Leptosphaeria maculans Alters Glucosinolate Profiles in Blackleg Disease–Resistant and -Susceptible Cabbage Lines

Abstract

Blackleg, a fungal disease caused by Leptosphaeria maculans, is one of the most devastating diseases of Brassica crops worldwide. Despite notable progress elucidating the roles of glucosinolates in pathogen defense, the complex interaction between B. oleracea (cabbage) and L. maculans infection that leads to the selective induction of genes involved in glucosinolate production and subsequent modulation of glucosinolate profiles remains to be fully understood. The current study was designed to identify glucosinolate-biosynthesis genes induced by L. maculans and any associated alterations in glucosinolate profiles to explore their roles in blackleg resistance in 3-month-old cabbage plants. The defense responses of four cabbage lines, two resistant and two susceptible, were investigated using two L. maculans isolates, 03–02 s and 00–100 s. A simultaneous increase in the aliphatic glucosinolates glucoiberverin (GIV) and glucoerucin (GER) and the indolic glucosinolates glucobrassicin (GBS) and neoglucobrassicin (NGBS) was associated with complete resistance. An increase in either aliphatic (GIV) or indolic (GBS and MGBS) glucosinolates was associated with moderate resistance. Indolic glucobrassicin (GBS) and neoglucobrassicin (NGBS) were increased in both resistant and susceptible interactions. Pearson correlation showed positive association between GER content with GSL-OH (Bol033373) expression. Expressions of MYB34 (Bol007760), ST5a (Bol026200), and CYP81F2 (Bol026044) were positively correlated with the contents of both GBS and MGBS. Our results confirm that L. maculans infection induces glucosinolate-biosynthesis genes in cabbage, with concomitant changes in individual glucosinolate contents. In resistant lines, both aliphatic and indolic glucosinolates are associated with resistance, with aliphatic GIV and GER and indolic MGBS glucosinolates particularly important. The association between the genes, the corresponding glucosinolates, and plant resistance broaden our molecular understanding of glucosinolate mediated defense against L. maculans in cabbage.

Altered Glucosinolate Profiles and Expression of Glucosinolate Biosynthesis Genes in Ringspot-Resistant and Susceptible Cabbage Lines

“…These data show that changes in the status of these genes correlate to the contents of individual GSLs in response to M. brassicicola infection. This result is consistent with a recent study for higher expression of MYB34 genes under L. maculans inoculation [ 36 ] and methyl jasmonate elicitation [ 50 ]. PCA showed a major difference between the accumulation of individual GSL components and gene expression in the R and S lines.…”

“…The concentration of aliphatic GIV and indolic GBS and MGBS GSLs increased in the R line after inoculation ( Figure 2 B), indicating that ringspot resistance in cabbage may be accomplished through an accumulation of both aliphatic and indolic GSLs. These results were consistent with some studies [ 36 ], but different from other studies [ 45 , 48 , 53 , 54 ]. These data suggest that increased levels of aliphatic GIV, and indolic GBS and MGBS may confer resistance to M. brassicicola in cabbage.…”

“…In vitro studies have reported the antifungal activity of MGBS, as well as SIN and GBS [ 58 ] and increased accumulation of MGBS conferred moderate resistance in cabbage plants to L. maculans . [ 36 ]. Here, we observed an increase in the levels of MGBS in the R line compared to the S line after M. brassicicola infection ( Figure 2 B) along with upregulation of CYP81F4-Bol032712 , CYP81F3-Bol032711 , CYP81F2-Bol026044 , and CYP81F4-Bol032714 , which are involved in methoxylation and the conversion of GBS to 4-MGBS.…”

“…A wide range of defense reactions can affect GSL content [ 18 , 33 , 34 , 35 ]. However, an association between GSL levels and resistance to various fungal pathogens in brassicas has not yet been established [ 36 ].…”

“…These findings on the association of resistance with the GSL profile of brassica species prompted us to examine plant–pathogen interactions at the molecular and biochemical levels. Upregulation of GSL biosynthesis genes was linked with increased contents of individual GSL compounds in B. oleracea inbred lines [ 36 , 49 , 50 , 51 ]. Here, we investigated the correlation between GSL profiles in resistant and susceptible cabbage lines, and the expression of GSL biosynthesis genes upon infection with the M. brassicicola IPO-99008 isolate.…”