Plant Proteases Target Pathogen Outer Membrane Proteins

McClelland, Alexander Joseph (2024) Plant Proteases Target Pathogen Outer Membrane Proteins. Doctoral thesis, University of East Anglia.

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Abstract

Citrus greening disease, or Huanglongbing (HLB), is the most devastating disease of citrus. Florida produces ~10% of the citrus it did before the introduction of HLB, which is caused by the phloem-residing, insect-vectored bacterium Candidatus Liberibacter asiaticus (CLas). There is no cure for HLB, and very little is known about how plants mount an immune response against phloem-residing pathogens.

Proteases were previously found to be induced in citrus phloem during CLas infection. CLas deploys an effector protein, SDE1, that inhibits papain-like cysteine protease (PLCP) enzymatic function and contributes to disease progression. This suggests that proteases are important hubs of immunity in the phloem. While proteases are known to regulate plant immunity, identifying relevant substrates of proteases is particularly challenging.

In this work, I identified novel substrates of a citrus PLCP, CsRD21a. CsRD21a interacts with and cleaves beta barrel outer-membrane porin-like proteins (OMPs) from bacteria in the Liberibacter genus, including CLasOMP1. CLasOMP1 is highly expressed and may have an essential role in bacterial survival and/or host colonization, making it an attractive target for host defense. Cleaved products of CLasOMP1 are detected in infected citrus and resemble CsRD21a-generated cleavage products detected using a semi-in vitro cleavage assay. Further, CsRD21a overexpression enhances tolerance to CLas in transgenic citrus. CLasOMP1 also interacts with two serine carboxypeptidases from citrus and therefore may be a substrate of many classes of host proteases. Lastly, cell surface-exposed beta barrel OMPs are found in all Gram-negative bacteria and might be common targets of proteases in other pathosystems.

As strategies to mitigate the spread of phloem-residing pathogens are limited, engineering proteases in the phloem may be an effective strategy to achieve resistance against economically destructive pathogens, including Liberibacters and Phytoplasmas. Together, this work has shed mechanistic insight into the defense functions of plant proteases.

Item Type:	Thesis (Doctoral)
Faculty \ School:	Faculty of Science > School of Biological Sciences
Depositing User:	Chris White
Date Deposited:	08 Jan 2025 12:54
Last Modified:	08 Jan 2025 12:54
URI:	https://ueaeprints.uea.ac.uk/id/eprint/98117
DOI:

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