Comparative Transcriptome and Targeted Metabolome Profiling Unravel the Key Role of Phenylpropanoid and Glucosinolate Pathways in Defense against Alternaria brassicicola in Broccoli

Alternaria brassicicola (Ab) can
cause a major yield and quality-limiting disease
of Brassica oleracea called black spot,
and the genetic resources conferring complete resistance against Ab have not been identified to date. Here, comparative transcriptome
and targeted metabolome analysis were performed utilizing a newly
identified resistant (R) line and a broccoli susceptible (S) line
at 6, 24, and 72 h post-inoculation (hpi). Kyoto encyclopedia of genes
and genomes pathway enrichment and the weighted gene co-expression
network analyses showed that the phenylpropanoid pathway regulates
the resistance to Ab in broccoli. One metabolite,
cinnamic acid, was significantly upregulated in the Ab_inoculated R line compared with the mock treatment but no significant
difference in the S line, indicating that the cinnamic acid may cause
the resistance difference between R and S lines. Our results also
revealed that three indolic glucosinolates of I3G, 4MI3G, and 1MI3G
were significantly increased in the Ab_inoculated
R line compared with the mock treatment, and some related genes were
differentially expressed between the R and S lines. These results
provided new insights into the mechanism of Ab defense
in B. oleracea and have laid a theoretical
foundation for effectively utilizing resistant germplasm resources
in broccoli breeding.