Molecular and cellular analysis of orange plants infected with Huanglongbing (citrus greening disease)

Huanglongbing (HLB, also commonly known as citrus greening disease) is caused by the bacterium Candidatus Liberibacter asiaticus (CLas) in the US. It is a serious plant disease causing tremendous economic damages and posing an existential threat to the citrus industry worldwide. CLas proliferates inside the phloem, clogs the transportation system, stunts plant growth and renders trees barren. Currently there is no cure or effective treatments for this disease. In this study, qPCR and scanning electron microscopy (SEM) were used to examine orange plants with or without HLB symptoms. We found that symptomatic HLB plants showed high levels of CLas DNA and its prophage in leaves and had heavily clogged phloem tissues. In contrast, the asymptomatic, healthy looking plants had detectable, low levels of CLas and prophage DNA by qPCR but the SEM imaging revealed phloem clear of clogging deposits. Transcription of several CLas phage genes was detected in both healthy and diseased plants, however, higher transcription levels were observed in the healthy plants than in the diseased plants. As part of our efforts to explore the possibility of using a plant-based antibacterial concoction named Agent G to mitigate HLB, NMR and GC/MS were used to assess its organic chemical composition and migration through the phloem tissue. It was found that the major compounds of Agent G were allyl polysulfides and cinnamaldehyde. The phloemic migrate rate in planta was estimated between 1.5 and 3.25 cm/h. Our studies suggest that a combination of qPCR test and SEM imaging may prove to be more informative for gauging growth status of citrus plants and for early HLB detection. We also showcase a feasible approach of treating HLB by delivering antibacterial reagents directly into the phloem tissue where most CLas reside.