Divergent regulation of Arabidopsis SAUR genes: a focus on the SAUR10-clade

Hilda van Mourik, Aalt D. J. van Dijk, Niek Stortenbeker, Gerco C. Angenent, Marian Bemer

Small Auxin-Upregulated RNA (SAUR) genes encode growth regulators that induce cell elongation. Arabidopsis contains more than 70 SAUR genes, of which the growth-promoting function has been unveiled in seedlings, while their role in other tissues remained largely unknown. Here, we focus on the regulatory regions of Arabidopsis SAUR genes, to predict the processes in which they play a role, and understand the dynamics of plant growth. In this study, we characterized in detail the entire SAUR10-clade: SAUR8, SAUR9, SAUR10, SAUR12, SAUR16, SAUR50, SAUR51 and SAUR54. Overexpression analysis revealed that the different proteins fulfil similar functions, while the SAUR expression patterns were highly diverse, showing expression throughout plant development in a variety of tissues. In addition, the response to application of different hormones largely varied between the different genes. These tissue-specific and hormone-specific responses could be linked to transcription factor binding sites using in silico analyses. These analyses also supported the existence of two groups of SAURs in Arabidopsis: Class I genes can be induced by combinatorial action of ARF-BZR-PIF transcription factors, while Class II genes are not regulated by auxin. SAUR10-clade genes generally induce cell-elongation, but exhibit diverse expression patterns and responses to hormones. Our experimental and in silico analyses suggest that transcription factors involved in plant development determine the tissue specific expression of the different SAUR genes, whereas the amplitude of this expression can often be controlled by hormone response transcription factors. This allows the plant to fine tune growth in a variety of tissues in response to internal and external signals.