed, especially in the presence of zinc. The hypothesis is put forward that Cd affects aspecifically the photosynthetic apparatus of spinach basal leaves, replacing other metal ions inside proteins. Plants do not react by specific mechanisms but localize Cd into the basal leaves by an over-production of phytochelatins which avoid Cd diffusion to expanding leaves.
Chlorosis develops in Spinacia Oleracea L. plants exposed to Cd, but symptoms differ from those seen during Fe deficiency, as they are prevalently localized in the basal leaves. A proteomic comparison of basal and apical leaves from Cd treated plants showed modified profiles that are different and complementary in the two locations. Total chlorophyll increased in apical leaves as did photosynthetic complexes and enzymes involved in CO2 fixation and carbohydrate metabolism. Thus apical leaves seem to supply the plant’s energy requirements and, consistent with this, remain green after 40 days. In contrast, basal leaves experienced reduced chlorophyll a synthesis and photosynthesis, and later on an over production of ROS, which induces a cell defence response, leading to senescence and cell death. There was also over production of GSH and phytochelatins, whose main role is in chelating Cd. These chelate–polypeptide complexes accumulate in the vacuole, limiting the distribution of Cd to apical leaves. On line we found that many proteins involved in carbon metabolism were less abundant, while proteins involved in remobilizing carbon from other energy sources were up-regulated. We suggest that phytochelatin production has priority in Cd stressed basal leaves and the nitrogen and sulphur metabolic pathways are activated for this purpose. Finally, as dead leaves detach from the plant they carry away the sequestered Cd, thereby removing it completely from the plant and preventing any future access to the apical leaves. These events may represent an active detoxification strategy in higher plants.
Additional information
Dottorato di ricerca in Genetica e biologia cellulare
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