The Endoplasmic Reticulum Is a Key Battleground between Phytoplasma Aggression and Host Plant Defense

Abstract

Phytoplasmas are intracellular plant pathogens that heavily rely on host cell nutrients for survival and propagation due to their limited ability to synthesize essential substrates. The endoplasmic reticulum (ER), which plays a vital role in various cellular processes, including lipid and protein biosynthesis, is an attractive target for numerous intracellular pathogens to exploit. This study investigated the impact of potato purple top (PPT) phytoplasma infection on the ER in tomato plants. Abnormal accumulation of ER-resident proteins, disrupted ER network structures, and formation of protein aggregates in the phloem were observed using confocal microscopy and transmission electron microscopy, indicating a phytoplasma-infection-induced disturbance in ER homeostasis. The colocalization of phytoplasmas with the accumulated ER-resident proteins suggests an association between ER stress, unfolded protein response (UPR) induction, and phytoplasma infection and colonization, with the ER stress response likely contributing to the host plant’s defense mechanisms. Quantitative real-time PCR revealed a negative correlation between ER stress/UPR activation and PPT phytoplasma titer, implying the involvement of UPR in curbing phytoplasma proliferation. Inducing ER stress and activating the UPR pathway effectively decreased phytoplasma titer, while suppressing the ER-resident protein, binding immunoglobulin protein (BiP) increased phytoplasma titer. These results highlight the ER as an intracellular battleground where phytoplasmas exploit host components for survival and multiplication, while host plants deploy defense mechanisms to counteract the invasion. Understanding the intricate interactions between phytoplasmas and plant hosts at the subcellular level, particularly within the ER, provides valuable insights for developing new strategies to control phytoplasma diseases.

Differential Symptomology, Susceptibility, and Titer Dynamics Manifested by Phytoplasma-Infected Periwinkle and Tomato Plants

“…This antibody is specific to PPT phytoplasma’s immunodominant membrane protein (IDP). IDP occupies a substantial portion of the phytoplasma cellular membrane, enabling visualization/detection of phytoplasma within infected plant samples, as documented in previous studies [ 8 , 14 ]. Western blot analysis with the anti-PPT-IDP antibody indirectly assessed the presence and the relative abundance of PPT phytoplasma in the infected plants.…”

“…To further assess the phytoplasma abundance, western blot analysis was employed as a double-blind confirmation method by using a polyclonal antibody, referred to as the anti-PPT-IDP antibody, which was generated in our previous study [ 8 ]. This antibody is specific to PPT phytoplasma’s immunodominant membrane protein (IDP).…”

“…The western blot analysis utilized the polyclonal anti-PPT-IDP antibody, specifically targeting the immunodominant protein of PPT phytoplasma. This antibody was produced in rabbits in our previous study [ 8 ]. First, proteins were extracted from distinct symptomatic tissues (100 mg) representing various infection stages of PPT phytoplasma-infected tomato and periwinkle plants, along with corresponding mock controls.…”

“…These bacteria possess small genomes, ranging from 530 to 1200 kb, which reflect their intracellular and parasitic lifestyle, as they depend heavily on host plants for nutrients [ 3 , 4 , 5 ]. Similar to other intracellular pathogens [ 6 , 7 ], phytoplasmas can exploit host cells for survival and multiplication while evading the host’s immune system [ 8 ]. Once phytoplasmas encounter a host plant and start to establish infection, the plant recognizes the pathogen and activates defense mechanisms.…”