Overview of post-harvest physiological deterioration (PPD) of cassava and strategies to delay it

The production of cassava, the most important staple root crop in the world, is constrained by the short shelf life of the cassava storage roots that are undergoing post-harvest physiological deterioration (PPD) shortly after harvest. PPD reduces starch quality and renders the roots unpalatable and unmarketable. PPD is a complex process involving enzymatic stress responses to wounding, changes in gene expression and protein synthesis as well as accumulation of secondary metabolites. PPD can be strongly influenced by environmental factors making the identification of genotypes with delayed PPD trait difficult. In the present review, we propose an integrative presentation of PPD phenomenon based on a comprehensive analysis of several key PPD studies. We discuss recent progress in the standardization of methods to assess and score PPD tolerance in cassava roots. Traditional and improved storage techniques to extend cassava shelf-life are presented and prospects of transgenic approaches to delay PPD are discussed. After an Introduction, the article analyzes the Post-harvest physiological deterioration (PPD), Assessment of PPD,? Metabolite changes associated with PPD (during the initiation and during the development of symptoms), Modulation of transcriptome and proteome during PPD, Strategies to delay PPD (included below), Plant breeding, Transgenic approaches, and Conclusions (also included below). Strategies to delay PPDTraditional methods have been developed and used by cassava farmers and consumers to delay PPD development (Booth, 1975;?Wenham, 1995;?Westby, 2002?; ?Salcedo and Siritunga, 2011). More advanced pre- and post-harvest techniques have emerged because of the need to extend storage of cassava roots for supply to industry as well as long distance transport (Fig. 3). Cassava genotypes or lines with delayed PPD trait have also been generated through plant breeding and transgenic approaches. Pre-harvest methodsThese include delaying cassava harvest until the storage roots are required for consumption. However, this ties up land that could otherwise be used for farming. Extended ground storage also decreases starch quality and content, and storage roots become more fibrous, which increases cooking time (Wenham, 1995?; ?Salcedo and Siritunga, 2011). Pre-harvest pruning (i.e. the removal of the aerial part of cassava plants before harvest) delays the onset of PPD by increasing the sugar/starch ratio content in the storage roots (Wheatley and Schawabe, 1985?; ?Van Oirschot?et al., 2000) and limiting the accumulation of scopoletin (Wheatley, 1980). This technique reduces the impact of PPD by 25% and appears to be genotype-independent (Van Oirschot et al., 2000). However, the time of pruning is critical because it can affect texture, flavour, as well as general consumer acceptability (Van Oirschot et al., 2000). Adoption of adequate pruning practices is dependent on several variables, including farmer education (Adesina and Chianu, 2002). Because pruning can negatively impact cassava yield (Ayoola and Agboola, 2004), its adoption by farmers is also dependent on the establishment of pruning practices performed at the right physiological stage to preserve yield and starch content (Curcelli et al., 2014).Post-harvest methodsPost-harvest approaches include storage of harvested cassava roots, enzyme inactivation, chemical application, and avoidance strategies, which can reduce losses due to wounding of the storage roots during harvest. Cassava storage roots and roots of other tropical crops such as yam transpire and loose moisture, which reduce their quality during storage (Uchechukwu-Agua et al., 2015). Storage under high humidity and limited oxygen can reduce water loss and oxidative stress (Marriott?et al., 1978?; ?Noon and Booth, 1977). Moreover, storing cassava at high humidity also helps wound periderm formation after wounding (Booth, 1975;?Marriott?et al., 1978?; ?Rickard, 1985). Using polyethylene bags, storage boxes and coating cassava storage roots with paraffin wax also maintain high humidity and exclude air. They have been successfully field-tested and are used in commercial production (Fadeyibi, 2012). Other approaches to reduce PPD include storage of cassava roots under low temperature (0?4??C) (Ravi et al., 1996) and hot water treatment at 54?56??C for 10?min combined with modified atmosphere packaging (Acedo and Acedo Jr, 2012). However, these approaches are costly and therefore only suitable for high premium cassava products (Ravi et al., 1996). Post-harvest chemical treatment of cassava storage roots can extend their shelf-life (reviewed in?Ravi et al., 1996). Applications of ethanol (?