Phytoplasma infection blocks starch breakdown and triggers autophagic degradation of chloroplasts, leading to premature leaf senescence, sucrose reallocation, and spatiotemporal redistribution of phytohormones

Authors: Wei, Wei; Inaba, Junichi; Zhao, Yan; Mowery, Joseph; Hammond, Rosemarie

Submitted to: International Journal of Molecular Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/3/2022
Publication Date: 2/5/2022
Citation: Wei, W., Inaba, J., Zhao, Y., Mowery, J.D., Hammond, R. 2022. Phytoplasma infection blocks starch breakdown and triggers autophagic degradation of chloroplasts, leading to premature leaf senescence, sucrose reallocation, and spatiotemporal redistribution of phytohormones. International Journal of Molecular Sciences. 23(3):1810. https://doi.org/10.3390/ijms23031810.
DOI: https://doi.org/10.3390/ijms23031810

Interpretive Summary: Phytoplasma are small, wall-less bacteria that cause a series of typical symptoms in plants, including witches'- broom (WB). WB is characterized by excessive initiation and outgrowth of lateral buds in each leaf axil. Foliage in the WB structure is chlorotic and smaller in size. In this study, ARS researchers at the Beltsville Agricultural Research Center investigated the sugar content in various plant tissues and temporal and spatial distribution of phytohormones. The researchers unveiled that the starch breakdown was inhibited and autophagic degradation of chloroplasts occurred in the infected source leaves, resulting in premature leaf senescence. Phytoplasma infection severely impaired sugar metabolism and disrupted sucrose transport through phloem, resulting in sucrose reallocation to new sink tissues (leaf axils and axillary buds). Elevated levels of cytokinins in the leaf axils also promoted the initiation of new lateral buds. Based on these results, a working model was proposed to elucidate how phytoplasma infection impairs sugar metabolism and transport in plants, leading to leaf chlorosis, little leaf, stunting, root system reduction and WB symptoms. The findings of this study will contribute to a better understanding of the axillary bud formation process of and the signaling pathways involved in the whole process. The information is of great importance to scientists, students, and university professors studying agronomic traits, plant growth and development, and pathogen-host interactions.

Technical 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. Employing potato purple top phytoplasma and its alternative host tomato (Solanum lycopersicum) (cv. Money Maker), and DR5::GUS (auxin) and ARR5::GUS (cytokinin) reporter lines, sugar metabolism and transportation, and spatiotemporal distribution of phytohormones were investigated. Transmission electron microscopy (TEM) analysis revealed that starch breakdown was inhibited, resulting in autophagic degradation of damaged chloroplasts, and in turn, premature leaf senescence. In the infected mature leaves, two marker genes inducing early leaf senescence, Sl-ASN and Sl-TPS, were significantly up-regulated, while the key gibberellin biosynthesis gene, Sl-KS, was suppressed. The assessment of sugar content in various infected tissues indicated that sucrose transportation through phloem was impeded, leading to sucrose reallocation into the leaf axils. Redistributed sucrose, as well as cytokinin, accumulated in each leaf axil from which an axillary bud was initiated. The expression profiles of Sl-SBP1, Sl-BRC1a and Sl-BRC1b genes, controlling axillary bud release, were determined by quantitative real-time PCR, indicating their roles in WB induction, however, their interactions with sugars and cytokinins requires 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.