根は多くの穀物やマメ科作物を宿主とし, 世界各地で農業に甚大な被害をもたらしている. 根種子は宿主植物の根から分泌される信号物質を感受してはじめて発芽する. これは宿主から独立しては生きられない根が持つ巧妙な生存戦略と考えられている. 根種子の発芽を刺激する物質について概説するとともに, 宿主認識における発芽刺激物質の重要性および発芽刺激物質を用いた根の防除の可能性についてまとめた.

抄録全体を表示

は他の植物に水分や栄養分を頼って暮らす植物である。と、宿主なしでは生活環を全うできない絶対がある。ハマウツボ科の絶対であるストライガやオロバンキは世界各地で甚大な農業被害をもたらしている。我々はの寄生の仕組みを理解することにより、の防除法を確立することを目指している。
彼岸の季節に低山帯でピンクの花を咲かせるコシオガマは、日本に自生するハマウツボ科の条件的である。単独でも生育できるため実験系として優れていると考え、我々は、コシオガマを用いて寄生過程における遺伝子発現解析を試みた。コシオガマではEST情報等はほぼ存在しないため、次世代シーケンサーを用いたde novo RNA sequencing解析をおこなった。独立栄養で生育させたコシオガマとイネに寄生したコシオガマ根からRNAを抽出し、454型およびillumina型シーケンサーで解析し、アセンブリにより約5万の重複のない塩基配列を得た。さらに、各サンプルのリード配列をアセンブリ配列にマップし、特異的発現遺伝子の同定を試みた。その結果、寄生時のコシオガマ根ではプロテアーゼなどの分解酵素群が特異的に発現していた。本発表では、次世代シーケンサーを用いたde novo 解析および遺伝子発現の定量化における方法論や問題点等も議論したい。

抄録全体を表示

は，他の植物に寄生し，水分や栄養を吸収して生きる植物である．ハマウツボ科の植物はほとんどがであるが，なかでも最も有名なストライガは，アフリカなどでイネ科穀物に寄生し，深刻な農業被害を引き起こしている．また，日本にもナンバンギセルやコシオガマなどのが存在し，多くの有用植物の生育が阻害されている．これらのことから，ハマウツボ科の駆除を目的とした寄生メカニズムの解明が行われている．
これまでに明らかにされている主な3例を挙げる．①1根に「吸器」と呼ばれる特殊な寄生器官を形成し，宿主の根に侵入し，維管束から水や栄養を奪う．②宿主との間にmRNAの移動を伴う．③同科ヤセウツボ(Orobanche minor)の生育培地に芳香族アミノ酸 (AAAs：チロシン，フェニルアラニン，トリプトファン)を添加すると，その発芽や幼根の成長が阻害される．本稿では，③の知見をもとに，関連遺伝子導入によりAAAs産生量を増加させたタバコ(Nicotiana tabacum)を作製し，エジプトハマウツボ(O. aegyptiaca)の成長抑制に及ぼす影響を検討したOlivaらの報告を紹介する．
なお，本稿は下記の文献に基づいて，その研究成果を紹介するものである.
1) Wakatake T. et al., BSJ-Review, 7, 241-249(2016).
2) Fernández A. M. et al., Front. Plant Sci., online 17 May 2017.
3) Oliva M. et al.，Front. Plant Sci., online 12 Jan. 2021.

抄録全体を表示

根

は重要農作物に寄生し収量を低下させるため，世界の食糧生産に深刻な被害を及ぼしている．近年，我々は放線菌Streptomyces ficellusの生産するノジリマイシン（NJ）が根種子の発芽を阻害することを見出した．本研究ではS. ficellusのNJ生産性向上を目指した培地改良，および未精製培養物の根防除剤としての適用可能性について検討した．従来のNJ生産培地に使用されていたPharmamedia™を他の汎用的な培地成分に置換したところ，マリンブロスによりNJ生産量が向上した．4日間培養を行ったところ，培地中のNJ含量は710 mg/Lに達し，従来の17倍まで向上した．得られた培養液を各種子に処理したところ，NJ 標準溶液と同等の発芽阻害活性を示した．本研究で示した当該培養法は根防除剤生産開発につながると期待される．

抄録全体を表示

Several parasitic plants in Orobanchaceae, such as Striga and Orobanche, cause devastating damage on agriculture worldwide. However, the molecular mechanisms of plant parasitism remain poorly understood. Orobanchaceae include species in a different range of parasitism, i.e. facultative and obligate parasites. Facultative parasites complete their life cycle without host plants, while obligate parasites are not able to survive without hosts in a natural condition. Although both parasites respond to quinone signals to develop infectious organs called haustoria, their shapes are distinct. Their responses to germination stimulants are also different. We are conducting genome and transcriptome analyses of an obligate parasite Striga asiatica and a facultative parasite Phtheirospermum japonicum to identify genes responsible for plant parasitism. A transformation protocol of P. japonicum was also established and used for functional characterization of parasitic plant genes.

