A slicing mechanism facilitates host entry by plant-pathogenic Phytophthora
Authors: Kiki Kots
Michiel Kasteel
Jesse Buijs
Jasper van der Gucht
Stijn van Veen
Francine Govers
Tijs Ketelaar
Jess M Clough
Joris Sprakel
Jochem Bronkhorst
Journal: Nature Microbiology (2021)
Discipline: MICROBIOLOGY
Keywords: Phytophthora species; oomycetes; plant pathogens; host invasion; biomechanics
Affiliations: Physical Chemistry and Soft Matter, Wageningen University and Research, Wageningen, The Netherlands., Laboratory of Phytopathology, Wageningen University and Research, Wageningen, The Netherlands., Laboratory of Cell Biology, Wageningen University and Research, Wageningen, The Netherlands., Physical Chemistry and Soft Matter, Wageningen University and Research, Wageningen, The Netherlands. .

Abstract:  Phytophthora species, classified as oomycetes, are among the most destructive plant pathogens worldwide and pose a substantial threat to food security. Plant pathogens have developed various methods to breach the cuticle and walls of plant cells. For example, plant-pathogenic fungi use a 'brute-force' approach by producing a specialized and fortified invasion organ to generate invasive pressures. Unlike in fungi, the biomechanics of host invasion in oomycetes remains poorly understood. Here, using a combination of surface-deformation imaging, molecular-fracture sensors and modelling, we find that Phytophthora infestans, Phytophthora palmivora and Phytophthora capsici slice through the plant surface to gain entry into host tissues. To distinguish this mode of entry from the brute-force approach of fungi that use appressoria, we name this oomycete entry without appressorium formation 'naifu' invasion. Naifu invasion relies on polarized, non-concentric, force generation onto the surface at an oblique angle, which concentrates stresses at the site of invasion to enable surface breaching. Measurements of surface deformations during invasion of artificial substrates reveal a polarized mechanical geometry that we describe using a mathematical model. We confirm that the same mode of entry is used on real hosts. Naifu invasion uses actin-mediated polarity, surface adherence and turgor generation to enable Phytophthora to invade hosts without requiring specialized organs or vast turgor generation. 

Retracted: No

PubPeer comments:
 The absence of visible hyphal swelling during oomycete host infection is documented in a 20-yr-old paper: Hardham, Australasian Plant Pathology 2001, figure 4a b below:  Figs 4A 4B. Cysts of Phytophthora cinnamomi have germinated on the surface of an onion root 1 h after inoculation. In many cases, the germ tube is not visible and is likely to have penetrated the root surface immediately beneath the cyst. In most cases where the germ tube is visible, it has penetrated the root along the anticlinal walls between the epidermal cells without changing diameter. In a few cases (arrowheads), the germ tube has penetrated through the periclinal wall of the epidermal cells. In these germlings, the hyphae show an appressorium-like swelling on the root surface. The spore with the appressorium-like swelling in the top right hand corner of Fig. 4B may be an exception as it appears to have penetrated the anticlinal wall. Both 750u00d7. I interpret the first case ( without changing diameter ) as the naifu infection described by the authors. Why are the authors not citing Hardham, 2007? And why are they ignoring the second case in their conclusion ( We find that these oomycetes invade hosts by a naifu mechanism )? , 
 Thank you for your comment and pointing to this relevant study; I agree that is a relevant study that we could have cited. Our work did not claim to have discovered that appressoria are not the invasive organs in oomycetes, since they by definition lack the machinery (septins, melanin) to build an appressorium, which we mention in the 2nd paragraph. We also cite several other studies in which live-cell microscopy images of the cyst during invasive force generation lacks an appresorium-like structure and we discuss the observations of appresorium-like swellings in our Discussion section and place these observations in the context of our findings. What we see, and claim, as the main novelty of our work is to answer the question how a pathogen can penetrate a host, from a mechanical point of view, when there is no appressorium to build enormous turgor. The naifu mechanism refers to this mechanical mechanism: that force application at an oblique angle can lead to host entry without large turgor by the redistribution of mechanical stress to the substrate. To the best of our knowlege we are the first to identify and quantify the oblique pressure application and to evidence how this can lead to appressorium-free penetration. We also did not ignore the appressorial-like swellings as you state: these are discussed in the Discussions section in the context of the uncovered mechanical mechanism. We argue that these swellings are not appressorium-like, since they play no role in force generation and are not melanized or orchestrated by septins, and thus naifu-invasion is most likely still at play. Rather, the hyphal swellings are probably inflated sections of the apical zone of the germ tube, which is well-established to be mechanically weakened, which inflates under turgor as it cannot extend in the longitudinal direction (as a non-invasive germ tube can) by elastic resistance at the contact with the substrate. , 
 Thanks for the brilliant work and discussion. Your last sentence would have been perfect in the discussion. ,