During our ongoing research into gaseous atmosphere effects on tissue cultures, we observed what appeared to be an anomaly. The existent literature suggests that lateral roots arise only from pre-existing roots, while adventitious roots develop from non-root tissues such as stem, petiole, and leaf tissues. The logical extension of this is the assumption that roots developed from callus cultures are adventitious in origin.
We compared the response of primary hypocotyl callus cultures (Murashige and Skoog media with IAA and Zeatin) of cv VFN8 and its isogenic mutant dgt (ethylene requiring / diageotropic / lateral root-less) to air, 1 ul/l ethylene, and 0.5% CO2 (constant flow at 2 l/hr). Ethylene completely inhibited callus growth when applied immediately after excision. CO2 appeared to stimulate the development of localized meristematic regions within the callus as compared to the air control. Callus derived from cv VFN8 developed many roots over the duration of the treatment period (2 weeks), while the dgt derived callus never developed roots. Anatomically and developmentally, dgt callus was identical to cv VFN8 callus in all treatments, with the exception of root formation.
Zobel (1974) demonstrated that the single gene mutant dgt was without lateral roots, being genetically unable to form lateral roots, but able to form adventitious, and basal roots. The absence of roots on dgt callus but present on cv. VFN8 callus strongly suggests that in this case the rooting in cv VFN8 callus was of the lateral root type, and not adventitious, or basal. If lateral rooting is truly restricted to pericyclic type meristematic cells, this then further suggests that extensive differentiation, and possibly embryo development has taken place in this callus.
We will follow up this research with studies using some of the other available root mutants.
Literature Cited:
Zobel, R.W. 1974 Control of morphogenesis in the ethylene-requiring tomato mutant, diageotropica. Canadian J. Bot. 52:735-741.