Since ancient times, grafting has been utilized to develop horticultural crops to increase plant vigor, yield potential, and tolerance to various environmental stresses. Grafting employs selected rootstocks of the same species or near relatives. Moreover, dwarfing is desirable for intensive cultivation and efficient orchard management. However, information on the regulatory mechanisms regulating rootstock-induced vigor remains unclear. Numerous studies comprehend the physiological and molecular mechanism of rootstock control scion vigor, which has significantly improved our understanding regarding vigor control processes in apple, litchi, pear, citrus, etc. This review summarizes the current knowledge on the impact of rootstocks on scion vigor and hormonal mechanisms involved in dwarfing, such as auxin (IAA), cytokinins (CK), abscisic acid (ABA), gibberellic acids (GAs), and brassinosteroids (BR). This study will provide fascinating information for future studies on rootstock-induced dwarfing effects and accelerate the breeding of dwarfing rootstocks.
Genetic modifications of floral organs are important in the breeding of Malus species. Flower-specific promoters can be used to improve floral organs specifically, without affecting vegetative organs, and therefore developing such promoters is highly desirable. Here, we characterized two paralogs of the Arabidopsis thaliana gene AGAMOUS (AG) from Malus domestica (apple): MdAG1 and MdAG2. We then isolated the second-intron sequences for both genes, and created four artificial promoters by fusing each intron sequence to a minimal 35S promoter sequence in both the forward and reverse directions. When transferred into tobacco (Nicotiana benthamiana) by Agrobacterium tumefaciens-mediated stable transformation, one promoter, rMdAG2I, exhibited activity specifically in flowers, whereas the other three also showed detectable activity in vegetative organs. A test of the four promoters’ activities in the ornamental species Malus micromalus by Agrobacterium-mediated transient transformation showed that, as in tobacco, only rMdAG2I exhibited a flower-specific expression pattern. Through particle bombardment transformation, we demonstrated that rMdAG2I also had flower-specific activity in the apple cultivar ‘Golden Delicious’. The flower-specific promoter rMdAG2I, derived from M. domestica, thus has great potential for use in improving the floral characteristics of ornamental plants, especially the Malus species.
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