Translational control in plant stress: the formation of messenger ribonucleoprotein particles (mRNPs) in response to desiccation of Tortula ruralis gametophytes

Changes in gene expression observed in vivo in response to desiccation and rehydration of the desiccation-tolerant bryophyte Tortula ruralis are regul ated by alterations in the pattern of selection of messenger RNAs, from a q ualitatively constant mRNA pool, by the translational machinery. When dryin g rates are slow, messenger ribonucleoprotein particles (mRNPs) are formed in the drying gametophytes. A representative rehydrin mRNA, Tr288, was sequ estered into these particles which were analysed using sucrose and CsCl gra dients. Quantitative RT-PCR analysis of the fractions from a low salt extra ction demonstrated that Tr288 mRNA migrated farther in the sucrose gradient , relative to those extractable in high salt, indicating that the transcrip t is associated with particles that are of higher density. RT-PCR analysis also demonstrated that the majority of Tr288 mRNA, from slowly desiccated g ametophytes, is associated with particles that have buoyant densities betwe en 1.44 and 1.64g cm (-3) which correspond to the buoyant density range rep orted for mRNP particles, mRNPs that are unique to drying T. ruralis gameto phytes form at least four size classes after in vivo UV cross-linking based upon FPLC analysis. This is the first report of mRNP formation in response to a vegetative water deficit in plants.