Transcription strength and halophytic lifestyle.

When plants experience elevated concentrations of NaCl only a few species survive and set seeds. However, survival specialists – termed halophytes or, generally, extremophiles – can be found in essentially every plant family, seemingly identifying genes determining the halophytic lifestyle as a legacy of all plants. We have indicated the importance of transcription control leading to ‘copy number variation’ (CNV), specifically strength of transcription and/or stability, as a crucial characteristic that distinguishes halophytes from their salt-sensitive (glycophytic) relatives [1,2]. Interspecies comparisons of expressed sequence tags (ESTs) over the years have shown it is probable that the tolerance phenotype is not, or not to a significant degree, based on global differences in protein/ enzyme complement or protein folding or stability characteristics, although examples in these categories are equally likely to exist. Stress-dependent gene expression programs can be particularly useful in defining differences. For example, comparisons by microarray hybridizations of the salt-sensitive Arabidopsis thaliana (Arabidopsis) with its relative Thellungiella salsuginea (previously T. halophila), an extremely salt-tolerant species, showed stressspecific, stress intensity-dependent, distinct responses in the halophyte. The Arabidopsis response, even at low stress levels, might best be characterized as chaotic and all-embracing [3]. Obviously, T. salsuginea is tolerant to higher concentrations of sodium ions, whereas Arabidopsis perceives a much lower concentration as stressful. However, if it is not the nature of regulated genes and proteins that determine plant salinity stress tolerance, the question is how more organized transcription programs might perform that service. One aspect distinguishing plant species, apart from gene copy number, appears to be the strength of transcript expression that is recorded as CNV [4,5]. In a comparison between Arabidopsis and T. salsuginea under control and salt stress conditions, we have shown that a fundamental difference exists in the steady-state amounts of SOS1 mRNAs in both conditions, and in the levels of induction during salt stress [1]. SOS1 has been well-studied and is known as a crucial component of the salt extrusion and intra-plant distribution systems [6]. Moreover, RNA-interference with the SOS1 gene in T. salsuginea, which reduced transcript amounts, converted the halophyte into a salt-sensitive species [1]. These observations have now been extended by including genome sequences and gene expression characteristics