Drought Responsive H1 Histone

The association of H1 histones with nucleosomal arrays is believed to be primarily responsible for the formation ofhigher orders of chromatin compaction, thus inactivating the transcription of genes in that chromatin region. The family of plant H1 histones can be divided into two groups based on size, amino acid sequence, and expression characteristics (As-cenzi and Gantt 1997). The larger group, equivalent to the somatic linker histones in animals, is comprised of the H1 histones associated with the bulk of the chromatin. The second group is populated with H1 variant or subtypes that have a slightly smaller carboxy terminal domain, and more importantly, the members of this group are drought responsive (Wei and O'Connell 1996; Ascenzi and Gantt 1997; Bray et al. 1999). These drought-responsive H1 histones formed a separate clade in a dendrogram of H1 hi-stones (O'Connell et al. 2007) and accumulated in the nucleus of drought-stressed plants (Ascenzi and Gantt 1997; Scippa et al. 2000). The abundance of the protein, however, is not sufficient to account for a general replacement of the H1 histones in the chromatin (Ascenzi and Gantt 1999b). Rather the abundance and pattern of expression of these variant H1 histones are likely to remodel the chromatin of selected regions of the genome, presumably around genes whose transcription is altered.

As mentioned above, repression of transcription of specific genes occurs during drought stress, notably the transcription of cab and ribulose-bisphosphate carboxylase small subunit (rbcS) (Bartholomew et al. 1991). Transcripts for the drought responsive H1 hi-stone accumulated prior to the repression of rbcS transcription in tomato leaves during a drought cycle (Wei and O'Connell 1996). These results are consistent with the hypothesis that the function of the drought induced H1 histone is to repress transcription of genes during drought stress. However there were no changes in transcription of selected drought responsive genes in transgenic Arabidopsis overex-pressing the drought-induced H1 histone (Ascenzi and Gantt 1999a). These plants did not have any alterations in water content during drought stress or any other measurable phenotypic differences. Scippa et al. (2004) used anti-sense constructs of the drought-induced H1 histone in tomato to investigate the function of this protein. They observed leaf developmental changes and an increased stomatal conductance rate in the transgenic plants with reduced expression of the drought-induced H1 histone. The authors concluded that this gene plays multiple roles during plant development associated with controlling plant water status.

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