論文の公開元へConstitutive and inducible defensive metabolites in Hordeum species and wheat
概要
（様式第１３号）
学 位 論 文 要 旨
氏名: 宇部 尚樹
題目: Constitutive and inducible defensive metabolites in Hordeum species and wheat
(オオムギ属植物とコムギにおける構成的および誘導性防御物質に関する研究）
Plants possess chemical defense systems to protect themselves against biotic stresses.
Antifungal compounds in plants, such as phytoanticipins and phytoalexins, contribute to the
rejection of pathogen infection. Phytoanticipins are defined as low molecular weight,
antimicrobial compounds that are present in plants before challenge by microorganisms or are
produced after infection solely from preexisting constituents. On the other hand, phytoalexins
are defined as low molecular weight, antimicrobial compounds produced by plants in response
to pathogen attack, and provide a chemical barrier against the invasion of pathogen.
In general, closely related species accumulate similar specialized metabolites. However,
in the genus Hordeum (Poaceae), which includes cultivated barley (Hordeum vulgare ssp.
vulgare), the occurrence of multiple defensive metabolites such as hordatines, benzoxazinones
(Bxs), and gramine has been reported. Hordeum species are classified into four clades, H, Xu,
Xa, and I. The H and Xu clades comprise a monophyletic group, while the I and Xa clades form
a separate monophyletic group. However, the correlation between phylogeny and the
distribution of defensive specialized metabolites in Hordeum has not been clarified. To reveal
the evolutionary changes in secondary metabolism in the genus Hordeum, the presence or
absence of defensive specialized metabolites was analyzed in representative Hordeum species in
all four clades. In the H clade, Hordeum vulgare accumulated hordatines but not Bxs, whereas H.
bulbosum accumulated neither compound. H. vulgare ssp. vulgare ‘Shurai’, H. vulgare ssp.
spontaneum, and H. murinum ssp. leporinum accumulated gramine at high concentrations, while
several species in I clade also accumulated it at low concentrations. Species in the clades I and
Xa accumulated Bxs without hordatines. In H. murinum, a Xu clade species, neither hordatines
nor Bxs were detected. Two hitherto undescribed compounds were found to commonly
accumulate in H. bulbosum in the H clade and in H. murinum in the Xu clade. On the basis of
spectroscopic analyses, they were identified as dehydrodimers of feruloylagmatine and were
designated murinamides A and B. These compounds showed antifungal activities against
pathogenic fungi, indicating their defensive roles. As hordatines are also dehydrodimers of
phenylamides with agmatine, both the H and Xu clade species are considered to accumulate the
same class of compounds. Thus, when the H/Xu clades split from the I/Xa clades during
evolution, the defensive metabolism shifted from the synthesis of Bxs to the synthesis of
dehydrodimers of phenylamides with agmatine plus gramine in the H/Xu clades.
Plants often activate multiple specialized metabolic pathways upon pathogen infection.
Poaceae species such as rice and maize accumulate phytoalexins with diverse chemical
structures including phenylamides, flavonoids, and terpenoid. On the other hands, in barley and
wheat, inducible defensive systems with specialized metabolites remain to be elucidated while
representative phytoanticipins such as hordatines and Bxs, respectively, have been identified.
Changes in specialized metabolites were analyzed in wheat leaves inoculated with Bipolaris
sorokiniana, the causal agent of spot blotch in Poaceae species. HPLC analysis detected the
accumulation of six compounds in B. sorokiniana-infected leaves. Of these, we purified two
compounds by silica gel and ODS column chromatography and preparative HPLC, and then
identified them as cinnamic acid amides, N-cinnamoyl-9-hydroxy-8-oxotryptamine and
N-cinnamoyl-8-oxotryptamine, by spectroscopic analyses. We named these compounds
triticamides A and B, respectively. The remaining four compounds were predicted to be
p-coumaric acid amides of hydroxyputrescine, hydroxyagmatine, hydroxydehydroagmatine, and
agmatine by mass spectrometry. To examine the localization of triticamides A and B in
pathogen-inoculated leaves, lesioned and healthy tissues were extracted separately and
subjected to HPLC analysis. Triticamides were detected at high concentrations in the lesioned
tissue of the leaves. The accumulation of two cinnamic acid amides was also induced by
Fusarium graminearum infection, and by treatment with CuCl2, jasmonic acid, and
isopentenyladenine. Antifungal activity of these amides was demonstrated by inhibition of
conidial germination and germ tube elongation of pathogenic fungi, such as Fusarium
graminearum and Alternaria brassicicola, thus indicating that they act as phytoalexins. The
accumulation of these amides was also detected in barley leaves treated with CuCl2. We
examined the accumulation of 25 phenylamides in B. sorokiniana-infected wheat leaves using
LC-MS/MS. Hydroxycinnamic acid amides of tryptamine, serotonin, putrescine, and agmatine
were induced after infection with B. sorokiniana. Thus, the induced accumulation of two groups
of phenylamides, cinnamic acid amides with indole amines and p-coumaric acid amides with
putrescine- and agmatine-related amines, represents a major metabolic response of wheat to
pathogen infection.
Metabolic changes in pathogen-infected barley were examined in addition to those in
wheat. HPLC analysis detected the induced accumulation of three compounds in barley roots
challenged by Fusarium culmorum, the causal agent of Fusarium root rot. The three compounds
were identified as triticamides A and B, and N-cinnamoyl-(1H-indol-3-yl)methylamine, which
we named triticamide C, by spectroscopic analysis. Triticamides A and B were also detected in
wheat, whereas triticamide C was detected only in barley. On the basis of their antimicrobial
activities, triticamides function as phytoalexins in barley. The accumulation of 25 phenylamides
in pathogen-infected root and leaves were also examined using LC-MS/MS. Accumulation of
phenylamides with putrescine, agmatine, tyramine, tryptamine, and serotonin was induced in the
pathogen-infected barley. The administration of deuterium-labeled N-cinnamoyl tryptamine
(CinTry) to barley roots resulted in the effective incorporation of the compound into triticamides
A and B, which suggested that they were synthesized through the oxidation of CinTry. Nine
putative tryptamine hydroxycinnamoyl transferase (THT)-encoding genes (HvTHT1–HvTHT9)
were identified by database search on the basis of homology to known THT gene sequences in
rice. Since HvTHT7 and HvTHT8 had the same sequences except one nucleotide, we measured
their expression levels in total by RT-qPCR. HvTHT7/8 were markedly upregulated in F.
culmorum-infected root and B. sorokiniana-infected leaves. The HvTHT7 and HvTHT8
enzymes preferred cinnamoyl- and feruloyl-CoAs as acyl donors and tryptamine as an acyl
acceptor, and (1H-indol-3-yl)methylamine was also accepted as an acyl acceptor. These findings
suggested that HvTHT7/8 are responsible for the induced accumulation of triticamides in barley.
In the present study, we found that barley and its related species in H and Xu clades in
the genus Hordeum accumulate HCAA dimers as phytoanticipins, whereas the species in I and
Xa clades accumulate Bxs in common with wheat. This distribution of different classes of
compounds suggests great metabolic changes that occurred in the evolution of species in H and
Xu clades. On the other hand, barley and wheat accumulated common phenylamide
phytoalexins, triticamides, in response to pathogen attacks. As these compounds have not been
described in other species to the best of our knowledge, triticamides are considered to be
characteristic compounds in these species. These differences and similarities of defensive
specialized metabolites among Triticeae species are considered to be a reflection of
“scrap-and-build” in specialized metabolism, which has occurred in evolution of Triticeae
species for survival under varying biological stresses.
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