Conservation and Diversification of the SHR-SCR-SCL23 Regulatory Network in the Development of the Functional Endodermis in Arabidopsis Shoots

Development of the functional endodermis of Arabidopsis thaliana roots is controlled, in part, by GRAS transcription factors, namely SHORT-ROOT (SHR), SCARECROW (SCR), and SCARECROW-LIKE 23 (SCL23). Recently, it has been shown that the SHR-SCR-SCL23 regulatory module is also essential for specification of the endodermis (known as the bundle sheath) in leaves. Nevertheless, compared with what is known about the role of the SHR-SCR-SCL23 regulatory network in roots, the molecular interactions of SHR, SCR, and SCL23 are much less understood in shoots. Here, we show that SHR forms protein complexes with SCL23 to regulate transcription of SCL23 in shoots, similar to the regulation mode of SCR expression. Our results indicate that SHR acts as master regulator to directly activate the expression of SCR and SCL23. In the SHR-SCR-SCL23 network, we found a previously uncharacterized negative feedback loop whereby SCL23 modulates SHR levels. Through molecular, genetic, physiological, and morphological analyses, we also reveal that the SHR-SCR-SCL23 module plays a key role in the formation of the endodermis (known as the starch sheath) in hypocotyls. Taken together, our results provide new insights into the regulatory role of the SHR-SCR-SCL23 network in the endodermis development in both roots and shoots.