Phloem Regeneration Is a Mechanism for Huanglongbing-Tolerance of “Bearss” Lemon and “LB8-9” Sugar Belle® Mandarin

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.

Dynamics of Candidatus Liberibacter asiaticus Movement and Sieve-Pore Plugging in Citrus Sink Cells

“…CLas infection is known to induce a reorganization of the citrus phloem. This reorganization includes the swelling of the middle lamina, callose deposition, P-protein accumulation, and phloem hyperplasia (Etxeberria et al, 2009;Achor et al, 2010;Folimonova and Achor, 2010;Deng et al, 2019). Here, we investigated different citrus sink tissues to gain further knowledge about the specific cellular reactions that are taking place in the sieve element.…”

“…In leaves, another phenotype associated with HLB is the accumulation of callose inside the sieve plate pores of the infected plant's phloem. Accumulation of callose was demonstrated both by aniline blue staining and by immunogold labeling (Kim et al, 2009;Achor et al, 2010;Deng et al, 2019;Granato et al, 2019). This accumulation is an early response that begins at early stages of the disease and probably leads, at more advanced disease stages, to collapse of the cells.…”

Citrus vascular proteomics highlights the role of peroxidases and serine proteases during Huanglongbing disease progression

“…All commercial citrus varieties are susceptible to CLas and current interventions such as D. citri control and infected tree removal have had little success in controlling the disease (1). Sweet orange varieties (Citrus sinensis) exhibit the fastest decline, while lemon (C. latifolia), mandarins (C. reticulata), and tangerines (C. reticulata) present delayed symptom progression but ultimately succumb to HLB (2,3). In the United States, HLB was first diagnosed in 2005 in Florida and has since spread to Texas, Louisiana, South Carolina, Georgia and California (4).…”

Heterologous Expression of CLIBASIA_03915/CLIBASIA_04250 by Tobacco Mosaic Virus Resulted in Phloem Necrosis in the Senescent Leaves of Nicotiana benthamiana

“…The inhibition of defense response is one of the strategies of CaLas to establish colonization in citrus. CaLas caused phloem anatomical changes such as phloem necrosis, phloem plugging, and phloem collapse [12]. The phloem of the flowering plant consists of companion cells and mature sieve elements to transport phloem sap from source to sink [13].…”