Applications of citrus shoot-tip grafting in vitro.
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.