BRASSINOLIDE IS A SELECTIVE RIBOSOMAL CISTRON REGULATOR IN ONION LEAFBASE TISSUE

Outer epidermal cells from the basal, equatorial, near-apical, and api cal regions of the third turgid onion (Allium cepa L. var. yellow, swe et Spanish) leaf base were treated (3 and 6 h in the dark = T-3 and T- 6, respectively) with brassinolide (Br, a brassinosteroid plant growth regulator; effects on excised pieces compared with those in water con trols: there were no statistical differences between the T-3 and T-6 r esults). Br induced increases in the volume and changes in morphologie s of the major nucleoli to a greater extent than observed;for major nu cleoli in basal through near-apical controls. No major nucleoli were a ctivated in control or Br-treated apical tissue. Minor nucleolar organ izer regions in control and Br-treated tissue remained inactive in all locations. We propose that Br is a major ribosomal cistron regulator.