Dissecting phytoplasma effector adaptation to plant targets (Bilateral BBSRC-FAPESP application)

Phytoplasmas are intracellular bacterial pathogens of plants that are transmitted by insect vectors. They induce a variety of symptoms in plants and crops, such as witches' brooms (increased lateral branching) and phyllody (flowers reverting into leaves that remain sterile). Phytoplasmas can negatively impact crop production in many regions of the world. They interfere with flower production and decline fruit/grain yields, a problem in maize, fruit trees, grapevines, coconut etc. Because phytoplasmas are insect-transmitted, their occurrence is expanding as the climate warms up and pesticide use is being restricted.

The Hogenhout (SH) group at The John Innes Centre has made significant progress with phytoplasma pathogenesis and their impact on host plants. This group found that phytoplasmas produce specific virulence proteins (effectors), which interact with conserved plant proteins leading to crinkled leaves, increased lateral branching (witches' brooms) and the downregulation of plant defence responses to the phytoplasma insect vectors. The latter leads to a greater number of insect vectors that transmit the phytoplasma to other plants. So far, the SH group has focused on phytoplasma interactions with the model plant Arabidopsis thaliana.

In this project, the SH group will collaborate with the Spotti-Lopes (JSL) group at The University of Sao Paulo in Brazil who has studied the ecology and epidemiology of an important phytoplasma disease agent of maize. This pathogen causes maize bushy stunt disease and is transmitted by the corn leafhopper, which builds up to high population levels in maize fields in Brazil, Argentina and Mexico, therefore triggering severe phytoplasma epidemics. Maize bushy stunt phytoplasma and the corn leafhopper have co-evolved with the domestication of maize from its wild progenitor, teosinte, and hence are well adapted to colonize maize. Phytoplasma-infected maize plants typically produce more primary and secondary lateral branches (hence, the name maize bushy stunt phytoplasma), fewer ears and lower grain yields. These infected plants may support a greater number of corn leafhoppers that transmit maize bushy stunt phytoplasma to other maize plants.

Together the SH and JSL group will translate the knowledge from the phytoplasma-Arabidopsis system to investigate maize bushy stunt phytoplasma and the corn leafhopper in maize. More specifically, we will determine if a maize bushy stunt phytoplasma effector protein induces the increased branching and reduced grain yield symptoms typically observed in phytoplasma-infected maize. Moreover, we will determine of this effector suppresses plant immunity to the corn leafhopper leading to greater leafhopper populations. We will also investigate the genomic variation of maize bushy stunt phytoplasmas in Brazil. Finally, we will assess if we can generate maize varieties that are not targeted by the phytoplasma effector protein leading to a reduction in maize bushy stunt symptoms and increased grain production during phytoplasma epidemics. We will collaborate with the agricultural company Dow Agrosciences. This project will provide exchange visits and a learning platform for all staff involved. Overall this work will increase our fundamental understanding of how pathogen virulence factors (effectors) adapt to host targets thereby facilitating pathogen colonization of these hosts and pathogen dispersal by insect vectors.

Phytoplasmas are intracellular bacterial pathogens of plants that are transmitted by insect vectors, such as leafhoppers. They induce a variety of symptoms in plants and crops, such as witches' brooms (increased lateral branching) and phyllody (flowers reverting into leaves that remain sterile).

The Hogenhout group at The John Innes Centre has identified the virulence protein (effector) SAP11 from Aster Yellows Witches' Broom phytoplasma (AY-WB) that destabilize class II CIN members of the TCP transcription factor family, which are conserved among plants. SAP11-mediated CIN-TCP destabilization induces crinkled leaves and reduces synthesis of the defense plant hormone jasmonic acid in Arabidopsis. This leads to an increase in the number of aster leafhoppers that transmit AY-WB to other plants. We have also evidence that SAP11 interacts with class II CYC/tb1 TCPs that include Arabidopsis Branched 1 (BRC1), possibly resulting in increased branching (witches' brooms) symptoms of AY-WB-infected plants.

