Differential gene regulatory pathways and co-expression networks associated with fire blight infection in apple (Malus × domestica)

Abstract

Apple cultivars with durable resistance are needed for sustainable management of fire blight, the most destructive bacterial disease of apples. Although studies have identified genetic resistance to fire blight in both wild species and cultivated apples, more research is needed to understand the molecular mechanisms underlying host-pathogen interaction and differential genotypic responses to fire blight infection. We have analyzed phenotypic and transcriptional responses of 'Empire' and 'Gala' apple cultivars to fire blight by infecting them with a highly aggressive E. amylovora strain. Disease progress, based on the percentage of visual shoot necrosis, started showing significant (p < 0.001) differences between 'Empire' and 'Gala' 4 days after infection (dai). 'Empire' seems to slow down bacterial progress more rapidly after this point. We further compared transcriptome profiles of 'Empire' and 'Gala' at three different time points after fire blight infection. More genes showed differential expression in 'Gala' at earlier stages, but the number of differentially expressed genes increased in 'Empire' at 3 dai. Functional classes related to defense, cell cycle, response to stress, and biotic stress were identified and a few co-expression gene networks showed particular enrichment for plant defense and abiotic stress response genes. Several of these genes also co-localized in previously identified quantitative trait locus regions for fire blight resistance on linkage groups 7 and 12, and can serve as functional candidates for future research. These results highlight different molecular mechanisms for pathogen perception and control in two apple cultivars and will contribute toward better understanding of E. amylovora-Malus pathosystem.

Comparative evaluation of lateral flow immunoassays, LAMP, and quantitative PCR for diagnosis of fire blight in apple orchards

“…Young plants were inoculated with different E. amylovora strains, once shoot length of the majority (90%) of the plants was above 15 cm. Inoculum was prepared with a concentration of 10 9 as described earlier (Silva et al 2019 ; Khan et al 2006 ). Scissors were dipped in the inoculum and used to cut the youngest leaf (1–2 cm size) for inoculation at approximately the start of the broadest part of the leaf.…”

Root system traits impact early fire blight susceptibility in apple (Malus × domestica)

“…Upon analysis of RACs of non-grafted 'M.7' rootstocks on fire blight susceptibility of above-ground leaf tissues, it is observed that increased root surface area contributed to decreased fire blight susceptibility in these above-ground leaf tissues, and the reverse is found to be true as well. These observed root-dependent differences in levels of fire blight disease susceptibility may be attributed to presence of multiple defense mechanisms [11,25,38,39,40] that interact with the pathways related to nutrient status of a plant. Indeed, involvement of diverse molecular pathways related to plant metabolism, cell cycle, oxidation-reduction, and stress response suggests presence of systemic regulation of fire blight infection in apples [11,38].…”

“…These observed root-dependent differences in levels of fire blight disease susceptibility may be attributed to presence of multiple defense mechanisms [11,25,38,39,40] that interact with the pathways related to nutrient status of a plant. Indeed, involvement of diverse molecular pathways related to plant metabolism, cell cycle, oxidation-reduction, and stress response suggests presence of systemic regulation of fire blight infection in apples [11,38]. Moreover, rootstock genotypes can significantly contribute to scion tolerance to fire blight susceptibility via rootstock-regulated gene expression patterns [11].…”

“…Potted young trees were inoculated with either bacteria (treatment) or water (control; only in second experiment) for fire blight evaluation. The youngest unfolded leaf of an actively growing shoot of a potted young tree was inoculated by bisecting across the midribs using scissors dipped in the bacterial suspension, as described earlier [38,57]. Deionized water was used to bisect midribs of leaves of control plants in the second experiment.…”

“…Leaf tissues from M.7 rootstocks of contrasting initial root surface areas (cm 2 ), in the second experiment, were used for RNA extraction and for gene expression analysis. Leaves were collected at 4 and 8 dai from two biological replicates of control and three biological replicates of bacterialinoculated young trees of two contrasting RACs as described earlier [38,57]. Leaf tissues were immediately immersed in liquid nitrogen, and stored at -80 0 C until used for RNA extraction.…”