Functional Analysis of Drought-Induced Transcriptional Regulatory Factor CLMYB1 in the Roots of Wild Watermelon

Wild watermelon (Citrullus lanatus L.) inhabiting the Kalahari Desert is tolerant to severe drought condition; it stimulates root growth in response to the onset of drought stress, thereby reaching deep water resources for survival under water deficit condition. To understand the molecular mechanisms for developing deep root systems, we have performed DNA microarray analysis of wild and domesticated watermelons roots. This analysis has identified two transcriptional factor genes that were specifically up-regulated prior to the root growth stimulation in wild watermelon, a zinc finger-type gene (CLZFB1) and a R2-R3-type MYB gene (CLMYB1). Subsequent functional analysis has demonstrated that CLZFB1 is a positive regulator of root development. In this study, we performed functional analyses of CLMYB1. A 2.3-kb promoter region of CLMYB1 was fused to GUS gene and introduced into wild watermelon by A. rhizogenes-mediated hairy root transformation. Consequently, the GUS signals were detected in the stele region of the roots, especially in the regions from which lateral root primordia were formed. These observations suggest that CLMYB1 may be involved in meristematic functions in the lateral roots, such as lateral root initiation, formation and/or growth. Subsequently, we generated two types of transgenic hairy roots, one for the over-expression of CLMYB1, and the other for the functional suppression of CLMYB1 by CRES-T method, both driven by aβ-estradiol- inducible promoter. As a result, growth of hairy roots were severely suppressed byβ-estradiol application in both cases. These results suggest that CLMYB1 might function as a negative regulator in root growth.