Auxin represses stomatal development in dark-grown seedlings via Aux/IAA proteins
M. Balcerowicz, A. Ranjan, L. Rupprecht, G. Fiene, U. Hoecker
Stomatal development is tightly regulated through internal and external factors that are integrated by a complex signalling network. Light represents an external factor that strongly promotes stomata formation. Here, we show that auxin-resistant aux/iaa mutants, e.g. axr3-1, exhibit a de-repression of stomata differentiation in dark-grown seedlings. The higher stomatal index in dark-grown axr3-1 mutants when compared with the wild type is due to increased cell division in the stomatal lineage.
... xcessive stomata in dark-grown seedlings were also observed in mutants defective in auxin biosynthesis or auxin perception and in seedlings treated with the polar auxin transport inhibitor NPA. Consistent with these findings, exogenous auxin repressed stomata formation in light-grown seedlings. Taken together, these results indicate that auxin is a negative regulator of stomatal development in dark-grown seedlings. Epistasis analysis revealed that axr3-1 acts genetically upstream of the bHLH transcription factors SPCH, MUTE and FAMA, as well as the YDA MAP kinase cascade, but in parallel with the repressor of photomorphogenesis COP1 and the receptor-like protein TMM. The effect of exogenous auxin required the ER family of leucine-rich repeat receptor-like kinases, suggesting that auxin acts at least in part through the ER family. Expression of axr3-1 in the stomatal lineage was insufficient to alter the stomatal index, implying that cell-cell communication is necessary to mediate the effect of auxin. In summary, our results show that auxin signalling contributes to the suppression of stomatal differentiation observed in dark-grown seedlings. DEVELOPMENT Auxin inhibits stomata formation The stomata-overproducing phenotype of the auxin-insensitive mutants axr2-1, axr3-1 and shy2-2 suggests that auxin is involved in the regulation of stomatal development. To further test this hypothesis, we investigated the effect of exogenous auxin on stomata formation. In wild-type seedlings, concentrations of 10 µM or higher of the synthetic auxin 1-Naphthaleneacetic acid (NAA) strongly reduced the SI, an effect that was much weaker in axr2-1 3166 RESEARCH ARTICLE