Colletotrichum orbiculare WHI2, a Yeast Stress-Response Regulator Homolog, Controls the Biotrophic Stage of Hemibiotrophic Infection Through TOR Signaling

The Whi2-Psr1-Psr2 complex selectively regulates TORC1 and autophagy under low leucine conditions but not nitrogen depletion

Amino acids and ammonia serve as sources of nitrogen for cell growth and were previously thought to have similar effects on yeast. Consistent with this idea, depletion of either of these two nitrogen sources inhibits the target of rapamycin complex 1 (TORC1), leading to induction of macroautophagy/autophagy and inhibition of cell growth. In this study, we show that Whi2 and the haloacid dehalogenase (HAD)-type phosphatases Psr1 and Psr2 distinguish between these two nitrogen sources in Saccharomyces cerevisiae, as the Whi2-Psr1-Psr2 complex inhibits TORC1 in response to low leucine but not in the absence of nitrogen. In contrast, a parallel pathway controlled by Npr2 and Npr3, components of the Seh1-associated complex inhibiting TORC1 (SEACIT), suppress TORC1 under both low leucine- and nitrogen-depletion conditions. Co-immunoprecipitations with mutants of Whi2, Psr1, Psr2 and fragments of Tor1 support the model that Whi2 recruits Psr1 and Psr2 to TORC1. In accordance, the interaction between Whi2 and Tor1 appears to increase under low leucine but decreases under nitrogen-depletion conditions. Although the targets of Psr1 and Psr2 phosphatases are not known, mutation of their active sites abolishes their inhibitory effects on TORC1. Consistent with the conservation of HAD phosphatases across species, human HAD phosphatases CTDSP1 (CTD small phosphatase 1), CTDSP2, and CTDSPL can functionally replace Psr1 and Psr2 in yeast, restoring TORC1 inhibition and autophagy activation in response to low leucine conditions.

TORC1 Signaling in Fungi: From Yeasts to Filamentous Fungi

Target of rapamycin complex 1 (TORC1) is an important regulator of various signaling pathways. It can control cell growth and development by integrating multiple signals from amino acids, glucose, phosphate, growth factors, pressure, oxidation, and so on. In recent years, it has been reported that TORC1 is of great significance in regulating cytotoxicity, morphology, protein synthesis and degradation, nutrient absorption, and metabolism. In this review, we mainly discuss the upstream and downstream signaling pathways of TORC1 to reveal its role in fungi.

The Rheb GTPase promotes pheromone blindness via a TORC1-independent pathway in the phytopathogenic fungus Ustilago maydis

The target of the rapamycin (TOR) signaling pathway plays a negative role in controlling virulence in phytopathogenic fungi. However, the actual targets involved in virulence are currently unknown. Using the corn smut fungus Ustilago maydis, we tried to address the effects of the ectopic activation of TOR on virulence. We obtained gain-of-function mutations in the Rheb GTPase, one of the conserved TOR kinase regulators. We have found that unscheduled activation of Rheb resulted in the alteration of the proper localization of the pheromone receptor, Pra1, and thereby pheromone insensitivity. Since pheromone signaling triggers virulence in Ustilaginales, we believe that the Rheb-induced pheromone blindness was responsible for the associated lack of virulence. Strikingly, although these effects required the concourse of the Rsp5 ubiquitin ligase and the Art3 α-arrestin, the TOR kinase was not involved. Several eukaryotic organisms have shown that Rheb transmits environmental information through TOR-dependent and -independent pathways. Therefore, our results expand the range of signaling manners at which environmental conditions could impinge on the virulence of phytopathogenic fungi.

