Identification and functional characterization of Candida albicans CDC4

Influence of Ubiquitin Mutations over Molecular Markers Contributing to Pathogenesis of Candida albicans

The emergence of Candida albicans as a life-threatening pathogen and its resistance to available antifungal drugs is a significant global health concern. Previous findings have established that the ubiquitin mutations UbEP42, UbL50P and UbI61T interfere with morphogenesis of C. albicans from commensal yeasts to pathogenic hyphae. The main objective of this study is to investigate the influence of ubiquitin mutations on the molecular markers of morphogenesis and virulence of C. albicans to identify potential targets for therapeutic intervention. The auxotrophic strain BWP17 of C. albicans was transformed by the wild-type ubiquitin gene (UbWT) and its mutants UbEP42, UbS20F, UbA46S, UbL50P and UbI61T cloned under the doxycycline-inducible promoter in the integrative plasmid pTET25-MNC. Induced expression of the mutant forms UbEP42, UbL50P, and UbI61T while inhibiting morphogenesis, reduced chitin deposition, increased β-glucan exposure on the cell wall, decreased the secretion of aspartyl protease, caused the differential expression of cyclins Cln3, Ccn1, Clb2 and certain key transcription factors compared to UbWT. However, UbS20F and UbA46S did not have any influence. These findings demonstrating the disruptive influence of UbEP42, UbL50P, and UbI61T on the levels of molecular markers of morphology and pathogenesis of C. albicans, highlight the importance of a functional ubiquitination system.

Molecular Insights into the Role of the MET30 Protein and Its WD40 Domain in Colletotrichum gloeosporioides Growth and Virulence

Colletotrichum gloeosporioides is a major phytopathogen responsible for anthracnose in Capsicum annuum (pepper) which leads to significant yield losses. At present, the molecular mechanism of C. gloeosporioides pathogenesis is not very clear. In this study, we focused on the MET30 protein and its key WD40 domain, with an emphasis on its role in the biological functions of C. gloeosporioides. Bioinformatics analysis revealed that the MET30 protein contains a conserved F-box domain and multiple WD40 repeats, which interact with other proteins to participate in various cellular processes, including nutrient acquisition, stress responses, and pathogenicity. Gene knockout and complementation experiments demonstrated that deleting the MET30 protein or its WD40 domain significantly reduced the rates of spore production and hyphal growth while increasing tolerance to environmental stresses such as high salinity and oxidative stress. Furthermore, pathogenicity assays revealed that the WD40 domain of the MET30 protein is crucial for regulating fungal pathogenicity, as mutants lacking WD40 domains presented increased virulence on pepper leaves. These findings suggest that the WD40 domain, in synergy with the MET30 protein, regulates the pathogenicity and stress response of C. gloeosporioides, provides new insights into the molecular mechanisms of anthracnose, and offers potential strategies for effective disease control.

Multiple F-Box Proteins Collectively Regulate Cell Development and Pathogenesis in the Human Pathogen Cryptococcus neoformans

The ubiquitin-proteasome system (UPS) mediates intracellular proteins degradation that influences various cellular functions in eukaryotic cells. The UPS is also involved in the development and virulence of pathogenic fungi. F-box proteins, which are part of the SCF (Skp1-Cullin-F-box protein) ligase, are a key component of UPS and are essential for the recognition of specific substrates. In this study, we identified 20 F-box proteins in C. neoformans and obtained deletion mutants for 19 of them. A comprehensive phenotypic analysis of these mutants revealed the diverse function of F-box proteins in stress response, cell size regulation, sexual reproduction, antifungal drug resistance, and fungal virulence in C. neoformans. The importance of three F-box proteins: Fbp4, Fbp8, and Fbp11, in these cellular functions were characterized in detail. This study provides an overall view of the F-box gene family in C. neoformans, which will lead to a better understanding of the function of fungal SCF E3 ligase-mediated UPS in fungal development and pathogenesis.