抄録全体を表示

Parasitic plants obtain nutrients and water from their angiosperm hosts via the infecting organ called haustorium.　Striga spp. are devastating parasitic weeds that parasitize important crops, such as maize, rice and sorghum, and therefore cause significant yield losses that are estimated as billion dollars annually.　Recent completion of Striga asiatica genome sequence provided us insights into evolution of parasitic plants.　Whole genome duplication in the Striga lineage and co-option of lateral root development programs may have brought innovation of haustorium formation.　The KAI2 genes encoding strigolactone receptors were locally duplicated in the Striga genome.　Furthermore, the large genome segments were horizontally transferred to Striga genome from their Poaceae hosts.　This article summarizes the Striga genome evolution together with recently-sequenced holoparasitic Cuscuta genomes.

抄録全体を表示

In angiosperms, approximately 1%, ca, 4,500 species are parasitic. Parasitic plants can be classified as to their photosynthesis status. Hemiparasites retain photosynthetic ability. On the other hand, holoparasites lack photosynthetic ability and obtain all their carbon from their hosts. In these parasitic species, selective pressure to maintain photosynthetic ability had been released. Photoresponses of plants which are important to optimize photosynthesis for green plants are not necessary in holoparasites. Photoresponses are also involved in the regulation of plant development such as germination process. Studies on photoresponses of parasitic plants, Orobanche minor and Striga hermonthica revealed that the pattern of their photoresponses are unique and had been modified for adaptation to its parasitic life cycle. Molecular analysis of their red/far-red light photoreceptor phytochrome A (phyA) also demonstrated that the alteration of their amino acid sequences and associated molecular function. Elucidation of altered components in phy signaling in the parasitic plants will provide useful information not only about its physiological characteristics but also about general plant photoreception systems.

抄録全体を表示

Cytokinins are widely recognized as crucial phytohormones regulating plant growth and development by promoting cell division and differentiation. They are also known to be involved in inter-organism interactions, including the interaction between parasitic plants and host plants. Orobanchaceae parasitic plants form prehaustoria, primitive haustorial structures before host invasion, when in close proximity to the hosts. The induction of prehaustoria requires host-derived compounds, namely haustorium-inducing factors (HIFs), including quinones, phenolics and flavonoids, with structural similarity to lignin monomers. Notably, recent reports showed that cytokinins are capable to induce prehaustoria in the obligate parasitic plants in Orobanchaceae. Once prehaustoria invade into host roots, they become mature haustoria, which efficiently extract water and nutrients from the host. Cytokinins are also involved in this stage, as they are synthesized by parasitic plants and transported to host plants, causing hypertrophy and promoting efficient parasitism. In this review, we summarize the roles of cytokinins in the interaction between parasitic plants and host plants, with a specific focus on current understanding of prehaustorium induction, hypertrophy and germination of obligate parasites.

抄録全体を表示

他の植物に寄生し養分を奪って生活するは、全被子植物種の約1％を占める。ハマウツボ科の根ストライガ(Striga hermonthica)は、モロコシやトウモロコシ、イネなどの主要な穀物を宿主とし、アフリカ半乾燥地域を中心に甚大な農業被害をもたらしている。しかし、の感染機構に関しては未知の点が多く、根本的な解決策は見つかっていない。ストライガは、その発芽に宿主植物から分泌されるストリゴラクトンを必要とするだけでなく、侵入・寄生器官である吸器の形成にも宿主由来のシグナルを必要とする。宿主根に侵入したストライガ吸器は宿主と自身の維管束系を連結させ宿主から養分を得て生活する。寄生成立まで一連の過程で特異的な分子プログラムが働いていると考えられるが、ストライガ感染の分子レベルでの解析はあまり進んでいない。
我々は、ストライガの分子生物学的解析のための基盤構築をおこなっている。本発表では、1）ストライガ感染系の確立と感染過程の観察、2）ストライガ宿主特異性の解析、3）全長cDNAライブラリーの構築とEST解析によって明らかになったストライガゲノムの特性について紹介する。また、日本在来種である半コシオガマ(Phtheirospermum japonicum)を用いた解析の新しい試みについても紹介したい。

