Environmental cues influence the development of stomata on the leaf epidermis, and allow plants to exert plasticity
in leaf stomatal abundance in response to the prevailing growing conditions. It is reported that Arabidopsis thaliana
‘Landsberg erecta’ plants grown under low relative humidity have a reduced stomatal index and that two genes in
the stomatal development pathway, SPEECHLESS and FAMA, become de novo cytosine methylated and
transcriptionally repressed. These environmentally-induced epigenetic responses were abolished in mutants lacking
the capacity for de novo DNA methylation, for the maintenance of CG methylation, and in mutants for the production
of short-interfering non-coding RNAs (siRNAs) in the RNA-directed DNA methylation pathway. Induction of
methylation was quantitatively related to the induction of local siRNAs under low relative humidity. Our results
indicate the involvement of both transcriptional and post-transcriptional gene suppression at these loci in response
to environmental stress. Thus, in a physiologically important pathway, a targeted epigenetic response to a specific
environmental stress is reported and several of its molecular, mechanistic components are described, providing
a tractable platform for future epigenetics experiments. Our findings suggest epigenetic regulation of stomatal
development that allows for anatomical and phenotypic plasticity, and may help to explain at least some of the
plant’s resilience to fluctuating relative humidity