Role of F-box Protein Cdc4 in Fungal Virulence and Sexual Reproduction of Cryptococcus neoformans

Cryptococcus neoformans is an opportunistic yeast-like pathogen that mainly infects immunocompromised individuals and causes fatal meningitis. Sexual reproduction can promote the exchange of genetic material between different strains of C. neoformans, which is one of the reasons leading to the emergence of highly pathogenic and drug-resistant strains of C. neoformans. Although much research has been done on the regulation mechanism of Cryptococcus sexual reproduction, there are few studies on the sexual reproduction regulation of Cryptococcus by the ubiquitin-proteasome system. This study identified an F-box protein, Cdc4, which contains a putative F-box domain and eight WD40 domains. The expression pattern analysis showed that the CDC4 gene was expressed in various developmental stages of C. neoformans, and the Cdc4 protein was localized in the nucleus of cryptococcal cells. In vitro stress responses assays showed that the CDC4 overexpression strains are sensitive to SDS and MMS but not Congo red, implying that Cdc4 may regulate the cell membrane integrity and repair of DNA damage of C. neoformans. Fungal virulence assay showed that although the cdc4Δ mutant grows normally and can produce typical virulence factors such as capsule and melanin, the cdc4Δ mutant completely loses its pathogenicity in a mouse systemic-infection model. Fungal mating assays showed that Cdc4 is also essential for fungal sexual reproduction in C. neoformans. Although normal mating hyphae were observed during mating, the basidiospores' production was blocked in bilateral mating between cdc4Δ mutants. Fungal nuclei development assay showed that the nuclei failed to undergo meiosis after fusion inside the basidia during the bilateral mating of cdc4Δ mutants, indicating that Cdc4 is critical to regulating meiosis during cryptococcal mating. In summary, our study revealed that the F-box protein Cdc4 is critical for fungal virulence and sexual reproduction in C. neoformans.

Filament Negative Regulator CDC4 Suppresses Glycogen Phosphorylase Encoded GPH1 that Impacts the Cell Wall-Associated Features in Candida albicans

We have previously identified Candida albicans GPH1 (orf19.7021) whose protein product was associated with C. albicans Cdc4. The GPH1 gene is a putative glycogen phosphorylase because its Saccharomyces cerevisiae homolog participates in glycogen catabolism, which involves the synthesis of β-glucan of the fungal cell wall. We made a strain whose CaCDC4 expression is repressed, and GPH1 is constitutively expressed. We established a GPH1 null mutant strain and used it to conduct the in vitro virulence assays that detect cell wall function. The in vitro virulence assay is centered on biofilm formation in which analytic procedures are implemented to evaluate cell surface hydrophobicity; competence, either in stress resistance, germ tube formation, or fibronection association; and the XTT-based adhesion and biofilm formation. We showed that the constitutively expressed GPH1 partially suppresses filamentation when the CaCDC4 expression is repressed. The C. albicans Gph1 protein is reduced in the presence of CaCdc4 in comparison with the absence of CaCdc4. Compared with the wild-type strain, the gph1Δ/gph1Δ mutant displayed a reduction in the capability to form germ tubes and the cell surface hydrophobicity but an increase in binding with fibronectin. Compared with the wild-type strain, the gph1Δ/gph1Δ mutant showed a rise in adhesion, the initial stage of biofilm formation, but displayed a similar capacity to form a mature biofilm. There was no major impact on the gph1Δ/gph1Δ mutant regarding the conditions of cell wall damaging and TOR pathway-associated nutrient depletion. We conclude that GPH1, adversely regulated by the filament suppressor CDC4, contributes to cell wall function in C. albicans.

Transcriptional Control of Hypoxic Hyphal Growth in the Fungal Pathogen Candida albicans

The ability of Candida albicans, an important human fungal pathogen, to develop filamentous forms is a crucial determinant for host invasion and virulence. While hypoxia is one of the predominant host cues that promote C. albicans filamentous growth, the regulatory circuits that link oxygen availability to filamentation remain poorly characterized. We have undertaken a genetic screen and identified the two transcription factors Ahr1 and Tye7 as central regulators of the hypoxic filamentation. Both ahr1 and tye7 mutants exhibited a hyperfilamentous phenotype specifically under an oxygen-depleted environment suggesting that these transcription factors act as negative regulators of hypoxic filamentation. By combining microarray and ChIP-chip analyses, we have characterized the set of genes that are directly modulated by Ahr1 and Tye7. We found that both Ahr1 and Tye7 modulate a distinct set of genes and biological processes. Our genetic epistasis analysis supports our genomic finding and suggests that Ahr1 and Tye7 act independently to modulate hyphal growth in response to hypoxia. Furthermore, our genetic interaction experiments uncovered that Ahr1 and Tye7 repress the hypoxic filamentation via the Efg1 and Ras1/Cyr1 pathways, respectively. This study yielded a new and an unprecedented insight into the oxygen-sensitive regulatory circuit that control morphogenesis in a fungal pathogen.

