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Anthracnose fungi secrete ammonia to increase virulence
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TRENDS in Plant Science Vol.6 No.11 November 2001
503
Methylation of a different kind In plants, cytosine methylation is important for gene silencing and maintenance of genome integrity, and is found in symmetrical CpG and CpNpG sites (where N represents any base) and at nonsymmetrical sites. Previous work established that methylation of CpG sites originates from DNA methyltransferase (MT), an enzyme known for its ten conserved domains. However, the enzyme does not seem to methylate CpNpG sites. Reduction in CpG methylation in Arabidopsis leads to developmental consequences, as shown in genetic backgrounds deficient in methylation. Until recently, it was unclear what enzyme methylated CpNpG sites and what role CpNpG methylation had in plant development. Three reports converged recently to identify chromomethylase (CMT) as necessary for CpNpG methylation. CMTs have similar domains to MTs, but contain an additional chromodomain, a motif found in several proteins involved in chromatinbased gene silencing. The groups took two separate strategies: Anders Lindroth et al.1 and Lisa Bartee et al.2 used forward genetics in Arabidopsis, whereas Charles Papa and colleagues3 used reverse genetics in maize. Lindroth et al. used a marker locus, SUPERMAN (SUP ), previously shown to be a target sensitive to methylation changes1. Methylation silences SUP and results in
increased stamen numbers. The group used lines with a stably methylated SUP allele and generated mutants with decreased numbers of stamen. Their screen identified CHROMOMETHYLASE3 (CMT3), a locus important for maintenance and silencing of the SUP gene and of at least one retroelement. Using a sequencing technique to detect methylated cytosines, Lindroth et al. showed that cmt3 mutants have specifically decreased CpNpG methylation. Bartee et al.2 used another marker gene responsive to methylation differences, coding phosphoribosylanthranilate isomerase2 (PAI2), to screen for methylation mutants. In their system, the PAI2 gene is normally methylated and silenced. In Arabidopsis, compromising the PAI pathway results in plants that are fluorescent. Bartee et al. screened for non-fluorescent plants that derepressed the PAI2 gene. They found that CMT3 is necessary to maintain CpNpG methylation and silencing of the PAI2 gene. Papa identified the gene encoding Zea methyltransferase2 (Zmet2) in maize, which has sequence similarity to a CMT (Ref. 3). The group used a transposable element to knock out the Zmet2 gene and found that total 5-methylcytosine levels decreased by ~13%. When they specifically examined the
methylation of a known methylated sequence, they found that the Zmet2 mutant only lacked CpNpG methylation and retained other methylation. Thus, Zmet2 encodes a CpNpG-specific MT. ‘…CMT3 is necessary to maintain CpNpG methylation and silencing of the PAI2 gene.’ The three separate studies reveal a new enzyme important in epigenetic gene regulation: CMT. Surprisingly, the cmt mutants did not show gross morphological changes, unlike Arabidopsis plants lacking CpG methylation that exhibit a variety of developmental abnormalities. Gene-chip experiments should be useful to determine how many genes are controlled by CMT-mediated methylation. 1 Lindroth, A.M. et al. (2001) Requirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation. Science 292, 2077–2080 2 Bartee, L. et al. (2001) Arabidopsis cmt3 chromomethylase mutations block non-CG methylation and silencing of an endogenous gene. Genes Dev. 15, 1753–1758 3 Papa, C.M. et al. (2001) Maize chromomethylase Zea methyltransferase2 is required for CpNpG methylation. Plant Cell 13, 1919–1928
Trevor Stokes [email protected]
Anthracnose fungi secrete ammonia to increase virulence Resistance of unripe fruits to anthracnose fungi usually depends on the presence of either preformed antifungal compounds that decline in ripening fruits, or on inducible antifungal compounds. Few reports have described the possibility that the susceptibility of fruit to attack is dependent on the activation of pathogenicity factors that fungi use to increase virulence. Pectate lyase, a key virulence factor in the development of Colletotrichum gloeosporioides on avocado (Persea americana), is secreted at pH values higher than 5.8, but its gene, pelB, is expressed at pH 5.1. This, together with other data, suggests that whereas pelB is expressed and translated, pectate lyase remains in the mycelium until a secretion-permissive pH level is reached, that is, 5.7 or higher. Can the pH of the environment of the infection court be controlled by the pathogen? http://plants.trends.com
Dov Prusky et al.1 have now shown that three species of anthracnose fungi secrete ammonia locally into their host, resulting in a pH increase that enables secretion of pectate lyase and enhanced virulence. This is the first report to suggest that local alkalinization as a result of ammonia increase is a virulence factor. The fungi, C. gloeosporioides, C. acutatum and C. coccodes, were all able to secrete ammonia and raise the ambient pH of an acidified yeast extract medium and on host tissues, namely avocado, apple and tomato, respectively. Even when the pelB gene was disrupted in C. gloeosporioides so that pectate lyase was not produced, the mutant was able to increase ammonia secretion and pH just as the wild type does, suggesting that production of ammonia is independent of pelB expression.
Fungi can use a surprisingly diverse array of compounds as nitrogen sources for the production of ammonia, and are capable of expressing on demand the catabolic enzymes of many different pathways. However, the activation of pectate lyase secretion as a result of ammonia accumulation and elevated pH is a newly described mechanism of broad significance, enabling nitrogen metabolism to affect pathogenicity. Indeed, the authors suggest that the discovery could be used in plant-breeding programs to control decay occurring in post-harvest pathosystems. 1 Prusky, D. et al. (2001) Local modulation of host pH by Colletotrichum species as a mechanism to increase virulence. Mol. Plant–Microbe Interact. 14, 1105–1113
Richard C. Staples [email protected]
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.
