Phytoplasma Infection Blocks Starch Breakdown and Triggers Chloroplast Degradation, Leading to Premature Leaf Senescence, Sucrose Reallocation, and Spatiotemporal Redistribution of Phytohormones

Abstract

Witches’-broom (WB, excessive initiation, and outgrowth of axillary buds) is one of the remarkable symptoms in plants caused by phytoplasmas, minute wall-less intracellular bacteria. In healthy plants, axillary bud initiation and outgrowth are regulated by an intricate interplay of nutrients (such as sugars), hormones, and environmental factors. However, how these factors are involved in the induction of WB by phytoplasma is poorly understood. We postulated that the WB symptom is a manifestation of the pathologically induced redistribution of sugar and phytohormones. Employing potato purple top phytoplasma and its alternative host tomato (Solanum lycopersicum), sugar metabolism and transportation, and the spatiotemporal distribution of phytohormones were investigated. A transmission electron microscopy (TEM) analysis revealed that starch breakdown was inhibited, resulting in the degradation of damaged chloroplasts, and in turn, premature leaf senescence. In the infected source leaves, two marker genes encoding asparagine synthetase (Sl-ASN) and trehalose-6-phosphate synthase (Sl-TPS) that induce early leaf senescence were significantly up-regulated. However, the key gibberellin biosynthesis gene that encodes ent-kaurene synthase (Sl-KS) was suppressed. The assessment of sugar content in various infected tissues (mature leaves, stems, roots, and leaf axils) indicated that sucrose transportation through phloem was impeded, leading to sucrose reallocation into the leaf axils. Excessive callose deposition and the resulting reduction in sieve pore size revealed by aniline blue staining and TEM provided additional evidence to support impaired sugar transport. In addition, a spatiotemporal distribution study of cytokinin and auxin using reporter lines detected a cytokinin signal in leaf axils where the axillary buds initiated. However, the auxin responsive signal was rarely present in such leaf axils, but at the tips of the newly elongated buds. These results suggested that redistributed sucrose as well as cytokinin in leaf axils triggered the axillary bud initiation, and auxin played a role in the bud elongation. The expression profiles of genes encoding squamosa promoter-binding proteins (Sl-SBP1), and BRANCHED1 (Sl-BRC1a and Sl-BRC1b) that control axillary bud release, as determined by quantitative reverse transcription (qRT)-PCR, indicated their roles in WB induction. However, their interactions with sugars and cytokinins require further study. Our findings provide a comprehensive insight into the mechanisms by which phytoplasmas induce WB along with leaf chlorosis, little leaf, and stunted growth.

Molecular characterization of ‘Candidatus Phytoplasma phoenicium’ infecting almond (Prunus dulcis) and evaluation of biochemical defenses produced in the plants

“…Wei et al. (2022) stated that chloroplasts were impaired by potato purple top (16SrII‐M) phytoplasma infection in tomato. Moreover, Buoso et al.…”

Phytoplasma: A plant pathogen that cannot be ignored in agricultural production—Research progress and outlook

“…Phytoplasmas have poor metabolic capacity, but their transport and secretion systems are relatively active (Chen et al., 2014; Saccardo et al., 2012). Most phytoplasmas are predicted to contain more than a dozen or dozens of secreted proteins, probably functioning as effectors (Oshima et al., 2004; Wei et al., 2022). In the past 15 years, the functions of many effectors have been revealed.…”

Bacteria–organelle communication in physiology and disease

“…In addition to Pseudomonas and Xanthomonas , another group of bacteria targeting chloroplast function is Phytoplasms . Once inside the plant, they infect chloroplasts, causing a range of symptoms, such as yellowing of leaves, reduced yield, dwarfing, and stunted growth ( Wei et al, 2022 ).…”