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Protecting fruit from phytoplasma

Phytoplasma are highly specialised intracellular bacterial pathogens that are transmitted by insects as they feed on the sap of plants. The pathogen infects many important crops, reducing both the quality and quantity of fruit harvests by interfering with the flowering and fruiting of infected orchards.
Protecting fruit from phytoplasma
Although phytoplasma has caused significant damage to vineyards and apple orchards in Europe, scientists had a limited understanding of how phytoplasma–host interactions. Therefore, the project 'A characterisation of the effectors of a plant pathogen' (AY-WB EFFECTORS) was established to investigate the mechanisms by which phytoplasma infect plants and cause disease. The underlying hypothesis posed by AY-WB EFFECTORS was that phytoplasma use an effector, or virulence protein, which actively changes the development of infected plants.

Researchers tested their theory by investigating how the effector genes were regulated and by characterising possible phytoplasma effectors to discover whether these proteins can change plant phenotype. The genome of Aster Yellows phytoplasma strain Witches' Broom (AY-WB) was predicted to encode 56 effector genes. In order to study the regulation of these genes, scientists determined their expression levels in insect- and plant-colonising AY-WB. Researchers selected six candidate AY-WB effectors to determine whether these proteins possessed the ability to change plant development.

Scientists found that expression of the effector SAP54 in the model plant Arabidopsis induced the growth of leaf-like flowers with green petals, a process known as phyllody, to closely resemble flowers from phytoplasma-infected plants. The consortium further characterised SAP54 using a yeast two-hybrid screen against an Arabidopsis library to identify plant proteins that are recognised and bound by the bacterial effector. The technique was successfully used in the study of another effector protein, SAP11.

Results showed that SAP54 interacts with the MADS-domain gene family, which plays a key role in regulating the timing of flowering and the growth of sepals and petals. The analysis was extended to cover all 106 MADS-domain proteins encoded in the Arabidopsis genome.

Project partners postulated that the interaction between SAP54 and MADS-domain proteins was related to phyllody in plants infected with phytoplasma. They observed that SAP54 induces the degradation of the MADS-domain proteins when expressed in plants. Furthermore, this degradation involved the host ubiquitin-26S proteasome system. The yeast two-hybrid screen revealed that SAP54 interacts with Arabidopsis proteins RAD23C and RAD23D.

The work of AY-WB EFFECTORS will benefit farmers throughout Europe and the rest of the world through the development of phytoplasma-resistant crops, thereby contributing to global food security.

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