Diverse Targets of Phytoplasma Effectors: From Plant Development to Defense Against Insects

Abstract

Phytoplasma research begins to bloom (75). Indeed, this review shows that substantial progress has been made with the identification of phytoplasma effectors that alter flower development, induce witches' broom, affect leaf shape, and modify plant-insect interactions. Phytoplasmas have a unique life cycle among pathogens, as they invade organisms of two distinct kingdoms, namely plants (Plantae) and insects (Animalia), and replicate intracellularly in both. Phytoplasmas release effectors into host cells of plants and insects to target host molecules, and in plants these effectors unload from the phloem to access distal tissues and alter basic developmental processes. The effectors provide phytoplasmas with a fitness advantage by modulating their plant and insect hosts. We expect that further research on the functional characterization of phytoplasma effectors will generate new knowledge that is relevant to fundamental aspects of plant sciences and entomology, and for agriculture by improving yields of crops affected by phytoplasma diseases.

Transgenic Plants That Express the Phytoplasma Effector SAP11 Show Altered Phosphate Starvation and Defense Responses

“…Owing to the significant reduction in the genome of phytoplasmas, they cannot be cultured in vitro and are restricted to phloem sieve cells with a rich growth environment (Oshima et al, 2004;Bai et al, 2006). Although phytoplasmas depend on their plant host for survival, they are able to manipulate the plant's physiology through their secreted effectors (Sugio et al, 2011a).…”

Phytoplasma Effector SAP54 Induces Indeterminate Leaf-Like Flower Development in Arabidopsis Plants

“…Aster Yellows phytoplasma strain Witches' Broom (AY-WB; Candidatus Phytoplasma asteris) is vectored by the polyphagous aster leafhopper Macrosteles quadrilineatus, which readily transmits this pathogen to a wide range of plants, including members of the Solanaceae and Brassicaceae families (Sugio et al, 2011). Phytoplasmas such as AY-WB induce a variety of symptoms in plants that are indicative of an abnormal development of host tissues, including the formation of witches' broom (a dense mass of shoots originating from a single point), phyllody (conversion of floral organs into leaves), virescence (green pigmentation of tissues such as flower petals), and bolting (growth of elongated stalks).…”

“…Phytoplasmas such as AY-WB induce a variety of symptoms in plants that are indicative of an abnormal development of host tissues, including the formation of witches' broom (a dense mass of shoots originating from a single point), phyllody (conversion of floral organs into leaves), virescence (green pigmentation of tissues such as flower petals), and bolting (growth of elongated stalks). We hypothesized that phytoplasmas secrete virulence pro-teins (effectors) to induce such alterations in phenotype of host plants Bai et al, 2009;Sugio et al, 2011).…”

“…As phytoplasmas inhabit the cytoplasm of the immature and mature sieve cells that constitute the phloem, these bacteria are intracellular parasites and hence do not require an elaborate secretion system for delivering effectors across the plant cell membranes. Rather, phytoplasmas secrete effectors directly into the host cytoplasm of sieve cells via the Sec-dependent protein translocation pathway, and the effectors then unload from the phloem to target other plant cells by symplastic transport (Bai et al, 2009;Hoshi et al, 2009;Sugio et al, 2011). Putative secreted proteins can be identified by the presence of a signal peptide (Bendtsen et al, 2004) that is cleaved during export to yield a mature protein (Kakizawa et al, 2004).…”

Post-translational cleavage and self-interaction of the phytoplasma effector SAP11

“…3 The identification of phytoplasma-secreted proteins has revealed that phytoplasma effectors are virulence factors responsible for inducing morphological changes in infected plants. 4,5 For example, SAP11 secreted by the Aster Yellows phytoplasma strain Witches' Broom (AY-WB) and TENGU secreted by the Onion Yellows phytoplasma strain M (OY-M) induce the proliferation of branches similar to the witches' broom symptom.…”