Does the different proteomic profile found in apical and basal leaves of spinach reveal a strategy of this plant toward cadmium pollution response?

Chlorosis develops in Spinacia oleracea L. plants exposed to Cd and is 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 defense response, leading to senescence and cell death. There was also an 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. In line, we found that many proteins involved in carbon metabolism were less abundant, whereas 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 sulfur 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.