Pandey Sheo Shankar, Li Jinyun, Oswalt Chris, Wang Nian
Citrus Research and Education Center (CREC), Department of Microbiology and Cell Science, Institute of Food and Agricultural Sciences, University of Florida, Lake Alfred, FL 33850, U.S.A.
Current affiliation: Life Sciences Division, Institute of Advanced Study in Science and Technology (IASST), Guwahati 781035, India.
Phytopathology. 2024 May;114(5):961-970. doi: 10.1094/PHYTO-08-23-0294-KC. Epub 2024 May 6.
Citrus Huanglongbing (HLB) caused by ' Liberibacter asiaticus' (CLas) is the most devastating citrus disease worldwide. CLas induces systemic and chronic reactive oxygen species (ROS) production, which has been suggested to be a primary cause of cell death in phloem tissues and subsequent HLB symptoms. Mitigating oxidative stress caused by CLas using horticultural approaches has been suggested as a useful strategy to reduce HLB damages. To provide information regarding the application timing to mitigate ROS, we investigated monthly dynamics of CLas concentration, CLas-triggered ROS, and phloem cell death in the bark tissues of asymptomatic and symptomatic branches of HLB-positive Hamlin and Valencia sweet orange trees in the field. Healthy branches in the screenhouse were used as controls. CLas concentration exhibited significant variations over the course of the year, with two distinct peaks observed in Florida citrus groves-late spring/early summer and late fall. Within both Hamlin and Valencia asymptomatic tissues, CLas concentration demonstrated a negative correlation with the deviation between the monthly average mean temperature and the optimal temperature for CLas colonization in plants (25.7°C). However, such a correlation was not evident in symptomatic tissues of Hamlin or Valencia sweet oranges. ROS levels were consistently higher in symptomatic or asymptomatic branches than in healthy branches in most months. ROS concentrations were higher in symptomatic branches than in asymptomatic branches in most months. CLas triggered significant increases in ion leakage in most months for asymptomatic and symptomatic branches compared with healthy controls. In asymptomatic branches of Hamlin, a positive correlation was observed between CLas concentration and ROS concentrations, CLas concentration and ion leakage levels, as well as ROS and ion leakage. Intriguingly, such a relationship was not observed in Valencia asymptomatic branches or in the symptomatic branches of Hamlin and Valencia. This study sheds light on the pathogenicity of CLas by providing useful information on the temporal dynamics of ROS production, phloem cell death, and CLas growth, as well as provides useful information in determining the timing for application of antioxidants and antimicrobial agents to control HLB.
由亚洲韧皮杆菌（CLas）引起的柑橘黄龙病（HLB）是全球最具毁灭性的柑橘病害。CLas会诱导系统性和慢性活性氧（ROS）的产生，这被认为是韧皮部组织细胞死亡及随后出现HLB症状的主要原因。有人提出采用园艺方法减轻由CLas引起的氧化应激是减少HLB损害的一种有效策略。为了提供有关减轻ROS的施用时机的信息，我们调查了田间HLB阳性哈姆林和巴伦西亚甜橙树无症状和有症状枝条树皮组织中CLas浓度、CLas引发的ROS以及韧皮部细胞死亡的月度动态变化。温室中的健康枝条用作对照。CLas浓度在一年中呈现出显著变化，在佛罗里达柑橘园中观察到两个明显的峰值——春末/初夏和深秋。在哈姆林和巴伦西亚的无症状组织中，CLas浓度与月平均温度与CLas在植物中定殖的最佳温度（25.7°C）之间的偏差呈负相关。然而，这种相关性在哈姆林或巴伦西亚甜橙的有症状组织中并不明显。在大多数月份，有症状或无症状枝条中的ROS水平始终高于健康枝条。在大多数月份，有症状枝条中的ROS浓度高于无症状枝条。与健康对照相比，在大多数月份，CLas会使无症状和有症状枝条的离子渗漏显著增加。在哈姆林的无症状枝条中，观察到CLas浓度与ROS浓度、CLas浓度与离子渗漏水平以及ROS与离子渗漏之间呈正相关。有趣的是，在巴伦西亚无症状枝条或哈姆林和巴伦西亚的有症状枝条中未观察到这种关系。这项研究通过提供有关ROS产生、韧皮部细胞死亡和CLas生长的时间动态的有用信息，揭示了CLas的致病性，同时也为确定施用抗氧化剂和抗菌剂以控制HLB的时机提供了有用信息。