抄録全体を表示

ゴマノハグサ科（Orobanchacea）に属するストライガ(Striga)とオロバンキ(Orobanche)は代表的な根であり、世界の農業生産に甚大な被害を与えている。これらの根は絶対寄生であり、宿主に寄生することによってのみ生存できる。根の種子は極めて微小で貯蔵養分も限られているため、発芽後数日間で宿主の根に寄生できない場合には死滅する。そこで根は、宿主の根の存在を検出し、根の近傍の種子だけが発芽するという巧妙な生存戦略を備えている。この宿主認識には、植物の根から分泌されるストリゴラクトンが関与している。最初に単離されたストリゴラクトンであるstrigolを始めとして10種類以上のストリゴラクトンが報告されている。ストリゴラクトンは化学的に不安定で、土壌中では速やかに消失するため、根の近傍の種子だけが発芽する。植物は単独ではなく複数のストリゴラクトンの混合物を分泌しており、ストリゴラクトンの種類、その組み合わせは同種の植物でも品種によって異なっている。また、生育ステージや生育条件によって分泌されるストリゴラクトンの質的／量的変化が起きる。これらの違いが発芽刺激活性の変化を介して根の宿主認識に関与しているのであろう。また、発芽以降の過程における宿主と根の相互作用も宿主認識に寄与していると考えられている。

抄録全体を表示

Parasitic plants have a very close association with their host. The physiological interactions between parasite and host and the signals required for parasite development are only beginning to be understood. Because of its small genome and rapid life cycle, Lotus has become a model legume. We have demonstrated that Lotus is susceptible to Orobanche crenata, a devastating pathogen of dicotyledonous crops. Lotus roots produce germination stimulant, allow haustorium formation, and support attached O. crenata plants, which develop normally to the point of flowering. Lotus was not however susceptible to other parasitic plant species examined, including O. aegyptiaca, O. minor, Striga gesnerioides and S. hermonthica. The finding that Lotus is highly susceptible to O. crenata provides an excellent opportunity to study different aspects of plant-parasite interactions, including host-parasite communication and host resistance mechanisms.

抄録全体を表示

Holoparasitic plants such as Orobanche spp. have lost their photosynthetic ability. On the other hand, hemiparasites retain photosynthetic ability. In the holoparasitic species, selective pressure to maintain photosynthetic ability had been released. Photoresponses of plants which are important to optimize photosynthesis for green plants are not necessary in holoparasites. Photoresponses are also involved in the regulation of plant development such as germination process. Studies on photoresponses of parasitic plants revealed that the pattern of their photoresponses had been modified for adaptation to its parasitic life cycle. Molecular analysis of their photoreceptors also demonstrated that the alteration of amino acid sequences and associated molecular function. Elucidation of altered components in these parasitic plants will provide useful information not only about its physiological characteristics but also about general plant photoreception systems and evolution.

抄録全体を表示

In angiosperms, approximately 1%, ca, 4,500 species are parasitic. Parasitic plants can be classified as to their photosynthesis status. Hemiparasites retain photosynthetic ability. On the other hand, holoparasites lack photosynthetic ability and obtain all their carbon from their hosts. In these parasitic species, selective pressure to maintain photosynthetic ability had been released. Photoresponses of plants which are important to optimize photosynthesis for green plants are not necessary in holoparasites. Photoresponses are also involved in the regulation of plant development such as germination process. Studies on photoresponses of parasitic plants revealed that the pattern of their photoresponses had been modified for adaptation to its parasitic life cycle. Molecular analysis of their red/far-red light photoreceptor phytochrome also demonstrated that the alteration of thier amino acid sequences and associated molecular function.

抄録全体を表示

ストライガやオロバンキといったは主要作物に寄生するため、アフリカを中心に世界中で甚大な農業被害を引き起こしている。これらはホストの根から地中に放出されるストリゴラクトンを認識して発芽を起こす事が知られている。このステップがどのようなメカニズムで起こるかを理解することはを有効に防除する方法を確立するのに欠かすことはできないものの、寄生というライフスタイルからも遺伝解析は非常に難しいため、現時点では極めて限られた情報しか得られていない。今回我々はシロイヌナズナを使った解析から、発芽を含めた光応答にストリゴラクトンが重要な役割を担っていることを報告する。 新たに得られた突然変異株の解析から、ストリゴラクトンがシロイヌナズナに対しても発芽促進活性を示し、胚軸の伸張阻害活性を持つことが明らかとなった。さらに詳細な解析から、ストリゴラクトンの生合成は光によって刺激され、光シグナル伝達の鍵となるHY5転写因子の安定化を経由して胚軸の伸張を阻害する事が明らかとなった。必ずしも光に応答する必要のないは、他の植物が放出するストリゴラクトンを光として受け取っているとも考えられ、のストリゴラクトン応答を利用した薬剤の開発の両方にとって光応答は重要なターゲットとなりうると考えられる。

