Bacterial Endophytes As Indicators of Susceptibility To Cercospora Leaf Spot (CLS) Disease In Beta vulgaris L.

Abstract

Background: The fungus Cercospora beticola causes Cercospora Leaf Spot (CLS) of sugar beet (Beta vulgaris L.). Despite the global importance of this disease, durable resistance to CLS has still not been obtained. Therefore, the development of tolerant hybrids is still a major goal for sugar beet breeding. Although recent studies have suggested that the leaf microbiome composition can offer useful predictors to assist plant breeders, this is an untapped resource in sugar beet breeding efforts. Methods: Using Ion GeneStudio S5 technology to sequence amplicons from seven 16S rRNA hypervariable regions, the most recurring endophytes discriminating CLS-symptomatic and symptomless sea beets (Beta vulgaris L.ssp. maritima) were identified. This allowed the design of taxon-specific primer pairs to quantify the abundance of the most representative endophytic species in large naturally occurring populations of sea beet and subsequently in sugar beet breeding genotypes under either CLS symptomless or infection stages using qPCR. Results: Among the screened bacterial genera, Methylobacterium and Mucilaginibacter were found to be significantly (p<0.05) more abundant in symptomatic sea beets with respect to symptomless. In cultivated sugar beet material under CLS infection, the comparison between resistant and susceptible genotypes confirmed that the susceptible genotypes hosted higher contents of the above-mentioned bacterial genera. Conclusions: These results suggest that the abundance of these species can be correlated with increased sensitivity to CLS disease. This evidence can further prompt novel protocols to assist plant breeding of sugar beet in the pursuit of improved pathogen resistance.

Understanding the sugar beet holobiont for sustainable agriculture

“…Bacterial phyllosphere communities differ between petioles and laminae: Phyllobacterium, Methylobacterium, and Sphingomonas are higher-abundant, while Pseudomonas and Enterobacteriaceae are lower-abundant in petioles (Okazaki et al, 2021). Phyllosphere abundance of Methylobacterium and Mucilaginibacter was further linked to tolerance toward leaf pathogen Cercospora beticola in both sea beets and sugar beets (Broccanello et al, 2022). In contrast to bacteria, fungi isolated from sugar beet leaves usually belong to opportunistic plant pathogens (Alternaria, Aspergillus, Cladosporium, Acremonium, Fusarium, Penicillium, Phoma, Plectosphaerella, Pleospora, Pythium) but also saprobionts (Cryptococcus, Epicoccum, Sporobolomyces, Stemphylium) (Thompson et al, 1993;Larran et al, 2000;Shi et al, 2009a;Pusenkova et al, 2016).…”

“…With currently available data, it appears that phyllosphere bacterial diversity shifted and increased during domestication, while rhizosphere diversity decreased (see Section 2.6); sea beet phyllospheres are strongly dominated by Sphingomonas and Methylobacterium, together accounting for ca. 84% relative abundance (Broccanello et al, 2022). This however could as well refer to fertilization regimes, since bacterial phyllosphere microbiomes in sugar beet respond stronger upon NPK fertilization than in bacterial endosphere communities (Okazaki et al, 2021).…”