Pathogenic fungi invade plants in four main steps: adhesion on host surface, form infection structure, invasion of host, colonization and expansion within host. Some pathogenic fungi can even produce metabolites that are toxic to their hosts, and these substances are considered to be one of the main causes of plant diseases. Different pathogenic genes cause different infection processes and metabolic regulation modes. The interactions between plants and pathogenic fungi can be divided into incompatibility and affinity. In non-compatible interactions, local necrotic spots with obvious differences from surrounding healthy tissues are formed at the infection site, namely hyper-sensitive reaction (HR)
[1]. In affinity interaction, some fungi take advantage of stomata or trauma on the host surface to invade, usually producing infection structures formed by specialized hyphae. Infection cushion, appressorium, and haustorium help pathogenic fungi invade and establish parasitic relationships with hosts, resulting in plant infection
[2].
The innate immune system of plants consists of two main immune responses
[3]. One is nonspecific defense response: pattern recognition receptors (PRRs) of plants can recognize highly conserved macromolecular substances common to pathogenic microorganisms which are called pathogen-associated molecular patterns (PAMPs), such as flagella and polysaccharides
[4][5]. When PAMPs are recognized by PRRs, relative signal transduction pathways are activated and then induce defense response to limit the invasion of pathogenic microorganisms. This process is called PAMP-triggered immunity (PTI) response
[6]. The second is specific defense response: In order to successfully infect plants, pathogenic microorganisms have evolved effector proteins to inhibit the immune response induced by PAMPs. At the same time, plants have evolved R genes to monitor and identify effectors, cause hypersensitive response (HR) and limit the invasion of pathogens. This resistance is called effector-triggered immunity (ETI)
[3].
2. Signal Recognition of Pathogenic Fungi Infection Process
Signaling pathway refers to a series of enzymatic reaction pathways that can transmit extracellular signals into the cell through the cell membrane. Receptors on the cell membrane sense external signals. In organisms, these receptor proteins include ion channel receptors, G-protein-coupled receptors (GPCR), tyrosine kinase receptors, and receptors that regulate gene expression. In fungi, heterotrimer G protein participates in the regulation of vegetative growth, pathogenicity, sporulation, and differentiation of infection structure by regulating the activities of adenylate cyclase and phospholipase and ion channels
[7][8][9]. In
Ustilago maydis, heterotrimer G proteins and GPCR are involved in mating and pathogenicity by regulating hormone response and cAMP-dependent signaling pathways
[10][11]. In
Aspergillus fumigatus, the Gα subunit GpaB positively regulates conidia survival and PksP expression in macrophages
[12]. Phenotypes of the
gpaB mutant and the adenylate cyclase mutant
acyA suggest that a gpaB mediated cAMP-dependent signaling pathway is involved in the pathogenesis of
A. fumigatus [12][13]. In
Cryphonectria parasitica, Gα protein Cpg-1 is essential for pathogenesis. Cpg-1 is involved in the growth and sporulation of trophic mycelia, and the pathogenic process is regulated by a pathway independent of Gβγ
[14][15][16]. In contrast, Gβ subunit Cpgb-1 positively regulates pathogenicity but does not affect vegetative growth
[17]. In
Cryptococcus neoformans, adenylate cyclase Cac1 positively regulates the formation and toxicity of the capsule
[18][19]. On the other hand, Gβ subunit Gpb1 is not required for pod formation, pigment synthesis, or toxicity
[20]. Although defective capsular formation is observed in the
gpr4 GPCR mutant, pigment and toxicity remained unchanged
[19].
