The Meristem and Quiescent Centre in Cultured Root Apices of the gib-l Mutant of Tomato (Lycopersicon esculentum Mill.)*

Cultured root apices of tomato bearing the gib-I mutation, which reduces the levels of endogenous gibberellins, grew slower and were thicker than wild-type contols. This was the result of shorter and broader cells in the menstem of the mutant. Cells of both cortex and stele were affected, but this did not cause any alteration to the volume fraction occupied by these two tissues in the root meristem. Root caps were longer in the mutant and there were also more layers of rhizodermis. All these effects could be reproduced in wild-type roots by addition of 0.1μM 2S, 3S paclobutrazol (an inhibitor of gibberellin biosynthesis) to the culture medium and could be normalized in mutant roots by 0.1 μM GA3. Cell doubling times in the proximal region of the meristem were similar in mutant and wild-type roots, but were faster in both the quiescent centre (QC) and the cap meristem of the mutant. This latter feature of the mutant roots is likely to be the cause of their longer caps, while the faster rate of division in the QC accounts for the additional tiers of cells that were found to build up in the cortical portion of this zone These additional tiers failed to form in mutant roots grown in GA3, but they could be induced in wild-type roots by 2S, 3S paclobutrazol. These results suggest that endogenous gibberellins may be partly responsible for the slow rate of cell growth and proliferation in the QC.