Page 58 - Plant Canada 2024 Proceeding

PLANT CANADA 2024

Monday, July 8

Dr. Louis-Philippe Hamel

“Understanding plant molecular responses to the
production of enveloped VLPs leads to the
improvement of a molecular farming expression
platform”

Abstract: In plants, the production of COVID-19
vaccines can be achieved via transient expression of
the Spike (S) protein from Severe Acute Respiratory
Syndrome Coronavirus 2 (SARS-CoV-2). Relying on
bacterial vector Agrobacterium tumefaciens, this
process is favored by co-expression of silencing
suppressor P19. During expression, the S protein is produced and matured through the
secretory pathway of plant cells, before being trafficked to the plasma membrane where
it induces formation of coronavirus-like particles (CoVLPs). Using time course sampling,
we characterized molecular responses of Nicotiana benthamiana leaf cells expressing
P19 only, or co-expressing P19 and a recombinant S protein. This revealed expression
of the latter to trigger early but transient activation of the unfolded protein response, in
addition to distinct waves of transcription factor genes involved in immunity. Accordingly,
defense genes were induced with different kinetics, including those promoting oxidative
stress and cell wall lignification. Crosstalk between stress hormone pathways was also
denoted, including rapid repression of jasmonic acid biosynthesis genes after
agroinfiltration, and later dampening of salicylic acid responses upon S protein
accumulation. Further analysis of the data revealed CoVLP production to induce leaf
senescence, as revealed by up-regulation of transcription factor and senescence-
associated genes, accumulation of the senescence-specific protease SAG12, and
concomitant down-regulation of genes involved in photosynthesis and nitrogen
assimilation. In a molecular farming context, these combined responses were
hypothesized to restrain foreign protein accumulation and strategies were thus developed
to improve molecular farming efficacy. This includes the co-expression of helper proteins
that reduce stress symptoms or that improve yields in planta.

Bio: Dr. Louis-Philippe Hamel is an expert of the plant innate immune system with a unique
expertise in the biopharmaceutical industry and in the field of plant molecular farming. Formerly
employed by Medicago, his most recent work aims at understanding plant’s responses
to Agrobacterium-mediated expression of recombinant proteins in plants, including for the
production of plant-made vaccines and antibodies. In addition to these fundamental aspects, his
research focuses on the development of genetic and non-genetic approaches to improve plant
molecular farming practices. During his Ph.D and as a postdoc fellow at the University of
Sherbrooke and at Harvard University, Dr. Hamel worked on intracellular signaling pathways
that lead to the activation of plant defense, including downstream of resistance proteins and
mitogen-activated protein kinase cascades. His work highlighted several defense activation
routes, including through the degradation of defense repressors that inhibit resistance
mechanisms in the absence of stress.

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