Reactive oxygen species (ROS) play a major role in plant defense against pathogens, but the evidence for their role in defense against insects is still limited. In this study, HN16 and its mutated line (asm1) were used as research objects, and the potential role of H 2 O 2 in sorghum against aphid was examined. Constitutive H 2 O 2 was considered the main factor associated with increased aphid tolerance, although aphid feeding also induced an increase in the H 2 O 2 concentration. By studying ROS scavenging enzymes, it was found that APX and GPX were closely related, but SOD, POX and CAT were not involved in RMES1-mediated resistance. DEGs involved in the detoxification of ROS in HN16 worked through different means. Analysis of three DEGs encoding APX and GPX revealed that although the expression changes of SbAPx1 were consistent and those of SbGPx1 and SbGPx2 were inconsistent with enzyme activity, they all played an important role in RMES1-mediated resistance to aphids.
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.
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