Monika Raman, PSG College of Technology, Coimbatore
Many fungal pathogens evolve specialized structures to disrupt their plant or animal hosts – thick outer layers to cause infection. In several fungal species, these infection cells, known as appressoria, are widely researched. Pathogens must also penetrate newer cells and cross host cell intersections once they are within the host tissue. How they invade & transverse host cell junction has received much less attention yet.
But new data from a rice blast fungal investigation indicates the formation of a specific infection structure in a cell invasion within a host plant. A dome-shaped melanin-pigmented appressorium is used for entrance into a plant by Magnaporthe oryzae, the blast fungus. It causes disease responsible for rice blast and annually destroys sufficient rice to feed 60 million people.
Researchers from the Sainsbury Laboratory, University of East Anglia, Norwich, UK – compare the developmental biology of invasive growth of the rice blast fungus at different phases of plant infection. They also elucidate how pathogens invade new cells and traverse host cell junctions.
Prerequisites for appressorium morphogenesis:
A conidium germinates on the surface of the leaves to produce a polarised germ tube which, after 4 – 6 hours, develops into an appressorium. There are three main requirements for morphogenesis of appressorium:
- A hard hydrophobic surface should be recognized by M. oryzae, requiring Pmk1 Mitogen-Activated Protein Kinases (MAPK) signaling pathway. In the absence of Pmk1, the fungus cannot develop an appressorium and hence cannot cause an infection.
- The 3-celled conidium germinating cell must undergo mitosis. For initiating appressorium development, an S-phase checkpoint is essential. The nucleus must transit through G2-M to permit the maturation of appressorium.
- Before its contents are transported into appressorium, three cell conidium undergo the autophagy and iron-dependent programmed cell death process termed ferroptosis.
The primary author of the study, Neftaly Cruz-Mireles, stated: “The autophagy initiation needs both Pmk1 and the cell cycles progression.” But it is also related to starvation stress because appressoria only grow in the absence of exogenous nutrients.
Appressorium function:
When the appressorium gets created, it clings firmly to the leaf’s cuticle and produces huge turgor. This enormous pressure is produced of high glycerol and other polyols accumulation that osmosis the water into the cell.
The appressorium contains a differentiated cell wall rich in melanin that decreases the porosity of the cell walls, limiting polyol emigration and permitting continued water input. The appressorium melanization is necessary to generate turgor. Mutants not capable of synthesizing dihydroxy naphthalene melanin are unable to cause blast disease.
Turgor used as a mechanical force on the base of the appressorium enables a stiff narrow penetration hypha to rupture the rice cuticle. Once inside a plant cell, the penetration hypha differentiates into bulbous and fills up the cell’s interior rapidly.
Once the first epidermal cell has been fully colonized, does the fungus infect neighboring cells? “Yes, it often invades neighboring cells in a very synchronized manner, spreads from cell to cell, and quickly occupies the tissue of the host,” said Eseola, a research co-author.
What is transpressorium?
The formation of appressorium and cell to cell migration seem morpho-genetically connected. Both include an isotropic growth of a hyphal spike to rupture the cuticle or plant cell wall at pit fields, followed by a considerably smaller infection peg.
Superior observers of plant-fungal interactions who coined the term “Transpressorium” to describe structures of planta infections made up of fungi to move between host cells have previously noticed this similarity. When they studied Ceratocystis sp. of Pinus strobus, Liese and Schmid were the first to describe transpressoria.
“In conclusion, we review data showing that the invasive growth needs a specialized infection cell — transpressorium,” stated Eseola. Many similar characteristics between appressoria and transpressoria, were also discovered.
The processes of invasive growth by pathogenic fungi have much to know, but the significance of the transpressorium—which has been mostly unidentified so far—can be as crucial as that of the appressorium for fungal pathogenesis.
Given that approximately 3.7 billion of the world’s population – rice is the staple food, blast disease represents a clear and current threat to global food security. Magnaporthe oryzae blast fungus may also infect over 50 different kinds of grass, including other cereals, such as barley, oats, finger millet, and wheat. Therefore, it is vital to understand the biology of blast conditions for developing novel disease control techniques.
Also read: Extreme heat increases tree mortality in the sub-alpine forest
Reference:
- Cruz-Mireles, N., Eseola, A. B., Osés-Ruiz, M., Ryder, L. S., & Talbot, N. J. (2021). From appressorium to transpressorium—Defining the morphogenetic basis of host cell invasion by the rice blast fungus. PLOS Pathogens, 17(7), e1009779. https://doi.org/10.1371/journal.ppat.1009779
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Author info:
Monika Raman is an undergraduate student pursuing her final year B. Tech in Biotechnology. She is an enthusiastic Biotech student aspiring for an opportunity to develop skills and grow professionally in the research field. Extremely motivated and possess strong interpersonal skills. Read some of her published articles at BioXone:
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