A Gibberellin-Deficient Brassica Mutant—rosette

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Abstract
A single-gene mutant (rosette [ros/ros]) in which shoot growth and development are inhibited was identified from a rapid cycling line of Brassica rapa (syn campestris). Relative to normal plants, the mutant germinated slowly, had delayed or incomplete floral development, and reduced leaf, petiole, and internode growth. The exogenous application of GA3 by foliar spray or directly to the shoot tip of rosette resulted in rapid flowering, bolting (shoot elongation), and viable seed production. Shoots of rosette contained endogenous levels of total gibberellin (GA)-like substances (`Tan-ginbozu' dwarf rice assay) of about one-tenth of that of the normal rapid-cycling line of B. rapa which consisted almost entirely of a very nonpolar, GA-like substance which yielded GA1 and GA3 upon mild acid hydrolysis. In a normal rapid-cycling B. rapa line, the nonpolar putative GA1 and GA3 conjugates were present, but additionally, free GA1 and GA3 were abundant and identified by gas chromatography-mass spectrometry-selected ion monitoring. The quantities of free GA1 and GA3 in the normal line and in rosette were quantified by GC-MS-SIM using [2H2]GA1 as an internal standard. Fourteen-day-old rosette and normal seedlings contained 5.3 and 23.2 ng GA1 per plant, respectively. At day 21 the rosette plants contained 7.7 and 26.1 nanograms per plant of GA1 and GA3, while normal plants contained 31.1 and 251.5 nanograms per plant, respectively. Thus, normal plants contained from four to ten times higher levels of total GA-like substances, GA1, or GA3, than rosette. The ros allele results in reduced GA level, yielding the rosette phenotype whose delayed germination and flowering, and reduced shoot growth responses indicate a probable role for endogenous GA1 and GA3 in the regulation of these processes in Brassica.