**1. Introduction**

Peroxidases (EC 1.11.1.X) form a large family of enzymes that are widely distributed in living organisms and catalyze the oxidoreduction reaction between hydrogen peroxide (H2O2) as an electron acceptor and diverse electron donors, such as auxin, phenolic compounds, or secondary metabolites [1,2]. According to their protein sequences and structure, peroxidases are classified as either non-heme peroxidases or heme peroxidases [3]. The majority of heme peroxidase members are divided into animal and non-animal groups [4]. On the basis of their amino acid sequences and catalytic properties, non-animal heme peroxidases are assigned to one of three large families: class I, II, or III [3,5]. The class

III peroxidases (EC 1.11.1.7) are plant-specific oxidoreductases and have various abbreviations (POX, POD, Px, PER, and Prx) [2].

There are many multigenic class III peroxidasesin land plants, which is commonly secreted into the vacuole and cell wall [4–7]. The structures and weights of Prx proteins are highly conserved between paralogs and orthologs [1,3]. The class III plant peroxidases contain 10–12 conserved α-helices and two short β-strands [2,7–10], and they mainly participate in the peroxidative cycle and hydroxylic cycle to reduce the production of hydrogen peroxide and the formation of reactive oxygen species (ROS) [4,5,11,12]. Prx proteins are involved in a variety of physiological processes, such as the cross-linking of cell wall components, salt tolerance, defense against pathogen attack, the oxidation of toxic reductants, and the metabolism of phytohormones [2,3,13–17].

Some genetic evidence supports Prx proteins' role in the plant response to biotic and abiotic stresses. Overexpression of *AtPrx64* was able to enhance tolerance to aluminum stress in transgenic tobacco plants [18]. *AtPrx3* was shown to positively regulate plant tolerance to drought and salt stresses in Arabidopsis [19]. Overexpression of the *Catharanthus roseus* genes *CrPrx* and *CrPrx1* in tobacco led to enhanced chilling resistance and increased germination rates under dehydration and salt treatments, respectively [20]. Repressing the expression of *Ep5C* in tomato resulted in reduced susceptibility to bacterial speck caused by the pathogen *Pseudomonas syringae* pv *tomato* [21]. *CaPO2* gene-silenced pepper plants were shown to be susceptible to infection by *Xanthomonas campestris* pv *vesicatoria*, whereas overexpression of *CaPO2* in transgenic *Arabidopsis thaliana* conferred bacterial disease resistance [22]. Transgenic carrot plants overexpressing *OsPrx114* exhibited enhanced resistance to necrotrophic fungal pathogens [23]. Together, these previous studies reveal the positive role of class III plant peroxidases in the response to biotic and abiotic stresses.

To date, the peroxidase (POD) family members have been characterized by whole-genome analyses in several plants, including 73 PODs in Arabidopsis [24–26], 138 PODs in rice [9], 93 PODs in *Populus trichocarpa* [27], 102 PODs in *Medicago sativa* [28], 119 PODs in maize [29], and 94 PODs in *Pyrus bretschneideri* [30]. However, there is less known about the POD family in cassava, a major tropical crop. Cassava is the third most valuable crop after maize and rice in Africa, Latin America, and Asia, supplying a carbohydrate source to 600 million people in tropical and subtropical regions [31]. Cassava can efficiently use water, heat, and light resources, and it is resistant to dehydration stress and lower-fertility soils [32,33]. Unfortunately, the potential of cassava as a food and industrial crop is restricted because its storage roots deteriorate within 72 h of its harvest [34]. ROS production is an early event that leads to the postharvest physiological deterioration (PPD) of cassava storage roots [35]. The mechanisms underlying cassava's resistance to drought and sensitivity to PPD are not well understood. POD proteins function by reducing the production of H2O<sup>2</sup> and formation of ROS, which are involved in various physiological processes. Systematic investigations of the cassava POD family would provide novel insights into the POD-mediated stress response and regulation of root deterioration.

## **2. Results**

#### *2.1. Genome-Wide Identification of PODs in Cassava*

According to the 211 POD protein sequences from Arabidopsis and rice genome databases, 91 POD members were predicted from the cassava genome using BLAST and HMMER methods. After conserved domain detection was confirmed, these cassava POD proteins (MePODs) were named MePOD01 to MePOD91. The full length of these putative cassava POD proteins ranges from 153 (MePOD63) to 422 (MePOD46) amino acid residues, and their relative molecular weight varies from 16.64 (MePOD63) to 46.12 kDa (MePOD46), with isoelectric points ranging from 4.43 (MePOD26) to 9.63 (MePOD39) (Table S1).
