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

Detection and Identification of Diverse Phytoplasmas in Declining Persimmon Plants

Persimmon (Diospyros kaki) plants showing yellowing, reddening, die-back, and decline symptoms were observed in Mehriz (Yazd province), Iran. Total DNAs, extracted from samples collected from symptomatic and symptomless plants, were subjected to direct and nested PCR, amplifying the 16S rRNA gene of phytoplasmas using specific primer pairs. PCR amplicons of expected lengths were obtained, mainly from nested PCR, and only from samples collected from symptomatic plants. Real and virtual RFLP, phylogenetic, and DNA identity analyses of the partial 16S rRNA gene sequences suggested the presence of diverse phytoplasmas in the analyzed samples. The identified phytoplasmas were referable to 'Candidatus Phytoplasma omanense' (16SrXXIX group) and 'Ca. P. australasiae = australasiaticum' (16SrII-D subgroup). The results of the sampling and testing highlight the urgent need for an accurate survey to verify the presence and identity of phytoplasmas in symptomatic fruit trees in Iran, in order to be able to plan appropriate management strategies. Further investigations of the possible role of 'Ca. P. omanense' strains as an emerging threat to fruit orchards in Iran should also be performed.

The phytoplasma SAP54 effector acts as a molecular matchmaker for leafhopper vectors by targeting plant MADS-box factor SVP

Obligate parasites often trigger significant changes in their hosts to facilitate transmission to new hosts. The molecular mechanisms behind these extended phenotypes - where genetic information of one organism is manifested as traits in another - remain largely unclear. This study explores the role of the virulence protein SAP54, produced by parasitic phytoplasmas, in attracting leafhopper vectors. SAP54 is responsible for the induction of leaf-like flowers in phytoplasma-infected plants. However, we previously demonstrated that the insects were attracted to leaves and the leaf-like flowers were not required. Here, we made the surprising discovery that leaf exposure to leafhopper males is required for the attraction phenotype, suggesting a leaf response that distinguishes leafhopper sex in the presence of SAP54. In contrast, this phytoplasma effector alongside leafhopper females discourages further female colonization. We demonstrate that SAP54 effectively suppresses biotic stress response pathways in leaves exposed to the males. Critically, the host plant MADS-box transcription factor short vegetative phase (SVP) emerges as a key element in the female leafhopper preference for plants exposed to males, with SAP54 promoting the degradation of SVP. This preference extends to female colonization of male-exposed svp null mutant plants over those not exposed to males. Our research underscores the dual role of the phytoplasma effector SAP54 in host development alteration and vector attraction - integral to the phytoplasma life cycle. Importantly, we clarify how SAP54, by targeting SVP, heightens leaf vulnerability to leafhopper males, thus facilitating female attraction and subsequent plant colonization by the insects. SAP54 essentially acts as a molecular 'matchmaker', helping male leafhoppers more easily locate mates by degrading SVP-containing complexes in leaves. This study not only provides insights into the long reach of single parasite genes in extended phenotypes, but also opens avenues for understanding how transcription factors that regulate plant developmental processes intersect with and influence plant-insect interactions.

Virulence effectors encoded in the rice yellow dwarf phytoplasma genome participate in pathogenesis

Bacteria-like phytoplasmas alternate between plant and insect hosts, secreting proteins that disrupt host development. In this study, we sequenced the complete genome of "Candidatus Phytoplasma oryzae" strain HN2022, associated with rice yellow dwarf (RYD) disease, using PacBio HiFi technology. The strain was classified within the 16Sr XI-B subgroup. Through SignalP v5.0 for prediction and subsequent expression analysis of secreted proteins in Nicotiana benthamiana and rice (Oryza sativa L.), we identified the key virulence effector proteins RY348 and RY378. RY348, a homolog of Secreted Aster Yellows Phytoplasma Effector 54 (SAP54), targets and degrades the MADS-box transcription factors MADS1 and MADS15, causing pollen sterility. Meanwhile, RY378 impacts the strigolactone and auxin signaling pathways, substantially increasing tillering. These findings offer insights into the interactions between plants and phytoplasmas.

