Sorghum Stalk Rot Pathogens Affect Transcription of Key Genes Involved in Chlorophyll Metabolism of a Non-Staygreen, Disease Susceptible Genotype.

Fusarium stalk rot (Fusarium thapsinum, FT) and charcoal rot (Macrophomina phaseolina, MP) are high priority sorghum diseases worldwide. The staygreen trait (delayed post-flowering senescence) is generally regarded as a physiological means of stalk rot resistance and has been shown to be highly correlated with leaf chlorophyll content. The objectives of this study were to (i) investigate the differential expression of chlorophyll metabolism-related genes among staygreen and non-staygreen sorghum lines in response to FT and MP inoculation and (ii) to study pathogen effects on leaf chlorophyll content (measured by Soil and Plant Analytical Development meter, SPAD) at three maturity stages. SC599 (stalk rot resistant, staygreen) and TX7000 (stalk rot susceptible, non-staygreen) sorghum lines were grown in the greenhouse and inoculated with FT and MP. Control plants were mock-inoculated with phosphate-buffered saline. RNA was extracted from 3 biological replicates at 2, 7, and 30 days post-inoculation (d.p.i.) from stem tissues and subjected to RNA-Seq. Analysis for differentially expressed genes (DEG) was performed with DESeq2 and pathway analysis was performed to explore DEG. At 7 d.p.i., a gene responsible for chlorophyll degradation, chlorophyllase-2 (Sb02g012300), was significantly upregulated in Tx7000 in response to MP, while two key genes in chlorophyll biosynthesis, magnesium-protoporphyrin O-methyltransferase (Sb10g002100) and magnesium-chelatase (Sb08g004300), were significantly down-regulated. The resistant staygreen line did not show a significant differential expression. However, the field experiment conducted in relation to the second objective did not reveal a significant SPAD reduction with TX7000 compared to SC599 at soft dough, hard dough, and physiological maturity upon inoculation. Although expression in stalk tissues was measured, it is possible that TX7000 leaves remain green under pathogen-induced chlorophyll degradation as chlorophyllide, the result of chlorophyllase-2-mediated chlorophyll degradation, may contribute to the maintenance of leaf greenness.