Tackling food security: Understanding Plant and Oomycete Pathology to Improve Rice Crop Production
Introduction
Global food security and production is currently one of the most concerning problems correlating with the increasing global population. Many biotechnology companies and labs are aiming to improve food production and security whilst also ensuring that they meet sustainable and quality standards of the food industry. However, food security is greatly threatened by plant pathogenic fungi and oomycetes. For example, the rice blast fungus Magnaporthe oryzae is responsible for annual destruction of 30% rice crops (Fernandez and Orth, 2018). This contributes to decreased crop production and increased hunger in African and Asian continents. This article aims to discuss the nature, form and function of specialised infection structures of plant pathogenic fungi and oomycetes and the importance of understanding the pathological roles of these structures for therapeutic intervention. These specialised infection structures consist of the appressorium, haustorium, infection cushion, germ tubes and penetration pegs. This article, using M. oryzae as an example, will mainly focus on the appressorium and haustorium structures as these structures are widely conserved for plant pathogen and oomycete infection and virulence.
Melanisation of appressoria is required for penetration in host plant cells to facilitate infection
Morphological changes in plant pathogenic fungi and oomycetes are important for penetration and colonisation of host plant tissues. Zoospores from oomycetes land on host leaf surfaces and begin to form germ tubes to locate stomatal, lenticels or other openings in host tissue to begin the infection process. An appressorium is formed once an opening is located (Figure 1). The appressoria is the most important infection structure in plant pathogenic fungi like M. oryzae and oomycetes such as Phytophothora infestans. Appressoria is involved in direct and active penetration of host tissue for invasion and infection (Ryder et al., 2022).
Figure 1: The infection process of the rice blast fungus. Zoospores (S) of the rice blast fungus are released and these spores land on leaf surfaces. The zoospores start to form germ tubes (GT) to locate openings in the plant host and facilitate infection into plant tissue. An appressorium forms above the stomata and secretes a proteinaceous glue for adhesion to host lead surfaces. An immense turgor pressure is produced inside the appressoria, allowing for a forceful penetration into the host. Following this, penetration tubes will be formed from the appressoria for host tissue colonisation. Haustorium is then produced which induces formation of the extrahuastorial membrane (EHM) allowing for immune masking from plant host defences. Figure taken and modified from MC4314 Lectures, Prof Pieter van West, University of Aberdeen, 2023.
This invasion process is fungus-driven and is the dominant route involving only hyphae, unlike the commensal Candida albicans, in which its invasion is host-driven. Appressoria are bulbous or flattened structures, often melanised for protection against environmental stresses and extreme pressures (Ryder et al., 2022).
The appressorium forms above the stomata and secretes a proteinaceous glue for adhesion to the host leaf surface (Figure 1). The appressoria then produces an immense pressure of 6.0 - 8.0 MPa. de Jong et al 1997 showed that this hydrostatic turgor pressure is generated via glycerol accumulation. Glycerol builds this turgor pressure by driving water into the appressoria by osmosis. de Jong et al extracted contents of the appressoria and biochemically characterised them to identify a solute responsible for turgor pressure generation. M. oryzae spores were grown in water drops on hydrophobic plastic membranes, providing an ideal environment for appressoria formation. The mixture was then analysed by gas-liquid chromatography, which identified glycerol as the most abundant solute in the appressoria. They also observed that intracellular glycerol increases with turgor generation (de Jong et al.,1997).
Following this, the research team also investigated whether there was a relationship between glycerol accumulation and appressoria melanisation. Appressoria melanisation is a key virulence trait of Magnaporthe. de Jong et al observed lower levels of glycerol to be correlated with non-melanised strains harbouring single mutations affecting genes encoding for melanin biosynthesis. A similar effect was also observed when M. oryzae was treated with melanin biosynthesis inhibitors (MBI). They showed that M. oryzae with non-melanised appressoria are unable to penetrate host tissue. This is strong evidence that melanisation of appressoria is essential for M. oryzae virulence and penetration into host tissue. This further supports that the appressoria is a very important infection structure for M. oryzae. Additionally, MBIs could be promising therapeutics to treat M. oryzae plant infections (de Jong et al.,1997).
Appressoria formation is regulated by the highly conserved MAPK cascade. There are many kinases which contribute to appressoria formation, among which is Pmk1. Nick Talbot’s group has shown that deletion of the Pmk1 kinase in M. oryzae will deem the fungus non-pathogenic. M. oryzae is unable to form appressoria without Pmk1 and therefore is unable to invade host tissue. This study also demonstrates that appressoria is a crucial infection structure. The appressoria is conserved in other oomycetes, plant and also animal pathogenic fungi (Qu et al., 2021).
Besides pressure generation, the external matrix of the appressoria also contains cutinase and cellulase which aid the pathogen in softening the cuticle of plants. The external matrix also allows appressoria to withstand the extreme turgor pressure generated. The ultimate goal is to induce formation of penetration pegs; finger-like projections to further facilitate adhesion and penetration into plant tissue. Alternatively, some plant fungal pathogens are also able to form infection cushions to enhance adherence of the pathogen (Qu et al., 2021).
