Auxin-induced mRNA species in tobacco cell cultures

Using a 2,4-dichlorophenoxyacetic acid (2,4-D)-dependent tobacco cell-suspension culture we analyzed early hormone-induced molecular events preceding cell division. By differential screening of a cDNA library to mRNAs derived from hormone-starved cells treated with 2,4-D for 4 h, seven non-cross hybridizing cDNA clones to 2,4-D-induced mRNAs were obtained. Accumulation of these mRNAs started as early as 15-30 min or less after 2,4-D application. The lowest 2,4-D concentration necessary to induce the mRNAs varied between less than 2.2 × 10(-8) M and 2.2 × 10(-6) M, one mRNA being induced to nearly maximal values at 2.2 × 10(-6) M. Generally, 2,4-D was the most active compound to induce mRNA accumulation, followed by naphthalene-1-acetic acid (NAA). The level of 4 mRNAs increased independently from protein synthesis. Run-off transcription studies showed that the accumulation of some mRNAs was at least partly due to enhanced transcription rates. In different organs of the tobacco plant, the levels of the mRNAs were about as low as in hormone-starved cells. A similar low level of the 2,4-D-induced mRNAs was observed in cells growing in mid-log phase on 2,4-D-containing medium. Only quiescent cells that were triggered to undergo cell division, accumulate these mRNAs transiently.