[초청강연] Regulation of developmental timing by miRNAs and trans-acting siRNAs

Many organisms undergo programmed changes in their morphology and physiology during development. In plants, this occurs most obviously during the transition from vegetative to reproductive growth, when the shoot stops making leaves and begins to manufacture flowers or other types of reproductive structures. This event is preceded by the transition from a juvenile to an adult phase of vegetative development (vegetative phase change), which is marked by changes in leaf anatomy and morphology and by an increase in reproductive competency. Genetic analysis of vegetative phase change in maize and Arabidopsis has revealed that this phenomenon is regulated by several endogenous small (21 nt) RNAs. These include the trans-acting siRNA, tasi-ARF, and two miRNAs—miR156 and miR172. tasi-ARF represses the expression of the auxin-related transcription factors ETTIN/ARF3 and ARF4. miR156 and miR172 have complementary temporal expression patterns and target different families of transcription factors. miR156 targets members of the SBP/SPL family whereas miR172 targets AP2-like transcription factors. These genes operate in a pathway that regulates epidermal identity and flowering time. ETT/ARF3 and ARF4 regulate the amplitude of SPL gene expression, but are not part of the switch mechanism. The structure and developmental functions of the pathways in which these genes are involved will be discussed.