Expression Level of a Phenylalanine Ammonia-Lyase Gene in Poinsettia Is Negatively Correlated with Poinsettia Branch-Inducing Phytoplasma Titer

Poinsettia is an important ornamental cultivated worldwide. Commercial poinsettias are almost universally infected with a pathogen known as the poinsettia branch-inducing phytoplasma (PoiBI), which can increase the level of branching in host plants and make the plants more desirable to consumers. Despite PoiBI’s crucial role in poinsettia production, little is known about PoiBI-poinsettia interactions in regard to the pathogen’s in planta population dynamics. The expression profiles of a phenylalanine ammonia-lyase gene (Euphorbia pulcherrima PAL [EpPAL]) and the PoiBI titers in poinsettia tissues were investigated. Differential gene expression analyses using quantitative PCR (qPCR) showed that EpPAL expression levels differed significantly across tissue types. The highest expression levels were detected in stems, followed by root. Lower EpPAL expression levels were detected in leaf tissues, particularly in source leaves closer to the base; the average expression level in these leaves was only one-seventh of that detected in stems. Phytoplasma concentrations in source leaves close to the base were significantly greater than the other tissue types; the average value was 7.6-fold of that detected in stem tissues, which had the lowest phytoplasma titers. A negative correlation between EpPAL expression level and PoiBI load was detected, suggesting that the products of EpPAL-associated pathways or other genes indirectly associated with EpPAL may interfere with PoiBI’s growth. While additional studies are needed to validate these interpretations, the results from this work provide new insights into PoiBI-poinsettia interaction and showed that correlations between pathogen load and defense-related genes could be detected in phytoplasma-associated pathosystems. IMPORTANCE Phytoplasma-plant interactions are interesting subjects for fundamental and applicative research. Although many studies have characterized molecular interplays between these pathogens and hosts, knowledge on relationships between phytoplasmas’ in planta population dynamics and host gene expression remains scarce. By using the poinsettia branch-inducing phytoplasma (PoiBI) and poinsettia as a model system, a negative correlation was observed between the expression level of a plant defense-related gene and the pathogen’s titer. The findings provide potential explanations to PoiBI’s distribution patterns in the plant and highlight the importance of studying phytoplasma-plant interactions in regard to the pathogen’s population dynamics in other pathosystems.