MoWhi2 Mediates Mitophagy to Regulate Conidiation and Pathogenesis in Magnaporthe oryzae

Mitophagy refers to the specific process of degrading mitochondria, which is an important physiological process to maintain the balance of mitochondrial quantity and quality in cells. At present, the mechanisms of mitophagy in pathogenic fungi remain unclear. Magnaporthe oryzae (Syn. Pyricularia oryzae), the causal agent of rice blast disease, is responsible for the most serious disease of rice. In M. oryzae, mitophagy occurs in the foot cells and invasive hyphae to promote conidiation and infection. In this study, fluorescent observations and immunoblot analyses showed that general stress response protein MoWhi2 is required for mitophagy in M. oryzae. In addition, the activation of the autophagy, pexophagy and cytoplasm-to-vacuole targeting (CVT) pathway upon nitrogen starvation was determined using the GFP-MoATG8, GFP-SRL and MoAPE1-GFP strains and the ΔMowhi2 mutant in these backgrounds. The results indicated that MoWhi2 is specifically required for mitophagy in M. oryzae. Further studies showed that mitophagy in the foot cells and invasive hyphae of the ΔMowhi2 was interrupted, leading to reduced conidiation and virulence in the ΔMowhi2 mutant. Taken together, we found that MoWhi2 contributes to conidiation and invasive growth by regulating mitophagy in M. oryzae.

MoWhi2 regulates appressorium formation and pathogenicity via the MoTor signalling pathway in Magnaporthe oryzae

Magnaporthe oryzae causes rice blast disease, which seriously threatens the safety of food production. Understanding the mechanism of appressorium formation, which is one of the key steps for successful infection by M. oryzae, is helpful to formulate effective control strategies of rice blast. In this study, we identified MoWhi2, the homolog of Saccharomyces cerevisiae Whi2 (Whisky2), as an important regulator that controls appressorium formation in M. oryzae. When MoWHI2 was disrupted, multiple appressoria were formed by one conidium and pathogenicity was significantly reduced. A putative phosphatase, MoPsr1, was identified to interact with MoWhi2 using a yeast two-hybridization screening assay. The knockout mutant ΔMopsr1 displayed similar phenotypes to the ΔMowhi2 strain. Both the ΔMowhi2 and ΔMopsr1 mutants could form appressoria on a hydrophilic surface with cAMP levels increasing in comparison with the wild type (WT). The conidia of ΔMowhi2 and ΔMopsr1 formed a single appressorium per conidium, similar to WT, when the target of rapamycin (TOR) inhibitor rapamycin was present. In addition, compared with WT, the expression levels of MoTOR and the MoTor signalling activation marker gene MoRS3 were increased, suggesting that inappropriate activation of the MoTor signalling pathway is one of the important reasons for the defects in appressorium formation in the ΔMowhi2 and ΔMopsr1 strains. Our results provide insights into MoWhi2 and MoPsr1-mediated appressorium development and pathogenicity by regulating cAMP levels and the activation of MoTor signalling in M. oryzae.

CoGRIM19 is required for invasive hyphal growth of Colletotrichum orbiculare inside epidermal cells of cucumber cotyledons

Colletotrichum orbiculare, an anthracnose disease fungus of cucurbit plants, extends penetration hyphae inside the epidermal cells of host plants. Unlike vegetative hyphae formed on a nutrient rich medium, this pathogen initially develops biotrophic penetration hyphae, which acquire nutrient resources from living host cells and secret effector proteins to suppress host defense responses. Subsequently, the nature of penetration hyphae changes from biotrophy to necrotrophy in response to the interaction with a host plant. Hence, controlling the extension of penetration hyphae is crucial for C. orbiculare infection. Here, we identified CoGRIM19 encoding Nadh-ubiquinone oxidoreductase subunit as a pathogenicity gene. Pathogenicity assays showed that the cogrim19 mutant caused no visible symptoms on cucumber cotyledons. Microscopic observations revealed that the cogrim19 mutant developed an appressorium and penetration hyphae under artificial conditions such as on coverslips or cellulose membranes, but the penetration hyphae of the mutant were retarded in the cucumber cotyledons. Microscopic observations of biotrophy-specific expression fluorescent signals revealed that the biotrophic stage was maintained in the retarded penetration hyphae of the cogrim19 mutant as the penetration of the wild type. In addition to cytological observations, pathogenicity assays using wounded leaves showed that the cogrim19 mutant had an attenuated pathogenesis. Taking our results together, CoGRIM19 is required for invasive hyphal growth inside the epidermal cells of cucumber cotyledons in C. orbiculare.

