Effects of Candidatus Liberibacter asiaticus on assimilate accumulation and mineral nutrient transport in citrus leaves

Objectives Huanglongbing is one of the most devastating citrus bacterial diseases worldwide, mainly caused by Candidatus Liberibacter asiaticus (C. Las) infection. We investigated the relationship between leaf mineral elements and the development of Huanglongbing to provide a theoretical basis for revealing the pathogenesis of the disease.

Methods The surveyed citrus orchard was located in the Huanglongbing Disease Research Center, Yangshuo, Guangxi Province. Six healthy and diseased plants were selected, and the twigs in the middle part of the citrus trees with 6 to 8 leaves were chosen for leaf sampling. Leaf pathogen quantity, leaf temperature, branch assimilate transport and distribution were analyzed. X-ray fluorescence spectroscopy (XRF) method and ICP-MS method were used to measure to determine the nutrient contents and distribution in the leaves.

Results The C. Las mainly colonized in the vicinity of sieve tubes and companion cells in the leaves (i.e., midrib). The pathogen's cycle threshold (Ct value) in the midrib (P<0.05) decreased. Compared with healthy citrus leaves, the infected leaves were mottled yellow, with reduced chlorophyll content and increased leaf temperature. The starch, total soluble sugar and fructose (P<0.05) increased in the mesophyll and midribs, while the sucrose (P<0.05) increased in phloem. The accumulation of nutrient elements in different parts of the leaves significantly changed between the healthy and ill leaves. Ca and Fe accumulated in mesophyll, K in midrib, and Zn, Cu, and Mn were lacking in mesophyll. There was a (P<0.05) positive correlation between Cu and Zn in the mesophyll and a negative correlation between Ca and Mn and Cu in midrib. In the linear mixed effect model, the pathogen concentration had a (P<0.05) effect on K, Ca, Fe, Cu, Mn, and Zn, showing a positive effect on K, Cu, and Mn and a negative effect on Ca and Fe.

Conclusions The pathogen of Huanglongbing mainly colonized the midrib of leaves, blocked the transport of starch and total soluble sugar from mesophyll, midrib and phloem to xylem of the leaves. The mineral elements balance the nutrition through synergistic or antagonistic relationships to resist the growth of pathogens. The number of pathogens is positively correlated with K, Cu, and Mn content and negatively correlated with Ca and Fe in mesophyll. Therefore, Ca and Fe nutrition is essential for dealing with the Huanglong disease of citrus.