In tissue culture system, this is the main effect of cytokines

The correct option is adventitious shoot formation.

In plant tissue culture, the growth, development, and differentiation of tissues are primarily regulated by the balance and ratio of two major classes of plant hormones (phytohormones): auxins and cytokinins.

This hormonal regulation operates based on the following specific ratios:
1. High Cytokinin to Auxin Ratio: A relatively high concentration of cytokinins compared to auxins promotes organogenesis leading to the formation of shoots, specifically promoting adventitious shoot formation from the callus.
2. High Auxin to Cytokinin Ratio: Conversely, a high concentration of auxins relative to cytokinins stimulates adventitious root formation.
3. Balanced Ratio: An intermediate or balanced ratio of both hormones promotes the proliferation of unorganized cell masses, known as callus formation.

Therefore, the primary and main effect of cytokinins in a tissue culture system is to induce and promote the development of shoots, which corresponds to adventitious shoot formation.