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 投稿者:  投稿日:2022年 2月15日(火)15時13分14秒

Hard graft problem solved for key global food crops
Colin Turnbull & Sean Carrington
Grafting has long been used to join tissues of different plants in horticulture and research. Methods have now been devised to extend the technique to plants called monocotyledons, which include major crops such as cereals and bananas.

The technique of grafting together the shoot of one plant and the roots of another is immensely beneficial in a variety of contexts. However, efforts to use this approach have long failed for certain key crops. On page 280, Reeves et al.1 report success in developing a grafting method that can be used for plants called monocotyledons, or monocots.
Plant grafting has an ancient history, dating back to early civilizations. More than 2,000 years ago, De Agri Cultura (‘On Farm- ing’), a book written by the Roman senator Cato, details the grafting of vines and fruit trees, indicating it to be commonplace. Such grafting practices remain widespread today.
Yet one major group of plants, the mono- cots, have proved problematic for use in grafting. The name refers to the single leaf (a cotyledon) in the plant seed, a feature that distinguishes monocots from other flowering plant groups that have two cotyledons, and that are conventionally called dicotyledons, or dicots. Monocots abound in the global flora. They include all of the world’s cereals ? rice, wheat and maize (corn) ? which together pro- vide more than half the calories consumed by humans. Another key monocot is the banana, a staple food in many nations and the world’s most popular fruit after the tomato.
Despite many attempts to graft monocots, minimal success meant that grafting never became mainstream. Indeed, many experts viewed monocot grafting as a near-impossible feat2,3, often attributing failure to anatomical differences between monocots and dicots, especially monocots’ lack of a specialized inner cellular layer found in dicots called vascular cambium. However, paradoxically, there are witchweeds ? dicot plants of the genus Striga that live as devastating parasites attached to the roots of monocot crops such as maize and sorghum4. The parasite feeds through an interface that is, in essence, a nat- ural graft plumbed into the host’s transport (vascular) systems, proving that nature long ago accomplished a version of grafting that humans have struggled to achieve.

ある植物の新芽と別の植物の根を接ぎ木する技術は、さまざまな状況で非常に有益です。ただし、このアプローチを使用する取り組みは、特定の主要作物では長い間失敗しています。 280ページで、Reeves et al.1は、単子葉植物または単子葉植物と呼ばれる植物に使用できる接ぎ木法の開発に成功したことを報告しています。
植物の接ぎ木には、初期の文明にまでさかのぼる古代の歴史があります。 2、000年以上前、ローマの上院議員カトが書いた本、De Agri Cultura(「農業について」)は、ブドウの木と果樹の接ぎ木について詳しく説明しており、それが当たり前のことであることを示しています。そのような接ぎ木慣行は今日でも広まっています。

Reeves and colleagues present compelling evidence that monocot grafting is feasible, after all, and propose that the absence of vas- cular cambium is not a limiting factor. Instead, their work focuses on a feature shared by all plants: immature tissue that can be repro- grammed to make the essential connecting structures needed for a successful graft. The authors’ use of fine surgical tools enabled the precise assembly of grafts of young germinat- ing seedlings at a time in the plants’ develop- ment when the root is just emerging, whereas the same method tried in older plants proved much less successful.
One key to plants’ terrestrial dominance is linked to their extraordinary ability to recover from damage, whether arising from storms, herbivore grazing or simply humans mowing the grass. With a much greater regenerative capacity than that of most vertebrates, many types of plant cell are totipotent, enabling the replacement of missing parts. Some- times, this regenerative process requires the formation of a disorganized group of cells called a callus, from which tissues and organs emerge. In grafting, when two cut pieces are placed together, a wound-repair mechanism makes connections between the pieces, result- ing in a new whole plant. An essential feature of this process is connection of the plant’s ‘plumbing system’ ? the vascular highways of tissues called xylem and phloem that transport water, sugars, nutrients and other molecules throughout a plant (Fig. 1).
Reeves et al. report that, within days of making a monocot graft, they observe fluores- cent dyes (applied to the cut surface) moving in both directions across the graft. Vascular cells develop, and the graft is sufficiently strong to be picked up by hand.
The authors find that genes in cells around the graft junction are rapidly expressed, as a prelude to visible signs of graft formation. The expression of many of these genes is a hallmark of regenerative processes. The genes encode wound-repair factors, regulatory proteins and hormones, as well as components needed torestart the cell cycle and cell growth, remodel cell walls and create the essential vascular connections. Some of the gene-expression patterns observed by the authors mirror those seen during graft formation in dicots5, whereas others might be unique to monocots.
Intriguingly, Reeves et al. report that grafts could be made between strikingly different pairs of cereals, such as between wheat and sorghum, that cannot generate a hybrid offspring plant through a conventional pollination approach. The authors explored a spectacular spectrum of edible and ornamen- tal monocots, examining the sorts of plant that can be found by raiding the shelves of garden centres and tropical greenhouses. From palm to pineapple, agave to lily, cardamom to yam, all yielded grafts through cutting and connect- ing young seedlings.

Reeves etal。単子葉植物の移植片を作ってから数日以内に、彼らは(切断面に塗布された)蛍光染料が移植片を横切って両方向に動くのを観察すると報告している。血管細胞が発達し、移植片は手で拾うのに十分な強さです。
興味深いことに、Reeves etal。小麦とソルガムの間など、従来の受粉アプローチでは雑種の子孫植物を生成できない、著しく異なる穀物のペアの間で移植を行うことができると報告しています。著者は、園芸用品センターや熱帯温室の棚を襲撃することによって見つけることができる植物の種類を調べて、食用および観賞用の単子葉植物の壮大なスペクトルを調査しました。手のひらからパイナップル、アガベからユリ、カルダモンからヤムイモまで、すべてが若い苗を切ってつなぐことで移植片を生み出しました。

Figure 1 | Grafting of crop plants. Plants called monocotyledons, or monocots, include cereals and many tropical crops. Grafting of monocots ? the production of a plant that has roots and shoots from different plants ? has a history of failure. Reeves et al.1 report a successful monocot grafting method. a, This approach in cereals uses germinating seeds. The embryonic shoot (called the plumule) is excised and replaced by
a plumule from a donor seed, ensuring that transplanted material makes close contact with the radicle
(the embryonic region that forms the root). b, Immature cells at the interface of the grafted tissue, called the graft union, reprogram to join up the two parts. Communication and the movement of materials (such as water and nutrients) between the shoot and the root was possible because the xylem and phloem tissues of the transport system connected. Grafting offers a way to engineer plants that have beneficial characteristics, such as those aiding their ability to cope with diseases and stresses exacerbated by climate change.

図1 |作物の接ぎ木。単子葉植物、または単子葉植物と呼ばれる植物には、穀物や多くの熱帯作物が含まれます。単子葉植物の接ぎ木(さまざまな植物からの根と芽を持つ植物の生産)には、失敗の歴史があります。 Reeves et al.1は、単子葉植物の接ぎ木法の成功を報告しています。 a、穀物のこのアプローチは発芽種子を使用します。胚芽(小球と呼ばれる)は切除され、
(ルートを形成する胚の領域)。 b、グラフトユニオンと呼ばれるグラフト組織の界面にある未熟な細胞は、2つの部分を結合するように再プログラムします。輸送システムの木部組織と師部組織が接続されているため、シュートと根の間のコミュニケーションと物質(水や栄養素など)の移動が可能でした。接ぎ木は、気候変動によって悪化する病気やストレスに対処する能力を支援する植物など、有益な特性を持つ植物を設計する方法を提供します。


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