The SH group has identified a homolog of the SAP11 protein (SMP11) in the genome of Maize bushy stunt phytoplasma (MBSP). SMP11 induces increased branching but not crinkled leaves in Arabidopsis suggesting that SMP11 interacts with class II CYC/tb1 TCPs, but not the CIN-TCPs. It is also possible that SMP11 interacts with maize CIN-TCPs, but not Arabidopsis CIN-TCPs.

In this project we will collaborate with the Spotti-Lopes group at The University of Sao Paulo University in Brazil who are experienced with the ecology and epidemiology of MBSP, an important phytoplasma disease agent of maize in Brazil. We will (1) investigate if SMP11 and SAP11 interact with various classes of Arabidopsis and maize TCPs, (2) study genomic variations Brazilian MBSP isolates, and (3) Investigate the contributions of SMP11 and SAP11 to phytoplasma symptom development and leafhopper-plant interactions. Overall, this work increases our understanding of pathogen-host adaptation processes.

Maize can be used as food, animal feed or to make biofuels. It is the most widely grown crop in the Americas and has been introduced into many other countries worldwide. Corn is included in the focus of the European Biofuels Technology Platform to develop new technologies for industrial-scale production of Second Generation (2G) biofuels (advanced biofuels) from sustainable feedstocks, because corn stover (i.e. leaves and stalks of corn plants left in the field after harvesting the edible corn grain) could supply up to 25% of the biofuel crop needed by 2030.

Maize bushy stunt phytoplasma (MBSP) is a major pathogen associated with corn stunt disease in Brazil and throughout Latin America, which is transmitted by the corn leafhopper, Dalbulus maidis. It has a wide geographical distribution that spans much of South America, Central America and the southern United States. Maize plants diagnosed with corn stunt disease exhibit a variety of symptoms, including increased branching, development of several small ears, ears with loose kernels, and overall plant stunting. Maize fields with high incidences of this disease show severe yield losses (up to 100%), affecting the production of food for both livestock and human consumption. The work proposed in this project will advance the use of corn as a sustainable, advanced biofuel (second generation and beyond) that is a focus in the BBSRC funding priority area Crop Science.

Strategies to control phytoplasmas typically rely on the use of pesticides to manage insect vector populations, with limited success in preventing disease spread. Approximately 80% of corn grown the USA is genetically modified (mostly Bt maize hybrids) and use of Bt maize is increasing exponentially in South American countries (particularly in Brazil and Argentina). Thus, insecticide treatments that were previously used to control lepidopteran pests will now be reduced. As a result, corn stunt disease incidence is likely to increase. This work will be useful for the identification of maize lines that are more resistant to the induction of increased (lateral) branching by MBSP infection (through genetic modification, tilling or classical breeding approaches).

This project will involve the participation of Dow AgroSciences, Brazil. They will contribute in-kind to this project by coordinating the leaf sampling in three important maize growing regions that are frequently affected with MBSP infections in Brazil. Genetic variability and pathogenicity mechanisms have not yet been investigated for MBSP and this information will be of great value for breeders.

We also plan to present talks in conferences largely attended by maize growers and technical personnel, such as the 'Brazilian Congress of Maize and Sorghum' or the 'Annual Seminar of "Safrinha" (second season) Maize'.

This work fits with the BBSRC themes "research on insect pests that affect crop performance", "translation of research outputs into practical use and application by the agriculture and food industries" and "alternatives to chemical inputs" that are listed under the BBSRC funding priority area Crop Science. It also adheres to "pests and diseases of crops and livestock" listed under the BBSRC funding priority area Living with Environmental Change. This work also takes a systems approach by combining molecular and ecological aspects of bacterial adaptation to plants and broadly fits with the BBSRC theme "Systems based approach to biology". This project seeks support from multiple agencies (BBSRC and FAPESP). Moreover it translates fundamental research obtained from a model system to and agriculturally relevant pathosystem. Thus, this project contributes to BBSRC's mission to promote interdisciplinary and whole systems approaches to research on UK and global food supplies.