Degradation of chlorophylls during postharvest senescence of broccoli

MARTINEZ, GUSTAVO A ; CIVELLO, PEDRO MARCOS; GOMEZ LOBATO, MARIA EUGENIA

Brassica: Characterization, Functional Genomics and Health Benefits

Nova Scientific Publisher

Año: 2013; p. 93 - 110

Broccoli (Brassica oleracea L. Italica) is a floral vegetable rich in diverse compounds such as vitamins A and C, antioxidants, and anti-carcinogenic compounds. Floral heads of broccoli are composed of hundreds of florets arranged in whorls on top of stems. For consumption, they are harvested in an immature stage when male and female reproductive structures are still surrounded by petals and enclosed by chlorophyll-containing sepals. Harvesting causes heads to experience disruption of energy, nutrition, and hormone supplies, thus causing fast senescence and chlorophyll degradation in sepals. Catabolism of chlorophyll leads to yellowing, which is the main sign of quality deterioration in harvested broccoli. In recent years, a pathway of chlorophyll degradation that is active during senescence has been elucidated. Most of the genes and enzymes of this pathway have been characterized in Arabidopsis thaliana, although many of them have their orthologs in broccoli. In chloroplasts, chlorophyll molecules interact with several proteins forming light-harvesting complexes, which must be destabilized as a prerequisite for the subsequent degradation of chlorophyll. It has been described that a protein named SGR interacts with light-harvesting complex II, enhancing destabilization of these chlorophyll-apoprotein complexes. After that, phytol is hydrolyzed by the action of chlorophyllase or pheophytinase, and Mg2+ is removed by a metal-chelating substance. Then, the porphyrin ring of the pheophorbide is oxygenolytically opened by pheophorbide a oxygenase. The product of this reaction is red chlorophyll catabolite, which is site-specifically reduced by red chlorophyll catabolite reductase to yield the primary fluorescent chlorophyll catabolite, a product that is exported to vacuoles. Several attempts have been made to extend broccoli postharvest life, mainly by reducing the senescence rate and the loss of green color. To that end, refrigerated storage, controlled and modified atmospheres, heat treatments, UV applications, 1-methylcyclopropene and ethanol have been used. In this review, research on changes in enzyme activity and expression of genes, associated with chlorophyll catabolism during postharvest senescence of broccoli is reviewed. Furthermore, the effect of different hormonal and postharvest physical treatments on the expression of the mentioned genes and enzymes is examined.