Degradation of chlorophylls during postharvest senescence of broccoli

MARTINEZ GUSTAVO; CIVELLO PEDRO; GOMEZ LOBATO MARÍA EUGENIA

Brassica: Characterization, Functional Genomics and Health Benefits

Nova Scientific Publisher

Año: 2013; p. 45 - 55

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 stem. For consuming, 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 that heads experience disruption in energy, nutrition, and hormone supplies conducting to fast senescence and chlorophyll degradation in the sepals. Catabolism of chlorophylls 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 complex, which must be destabilized as a prerequisite for the subsequent degradation of chlorophylls. It was 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 done to extend broccoli postharvest life, mainly by reducing senescence rate and loss of green color. These treatments include the use of refrigerated storage, controlled and modified atmospheres, heat treatments, UV applications, 1-MCP and ethanol. In this chapter, it is reviewed the research about changes in enzyme activity and expression of genes associated with chlorophyll catabolism during postharvest senescence of broccoli. Furthermore, it is examined the effect of different hormonal and postharvest physical treatments on the expression of the mentioned genes and enzymes.