635 Applications of Citrus Shoot-Tip Grafting In Vitro José Juárez, Pablo Aleza and Luis Navarro * Centro de Protección Vegetal y Biotecnología, Instituto Valenciano de Investigación Agrarias (IVIA), Moncada, Valencia, Spain *Corresponding author: lnavarro@ivia.es Abstract The main application of shoot-tip grafting in vitro (STG) is to control graft- transmissible pathogens that requires the use of healthy trees in the new plantings. The routine application of STG using 0.1-0.2 mm shoot tips is very efficient for elimination of all citrus graft-transmissible pathogens from local or imported varieties. It has allowed the recovery of hundreds of healthy cultivars and the planting of hundreds of millions of healthy certified trees worldwide. Only in Spain about 140 million certified nursery plants propagated from micrografted plants have been planted. STG is also a very useful technique for regeneration of elite genotypes in several research areas. In vitro grafting for these purposes may be done using larger shoots (up to 1 cm). STG is being routinely used for the following purposes: (i) Regeneration of somatic hybrids from embryos difficult to germinate; (ii) Regeneration of plants from irradiated shoots to produce seedless varieties; (iii) Regeneration of plants from haploid embryos that are very difficult to germinate. STG was used to regenerate the ‘Clemenules’ haploid plant that has been used by the International Citrus Genome Consortium to sequence the whole citrus genome; (iv) Production of stable tetraploid plants of non-apomictic genotypes, which are very useful for triploid breeding; (v) Regeneration of transgenic plants from shoots that are very difficult to root in vitro. STG has become a routine application in citrus genetic transformation. Keywords: micrografting, certification of nursery plants, regeneration, somatic hybrids, irradiation, pathogen elimination, quarantine, haploid, tetraploid, genetic transformation INTRODUCTION Citrus graft-transmissible diseases produced by viruses, viroids, some bacteria, spiroplasmas, and phytoplasmas result in serious economic losses in most citrus growing regions of the world. They cause decline, loss of vigour and low yields and poor fruit quality. In addition, they restrict the use of some superior rootstocks susceptible to one or several diseases. Thus, they may potentially become the main limiting factors of production. Only preventive measures are efficient to control graft-transmissible pathogens, such as the utilization of tolerant or resistant germplasm, exclusion of potential diseases from the citrus area, and establishing new plantings using pathogen-free high quality nursery trees. Pathogen-free plants of many cultivars are often not available and it is necessary to recover healthy plants from infected ones. In this situation, a method able to recover citrus plant free of all graft-transmissible pathogens and without juvenile characters was required to produce healthy trees for commercial propagation. Murashige et al. (1972) were able to recover a few citrus plants by grafting shoot tips from diseased plants on young rootstock seedlings growing in vitro. Some of these plants were free of the exocortis viroid and did not have juvenile characters. This procedure was studied in detail by Navarro et al. (1975), who named it shoot tip grafting in vitro (STG), and developed a routine procedure that allowed a 30-50% incidence of successful grafts that were transplanted to soil, with over 95% survival rate. The resulting plants did not have juvenile characters, and most of them were free of graft-transmissible pathogens. Proc. XII th Intl. Citrus Congress Eds.: B. Sabater-Muñoz et al. Acta Hort. 1065, ISHS 2015