抄録全体を表示

Plants of the genera Striga and Orobanche are noxious root parasitic weeds on many food crops. Their life cycle is strongly cued to that of their hosts. To germinate the parasites need a moist pre-treatment (conditioning) at specified temperatures followed by exposure to an exogenous stimulant. On perception of a second signal the parasites produce haustoria that are the bridge for transfer of water, minerals and organic compounds from the hosts. I have been studying the interactions between the parasites and their hosts. Based on proposed structure of sorgolactone, the main germination stimulant isolated from sorghum root exudates, all eight stereoisomers were synthesized. Only those isomers possessing the same stereochemistry as the natural sorgolactone at two adjacent chiral centers exhibited significant germination stimulatory activity. The involvement of endogenous ethylene in germination of S. hermonthica was confirmed. Expression of ACC synthase and ACC oxidase genes was found to be responsive to germination stimulants and to conditioning, respectively. Lotus japonicus, a model legume for nodulation and mycorrhizal colonization, was compatible with O. aegyptiaca but not with O. minor, S. hermonthica and S. gesnerioides. O. aegyptiaca and S. hermonthica parasitism specifically induced the expression of genes involved in nodulation and phytoalexin biosynthesis, respectively. Reactions of the model legume and its mutants to O. aegyptiaca suggest that nodulation, mycorrhizal colonization and plant parasitism are modulated by similar mechanisms.

抄録全体を表示

Strigolactones (SLs) act as both plant growth regulators and as chemoattractants for arbuscular mycorrhizal fungi. Secreted SLs are also sensed by Orobanchaceae root parasitic plants as germination stimulants. Here we focus on the impact of exogenous SLs on Phtheirospermum japonicum, a facultative hemiparasite in the Orobanchaceae. The genome of P. japonicum encodes SL receptor candidates, and its roots exhibit chemotropism toward host-derived SLs. This chemotropism was also observed in the obligate hemiparasite Striga hermonthica but not in non-parasitic plants, suggesting that chemotropism to SLs might be limited to Orobanchaceae parasitic plants. Interestingly, under ammonium-rich conditions, where P. japonicum can grow without hosts, its chemotropism to SLs does not occur. Furthermore, SLs induced asymmetric auxin response in the root epidermis in P. japonicum, leading to asymmetric root growth. Our findings shed new light on the function of SLs as host-derived chemoattractants, providing valuable insights into the mechanisms behind parasitic plant interactions with their hosts.

抄録全体を表示

Effects of rhizobial symbiosis on Orobanche aegyptiaca parasitism to Lotus japonicus were studied. Approximately 30% of the parasite attached onto the host root formed tubercles, and root primordia developed from the tubercles. Concurrent inoculation of Mesorhizobium loti doubled the number of tubercles, suggesting some positive effects of rhizobial symbiosis on the parasite invasion. Inoculation of the rhizobium three weeks before attachment of the parasite did not affect the number of tubercles. However, the tubercles turned brown, and root development of the parasite delayed significantly, suggesting the negative effects of the symbiosis on the parasite growth.

抄録全体を表示

Parasitic plants, Orobanche spp. and Striga spp. cause serious damage to the African and Mediterranean agriculture. An effective strategy for the parasitic weed control is desired for economical and humanitarian reasons. Reception of strigolactones which are released from roots of host plants is necessary for the parasitic plants to germinate. Metabolites profiling with GC/MS revealed that activation of metabolism occurred three days after GR24, a synthetic strigolactone, treatment in seeds of Orobanche minor. One of the metabolites, allantoin was found to decrease after GR24 treatment and inhibitors of allantoin metabolism reduced the germination rate of O. minor. Allantoin is used for nitrogen transport in leguminous plants. We found that glutamine was increased in parallel with the allantoin degradation in O. minor seeds. In Arabidopsis seeds, arginine is pooled as a nitrogen source but no correlation was found between the amounts of arginine and glutamine in the seeds of O. minor. We hypothesize that allantoin is pooled in O. minor seeds as a nitrogen source. This results will provide a new target for the parasitic weed control.

抄録全体を表示