Targeting Cell Wall Formation in the Oomycete Phytophthora cinnamomi for Disease Control

Abstract

The oomycete Phytophthora genus comprises microorganisms that cause devastating plant diseases, such as late blight and root rot diseases, leading to significant agricultural economic losses, and causing extensive damages to ecosystems. To date, no practical method is available to prevent these diseases. Furthermore, current strategies to control Phytophthora-induced diseases are ineffective in the long term. These strategies currently rely on different classes of chemicals. However, repeated use of the same chemicals can lead to development of pesticide resistance in phytopathogens. This concern, combined with an increased awareness of alternative approaches that have minimal impact on biodiversity and human health, highlights that efficient methods for controlling diseases caused by Phytophthora are urgently required. Targeting cell wall biosynthesis is a promising strategy to combat these pathogens. Indeed, the inhibition of enzymes involved in carbohydrate biosynthesis affects the growth and survival of these pathogens, offering a promising avenue for the development of effective treatments. In recent years, plant antimicrobial peptides (AMPs) have been found to be effective against different phytopathogens. Well-known AMPs are plant defensins, a family of small cysteinerich peptides that can bind to chitin and cell wall glucans in fungi. However, knowledge about the inhibitory role of plant defensins in oomycetes is limited. As such, this work investigates the effects of the plant defensin NaD1 (Nicotiana alata defensin 1) on Phytophthora species, which may reveal novel opportunities for controlling plant diseases. Our findings demonstrate that NaD1 effectively inhibits the mycelial growth of Phytophthora cinnamomi, Phytophthora cambivora, Phytophthora nicotianae, and Phytophthora citricola. Exposure to NaD1 induced alterations in the growth and structure of P. cinnamomi, leading to suppressed apical dominance, hyper-branching, and changes in cell wall composition, likely due to disruption of calcium homeostasis. Transcriptomic analyses confirmed altered expression of genes involved in cellulose synthesis and calcium transport (Chapter 2), and uncovered changes in the transcriptome across the entire genome in hyphal cells exposed to NaD1, shedding light on the mechanism of action of this AMP. These differentially expressed genes can serve as candidates to study the efficacy of NaD1 against Phytophthora species (Chapter 4). In addition to NaD1, the effects of a chitin synthase inhibitor, nikkomycin Z, were also investigated. This study shows that nikkomycin Z causes strong growth inhibition of four Phytophthora species and induces abnormal hyphal growth. Exposure to this inhibitor decreases cellulose levels and affects the expression of genes related to vital functions such as cell wall biosynthesis, hexosamine biosynthesis and chitin deacetylation (Chapter 3). Altogether, the present work reveals critical information about the fundamental inhibitory mechanisms of NaD1 and nikkomycin Z on Phytophthora species, with a focus on cell wall biosynthesis. This work paves the way for the development of novel effective targets for oomycete disease control.