Phloem Production in Huanglongbing-affected Citrus Trees

Abstract

Citrus greening disease [Huanglongbing (HLB)] is the most significant and widespread threat to the citrus industry in recent history. A bacterium [Candidatus Liberibacter asiaticus (CLas)] vectored by the Asian citrus psyllid is the presumed causal agent of the disease, which results in the collapse of phloem tissue, leading to decreased productivity, chlorotic leaves, and bitter, misshapen fruit. Once infected, trees never fully recover and there currently is no cure, although foliar nutrient sprays and intensive irrigation appear to slow tree decline in some situations. Despite phloem necrosis in older tissue, new vegetative and reproductive growth occurs. Our current understanding of phloem collapse in citrus resulting from HLB is based on anatomical reports of trees in different stages of decline and does not explain the persistence of growth. Here, we present data that show new phloem cells are produced during the periodic flushes of vegetative growth and their subsequent collapse and plugging over a 6-month period. Cellular activity within the cambium and the ray parenchyma was diminished in HLB-affected petioles, suggesting an important link in the carbohydrate transport pathway is missing. Because of the short window of time during which the phloem appears healthy, the weeks immediately before and after the spring and summer flush are of critical importance for the management of citrus health.