Fundo de Defesa da Citricultura (Fundecitrus), Araraquara SP, Brazil.
Universidade Estadual Paulista (UNESP), Jaboticabal SP, Brazil.
Plant Dis. 2021 Jan;105(1):34-42. doi: 10.1094/PDIS-01-20-0158-RE. Epub 2020 Nov 17.
' Liberibacter asiaticus' (Las) is an unculturable, phloem-limited, insect-transmitted bacterium associated with the Asiatic form of huanglongbing (HLB), the most destructive citrus disease. In Asia and the Americas, it is transmitted by the Asian citrus psyllid ( Kuwavama). Despite considerable research, little is known about the processes involved in plant infection and colonization by Las. This study was conducted to determine whether the basal portion (below girdling) of the plant is an important route for Las to move laterally from a point of inoculation on a branch to pathogen-free branches elsewhere in the canopy, and to quantify the influence of actively growing tissues on vertical upward (acropetally) or downward (basipetally) movement of Las. Nongirdled and fully or partially girdled stems of potted plants of 'Pera' sweet orange, graft-inoculated above or below girdling, were sampled in distinct regions and assessed by qPCR, 6 months postinoculation. Las invaded all regions of partially and nongirdled plants but remained restricted to the inoculated regions of fully girdled plants, evidence that in planta bacterium movement is limited to the phloem. In fully girdled plants, starch accumulated above the girdling site, probably because of changes in flow of phloem sap. To study the influence of actively growing tissues, inoculated 'Valencia' sweet orange plants were kept intact or were top- or root-pruned to force production of new tissues, and sampled at 15-day intervals. Las migrated rapidly and most predominantly toward newly developing root and leaf tissues. The rapid and predominant movement of Las to newly developed shoots and roots would explain failures of canopy heat treatments and pruning to cure HLB-affected trees, and reinforces the need to protect rapidly growing new shoots from feeding by in order to minimize transmission and spread of the pathogen by the vector within and between orchards.
'Liberibacter asiaticus'（Las）是一种无法培养、局限于韧皮部、经昆虫传播的细菌，与亚洲黄龙病（HLB）有关，这是最具破坏性的柑橘病害。在亚洲和美洲，它由亚洲柑橘木虱（Kuwavama）传播。尽管进行了大量研究，但对 Las 感染和定植植物的过程知之甚少。本研究旨在确定植物的基部（环割以下）是否是 Las 从树枝上的接种点侧向移动到树冠其他无病原体分支的重要途径，并量化活跃生长组织对 Las 垂直向上（向顶）或向下（向基）运动的影响。对嫁接接种在环割上方或下方的盆栽'Pera'甜橙的未环割和完全或部分环割的茎进行采样，并在接种后 6 个月通过 qPCR 进行评估。Las 侵入了部分和未环割植物的所有区域，但仍局限于完全环割植物的接种区域，这表明植物内细菌运动仅限于韧皮部。在完全环割的植物中，环割部位上方的淀粉积累，可能是由于韧皮部汁液流动的变化。为了研究活跃生长组织的影响，将接种的'Valencia'甜橙植物保持完整或进行顶部或根部修剪，以迫使新组织的产生，并在 15 天的间隔内进行采样。Las 迅速且主要向新发育的根和叶组织迁移。Las 迅速且主要向新发育的嫩枝和根部移动，这将解释树冠热处理和修剪未能治愈 HLB 感染的树木的原因，并加强需要保护快速生长的新嫩枝免受的侵害，以最大程度地减少介体在果园内和之间传播和传播病原体。