The F-Box Protein Fbp1 Regulates Virulence of Cryptococcus neoformans Through the Putative Zinc-Binding Protein Zbp1

The ubiquitin-proteasome system (UPS) is the major protein turnover mechanism that plays an important role in regulating various cellular functions. F-box proteins are the key proteins of the UPS, responsible for the specific recognition and ubiquitination of downstream targets. Our previous studies showed that the F-box protein Fbp1 plays an essential role in the virulence of C. neoformans. However, the molecular mechanism of Fbp1 regulating the virulence of C. neoformans is still unclear. In this study, we analyzed the potential Fbp1 substrates using an iTRAQ-based proteomic approach and identified the zinc-binding protein Zbp1 as a substrate of Fbp1. Protein interaction and stability assays showed that Zbp1 interacts with Fbp1 and is a downstream target of Fbp1. Ubiquitination analysis in vivo showed that the ubiquitination of Zbp1 is dependent on Fbp1 in C. neoformans. Subcellular localization analysis revealed that the Zbp1 protein was localized in the nucleus of C. neoformans cells. In addition, both deletion and overexpression of the ZBP1 gene led to the reduced capsule size, while overexpression has a more significant impact on capsule size reduction. Fungal virulence assays showed that although the zbp1Δ mutants are virulent, virulence was significantly attenuated in the ZBP1 overexpression strains. Fungal load assay showed that the fungal burdens recovered from the mouse lungs decreased gradually after infection, while no yeast cells were recovered from the brains and spleens of the mice infected by ZBP1 overexpression strains. Thus, our results revealed a new determinant of fungal virulence involving the post-translational regulation of a zinc-binding protein.

More Than Just Cleaning: Ubiquitin-Mediated Proteolysis in Fungal Pathogenesis

Ubiquitin-proteasome mediated protein turnover is an important regulatory mechanism of cellular function in eukaryotes. Extensive studies have linked the ubiquitin-proteasome system (UPS) to human diseases, and an array of proteasome inhibitors have been successfully developed for cancer therapy. Although still an emerging field, research on UPS regulation of fungal development and virulence has been rapidly advancing and has generated considerable excitement in its potential as a target for novel drugs. In this review, we summarize UPS composition and regulatory function in pathogenic fungi, especially in stress responses, host adaption, and fungal pathogenesis. Emphasis will be given to UPS regulation of pathogenic factors that are important for fungal pathogenesis. We also discuss future potential therapeutic strategies for fungal infections based on targeting UPS pathways.

From Jekyll to Hyde: The Yeast-Hyphal Transition of Candida albicans

Candida albicans is a major fungal pathogen of humans, accounting for 15% of nosocomial infections with an estimated attributable mortality of 47%. C. albicans is usually a benign member of the human microbiome in healthy people. Under constant exposure to highly dynamic environmental cues in diverse host niches, C. albicans has successfully evolved to adapt to both commensal and pathogenic lifestyles. The ability of C. albicans to undergo a reversible morphological transition from yeast to filamentous forms is a well-established virulent trait. Over the past few decades, a significant amount of research has been carried out to understand the underlying regulatory mechanisms, signaling pathways, and transcription factors that govern the C. albicans yeast-to-hyphal transition. This review will summarize our current understanding of well-elucidated signal transduction pathways that activate C. albicans hyphal morphogenesis in response to various environmental cues and the cell cycle machinery involved in the subsequent regulation and maintenance of hyphal morphogenesis.

Genome Wide Analysis of WD40 Proteins in Saccharomyces cerevisiae and Their Orthologs in Candida albicans

The WD40 domain containing proteins are present in the lower organisms (Monera) to higher complex metazoans with involvement in diverse cellular processes. The WD40 repeats fold into β propeller structure due to which the proteins harbouring WD40 domains function as scaffold by offering platform for interactions, bring together diverse cellular proteins to form a single complex for mediating downstream effects. Multiple functions of WD40 domain containing proteins in lower eukaryote as in Fungi have been reported with involvement in vegetative and reproductive growth, virulence etc. In this article insilico analysis of the WDR proteins in the budding yeast Saccharomyces cerevisiae was performed. By WDSP software 83 proteins in S. cerevisiae were identified with at least one WD40 motif. WD40 proteins with 6 or more WD40 motifs were considered for further studies. The WD40 proteins in yeast which are involved in various biological processes show distribution on all chromosomes (16 chromosomes in yeast) except chromosome 1. Besides the WD40 domain some of these proteins also contain other protein domains which might be responsible for the diversity in the functions of WD40 proteins in the budding yeast. These proteins in budding yeast were analysed by DAVID and Blast2Go software for functional and domains categorization. Candida albicans, an opportunistic fungal pathogen also have orthologs of these WD40 proteins with possible similar functions. This is the first time genome wide analysis of WD40 proteins in lower eukaryote i.e. budding yeast. This data may be useful in further study of the functional diversity of yeast proteomes.

