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植物科学:単子葉類でも接木は可能 続き

 投稿者:  投稿日:2022年 2月15日(火)15時16分34秒
  Banana proved amenable, even though most cultivated varieties do not make seeds. To succeed with this fruit, Reeves and col- leagues grafted microscopic banana shoots grown in laboratory cultures to genetically different roots. These rapidly developed into normal-looking banana plants. Given that the world’s most popular banana, a variety called Cavendish, is at risk of being wiped out by the fungus that causes the deadly Panama disease, or banana wilt6, grafting bananas onto disease-resistant roots might present an opportunity to combat this threat. Indeed, wheat plants readily succumb to the soil fungus that causes an infection called take-all disease, but Reeves et al. found that the plants were protected if wheat shoots were grafted onto disease-resistant oat rootstocks.
Similar approaches to tackle plant disease already exist for dicots. For example, in the nineteenth century, grapevines were devas- tated by the insect pest phylloxera. Grafting saved the day back then, and most grapes cul- tivated worldwide continue to be grown on phylloxera-resistant rootstocks7. For monocot root crops such as yam, where high-yielding varieties might be limited by shoot suscepti- bility to disease, grafting of plants to disease- resistant shoots could be a game-changer.
Reeves and colleagues’ demonstration that grafting is feasible in monocots opens the door to the investigation of many questions for research, some of which have been explored only in dicots. Previous grafting work has already led to insights into how different parts of a plant communicate with each other. Moreover, grafting offers a way of altering plant development, such as accelerating the start of flowering8, changing plant height8 and modifying the number of branches. With regard to branching regulation, Reeves and colleagues confirmed that monocot roots transmit the same signals as those used by dicots, with grafted roots being able to suppress branching of mutant shoots that lack the ability to make the branch-inhibitinghormone strigolactone9,10. Optimizing the number of stems or branches on a plant is valuable in agriculture. Too few can mean low numbers of the flowers that develop into grains or fruits, but too many create crowded, spindly, unproductive shoots.
Grafting can reveal the types of molecule that travel through the plant vasculature. For example, when a disease-causing agent invades a leaf, local defences are activated, and signals might then travel to the rest of the plant, providing a spreading immunity that provides protection ahead of further attacks. Antimicrobial metabolite molecules can move across graft sites, as can many defence and stress hormones, and other larger signal- ling molecules, such as RNA, peptides and proteins11.
It might not be possible for grafting to be adopted across the millions of hectares currently devoted to cereals ? it would be impractical to graft every seedling before it is transplanted into fields. But there could be huge benefits in grafting high-value crops and longer-lived species such as palm trees, analo- gous to the widely deployed use of grafting of dicot tree fruits, vines, melons and tomatoes. Grafting could offer a way to develop special- ized root systems tailored to local challenges from pests and diseases. Alternatively, it could help plants to cope with drought or adapt to soils that lack nutrients or have high levels of salt. Climate change and certain forms of intensive agriculture are exacerbating many such problems.
Grafting might be superior to using conven- tional breeding to introduce stress-response factors into elite plant varieties. This is especially pertinent if the factors originate from wild undomesticated relatives, because the unpredictable mixing of genomes throughconventional breeding often leads to com- promises in the quality and quantity of crop production.
The next few years should see the refine- ment and expansion of monocot grafting. This should pave the way for the technique to become commercially viable. Monocot graft- ing would add a new weapon to the armoury that could enable plants to remain robust in the face of the many pressures that climate change will continue to exert on the planet’s ecosystems.


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