Effects of Amino Acids on Callus Proliferation and Somatic Embryogenesis in <i>Litchi chinensis</i> cv. ‘Feizixiao’

Amino acids (AAs) are important regulators of morphogenesis during somatic embryogenesis. This study investigated the roles of AAs in callus proliferation and embryo induction in <i>Litchi chinensis</i> Sonn. cv. ‘Feizixiao’ to improve the current in vitro regeneration protocols toward supporting efficient commercial litchi propagation and breeding programs. During callus proliferation, the total AAs ranged from 4627.9 μg·gFW⁻¹ on day 3 to 7827.8 μg·gFW⁻¹ on day 21, with an average content of 5994.0 μg·gFW⁻¹. The total AA content continuously decreased to 3390.1 μg·gFW⁻¹ on day 10 and slightly increased between days 10 and 20 during embryo induction, with an average content of 4849.7 μg·gFW⁻¹. At both stages, γ-aminobutyric acid, arginine, alanine, and glutamine accounted for more than 10% of the total AAs, and methionine, tryptophan, glycine, and cystine contents were lower than 1%, except for the tyrosine content, which was also lower than 1% at the embryo induction stage. The callus proliferated on medium supplemented with a low concentration of AMO1 solution, a mixture of 21 amino acids prepared according to the average proportion of each amino acid during callus proliferation. The calli were small, yellow, and uniform, with deep Ehrlich hematoxylin staining. When cultured on medium containing a high concentration of AMO1, the callus gradually turned brown, with more dark yellow proembryos, and cell lysis and apoptosis occurred. The combination of adding 4× AMO1 solution to the proliferation medium and adding 1× AMO2 solution, which was mixed with 21 amino acids according to their average proportion during embryo induction, to the embryo induction medium had the best regeneration efficiency, with about 396 embryos and 88 regenerated plantlets per gram of callus. The results provide a basis for the rational combined application of AAs during the in vitro regeneration of litchi to achieve greater somatic embryogenesis efficiency.