Brassinolide is a selective ribosomal cistron regulator in onion leaf base tissue.

Outer epidermal cells from the basal, equatorial, near-apical, and apical regions of the third turgid onion (Allium cepa L. var. yellow, sweet Spanish) leaf base were treated (3 and 6 h in the dark = T3 and T6, respectively) with brassinolide (Br, a brassinosteroid plant growth regulator; effects on excised pieces compared with those in water controls: there were no statistical differences between the T3 and T6 results). Br induced increases in the volume and changes in morphologies of the major nucleoli to a greater extent than observed for major nucleoli in basal through near-apical controls. No major nucleoli were activated in control or Br-treated apical tissue. Minor nucleolar organizer regions in control and Br-treated tissue remained inactive in all locations. We propose that Br is a major ribosomal cistron regulator.