GENOME-WIDE ANALYSIS OF POLYPHENOL OXIDASE GENES AND THEIR TRANSCRIPTIONAL PATTERNS DURING GRAIN DEVELOPMENT IN SORGHUM

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.