>20%), sodium sulphite (10%), sodium dithiocarbamate (10%), saturated sodium chloride, benomyl (500?ppm) and dicloran (1000?ppm) can delay the onset of PPD for several days (Booth, 1976?; ?Booth, 1977). However, the mode of action of the above chemicals has not been further investigated or tested on field-grown cassava roots, and their potential toxicities could also restrain their broad use. Antioxidant molecules are alternatives for treatment of storage roots to delay PPD as suggested by the longer shelf-life of cassava genotypes with high antioxidant contents (S?nchez et al., 2006). Application of melatonin (500?ppm) to cassava storage roots could attenuate vascular discoloration of root slices. Exogenous melatonin leads to a moderate but significant reduction of PPD symptoms (Ma et al., 2016). Experimental data showed that exogenous melatonin acts by reducing H2O2content and improving activities of catalase and peroxidase (Hu et al., 2016). The reduction of vascular discoloration might also be due to the antioxidant properties of melatonin (Reiter et al., 2002). Processing of cassava storage roots immediately after harvest into more stable traditional food or industrial products avoids the PPD problem (reviewed in?Wenham, 1995?; ?Reilly?et al., 2004). While this approach is practical at rural or local scale with appropriate infrastructure, PPD remains a challenge when the time and distance between harvesting and processing cannot be shortened (Reilly et al., 2004). The Dutch Agricultural Development & Trading Company (DADTCO) recently presented a new concept to bring the cassava-processing factory to the farmers by using the patented Autonomous Mobile Processing Unit (AMPU,?http://www.dadtco.nl/). With this technology, cassava storage roots can be processed into either a stable and easily transportable intermediate product with a long shelf-life or immediately into a commercial cassava starch flour. Despite its rapid implementation in several cassava-growing regions in Africa, investment costs and poor road infrastructures in remote cassava-growing regions limit the wide adoption of AMPU technology in the cassava value chain. ConclusionsTranscriptome (Huang?et al., 2001?; ?Reilly?et al., 2007), proteome (Owiti?et al., 2011?; ?Vanderschuren?et al., 2014) and metabolome (Uarrota?et al., 2014?; ?Uarrota and Maraschin, 2015) studies have now provided important new information about PPD triggers and symptoms that had been earlier studied at phenotypic levels. Comparative transcriptomics analysis has also revealed the actions of melatonin in delaying PPD (Hu et al., 2016). In combination with comparative proteomics between two PPD contrasting genotypes (Qin et al., 2017), these approaches could suggest some molecular determinants of a PPD tolerance trait that can be used as molecular markers in large segregating populations. Further knowledge of molecular mechanisms will also open new opportunities to genetically engineer PPD tolerance in farmer- and industry-preferred genotypes for which trait introgression would be a lengthy processSource Cassava post-harvest physiological deterioration: From triggers to symptomsIma M. Zainuddin (a,?b), Ahmad Fathoni (b,?c), Enny Sudarmonowati (b), John R. Beeching (c), Wilhelm Gruissem (a),?Herv? Vanderschuren (a,?d)a?Department of Biology, Plant Biotechnology, Eidgen?ssische Technische Hochschule (ETH) Zurich, Universit?tstrasse 2, Zurich, 8092, Switzerlandb?Research Center for Biotechnology, Indonesian Institute of Sciences (LIPI), Cibinong, 16911, Indonesiac?Department of Biology and Biochemistry, University of Bath, Claverton Down Rd, Bath, North East Somerset, BA2 7AY, United Kingdomd?AgroBioChem Department, Plant Genetics, University of Li?ge,Passage des D?port?s 2, Gembloux, 5030, Belgium?Postharvest Biology and Technology,?Available online 16 October 2017https://doi.org/10.1016/j.postharvbio.2017.09.004 Science Direct,?http://www.sciencedirect.com/science/article/pii/S0925521417300613 ? Pictures1 - Corresponds of Picture 1 of the original article,?PPD in field-grown cassava storage roots: A. root slice at harvest, B. root slice at 2 dph (days post-harvest), C. root slice at 4 dph, D. root slice at 7 dph. Black coloured bar?=?11.6?mm.2 - Corresponds to Picture 3 of the oriingal article?Summary of cassava shelf-life extension by traditional and improved storage techniques (Etejere and Bhat, 1986;?Karim?et al., 2009;?Thanh, 1974;?Bancroft and Crentsil, 1995;?Rickard and Coursey, 1981;?Knoth, 1993;?Balagopal and Padmaja, 1985?; ?de Buckle Teresa?et al., 1973).Legends of picture 3Summary of cassava shelf-life extension by traditional and improved storage techniquesFew days- Piled into heaps & watered daily- Coated with a paste of earth or mud- Silos or under covered roofs- Pre-harvest pruning1 week - Plastic box storage- Trench containing wet sawdust2 weeks - Polyethylene bags with thiabendazole- Recycled rice sacks with thiabendazole4 weeks- Clamps- Refrigeration (3?C)2 months- Paraffin wax- Pits containing sand / soil