THR1 mediates GCN4 and CDC4 to link morphogenesis with nutrient sensing and the stress response in Candida albicans

Candida albicans (C. albicans) CDC4 (CaCDC4), encoding the F-box protein for the substrate specificity of the Skp1-cullin-F-box E3 ubiquitin ligase complex, suppresses the yeast-to-filament transition in C. albicans. In our previous study, Thr1 was identified as a CaCdc4-associated protein using affinity purification. THR1 encodes a homoserine kinase, which is involved in the threonine biosynthesis pathway. The present study generated a strain with repressible CaCDC4 expression and continuous THR1 expression. Colony and cell morphology analyses, as well as immunoblotting, revealed that the Thr1 protein was detectable under conditions in which the expression of CaCDC4 was repressed and that the filaments resulting from the repressed expression of CaCDC4 were suppressed by the constitutive expression of THR1 in C. albicans. Additionally, by using the CaSAT1-flipper method, the present study produced null mutants of THR1, GCN4, and CaCDC4. The phenotypic consequences were evaluated by growth curves, spotting assays, microscopic analysis, reverse transcription-polymerase chain reaction and XTT-based biofilm formation ability. The results revealed that fewer cells lacking THR1 entered the stationary phase but had no apparent morphological alteration. It was observed that the expression of THR1 was upregulated concurrently with GCN4 during nutrient depletion and that cells lacking GCN4 rescued the lethality of cells in the absence of THR1 in conditions accumulating homoserine in the threonine biosynthesis pathway. Of note, it was found that cells with either CaCDC4 or THR1 loss were sensitive to oxidative stress and osmotic stress, with those with THR1 loss being more sensitive. In addition, it was observed that cells with loss of either CaCDC4 or THR1 exhibited the ability to increase biofilm formation, with those lacking CaCDC4 exhibiting a greater extent of enhancement. It was concluded that CaCDC4 is important in the coordination of morphogenesis, nutrient sensing, and the stress response through THR1 in C. albicans.

Regulation of the Candida albicans Hypha-Inducing Transcription Factor Ume6 by the CDK1 Cyclins Cln3 and Hgc1

The ability to switch between proliferation as yeast cells and development into hyphae is a hallmark of Candida albicans. The switch to hyphal morphogenesis depends on external inducing conditions, but its efficiency is augmented in stationary-phase cells. Ume6, a transcription factor that is itself transcriptionally induced under hypha-promoting conditions, is both necessary and sufficient for hyphal morphogenesis. We found that Ume6 is regulated posttranslationally by the cell cycle kinase Cdc28/Cdk1, which reduces Ume6 activity via different mechanisms using different cyclins. Together with the cyclin Hgc1, Cdk1 promotes degradation of Ume6 via the SCFCDC4 ubiquitin ligase. Since HGC1 is a key transcriptional target of Ume6, this results in a negative-feedback loop between Hgc1 and Ume6. In addition, we found that Cln3, a G1 cyclin that is essential for cell cycle progression and yeast proliferation, suppresses hyphal morphogenesis and that Cln3 suppresses Ume6 activity both in the heterologous Saccharomyces cerevisiae system and in C. albicans itself. This activity of Cln3 may provide the basis for the antagonistic relationship between yeast proliferation and hyphal development in C. albicans. IMPORTANCE The yeast to hypha (mold) morphogenetic switch of Candida albicans plays a role in its virulence and constitutes a diagnostic trait for this organism, the most prevalent systemic fungal pathogen in industrialized countries. It has long been known that hyphae are most efficiently induced from stationary cultures. Here, a molecular basis for this observation is provided. The G1 cyclin Cln3, an essential promoter of yeast proliferation, was found to suppress hyphal induction. Suppression of hyphal induction is achieved by inhibition of the activity of the central activator of hyphal morphogenesis, the transcription factor Ume6. Thus, levels of Cln3 control the switch between proliferation of C. albicans as individual yeast cells and development into extended hyphae, a switch that may preface the proliferation/differentiation switch in multicellular organisms.

