Abstract
BackgroundCitrus Huanglongbing (HLB), which is linked to the bacterial pathogen ‘Ca. Liberibacter asiaticus’ (Las), is the most devastating disease of citrus plants, and longer-term control measures via breeding or genetic engineering have been unwieldy because all cultivated citrus species are susceptible to the disease. However, the degree of susceptibility varies among citrus species, which has prompted efforts to identify potential Las resistance/tolerance-related genes in citrus plants for application in breeding or genetic engineering programs. Plant exposure to one form of stress has been shown to serendipitously induce innate resistance to other forms of stress and a recent study showed that continuous heat treatment (40 to 42 °C) reduced Las titer and HLB-associated symptoms in citrus seedlings. The goal of the present study was to apply comparative proteomics analysis via 2-DE and mass spectrometry to elucidate the molecular processes associated with heat-induced mitigation of HLB in citrus plants. Healthy or Las-infected citrus grapefruit plants were exposed to room temperature or to continuous heat treatment of 40 °C for 6 days.ResultsAn exhaustive total protein extraction process facilitated the identification of 107 differentially-expressed proteins in response to Las and/or heat treatment, which included a strong up-regulation of chaperones including small (23.6, 18.5 and 17.9 kDa) heat shock proteins, a HSP70-like protein and a ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO)-binding 60 kDa chaperonin, particularly in response to heat treatment. Other proteins that were generally down-regulated due to Las infection but up-regulated in response to heat treatment include RuBisCO activase, chlorophyll a/b binding protein, glucosidase II beta subunit-like protein, a putative lipoxygenase protein, a ferritin-like protein, and a glutathione S-transferase.ConclusionsThe differentially-expressed proteins identified in this study highlights a premier characterization of the molecular mechanisms potentially involved in the reversal of Las-induced pathogenicity processes in citrus plants and are hence proposed targets for application towards the development of cisgenic Las-resistant/tolerant citrus plants.Electronic supplementary materialThe online version of this article (doi:10.1186/s12870-016-0942-x) contains supplementary material, which is available to authorized users.
Highlights
Citrus Huanglongbing (HLB), which is linked to the bacterial pathogen ‘Ca. Liberibacter asiaticus’ (Las), is the most devastating disease of citrus plants, and longer-term control measures via breeding or genetic engineering have been unwieldy because all cultivated citrus species are susceptible to the disease
These results are consistent with those from Hoffman et al [14] and Yang et al [15], which showed thermotherapy-induced reduction in Las titer of HLB-affected citrus plants. Both Las infection and heat treatment confer significant effects on citrus leaf proteomics The exhaustive total protein extraction method used in this study produced an average protein yield of over 20 mg g−1 from citrus grapefruit leaves irrespective of Las or heat treatment (Table 1), which is higher than the mean protein yield of approximately 13 mg g−1 from citrus grapefruit leaves in our a high resolution of total protein separation in a 4–7 Isoelectric point (pI) range and 10–150 kDa molecular mass was observed in Two dimensional electrophoresis (2-DE) gels of total leaf proteins from citrus grapefruit plants (Fig. 2)
HLB, which is etiologically-linked to Las, is currently the most destructive disease of citrus and all commercially grown citrus species/relatives are susceptible to the disease
Summary
Citrus Huanglongbing (HLB), which is linked to the bacterial pathogen ‘Ca. Liberibacter asiaticus’ (Las), is the most devastating disease of citrus plants, and longer-term control measures via breeding or genetic engineering have been unwieldy because all cultivated citrus species are susceptible to the disease. Koch’s postulate has yet to be fulfilled, HLB is etiologically-linked to three species of insect-transmissible fastidious, phloem-restricted α-proteobacteria: ‘Candidatus Liberibacter asiaticus’ (Las), ‘Ca. L. africanus’ (Laf ), and ‘Ca. L. americanus’ (Lam) [2]. Among these three Liberibacter species, Las has the largest geographical distribution and is the species present in the U.S [1, 2]. The steep financial burden concomitant with current HLB control measures, especially for small scale growers, coupled with the increased incidence and severity of HLB around the world underscores the need for more effective control measures
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