TRENDS in Plant Science Vol.6 No.11 November 2001
503
Methylation of a different kind In plants, cytosine methylation is important for gene silencing and maintenance of genome integrity, and is found in symmetrical CpG and CpNpG sites (where N represents any base) and at nonsymmetrical sites. Previous work established that methylation of CpG sites originates from DNA methyltransferase (MT), an enzyme known for its ten conserved domains. However, the enzyme does not seem to methylate CpNpG sites. Reduction in CpG methylation in Arabidopsis leads to developmental consequences, as shown in genetic backgrounds deficient in methylation. Until recently, it was unclear what enzyme methylated CpNpG sites and what role CpNpG methylation had in plant development. Three reports converged recently to identify chromomethylase (CMT) as necessary for CpNpG methylation. CMTs have similar domains to MTs, but contain an additional chromodomain, a motif found in several proteins involved in chromatinbased gene silencing. The groups took two separate strategies: Anders Lindroth et al.1 and Lisa Bartee et al.2 used forward genetics in Arabidopsis, whereas Charles Papa and colleagues3 used reverse genetics in maize. Lindroth et al. used a marker locus, SUPERMAN (SUP ), previously shown to be a target sensitive to methylation changes1. Methylation silences SUP and results in
increased stamen numbers. The group used lines with a stably methylated SUP allele and generated mutants with decreased numbers of stamen. Their screen identified CHROMOMETHYLASE3 (CMT3), a locus important for maintenance and silencing of the SUP gene and of at least one retroelement. Using a sequencing technique to detect methylated cytosines, Lindroth et al. showed that cmt3 mutants have specifically decreased CpNpG methylation. Bartee et al.2 used another marker gene responsive to methylation differences, coding phosphoribosylanthranilate isomerase2 (PAI2), to screen for methylation mutants. In their system, the PAI2 gene is normally methylated and silenced. In Arabidopsis, compromising the PAI pathway results in plants that are fluorescent. Bartee et al. screened for non-fluorescent plants that derepressed the PAI2 gene. They found that CMT3 is necessary to maintain CpNpG methylation and silencing of the PAI2 gene. Papa identified the gene encoding Zea methyltransferase2 (Zmet2) in maize, which has sequence similarity to a CMT (Ref. 3). The group used a transposable element to knock out the Zmet2 gene and found that total 5-methylcytosine levels decreased by ~13%. When they specifically examined the
methylation of a known methylated sequence, they found that the Zmet2 mutant only lacked CpNpG methylation and retained other methylation. Thus, Zmet2 encodes a CpNpG-specific MT. ‘…CMT3 is necessary to maintain CpNpG methylation and silencing of the PAI2 gene.’ The three separate studies reveal a new enzyme important in epigenetic gene regulation: CMT. Surprisingly, the cmt mutants did not show gross morphological changes, unlike Arabidopsis plants lacking CpG methylation that exhibit a variety of developmental abnormalities. Gene-chip experiments should be useful to determine how many genes are controlled by CMT-mediated methylation. 1 Lindroth, A.M. et al. (2001) Requirement of CHROMOMETHYLASE3 for maintenance of CpXpG methylation. Science 292, 2077–2080 2 Bartee, L. et al. (2001) Arabidopsis cmt3 chromomethylase mutations block non-CG methylation and silencing of an endogenous gene. Genes Dev. 15, 1753–1758 3 Papa, C.M. et al. (2001) Maize chromomethylase Zea methyltransferase2 is required for CpNpG methylation. Plant Cell 13, 1919–1928
Trevor Stokes [email protected]
Anthracnose fungi secrete ammonia to increase virulence Resistance of unripe fruits to anthracnose fungi usually depends on the presence of either preformed antifungal compounds that decline in ripening fruits, or on inducible antifungal compounds. Few reports have described the possibility that the susceptibility of fruit to attack is dependent on the activation of pathogenicity factors that fungi use to increase virulence. Pectate lyase, a key virulence factor in the development of Colletotrichum gloeosporioides on avocado (Persea americana), is secreted at pH values higher than 5.8, but its gene, pelB, is expressed at pH 5.1. This, together with other data, suggests that whereas pelB is expressed and translated, pectate lyase remains in the mycelium until a secretion-permissive pH level is reached, that is, 5.7 or higher. Can the pH of the environment of the infection court be controlled by the pathogen? http://plants.trends.com
Dov Prusky et al.1 have now shown that three species of anthracnose fungi secrete ammonia locally into their host, resulting in a pH increase that enables secretion of pectate lyase and enhanced virulence. This is the first report to suggest that local alkalinization as a result of ammonia increase is a virulence factor. The fungi, C. gloeosporioides, C. acutatum and C. coccodes, were all able to secrete ammonia and raise the ambient pH of an acidified yeast extract medium and on host tissues, namely avocado, apple and tomato, respectively. Even when the pelB gene was disrupted in C. gloeosporioides so that pectate lyase was not produced, the mutant was able to increase ammonia secretion and pH just as the wild type does, suggesting that production of ammonia is independent of pelB expression.
Fungi can use a surprisingly diverse array of compounds as nitrogen sources for the production of ammonia, and are capable of expressing on demand the catabolic enzymes of many different pathways. However, the activation of pectate lyase secretion as a result of ammonia accumulation and elevated pH is a newly described mechanism of broad significance, enabling nitrogen metabolism to affect pathogenicity. Indeed, the authors suggest that the discovery could be used in plant-breeding programs to control decay occurring in post-harvest pathosystems. 1 Prusky, D. et al. (2001) Local modulation of host pH by Colletotrichum species as a mechanism to increase virulence. Mol. Plant–Microbe Interact. 14, 1105–1113
Richard C. Staples [email protected]
1360-1385/01/$ – see front matter © 2001 Elsevier Science Ltd. All rights reserved.