Molecular Variation and Phylogeny of Thymidylate Kinase Genes of Candidatus Phytoplasma ziziphi from Different Resistant and Susceptible Jujube Cultivars in China

The thymidylate kinase (tmk) gene is indispensable for the proliferation and survival of phytoplasma. To reveal the molecular variation and phylogeny of the tmk genes of Candidatus phytoplasma ziziphi, in this study, the tmk genes of 50 phytoplasma strains infecting different resistant and susceptible jujube cultivars from different regions in China were amplified and analyzed. Two sequence types, tmk-x and tmk-y, were identified using clone-based sequencing. The JWB phytoplasma strains were classified into three types, type-X, type-Y, and type-XY, based on the sequencing chromatograms of the tmk genes. The type-X and type-Y strains contained only tmk-x and tmk-y genes, respectively. The type-XY strain contained both tmk-x and tmk-y genes. The type-X, type-Y, and type-XY strains comprised 42%, 12%, and 46% of all the strains, respectively. The type-X and type-XY strains were identified in both susceptible and resistant jujube cultivars, while type-Y strain was only identified in susceptible cultivars. Phylogenetic analysis indicated that the tmk genes of the phytoplasmas were divided into two categories: phylo-S and phylo-M. The phylo-S tmk gene was single-copied in the genome, with an evolutionary pattern similar to the 16S rRNA gene; the phylo-M tmk gene was multi-copied, related to PMU-mediated within-genome transposition and between-genome transfer. Furthermore, the phylogenetic tree suggested that the tmk genes shuttled between the genomes of the Paulownia witches' broom phytoplasma and JWB phytoplasma. These findings provide insights into the evolutionary and adaptive mechanisms of phytoplasmas.

Heat Stress and Plant-Biotic Interactions: Advances and Perspectives

Climate change presents numerous challenges for agriculture, including frequent events of plant abiotic stresses such as elevated temperatures that lead to heat stress (HS). As the primary driving factor of climate change, HS threatens global food security and biodiversity. In recent years, HS events have negatively impacted plant physiology, reducing plant's ability to maintain disease resistance and resulting in lower crop yields. Plants must adapt their priorities toward defense mechanisms to tolerate stress in challenging environments. Furthermore, selective breeding and long-term domestication for higher yields have made crop varieties vulnerable to multiple stressors, making them more susceptible to frequent HS events. Studies on climate change predict that concurrent HS and biotic stresses will become more frequent and severe in the future, potentially occurring simultaneously or sequentially. While most studies have focused on singular stress effects on plant systems to examine how plants respond to specific stresses, the simultaneous occurrence of HS and biotic stresses pose a growing threat to agricultural productivity. Few studies have explored the interactions between HS and plant-biotic interactions. Here, we aim to shed light on the physiological and molecular effects of HS and biotic factor interactions (bacteria, fungi, oomycetes, nematodes, insect pests, pollinators, weedy species, and parasitic plants), as well as their combined impact on crop growth and yields. We also examine recent advances in designing and developing various strategies to address multi-stress scenarios related to HS and biotic factors.

Paulownia Witches' Broom Disease: A Comprehensive Review

Phytoplasmas are insect-transmitted bacterial pathogens associated with diseases in a wide range of host plants, resulting in significant economic and ecological losses. Perennial deciduous trees in the genus Paulownia are widely planted for wood harvesting and ornamental purposes. Paulownia witches' broom (PaWB) disease, associated with a 16SrI-D subgroup phytoplasma, is a destructive disease of paulownia in East Asia. The PaWB phytoplasmas are mainly transmitted by insect vectors in the Pentatomidae (stink bugs), Miridae (mirid bugs) and Cicadellidae (leafhoppers) families. Diseased trees show typical symptoms, such as branch and shoot proliferation, which together are referred to as witches' broom. The phytoplasma presence affects the physiological and anatomical structures of paulownia. Gene expression in paulownia responding to phytoplasma presence have been studied at the transcriptional, post-transcriptional, translational and post-translational levels by high throughput sequencing techniques. A PaWB pathogenic mechanism frame diagram on molecular level is summarized. Studies on the interactions among the phytoplasma, the insect vectors and the plant host, including the mechanisms underlying how paulownia effectors modify processes of gene expression, will lead to a deeper understanding of the pathogenic mechanisms and to the development of efficient control measures.

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

Phytoplasmas are phloem-restricted plant-pathogenic bacteria transmitted by insects. They cause diseases in a wide range of host plants, resulting in significant economic and ecological losses worldwide. Research on phytoplasmas has a long history, with significant progress being made in the past 30 years. Notably, with the rapid development of phytoplasma research, scientists have identified the primary agents involved in phytoplasma transmission, established classification and detection systems for phytoplasmas, and 243 genomes have been sequenced and assembled completely or to draft quality. Multiple possible phytoplasma effectors have been investigated, elucidating the molecular mechanisms by which phytoplasmas manipulate their hosts. This review summarizes recent advances in phytoplasma research, including identification techniques, host range studies, whole- or draft-genome sequencing, effector pathogenesis and disease control methods. Additionally, future research directions in the field of phytoplasma research are discussed.