Leaf epicuticular wax and hormone-mediated resistance to Alternaria brassicicola in broccoli

Authors: GANGURDE, SUNIL - University Of Georgia; KAUR, NAVJOT - University Of Georgia; Guo, Baozhu; DUTTA, BHABESH - University Of Georgia

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Broccoli is grown in both fall and spring growing seasons in Georgia and the production has been negatively impacted by several pests and diseases, such as Alternaria leaf blight and head rot (ABHR) caused by an Alternaria species complex. Routine outbreaks of ABHR occur in broccoli and in other brassicas across the eastern United States and has been routinely causing losses in yield and quality. Host resistance mediated by either plant physical structures or by production of secondary metabolites like phytoalexins provides first line of defense against any invading pathogens. Previous reports have suggested that cuticular wax biosynthetic genes were significantly induced by drought stress in broccoli. Host resistance is also related to plant maturity. This study established the age-related susceptibility of broccoli foliage to A. brassicicola and demonstrated empirically that older leaves are more susceptible to fungal infection compared with younger leaves. Further regression analysis showed a negative correlation between wax per unit area and disease severity. Differential gene expression showed upregulation of wax biosynthesis genes in the younger leaves. Hormones modulation was observed for abscisic acid, salicylic acid and jasmonic acid in the older leaves as compared to the younger inoculated leaf. In summary, these findings suggest that the older leaves of broccoli are highly susceptible to A. brassicicola, potentially associated with lower wax deposition and increased modulation in senescence hormone.

Technical Abstract: Alternaria leaf spot is a foliar disease of brassica crops caused by Alternaria brassicicola. This disease is a serious problem causing unmarketable crops and millions of dollars in losses in broccoli and other crucifers across the globe. This disease seems to appear first in lower (older) leaves that gradually progress upward to younger leaves and ultimately to the broccoli head. During the pathogenicity and whole plant inoculation assay, we observed significantly higher disease severity in the older leaves at the bottom, as compared to the younger leaves at the top. Epicuticular wax analysis showed a significantly higher amount of wax deposition on the younger leaves at the top as compared to the older leaves at the bottom. Further regression analysis showed a negative correlation between wax per unit area and disease severity. Differential gene expression analysis showed upregulation of key wax biosynthesis genes namely 3-ketoacyl-CoA synthase (BoI018447), alkane hydroxylase CYP96A15 (BoI016302) and O-acyltransferase WSD1 (BoI024738) in the younger leaves. Hormones modulation was observed for abscisic acid, salicylic acid and jasmonic acid in the older leaves as compared to the younger inoculated leaf. Transcriptome analysis uncovered large transcriptome reprogramming in the older leaves as compared to younger leaves. Senescence-associated genes such as senescence regulator S40 (BolC7t42093H), stress up-regulated Nod 19 (BolC2t12223H), and late embryogenesis abundant protein (BolC8t47646H) were upregulated in the older leaves. These findings suggest that the older (bottom) leaves of broccoli are highly susceptible to A. brassicicola, potentially associated with lower wax deposition and increased modulation in senescence hormone.