The EkSLK2-EkAACT regulatory module enhances drought and salt stress resistance in Euphorbia kansui by promoting triterpene synthesis.

Drought and salt stress are critical limiting factors for plants growth, development, and geographical distribution. Currently, plants have formed a variety of regulatory mechanisms to cope with these stresses. SEUSS protein is a transcription regulator involved in responses to multiple abiotic stresses via biomolecular condensation. Nevertheless, due to complexity of the protein function and regulatory mechanism in response to drought and salt stress, the function of SEUSS protein in plants still require further exploration. In this study, a SEUSS-LIKE gene EkSLK2 was isolated from Euphorbia kansui, which was widely expressed in different tissues. It was further validated that EkSLK2 overexpression can enhance the plant tolerance under PEG6000 and NaCl treatments in Arabidopsis. Subsequently, an EkAACT protein was characterized from E. kansui and was verified interaction with EkSLK2 by the yeast two-hybrid (Y2H) system. Overexpressing EkSLK2 as well as EkSLK2 & EkAACT in Arabidopsis demonstrated the interaction between EkSLK2 and EkAACT increases enzyme activity of EkAACT and then promotes triterpenoids biosynthesis. Moreover, analyses of EkSLK2 overexpressing E. kansui further confirmed EkSLK2 can up-regulate transcription level of related genes in mevalonate (MVA) pathway and triterpenoids content, thereby enhancing the plant tolerance to drought and salt stress. In summary, our findings offer new insights into the molecular mechanisms for EKSLK2, which is different from the canonical mechanism whereby SEUSS enhances plant drought and salt tolerance via molecular condensation.