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New breakthrough in the research of physiological deterioration of cassava storage roots

Cassava (Manihot esculenta Crantz) produces an acceptable yield under the adverse climatic and nutrient-poor soil conditions occurring in some tropical and subtropical regions; therefore, it is recognized as being an important food security crop. Nevertheless, the rapid postharvest physiological deterioration (PPD) of its storage roots, a unique phenomenon compared with other root crops, renders the roots unpalatable and unmarketable within 24 to 72 h of harvest, thereby adversely impacting farmers, processors, and consumers alike. Reactive oxygen species (ROS) increased very early during PPD .Cellular processes, including ROS turnover, programmed cell death, defense pathways, signaling pathways, and cell wall remodeling, have been shown to be active during the deterioration response. Therefore, while there is considerable evidence linking ROS accumulation to PPD, there is a need to confirm the intrinsic relationship between ROS accumulation and PPD occurrence and their regulation.
Prof. ZHANG Peng’s group from Shanghai Chenshan Plant Science Research Center using the fluorescent probe rhodamine-123 in vivo imaging of ROS, in cassava mesophyll protoplasts or DAB staining of leaves showed the improved tolerance of transgenic cassava cells to oxidative stress caused by H2O2 and MV.  MV treatment in the presence of light leads to the generation of superoxide radicals and H2O2 in chloroplasts. The increased SOD and CAT enzyme activities were able to rapidly scavenge ROS at the site of generation as well as prevent the formation of hydroxyl radicals.  
This study convincingly demonstrates that coexpression of the antioxidant enzymes SOD and CAT in transgenic cassava leads to a synergistic effect that not only reduces ROS levels but also delays cassava PPD.  In conclusion, coexpression of MeCu/ZnSOD and MeCAT1 in cassava could dramatically improve ROS-scavenging ability, leading to reduced H2O2 accumulation, improved abiotic stress resistance, and delayed PPD occurrence. It also confirms the current model of oxidative burst as a key player in initiating ROS and that enhanced ROS-scavenging capacity represses PPD occurrence.
The study has been published online in Plant Physiology and it has a great significant to the industrialization of cassava.
This work was supported by the National Natural Science Foundation of China, the 973 Program, the Earmarked Fund for China Agriculture Research System, the Shanghai Municipal Afforestation and City Appearance and Environmental Sanitation Administration, and the Bill and Melinda Gates Foundation (BioCassava Plus program grant to J. R. B. and P.Z.)
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