The MAPK cascade pathway is located in the center of the cell signal transmission network, and is involved in regulating cell growth and differentiation, photosynthesis, metabolism, synthesis and release of neurotransmitters, adaptation to adverse environment, infection of pathogens, and other physiological processes
[21][22][23][24][25]. In many pathogenic fungi, the HOG pathway mainly plays a role in adapting to high osmotic pressure in the external environment. In
Neurospora crassa, it is found that the HOG pathway mainly consists of osmotically responsive histidine kinase Os1, histidine phosphate group transfer proteins Hpt1, RRG1/2, and downstream phosphoric acid coupling systems of Os4, Os5, and Os2
[26][27][28]. Studies have found that the HOG pathway in
N. crassa is not only related to environmental stress, but also regulates the production of bacterial pigments, resistance to diimide fungicides, and pathogenicity of bacteria. The HOG pathway of
Botrytis cinerea is also reported to be sensitive to osmotic stress, DCFs, and pathogenicity of the pathogen to different hosts
[29][30]. However, Osm1 (homologous protein of Hog1), a key element of the HOG pathway in
Magnaporthe oryzae, is associated with drug sensitivity and osmotic stress, but is not closely associated with pathogen pathogenicity
[31][32]. Fus3/Kss1 pathway not only has an important relationship with the sexual reproduction of pathogenic fungi but also plays an important role in regulating the pathogenicity of pathogenic fungi. In
Aspergillus nidulans, although the scaffold protein Ste5 is missing, it can still form the complex AnSte50-AnSte11-AnSte7-AnFus3. After the complex is activated by upstream signals, AnFus3 will enter the nucleus and activate the activity of AnSteA (homologous protein of Ste12) and AnVeA. Activated AnSteA can regulate mycelial fusion and sexual reproduction of
A. nidulans, while activated AnVeA can regulate secondary metabolism of
A. nidulans [33][34][35]. In
M. oryzae, mutants of
pmk1 (homologous protein of Fus3) cannot produce appressorium and cannot penetrate the host surface
[36][37]. The CWI pathway is a kind of MAPK signaling pathway that has been studied well in pathogenic fungi. Kinase Slt2, a core component of the CWI pathway, has been found to regulate cell wall integrity in
Alternaria brassicicola,
A. nidulans, and
M. oryzae [38][39][40][41]. Slt2 has also been found to be significantly associated with the pathogenicity of many pathogens, such as
Candida albicans [42].
3. Regulation of Pathogenic Processes by Transcription Factors of Pathogenic Fungi
Transcription factors are the largest family of trans-acting factors. In a broad sense, all transcription-related proteins except RNA polymerase itself can be classified as transcription factors
[43]. The zinc finger protein family is the most widely distributed in the eukaryotic transcription factors family and can generally be divided into zinc finger and zinc cluster structure. Zn2Cys6 transcription factor is a kind of zinc finger protein peculiar to fungi.
Colletotrichum melanin regulation (CMR) and Pigment of
M. oryzae (PIG) encodes a protein that contains both zinc fingers and zinc clusters, both of which are involved in melanin synthesis
[44]. The AlcR protein of
A. nidulans contains Zn2Cys6 zinc clusters, and its main function is mainly related to ethanol metabolism
[45].
bZIP transcription factors in
A. nidulans are involved in the regulation of secondary metabolism, sexual reproduction, and stress response
[46]. The bZIP protein in
N.
crassa is associated with sulfur utilization and oxidative pressure reaction
[47][48]. The bZIP transcription factor in
A. fumigatus mainly regulates asexual reproduction, gelatoxin production, sulfur assimilation, and infection
[49][50]. FgAp1, a bZIP transcription factor in
Fusarium graminearum, is associated with oxidative pressure response and toxin synthesis. FoMeab, a bZIP protein from
Fusarium oxysporum, is involved in regulating nitrogen cycling pathways
[51]. In
M. oryzae, bZIP transcription factor genes coordinate the physiological processes such as growth and development, conidiation, appressorium formation, infection, and pathogenicity
[52].
Homologous heteromorphic box structure transcription factors proteins generally have two protein binding regions, and conformational changes after binding to regulatory proteins; thus, regulating DNA binding activity
[53]. In
U.
maydis, homologous genes are mainly involved in the regulation of linear growth, sexual reproduction, and infection of host plants
[54]. In
Podospora anseria, the homologous heteromorphic box gene PAH1 regulates mycelia extension growth and male spore production, and its gene deletion mutants grow slowly and the mycelia is compact
[55]. In
M. oryzae, eight homologous heteromorphic box genes are identified and named as MoHox1-MoHox8, in which Δ
Mohox1, Δ
Mohox4, and Δ
Mohox6 grew at a slower rate, and aerial hyphae were scarce. Δ
Mohox8 appressorium penetration decreased and pathogenicity decreased. Δ
Mohox7 does not produce functional appressorium in either germ tube or mycelium tip, resulting in a complete loss of pathogenicity
[56]. bHLH transcription factors are highly conserved transcription factors in eukaryotes. In
N.
crassa, bHLH transcription factor CHC-1 is associated with CO
2-mediated negative regulation of sporulation
[57]. In
A. nidulans, AnBH1 regulates penicillin synthesis, and DevR regulates sexual and asexual reproduction
[58][59]. SclR in
Aspergillus oryzae promotes sclerotia. EcdR is related to the early differentiation of conidiophore
[60][61].