Antifungal activity of rapamycin on Botryosphaeria dothidea and its effect against Chinese hickory canker

BACKGROUND

The trunk canker caused by Botryosphaeria dothidea is a devastating disease for Chinese hickory (Carya cathayensis) in China. Chemical fungicides are commonly and repeatedly applied to control the disease; however, fungicide application raises major environmental and food safety issues. Ecofriendly biocontrol alternatives were urgently needed. Herein, the antifungal activity of a natural secondary metabolite, rapamycin, against B. dothidea and the effect of a rapamycin-producing Streptomyces hygroscopicus on C. cathayensis canker were investigated.

RESULTS

The 50% effective concentrations (EC₅₀ ) of rapamycin against mycelial growth and spore germination on the potato dextrose agar were 5.1 × 10^-3 and 5.5× 10^-2 μg mL^-1 , respectively, which were much lower than the EC₅₀ values of thiophanate-methyl. The hyphae of B. dothidea exhibited premature aging and wrinkling after treatment with rapamycin at 5.0 × 10^-3 μg mL^-1 . A rapamycin-producing bacterium S. hygroscopicus LYJ637 was batch produced and formulated in a carboxymethylcellulose/poly (vinyl alcohol) (CMC/PVA) blend and used for testing the efficiency of the bacterium in controlling Botryosphaeria canker in C. cathayensis. S. hygroscopicus exhibited high stability in the CMC/PVA blend. Results of a 3-year field experiment suggested that rapamycin formation reduced the occurrence of both developed cankers and new cankers, with an efficacy comparable to the treatment with thiophanate-methyl.

CONCLUSION

The rapamycin-producing S. hygroscopicus LYJ637 carried in a CMC/PVA blend prevented effectively Botryosphaeria canker on Chinese hickory, which provides an alternative approach to chemical control strategies.

The Whi2p-Psr1p/Psr2p complex regulates interference competition and expansion of cells with competitive advantage in yeast colonies

Yeast form complex highly organized colonies in which cells undergo spatiotemporal phenotypic differentiation in response to local gradients of nutrients, metabolites, and specific signaling molecules. Colony fitness depends on cell interactions, cooperation, and the division of labor between differentiated cell subpopulations. Here, we describe the regulation and dynamics of the expansion of papillae that arise during colony aging, which consist of cells that overcome colony regulatory rules and disrupt the synchronized colony structure. We show that papillae specifically expand within the U cell subpopulation in differentiated colonies. Papillae emerge more frequently in some strains than in others. Genomic analyses further revealed that the Whi2p-Psr1p/Psr2p complex (WPPC) plays a key role in papillae expansion. We show that cells lacking a functional WPPC have a sizable interaction-specific fitness advantage attributable to production of and resistance to a diffusible compound that inhibits growth of other cells. Competitive superiority and high relative fitness of whi2 and psr1psr2 strains are particularly pronounced in dense spatially structured colonies and are independent of TORC1 and Msn2p/Msn4p regulators previously associated with the WPPC function. The WPPC function, described here, might be a regulatory mechanism that balances cell competition and cooperation in dense yeast populations and, thus, contributes to cell synchronization, pattern formation, and the expansion of cells with a competitive fitness advantage.

