Caracterização histológica da organogênese in vitro de Eucalyptus urophylla e Eucalyptus grandis

Abstract

Eucalyptus, a species native to Australia, has great commercial value for the pulp and paper, coal, and energy industries, as well as for the pharmaceutical and cosmetics industries. Brazil is the largest exporter of pulp and paper, and eucalyptus is the most widely cultivated crop in Brazilian forestry, occupying 77% of the planted area. In vitro propagation and cultivation methods are used in laboratories to develop eucalyptus seedlings with the desired resistance and productivity characteristics. Among these methodologies, callogenesis and regeneration are employed as a means of obtaining such seedlings through the appropriate use of plant growth regulators. Therefore, the objective of this study was to evaluate the origin of callus shoots in Eucalyptus grandis and Eucalyptus urophylla during the regeneration stage, induced by the use of different concentrations of the plant growth regulator Thidiazuron (TDZ). A callus culture was established from leaf, cotyledon, and apex explants taken from plants grown from seeds six weeks after germination. After 30 days of callus formation, the calli were allowed to regenerate. Histological sections were taken from calli of Eucalyptus grandis and Eucalyptus urophylla, which had two months of regeneration. Eucalyptus grandis was the most responsive genotype, presenting the highest number of shoots. The most effective TDZ concentration was 0.025 mg/L for Eucalyptus grandis and 0.025 mg/L and 0.050 mg/L for Eucalyptus urophylla. The apex was the best explant type used in this experiment, as it presented better regeneration than the others. Analysis of the histological section images showed that, as the sections progressed, the origin of the shoots became visible. It is concluded that the shoots present in the callus of both eucalyptus species arose from cells close to the conduction systems of the original explant, where young vascular bundles are located close to mature ones, providing structural support and local signaling that favor regeneration. These results yielded useful information on how choices of genotype, explant type, and growth regulator concentration influence the callus formation and shoot regeneration stages in eucalyptus. Histological analyses revealed the location of the shoots' origin, crucial information for subsequent genetic transformation protocols.