The WUSCHEL-related homeobox transcription factor OsWOX4 controls the primary root elongation by activating OsAUX1 in rice

Primary root is the basic component of root system and plays a key role in early seedling growth and survival in rice. However, the molecular mechanism of primary root elongation still needs to be well understood. Here, we showed that OsWOX4, a WUSCHEL-related homeobox (WOX) transcription factor, was involved in the primary root elongation in rice. Silencing of OsWOX4 by RNA interference (RNAi) greatly increased the primary root length, whereas its overexpression reduced primary root elongation significantly. Moreover, the size of meristem zone and epidermal cell length of mature zone in RNAi root tips were drastically enhanced, while they were reduced markedly in overexpression lines, in comparison with that of wild type. Further analysis showed that the accumulation of free IAA was slightly increased in RNAi roots, but drastically reduced in plants overexpressing OsWOX4. The expression of genes responsible for auxin biosynthesis and transport was also changed in OsWOX4 transgenic lines. Transient transcriptional activation and electrophoretic mobility shift assays showed that OsWOX4 directly regulated the transcription of OsAUX1 through binding to its promoter region. Collectively, our results indicated that OsWOX4 played a crucial role in the primary root elongation by regulating auxin transport, suggesting its importance in rice root system architecture.