KCTD: A new gene family involved in neurodevelopmental and neuropsychiatric disorders

The underlying molecular basis for neurodevelopmental or neuropsychiatric disorders is not known. In contrast, mechanistic understanding of other brain disorders including neurodegeneration has advanced considerably. Yet, these do not approach the knowledge accrued for many cancers with precision therapeutics acting on well-characterized targets. Although the identification of genes responsible for neurodevelopmental and neuropsychiatric disorders remains a major obstacle, the few causally associated genes are ripe for discovery by focusing efforts to dissect their mechanisms. Here, we make a case for delving into mechanisms of the poorly characterized human KCTD gene family. Varying levels of evidence support their roles in neurocognitive disorders (KCTD3), neurodevelopmental disease (KCTD7), bipolar disorder (KCTD12), autism and schizophrenia (KCTD13), movement disorders (KCTD17), cancer (KCTD11), and obesity (KCTD15). Collective knowledge about these genes adds enhanced value, and critical insights into potential disease mechanisms have come from unexpected sources. Translation of basic research on the KCTD-related yeast protein Whi2 has revealed roles in nutrient signaling to mTORC1 (KCTD11) and an autophagy-lysosome pathway affecting mitochondria (KCTD7). Recent biochemical and structure-based studies (KCTD12, KCTD13, KCTD16) reveal mechanisms of regulating membrane channel activities through modulation of distinct GTPases. We explore how these seemingly varied functions may be disease related.

Chloramphenicol inhibits eukaryotic Ser/Thr phosphatase and infection-specific cell differentiation in the rice blast fungus

Chloramphenicol (Cm) is a broad-spectrum classic antibiotic active against prokaryotic organisms. However, Cm has severe side effects in eukaryotes of which the cause remains unknown. The plant pathogenic fungus Magnaporthe oryzae, which causes rice blast, forms an appressorium to infect the host cell via single-cell differentiation. Chloramphenicol specifically inhibits appressorium formation, which indicates that Cm has a novel molecular target (or targets) in the rice blast fungus. Application of the T7 phage display method inferred that MoDullard, a Ser/Thr-protein phosphatase, may be a target of Cm. In animals Dullard functions in cell differentiation and protein synthesis, but in fungi its role is poorly understood. In vivo and in vitro analyses showed that MoDullard is required for appressorium formation, and that Cm can bind to and inhibit MoDullard function. Given that human phosphatase CTDSP1 complemented the MoDullard function during appressorium formation by M. oryzae, CTDSP1 may be a novel molecular target of Cm in eukaryotes.

Colletotrichum orbiculare MTF4 Is a Key Transcription Factor Downstream of MOR Essential for Plant Signal-Dependent Appressorium Development and Pathogenesis

The cucumber anthracnose fungus Colletotrichum orbiculare forms a specialized infection structure, called an appressorium. Appressorium differentiation relies on fungal perception of physical and biochemical signals at the plant surface. Our previous report showed that the morphogenesis-related NDR (nuclear Dbf2-related) kinase pathway (MOR) is crucial for translating plant-derived signals for appressorium development. Here, we focused on identifying transcriptional regulators downstream of MOR that are involved in plant signal sensing and transduction for appressorium development. Based on whole-genome transcript profiling, we identified a Zn(II)₂Cys₆ transcription factor, CoMTF4, as a potential downstream factor of MOR. CoMTF4 was expressed in planta rather than in vitro under the control of the NDR kinase CoCbk1. Phenotypes of comtf4 mutants, strains with constitutively active CoCbk1 and strains with constitutive overexpression of CoMTF4 suggested that CoMtf4 acts downstream of MOR. Furthermore, nuclear localization of CoMtf4 was dependent on the MOR and responsive to plant-derived signals that lead to appressorium morphogenesis. Thus, we conclude that CoMtf4 is a transcription factor downstream of MOR that is essential for appressorium morphogenesis and pathogenesis and is regulated in response to plant-derived signals. This study provides insights into fungal sensing of plant signals and subsequent responses critical for appressorium formation.