Candida albicans Dbf4-dependent Cdc7 kinase plays a novel role in the inhibition of hyphal development

Candida albicans is an opportunistic human fungal pathogen. The ability to switch among multiple cellular forms is key to its pathogenesis. The Dbf4-dependent protein kinase gene CDC7 is conserved due to its role in initiating DNA replication. Because a C. albicans Cdc7 (Cacdc7) homozygous null was not viable, we generated a C. albicans strain with a deleted C. albicans CDC7 (CaCDC7) allele and an expression-repressible allele. Surprisingly, cells of the strain grew as hyphae under the repressed conditions. The in vitro kinase assays confirmed that CaCdc7 (K232) and CaCdc7 (T437) are critical for catalytic and phosphoacceptor of activation activity, respectively. C. albicans cells formed hyphae when expressing either the catalytically inactive CaCdc7 (K232R) or the phosphoacceptor-deficient CaCdc7 (T437A). While CaCdc7 interacted with CaDbf4, cells of the strain in which CaCDC7 was repressed were not rescued by constitutively expressing C. albicans DBF4 or vice versa. We conclude that CaDBF4-dependent CaCDC7 is an essential gene suppressing the hyphal development.

A role of Candida albicans CDC4 in the negative regulation of biofilm formation

The CDC4 gene is nonessential in Candida albicans and plays a role in suppressing filamentous growth, in contrast to its homologues, which are involved in the G1-S transition of the cell cycle. While characterizing the function of C. albicans CDC4 (CaCDC4), we found that the loss of CaCDC4 resulted in a reduction in cell flocculation, indicating a possible role for CaCDC4 in biofilm formation. To elucidate the role of CaCDC4 in biofilm formation, Cacdc4 null mutant strains were constructed by using the mini-Ura-blaster method. To create a CaCDC4 rescued strain, the plasmid p6HF-ACT1p-CaCDC4 capable of constitutively expressing CaCDC4 was introduced into the Cacdc4 homozygous null mutant. To determine the biofilm formation ability, an in vitro XTT (2,3-bis-(2-methoxy-4-nitro-5-sulfophenyl)-5-[(phenylamino)carbonyl]-2H-tetrazolium-5-carboxanilide) reduction assay was used. Compared with the parental auxotrophic strain BWP17, the Cacdc4 homozygous null mutant was able to enhance biofilm formation significantly. This enhancement of biofilm formation in the Cacdc4 homozygous null mutant could be reversed by constitutively expressing CaCDC4. We conclude that CaCDC4 has a role in suppressing biofilm formation in C. albicans.

Dissection of the Candida albicans Cdc4 protein reveals the involvement of domains in morphogenesis and cell flocculation

Background

CDC4, which encodes an F-box protein that is a member of the Skp1-Cdc53/Cul1-F-box (SCF) ubiquitin E3 ligase, was initially identified in the budding yeast Saccharomyces cerevisiae as an essential gene for progression through G1-S transition of the cell cycle. Although Candida albicans CDC4 (CaCDC4) can release the mitotic defect caused by the loss of CDC4 in S. cerevisiae, CaCDC4 is nonessential and suppresses filamentation.

Results

To further elucidate the function of CaCDC4, a C. albicans strain, with one CaCDC4 allele deleted and the other under the repressible C. albicans MET3 promoter (CaMET3p) control, was made before introducing cassettes capable of doxycycline (Dox)-induced expression of various C. albicans Cdc4 (CaCdc4) domains. Cells from each strain could express a specific CaCdc4 domain under Dox-induced, but CaMET3-CaCDC4 repressed conditions. Cells expressing domains without either the F-box or WD40-repeat exhibited filamentation and flocculation similarly to those lacking CaCDC4 expression, indicating the functional essentiality of the F-box and WD40-repeat. Notably, cells expressing the N-terminal 85-amino acid truncated CaCdc4 partially reverse the filament-to-yeast and weaken the ability to flocculate compared to those expressing the full-length CaCdc4, suggesting that N-terminal 85-amino acid of CaCdc4 regulates both morphogenesis and flocculation.

Conclusions

The F-box and the WD40-repeat of CaCdc4 are essential in inhibiting yeast-to-filament transition and flocculation. The N-terminal region (1–85) of CaCdc4 also has a positive role for its function, lost of which impairs both the ability to flocculate and to reverse filamentous growth in C. albicans.

