Sugarcane aphid (Melanaphis sacchari) is a phloem-feeding insect that severely affects the growth and productivity of sorghum and other related crops. While a growing body of knowledge is accumulating regarding plant, and insect interactions, the role of reactive oxygen species (ROS) against aphid infestation in sorghum has not been established yet. Here, the involvement of H₂O₂ and ROS detoxification enzymes in host plant resistance to sugarcane aphid in sorghum was demonstrated. The H₂O₂ accumulation and expression patterns of selected ROS scavenging enzymes including ascorbate peroxidase (APX), glutathione S transferase (GST), superoxide dismutase (SOD), and catalase (CAT) in response to sugarcane aphid infestation at 3, 6, 9, and 12 days post infestation (dpi) in resistant (Tx2783) and susceptible (Tx7000) sorghum genotypes were assessed, respectively. A significant increase in H₂O₂ accumulation was observed in resistant genotypes at all time points studied as compared to susceptible plants. Furthermore, gene expression analysis revealed that in responding to attack by sugarcane aphid, antioxidant genes were induced in both genotypes, but much stronger in the resistant line. Furthermore, aphid survival and fecundity were significantly inhibited in resistant plants compared to susceptible plants. Taken together, our results suggest that the elevated accumulation of H₂O₂ and the strong upregulation of the antioxidant genes in sorghum may have contributed to host plant resistance in Tx2783 against sugarcane aphid but the weak expression of those antioxidant genes in Tx7000 resulted in the failure of attempting defense against sugarcane aphid. This report also provides the experimental evidence for the role of ROS involvement in the early defensive response to an attack by sugarcane aphid in sorghum.

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.25) plays a crucial role in plant adaptation to biotic and abiotic stresses. However, the current knowledge about PAL proteins in sorghum is essentially lacking. Thus, in this study we aimed to analyze the PAL family genes in sorghum using a genome-wide approach and to explore the role of PAL genes in host plant resistance to aphids via SA-mediated defense signaling. Here, we report gene structural features of 8 PAL (SbPAL) genes in sorghum (Sorghum bicolor), their phylogeny, protein motifs and promoter analysis. Furthermore, we demonstrated that the SbPAL genes were induced by sugarcane aphid (SCA) infestation and SbPAL exhibited differential gene expression in susceptible and resistant genotypes. PAL activity assays further validated upregulated expression of the SbPAL genes in a resistant genotype. In addition, exogenous application of SA reduced plant damage and suppressed aphid population growth and fecundity in susceptible genotype, suggesting that those SbPAL genes act as positive regulator of the SA-mediated defense signaling pathway to combat aphid pests in sorghum. This study provides insights for further examination of the defense role of PAL in sorghum against other pests and pathogens.

Jasmonate ZIM-domain (JAZ) proteins are the key repressors of the jasmonic acid (JA) signal transduction pathway and play a crucial role in stress-related defense, phytohormone crosstalk and modulation of the growth-defense tradeoff. In this study, the sorghum genome was analyzed through genome-wide comparison and domain scan analysis, which led to the identification of 18 sorghum JAZ (SbJAZ) genes. All SbJAZ proteins possess the conserved TIFY and Jas domains and they formed a phylogenetic tree with five clusters related to the orthologs of other plant species. Similarly, evolutionary analysis indicated the duplication events as a major force of expansion of the SbJAZ genes and there was strong neutral and purifying selection going on. In silico analysis of the promoter region of the SbJAZ genes indicates that SbJAZ5, SbJAZ6, SbJAZ13, SbJAZ16 and SbJAZ17 are rich in stress-related cis-elements. In addition, expression profiling of the SbJAZ genes in response to phytohormones treatment (JA, ET, ABA, GA) and sugarcane aphid (SCA) was performed in two recombinant inbred lines (RILs) of sorghum, resistant (RIL 521) and susceptible (RIL 609) to SCA. Taken together, data generated from phytohormone expression and in silico analysis suggests the putative role of SbJAZ9 in JA-ABA crosstalk and SbJAZ16 in JA-ABA and JA-GA crosstalk to regulate certain physiological processes. Notably, upregulation of SbJAZ1, SbJAZ5, SbJAZ13 and SbJAZ16 in resistant RIL during JA treatment and SCA infestation suggests putative functions in stress-related defense and to balance the plant defense to promote growth. Overall, this report provides valuable insight into the organization and functional characterization of the sorghum JAZ gene family.