Formation of germ tubes and haustoria play key roles in nutrient uptake, tissue colonisation and infection spread
M. oryzae enters the host leaf through penetration pegs. These penetration pegs are created by a GTP-binding protein, septin, located at the base of the appressorium. Septin is responsible for cytoskeletal remodelling within the fungus for formation of the penetration peg. Once inside the leaf, M. oryzae will continue to form germ tubes to colonise host tissue and spread the infection to other plant cells. Some germ tubes may penetrate into neighbouring plant cells through plasmodesmata or other openings between cells to induce haustoria formation for nutrient uptake, infection and masking from plant host defences. It is important to note that some fungal pathogens, like Cladosporium fulvum-tomato do not form haustoria, but rather acquire nutrients via formation of many germ tubes from the appressoria. Cladosporium facilitates its nutrient acquisition by only entering the intracellular space of the plant (Catanzariti et al., 2007).
M. oryzae will produce haustoria inside infected plant cells. The haustorium is a feeding infection structure which protrudes from a thick hyphae and induces formation of the extra-haustorial matrix (EHM). Interestingly, the EHM is a membrane created by the plant but induced by M. oryzae for purposes of immune masking (Catanzariti et al., 2007). This will avoid triggering of plant immune responses against the plant pathogenic fungi, permitting intracellular survival. Haustoria is responsible for secretion of effector proteins which inhibit transcription factors controlling host plant immune responses and resistance to the pathogen (Fishman and Shirasu, 2021). The secreted effector proteins can also inhibit plant cytoskeletal proteins, to prevent plant cell wall remodelling in response to the pathogen. Effector proteins can also assimilate plant metabolites to aid in M. oryzae growth and spread. One example of micronutrient assimilation would be a gene which encodes for an amino acid transporter protein in the rust fungus Uromyces fabae identified by Hahn et al 1997. This discovery confirmed the role of haustoria in nutrient assimilation and its importance as an infection structure. The haustoria, unlike the appressoria, is more difficult to study and develop potential therapeutics for because of its intimate interaction with the host plant. Current studies are aimed at enhancing our understanding of the role of haustoria in plant infection and methods to overcome or inhibit haustoria formation. Just like the appressoria, the haustoria is also conserved in other oomycetes, plant and animal pathogenic fungi, highlighting its importance as an infection structure (Voegele, 2006).
Conclusion
This article, using Magnaporthe oryzae as an example, has demonstrated the importance of various specialised infection structures formed by plant pathogenic fungi and oomycetes. The main takeaway is that formation of appressoria and haustoria are extremely crucial infection structures for infection, survival, replication and spreading of infection in susceptible plants. Appressoria has a key role in facilitating penetration and initial stages of infection. As discussed, the appressoria is a widely studied infection structure due to its evolutionary conservation in most plant pathogens and potential for therapeutic exploitation to combat early infection, preventing progression into late stages of infection and haustoria formation. This will ultimately prevent crop destruction. Interestingly, both the appressoria and haustoria infection structures are protected by a melanised external matrix and the extra-haustorial matrix (EHM) respectively. This is strong evidence that appressoria and haustoria are important specialised infection structures and should be widely studied for the development of novel and sustainable crop therapeutics to ensure global food security and zero hunger, which is consistent with Goal 2 of the United Nations Sustainable Development Goals (SDG) (SDG Goals, United Nations).
Article prepared by: Eldrian Tho Jiat Yang, MBIOS 23/24 Advisor, MBIOS 22/23 President, & Research and Development Director 21/22
If you enjoyed this article, do sign up to become a part of our MBIOS family and receive our monthly newsletter along with many more resources in the link below.
References
de Jong, J.C., McCormack, B.J., Smirnoff, N. and Talbot, N.J., 1997. Glycerol generates turgor in rice blast. Nature, 389(6648), pp.244-244.
Catanzariti, A.M., Dodds, P.N. and Ellis, J.G., 2007. Avirulence proteins from haustoria-forming pathogens. FEMS microbiology letters, 269(2), pp.181-188.
Fernandez, J. and Orth, K., 2018. Rise of a cereal killer: the biology of Magnaporthe oryzae biotrophic growth. Trends in microbiology, 26(7), pp.582-597.
Fishman, M.R. and Shirasu, K., 2021. How to resist parasitic plants: pre-and post-attachment strategies. Current Opinion in Plant Biology, 62, p.102004.
Hahn, M., Neef, U., Struck, C., Göttfert, M. and Mendgen, K., 1997. A putative amino acid transporter is specifically expressed in haustoria of the rust fungus Uromyces fabae. Molecular plant-microbe interactions, 10(4), pp.438-445.
Qu, Y., Wang, J., Huang, P., Liu, X., Lu, J. and Lin, F.C., 2021. PoRal2 is involved in appressorium formation and virulence via Pmk1 MAPK pathways in the rice blast fungus Pyricularia oryzae. Frontiers in plant science, 12, p.702368.
Ryder, L.S., Cruz-Mireles, N., Molinari, C., Eisermann, I., Eseola, A.B. and Talbot, N.J., 2022. The appressorium at a glance. Journal of Cell Science, 135(14), p.jcs259857.
SDG Goals, United Nations. Retrieved 11 August 2024 from: https://sdgs.un.org/goals
van West, Pieter., 2023. MC4314 Pathogenesis of Plant Pathogens & Oomycetes Lecture.
Voegele, R.T., 2006. Uromyces fabae: development, metabolism, and interactions with its host Vicia faba. FEMS Microbiology Letters, 259(2), pp.165-173.
Comments