Whi2: a new player in amino acid sensing

A critical function of human, yeast, and bacterial cells is the ability to sense and respond to available nutrients such as glucose and amino acids. Cells must also detect declining nutrient levels to adequately prepare for starvation conditions by inhibiting cell growth and activating autophagy. The evolutionarily conserved protein complex TORC1 regulates these cellular responses to nutrients, and in particular to amino acid availability. Recently, we found that yeast Whi2 (Saccharomyces cerevisiae) and a human counterpart, KCTD11, that shares a conserved BTB structural domain, are required to suppress TORC1 activity under low amino acid conditions. Using yeast, the mechanisms were more readily dissected. Unexpectedly, Whi2 suppresses TORC1 activity independently of the well-known SEACIT-GTR pathway, analogous to the GATOR1-RAG pathway in mammals. Instead, Whi2 requires the plasma membrane-associated phosphatases Psr1 and Psr2, which were known to bind Whi2, although their role was unknown. Yeast WHI2 was previously reported to be involved in regulating several fundamental cellular processes including cell cycle arrest, general stress responses, the Ras-cAMP-PKA pathway, autophagy, and mitophagy, and to be frequently mutated in the yeast knockout collections and in genome evolution studies. Most of these observations are likely explained by the ability of Whi2 to inhibit TORC1. Thus, understanding the function of yeast Whi2 will provide deeper insights into the disease-related KCTD family proteins and the pathogenesis of plant and human fungal infections.

Whi2 signals low leucine availability to halt yeast growth and cell death

Cells are exquisitely tuned to environmental ques. Amino acid availability is rapidly sensed, allowing cells to adjust molecular processes and implement short or long-term metabolic shifts accordingly. How levels of most individual amino acids may be sensed and subsequently signaled to inform cells of their nutrient status is largely unknown. We made the unexpected observation that small changes in the levels of specific amino acids can have a profound effect on yeast cell growth, leading to the identification of yeast Whi2 as a negative regulator of cell growth in low amino acids. Although Whi2 was originally thought to be fungi-specific, Whi2 appears to share a conserved structural domain found in a family of 25 largely uncharacterized human genes encoding the KCTD (potassium channel tetramerization domain) protein family. Insights gained from yeast Whi2 are likely to be revealing about human KCTDs, many of which have been implicated or demonstrated to cause disease when mutated. Here we report new evidence that Whi2 responds to specific amino acids in the medium, particularly low leucine levels. We also discuss the known pathways of amino acid signaling and potential points of regulation by Whi2 in nutrient signaling in yeast and mammals.

Whi2 is a conserved negative regulator of TORC1 in response to low amino acids

Yeast WHI2 was originally identified in a genetic screen for regulators of cell cycle arrest and later suggested to function in general stress responses. However, the function of Whi2 is unknown. Whi2 has predicted structure and sequence similarity to human KCTD family proteins, which have been implicated in several cancers and are causally associated with neurological disorders but are largely uncharacterized. The identification of conserved functions between these yeast and human proteins may provide insight into disease mechanisms. We report that yeast WHI2 is a new negative regulator of TORC1 required to suppress TORC1 activity and cell growth specifically in response to low amino acids. In contrast to current opinion, WHI2 is dispensable for TORC1 inhibition in low glucose. The only widely conserved mechanism that actively suppresses both yeast and mammalian TORC1 specifically in response to low amino acids is the conserved SEACIT/GATOR1 complex that inactivates the TORC1-activating RAG-like GTPases. Unexpectedly, Whi2 acts independently and simultaneously with these established GATOR1-like Npr2-Npr3-Iml1 and RAG-like Gtr1-Gtr2 complexes, and also acts independently of the PKA pathway. Instead, Whi2 inhibits TORC1 activity through its binding partners, protein phosphatases Psr1 and Psr2, which were previously thought to only regulate amino acid levels downstream of TORC1. Furthermore, the ability to suppress TORC1 is conserved in the SKP1/BTB/POZ domain-containing, Whi2-like human protein KCTD11 but not other KCTD family members tested.