Genome-Wide Analysis of Polyphenol Oxidase Genes and Their Transcriptional Patterns during Grain Development in Sorghum

Abstract

Polyphenol oxidase (PPO) plays multiple roles in plant growth and responses to environmental stimuli. However, there is still little molecular information on PPO genes in sorghum. Here, eight distinct PPO gene members, designated as SbPPO1-SbPPO8, were identified in two grain sorghum genotypes, i.e., BTx623 and Henong 16 (HN16). The eight members were located on sorghum chromosomes 3 (SbPPO1 and SbPPO2), 6 (SbPPO3 and SbPPO4), 7 (SbPPO5-SbPPO7), or 10 (SbPPO8) and contained zero (SbPPO6-SbPPO8), one (SbPPO1, SbPPO2, and SbPPO5), or two (SbPPO3 and SbPPO4) introns. In BTx623, the deduced proteins of SbPPO1-SbPPO8 all contained an intact binuclear copper center required for enzyme activity, as well as the tyrosine motif and its interacting residues involved in structural maintenance. SbPPO2-SbPPO6 were found to possess thylakoid transfer domains, whereas SbPPO1 and SbPPO8 were predicted to contain N-terminal signal peptides for targeting to the secretory pathway. In HN16, the SbPPO3 allele was a pseudogene, whereas the remaining seven SbPPO alleles showed high identities with their counterparts in BTx623. Phylogenetically, sorghum PPO genes generally exhibited relationships closer to those of maize, and the maize orthologues of SbPPO2-SbPPO8 could be recognized by synteny analysis. Finally, SbPPO5 was found to be the major expressed member at the early stage of grain growth, and the dynamic change of SbPPO5 transcript level paralleled that of the total PPO activity during the grain development in both BTx623 and HN16. Our study generated new information on the PPO members in sorghum, which may facilitate more detailed functional analysis of these important genes in further research.

Genome-wide investigation and expression profiling of polyphenol oxidase (PPO) family genes uncover likely functions in organ development and stress responses in Populus trichocarpa

“…Under normal circumstances, PPO is inactive and tightly binds to the inner capsule membrane; activated PPO plays an important role after tissue damage. In addition, several PPO genes have been identified in Musa acuminata , Malus domestica, Oryza sativa , Sorghum bicolor , Solanum tuberosum, Ananas comosus, and Cucumis sativus [ 11 , 14 – 19 ]. However, there are considerable differences in the distribution and function of PPO proteins in different plants.…”

Polyphenol oxidase genes in barley (Hordeum vulgare L.): functional activity with respect to black grain pigmentation

“…Some of these ancestral genes have been lost during evolution; for example, in rice ( O. sativa ), only two Ppo genes have been identified ( Yu et al., 2008 ). Alternatively, additional duplicated genes have been acquired; for instance, eight Ppo genes have been found in sorghum ( Cai et al., 2013 ). Barley genes Ppo1 and Ppo2 are known to have arisen via segmental duplication of the long arm of chromosome 2 in a common ancestor of the Triticeae tribe ( Taketa et al., 2010 ; Taranto et al., 2017 ).…”

“…Such differential expression of different members of one gene family in distinct tissues is a well-known phenomenon in plants. For example, among eight S. bicolor Ppo genes, only SbPpo5 and SbPpo7 were detected in the ovary or panicle tissues that developed under normal conditions, while the expression of the other genes ( SbPPO1 – SbPPO3 , SbPPO6 , and SbPPO8 ) was induced by stressful environments (involving a phytohormone or acid, alkaline, wounding, or heat shock treatments) ( Cai et al., 2013 ). Among 11 Ppo genes of artichoke, three ( Ppo6 , Ppo7 , and Ppo11 ) showed significant upregulation of transcripts in capitula tissues after cutting in comparison with the other genes; these genes were chosen as targets for CRISPR/Cas9-based inhibition of the enzymatic browning to maintain high quality of capitula after processing and during marketing ( Pompili et al., 2023 ).…”

Seedsvsfungi: an enzymatic battle in the soil seedbank

“…A highly conserved trait shared by all PPOs is that they are metalloenzymes with an active site consisting of a binuclear type-3 copper centre containing two copper ions (CuA and CuB), each bound to three histidine side chains (Marusek et al ., 2006; Aniszewski et al ., 2008). Additional common structures shared by PPOs include a signal peptide, which directs the enzyme to the thylakoid lumen of chloroplasts or to the vacuolar lumen; a highly conserved N-terminal containing the features for copper binding, substrate catalysis, and structural maintenance; a variable C-terminal domain; and a linker region between the C- and N-termini that is highly variable in size and structure (Marusek et al ., 2006; Tran and Constabel, 2011; Cai et al ., 2013).…”

“…Whether cresolase is always present in organisms that produce CO is debated in the literature (Marusek et al, 2006). PPOs are a highly diverse group of enzymes that differ widely in structure, amino acid sequence, function, temporal and spatial expression, and substrate specificity (Mayer, 2006;Cai et al, 2013). Given the latter characteristic, measured activity may differ if one substrate is used versus another.…”