Determination of the Cellular Mechanisms Regulating Thermo-Induced Stem Growth in Thlaspi arvense L

Field pennycress (Thlaspi arvense L.) is a species with a cold requirement for the initiation of reproductive development (thermoinduction). Work in this laboratory has been focused on elucidating the biochemical and molecular mechanisms underlying the bolting or rapid stem elongation response that is an intricate part of reproductive development in this species. In the present paper the cellular basis for thermo-induced stem growth was determined. Evidence is presented indicating that bolting results from the production of new cells that elongate to their original length before thermoinduction. This increase in cell division occurs in the pith and cortex approximately 0.5 to 5.0 millimeters below the stem apex. For at least the early stages of thermo-induced stem growth, enhanced cell elongation does not appear to be a factor because average lengths of pith cells from stems of thermo-induced plants were similar or less than noninduced controls. In addition, both the amount of increase in the production of new pith cells and stem growth were positively correlated with the length of the cold treatment. Two other lines of evidence are presented corroborating previous assertions (JD Metzger [1985] Plant Physiol 78: 8-13) that gibberellins mediate thermo-induced stem growth in field pennycress. First, treatment of noninduced plants with gibberellin A₃ completely mimicked the effects of a 4-week cold treatment on mitotic activity in the pith and cortex. Second, very little increase in the production of new cells was observed in the pith and cortex of thermo-induced plants of a gibberellin-deficient dwarf mutant of field pennycress. It is also shown that the influence of photoperiod on stem growth is mediated by an effect on the final length that cells ultimately attain.