Abstract
Huanglongbing (HLB) is an extremely destructive and lethal disease of citrus worldwide, presumably caused by phloem-limited bacteria, Candidatus Liberibacter asiaticus (CLas). The widespread invasiveness of the HLB pathogen and lack of natural HLB-resistant citrus cultivars have underscored the need for identifying tolerant citrus genotypes to support the current citrus industry’s survival and potentially to lead to future natural HLB resistance. In this study, transverse sections of leaf lamina and midribs were examined with light and epifluorescence microscopy to determine anatomical characteristics that underlie HLB-tolerant mechanisms operating among “Bearss” lemon, “LB8-9” Sugar Belle® mandarin, and its sibling trees compared with HLB-sensitive “Valencia” sweet orange. The common anatomical aberrations observed in all CLas-infected varieties are as follows: phloem necrosis, hypertrophic phloem parenchyma cells, phloem plugging with abundant callose depositions, phloem collapse with cell wall distortion and thickening, excessive starch accumulation, and sometimes even cambium degeneration. Anatomical distribution of starch accumulation even extended to tracheid elements. Although there were physical, morphological, and pathological similarities in the examined foliage, internal structural preservation in “Bearss” lemon and “LB8-9” Sugar Belle® mandarin was superior compared with HLB-sensitive “Valencia” sweet orange and siblings of “LB8-9” Sugar Belle® mandarin. Intriguingly, there was substantial phloem regeneration in the tolerant types that may compensate for the dysfunctional phloem, in comparison with the sensitive selections. The lower levels of phloem disruption, together with greater phloem regeneration, are two key elements that contribute to HLB tolerance in diverse citrus cultivars.
Highlights
Citrus Huanglongbing (HLB, previously called citrus greening and yellow dragon disease), an extremely destructive and lethal disease of citrus (Bové, 2006), was discovered in Guangdong province in south China in 1919 (Reinking, 1919)
A β-1,3 glucan, along with PP2 of filamentous appearance, were the main obstructive media directly involved in phloem plugging, instead of the Ca. L. asiaticus (CLas) bacterial itself, which is not found in sufficient numbers to cause phloem sieve element plugging (Kim et al, 2009; Achor et al, 2010; Fan et al, 2012; Fan et al, 2013; Johnson et al, 2014; Etxeberria and Narciso, 2015)
This study presents an abundance of evidence of decreased phloem destruction, together with more replacement phloem generation, that underlies the greater HLB-tolerance of “Bearss”
Summary
Citrus Huanglongbing (HLB, previously called citrus greening and yellow dragon disease), an extremely destructive and lethal disease of citrus (Bové, 2006), was discovered in Guangdong province in south China in 1919 (Reinking, 1919). The putative pathological agent of HLB, Candidatus Liberibacter spp., is a Gram-negative, thin-walled, phloemlimited bacterial genus belonging to α (alpha) subdivision of the class Proteobacteria (Jagoueix et al, 1994; Bové, 2006). It has been reported that HLB has swept through almost 100% of commercial citrus groves in Florida (Browning, 2015), and more than 80% of all citrus trees have been affected (Albrigo and Stover, 2015)
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