Acceleration of flowering in woody plants by grafting.

Unlike annual herbaceous species, trees have long juvenile periods, and several years or even decades have to be passed before their first flowering occurs, which is undesirable for both industrial and breeding purposes. This review describes various applications of grafting to reduce the time to first flowering in woody species. Dwarfing rootstocks commonly used in agriculture often induce precocious flowering in scions. Possible molecular mechanisms underlying rootstock-induced early flowering in grafted trees are considered. Another traditional approach is grafting juvenile scions onto mature trees, which has been used to induce precocity in a variety of woody species. The main drawbacks of grafting on both dwarfing and mature rootstocks are moderate degree of precocity and varying efficiency for different rootstock-scion combinations. Modern biotechnological approaches to induce early flowering are based on the overexpression of floral activators through either genetic transformation or recombinant viruses. Transgenic rootstocks expressing graft-mobile floral activators have been utilized to induce early flowering in non-transgenic scions of several woody species. Different transgrafting studies demonstrated contrasting efficiency of this approach, apparently due to an insufficient amount of floral inducer being provided by the transgenic rootstock to the scion in certain experiments. Methods for increasing the amount of floral inducer in grafted scions are thoroughly described. Another approach to induce precocity in trees is VIF, or virus-induced flowering, which is based on the use of viral vectors to systemically infect the plant and express floral activators. Grafting has promising yet largely unexplored potential in VIF and other applications that utilize viral vectors.