Abstract
Huanglongbing (HLB) is considered the most destructive disease in Citrus production and threatens the future of the industry. Microbial-derived defense elicitors have gained recognition for their role in plant defense priming. This work assessed a 5% (V/V) microbial fermentation application (MFA) and its role in the elicitation of defense responses in HLB-infected Citrus sinensis trees following a foliar application with a pump sprayer. Using a PCR detection method, HLB infection levels were monitored in healthy and infected trees for 20months. Nutrient analysis assessed N, P, K, Ca, Mg, Mn, Zn, Fe, B, and Cu concentrations in the trees. MFA significantly increased Cu concentrations in treated trees and resulted in the stabilization of disease index (DI) in infected trees. Initial real-time qPCR analysis of defense-associated genes showed a significant increase in pathogenesis-related protein 2 (PR2) and phenylalanine ammonia lyase (PAL) gene expression in healthy and HLB-infected trees in response to MFA. Gene expression of PR2 and PAL peaked 6h post-microbial fermentation application during an 8-h sampling period. A transcriptomic assessment using GeneChip microarray of the hour 6 samples revealed differential expression of 565 genes when MFA was applied to healthy trees and 909 genes when applied infected citrus trees when compared to their respective controls. There were 403 uniquely differentially expressed genes in response to MFA following an intersectional analysis of both healthy and infected citrus trees. The transcriptomic analysis revealed that several genes associated with plant development, growth, and defense were upregulated in response to MFA, including multiple PR genes, lignin formation genes, ROS-related genes, hormone synthases, and hormone regulators. This study provides further evidence that MFA may play an important role as a plant elicitor in an integrated pest management strategy in citrus and other agronomically important crops.
Highlights
Huanglongbing (HLB) is a phloem-limited bacterial disease that infects citrus trees of all kinds, caused by Candidatus liberibacter (Jagoueix et al, 1996; Hansen et al, 2008; Duan et al, 2009)
This study examined the impact of a microbial preparation, microbial fermentation application (MFA), against HLB in young citrus trees
The results presented here indicate that MFA increased the transcriptional activity of citrus defense mechanisms, elevated Cu concentrations in citrus leaf tissue, and stabilized HLB infection in treated plants (Figure 1, Table 1)
Summary
Huanglongbing (HLB) is a phloem-limited bacterial disease that infects citrus trees of all kinds, caused by Candidatus liberibacter (Jagoueix et al, 1996; Hansen et al, 2008; Duan et al, 2009). Elicitors upregulate the expression of defense proteins that in turn activate both systemic and local defense responses in treated plants (Montesano et al, 2003). In situations where this activity is observed, the reduction of diseases results in healthier plants and higher yielding crops. In Citrus limon, HLB infection facilitated the downregulation of catalases, chitinases, lectin-related proteins, and miraculin-like proteins (Nwugo et al, 2013). These metabolic changes lead to increased bacterial spread and susceptibility to other problematic pathogens. The routine activation of a plants defense in response to an elicitor could perhaps reduce the success of HLB in citrus
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