Embryogenic abnormalities and genotoxicity induced by 2,4-dichlorophenoxyacetic acid during indirect somatic embryogenesis in Coffea

Abstract

Indirect somatic embryogenesis (ISE) is a morphogenetic pathway in which somatic cells form callus and, later, somatic embryos. 2,4-dichlorophenoxyacetic acid (2,4-D) is a synthetic auxins that promote the proliferation and dedifferentiation of somatic cells, inducing the ISE. However, 2,4-D can cause genetic, epigenetic, physiological and morphological disorders, preventing the regeneration and/or resulting abnormal somatic embryos. We aimed to evaluate the toxic 2,4-D effect during the Coffea arabica and Coffea canephora ISE, assessing the somatic embryo morphology, global 5-methylcytosine levels (5-mC%) and DNA damage. Leaf explants were inoculated in media with different 2,4-D concentrations. After 90 days, the friable calli were transferred to the regeneration medium, and the number of normal and abnormal somatic embryos was monthly counted. The increase of the 2,4-D concentration increased the number of responsive explants in C. arabica and C. canephora. At 9.06, 18.08 and 36.24 µM 2,4-D, C. arabica presented the highest values of responsive explants, differing from C. canephora. Normal and abnormal somatic embryos regeneration increased in relation to the time and 2,4-D concentration. Global 5-mC% varied at different stages of the ISE in both Coffea. Furthermore, the 2,4-D concentration positively correlated with global 5-mC%, and with the mean number of abnormal somatic embryos. All abnormal somatic embryos of C. arabica and C. canephora exhibited DNA damage and showed higher global 5-mC%. The allotetraploid C. arabica exhibited greater tolerance to the toxic effect of 2,4-D than the diploid C. canephora. We conclude that synthetic 2,4-D auxin promotes genotoxic and phytotoxic disorders and promotes epigenetic changes during Coffea ISE.