Abstract
Phytoplasmas are bacterial pathogens that live mainly in the phloem of their plant hosts. They dramatically manipulate plant development by secreting effector proteins that target developmental proteins of their hosts. Traditionally, the effects of individual effector proteins have been studied by ectopic overexpression using strong, ubiquitously active promoters in transgenic model plants. However, the impact of phytoplasma infection on the host plants depends on the intensity and timing of infection with respect to the developmental stage of the host. To facilitate investigations addressing the timing of effector protein activity, we have established chemical-inducible expression systems for the three most well-characterized phytoplasma effector proteins, SECRETED ASTER YELLOWS WITCHES’ BROOM PROTEIN 11 (SAP11), SAP54 and TENGU in transgenic Arabidopsis thaliana. We induced gene expression either continuously, or at germination stage, seedling stage, or flowering stage. mRNA expression was determined by quantitative reverse transcription PCR, protein accumulation by confocal laser scanning microscopy of GFP fusion proteins. Our data reveal tight regulation of effector gene expression and strong upregulation after induction. Phenotypic analyses showed differences in disease phenotypes depending on the timing of induction. Comparative phenotype analysis revealed so far unreported similarities in disease phenotypes, with all three effector proteins interfering with flower development and shoot branching, indicating a surprising functional redundancy of SAP54, SAP11 and TENGU. However, subtle but mechanistically important differences were also observed, especially affecting the branching pattern of the plants.
Highlights
Phytoplasmas are obligate bacterial pathogens of both plants and insects [1]
Transgenic plants carrying the pBb7m34GW35S::XVE vector with no coding sequence cloned under control of the LexA operator, nor transgenic LexA::mGFP6 plants kept on non-inductive medium (NM), showed any green fluorescence signal (Figure 1a,b,d)
Phytoplasma cells secrete a whole cocktail of effector proteins into the phloem of their host plants, so that it is difficult to attribute the different effects on the phenotype to individual effector proteins
Summary
Phytoplasmas are obligate bacterial pathogens of both plants and insects [1]. Phytoplasmas are spread and introduced into the tissue of new host plants by insect vectors feeding on noninfected plants [1]. As with any tritrophic relationship, the dynamic of a phytoplasma outbreak and the spread from plant to plant depends on the spatiotemporal overlap of vulnerable host plants, pathogens, and vectors, each being influenced by many biotic and abiotic factors [3,4]. Phytoplasmas live in the phloem, and cause a plethora of morphological changes to the host plant, including bolting, witches’ broom, dwarfism, reddening or yellowing of leaves and stems, virescence, phyllody, and phloem necrosis [1]. One of the well-characterized phytoplasma effector proteins, SECRETED ASTER YELLOWS WITCHES’ BROOM PROTEIN
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