New Phytologist (Dec 2016)
Plant grafting: how genetic exchange promotes vascular reconnection
Jing Wang, Libo Jiang, Rongling Wu

Summary

Grafting has been widely used to improve horticultural traits. It has also served increasingly as a tool to investigate the long-distance transport of molecules that is an essential part for key biological processes. Many studies have revealed the molecular mechanisms of graft-induced phenotypic variation in anatomy, morphology and production. Here, we review the phenomena and their underlying mechanisms by which macromolecules, including RNA, protein, and even DNA, are transported between scions and rootstocks via vascular tissues. We further propose a conceptual framework that characterizes and quantifies the driving mechanisms of scion–rootstock interactions toward vascular reconnection and regeneration.

Phenotypic variation in grafted plants

Grafting commonly influences the phenotype of the grafted plants (Warschefsky et al., 2015), including changes in fruit quality, resistance to pests and pathogens, tolerance to adversity and stress, and other physiological disorders. The vegetative fruit quality of scions is commonly altered by the rootstocks after grafting. Taller et al. (1998) described a case in which two pepper scion cultivars acquired changes in fruit shape, color and pungency after grafting. The results also illustrated that several rootstock features were present in the progeny of the scion after self-pollination. Similarly, a graft of three watermelon cultivars and three hybrid squashes showed differences in shape, weight, yield, quality, rind thickness and pH among stocks (Turhan et al., 2012). Fruit trees, such as sweet cherry, apple and citrus, have also been shown to be influenced by grafting.

Grafting is widely used to improve resistance to pests and diseases. For instance, grafting can alleviate the development of post-harvest diseases in Hass avocado fruit (Willingham et al., 2001). Anthracnose, caused by the fungus Colletotrichum gloeosporioides, is the most severe post-harvest disease of avocado fruits. The rootstocks can significantly affect the post-harvest anthracnose resistance of scions, which is probably related to an increase in antifungal diene and improvement in mineral nutrients in the scions. Research on the resistance of pepper plants to both phytophthora blight and bacterial wilt also confirmed the effect of grafting. Five commercial stocks and nine breeding lines were used as rootstocks for the scion 'Nokkwang', three of which were selected for their greater resistance to phytophthora blight and bacterial wilt without reduction in productivity or fruit quality (Jang et al., 2012). Furthermore, a study using tomatoes revealed that cultivars grafted onto nematode-resistant rootstocks gained higher yields than did nongrafted ones (Lopez-Perez et al., 2006). Similar results have been found in eggplants (Ioannou, 2001), cucumbers (Gu et al., 2006) and peppers (Oka et al., 2004).

Grafting can also affect tolerance to abiotic stress. Experiments in cherry tomato found that grafting on drought-tolerant rootstocks resulted in higher fruit production (Sánchez-Rodríguez et al., 2012). Several studies have shown that salt stress in cucumber can be alleviated by grafting cucumber onto Cucurbita rootstocks. Alleviation of salt stress may be due to delaying photo-inhibition, which is caused by changes in nitrogen metabolism during salt penetration. Other examples, such as the tolerance of citrus to boron stress (Papadakis et al., 2004a,b) and the resistance of tomato to thermal stress (Rivero et al., 2003), provide more options for exploring the mechanisms of this phenomenon.

In addition to the cases mentioned above, related publications have indicated that physiological and morphological features can be altered by stock–scion interactions. The ability of rootstocks from certain fruit trees to dwarf their scions, which has been acknowledged for decades, is used in agriculture. A series of rootstocks used for dwarfing has been developed in apples, and genetic marker analysis linked to the dwarfing traits has been performed. In micrografting experiments in Arabidopsis, Turnbull et al. (2002) found that the wild-type (WT) stocks can effectively inhibit rosette branching of the increased branching mutants max1 (more axillary growth) and max3. Notably, when two shoots from the max1 and WT seedlings were simultaneously grafted onto a max1 rootstock, the mutant shoot showed increasing branching while the WT shoot did not. When the max1 rootstock was replaced with a WT rootstock, neither of the shoots branched profusely. The results indicated that branch signaling can spread from root to shoot but not from shoot to shoot.

*CR: Prince: Reciprocal Influence of Stock and Scion (1832)
I have now to state to you what I have never met with in any author, that the graft has an influence on the stock and root of the tree. The cherry tree when the thermometer in hard winters falls much below zero, is frequently killed by the severity of the frost. I had some years ago, 1821, a number of cherry trees killed, but the Weeping cherry, a native of Siberia, although budded some height from the ground, remained uninjured; this led me more minutely to examine their roots, and I found invariably, that the roots of all the weeping cherries differed from the roots of other cherry trees, although the stock was the same; the roots of the trees grafted or budded with the weeping cherry being much fuller of fine spreading fibres, and rooting much stronger. Mentioning this fact to a man who keeps a small apple nursery in this place, and on whose veracity I could depend, he told me that the graft of the Siberian crab apple trees, although grafted two feet from the ground, affected the roots, and caused them to become so wiry and hard, and so full of these fine tough fibrous roots, and that they were very different from the roots of other apple trees.

The majority of current research has been dedicated to using rootstocks to influence shoot phenotypes, but the root changes induced by scions have been seldom discussed,* probably due to the important role that scions play in agricultural and horticultural practices and also the relative difficulty in observing root phenotype changes given that they are below ground. However, the effects of the scion on stock growth and carbohydrate storage seem to be indisputable (Dahniya et al., 1982), and root–shoot interactions remain to be explored further.

Vegetative Hybrids

Stock / Scion Influence