Fbp1-mediated ubiquitin-proteasome pathway controls Cryptococcus neoformans virulence by regulating fungal intracellular growth in macrophages

Cryptococcus neoformans is a human fungal pathogen that often causes lung and brain infections in immunocompromised patients, with a high fatality rate. Our previous results showed that an F-box protein, Fbp1, is essential for Cryptococcus virulence independent of the classical virulence factors, suggesting a novel virulence control mechanism. In this study, we show that Fbp1 is part of the ubiquitin-proteasome system, and we further investigated the mechanism of Fbp1 function during infection. Time course studies revealed that the fbp1Δ mutant causes little damage in the infected lung and that the fungal burden in the lung remains at a low but persistent level throughout infection. The fbp1Δ mutant cannot disseminate to other organs following pulmonary infection in the murine inhalation model of cryptococcosis but still causes brain infection in a murine intravenous injection model, suggesting that the block of dissemination of the fbp1Δ mutant is due to its inability to leave the lung. The fbp1Δ mutant showed a defect in intracellular proliferation after phagocytosis in a Cryptococcus-macrophage interaction assay, which likely contributes to its virulence attenuation. To elucidate the molecular basis of the SCF(Fbp1) E3 ligase function, we analyzed potential Fbp1 substrates based on proteomic approaches combined with phenotypic analysis. One substrate, the inositol phosphosphingolipid-phospholipase C1 (Isc1), is required for fungal survival inside macrophage cells, which is consistent with the role of Fbp1 in regulating Cryptococcus-macrophage interaction and fungal virulence. Our results thus reveal a new determinant of fungal virulence that involves the posttranslational regulation of inositol sphingolipid biosynthesis.

Neddylation and CAND1 independently stimulate SCF ubiquitin ligase activity in Candida albicans

SCF (Skp1-cullin/Cdc53-F-box protein) ubiquitin ligases bind substrates via the variable F-box protein and, in conjunction with the RING domain protein Rbx1 and the ubiquitin-conjugating enzyme Ubc3/Cdc34, catalyze substrate ubiquitination. The cullin subunit can be modified covalently by conjugation of the ubiquitin-like protein Rub1/NEDD8 (neddylation) or bound noncovalently by the protein CAND1 (cullin-associated, neddylation-dissociated). Expression of the Candida albicans CAND1 gene homolog CaTIP120 in Saccharomyces cerevisiae is toxic only in the presence of CaCdc53, consistent with a specific interaction between CaTip120 and CaCdc53. To genetically analyze this system in C. albicans, we deleted the homologs of RUB1/NEDD8, TIP120/CAND1, and the deneddylase gene JAB1, and we also generated a temperature-sensitive allele of the essential CaCDC53 gene by knock-in site-directed mutagenesis. Deletion of CaRUB1 and CaTIP120 caused morphological, growth, and protein degradation phenotypes consistent with a reduction in SCF ubiquitin ligase activity. Furthermore, the double Carub1(-/-) Catip120(-/-) mutant was more defective in SCF activity than either individual deletion mutant. These results indicate that CAND1 stimulates SCF ubiquitin ligase activity and that it does so independently of neddylation. Our data do not support a role for CAND1 in the protection of either the F-box protein or cullin from degradation but are consistent with the suggested role of CAND1 in SCF complex remodeling.

Impaired colonization and infection of tomato roots by the Deltafrp1 mutant of Fusarium oxysporum correlates with reduced CWDE gene expression

The vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici efficiently invades roots and colonizes vascular tissues of its host tomato. For these processes, the F-box protein Frp1 is required. The Fusarium oxysporum Deltafrp1 mutant was characterized in detail to uncover the cause of its colonization defect. Using growth assays, we could attribute poor root colonization to reduced assimilation of organic acids, amino acids (except proline), or polysaccharides, singly or in combination. External root colonization by the Deltafrp1 mutant is restored by the addition of 0.1% glucose or proline but infection still does not occur. This is due to the inability of the Deltafrp1 mutant to penetrate the roots, as demonstrated by the lack of expression of SIX1 in the Deltafrp1 strain, which is a gene exclusively expressed inside roots, and loss of cell wall-degrading enzyme (CWDE) gene expression. Many of the metabolic defects of the Deltafrp1 strain can be attributed to reduced expression of the ICL1 (isocitrate lyase) gene. Strikingly, an Deltaicl1 mutant is still fully pathogenic and capable of external root colonization. We conclude that the inability of the Deltafrp1 strain to colonize and invade roots is not primarily due to metabolic defects but can be attributed to reduced expression of several CWDE genes.