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Leaves of many plant species open during the day and fold at night. Diurnal leaf movement, named nyctinasty, has been of great interest to researchers since Darwin's time. Nyctinastic leaf movement is generated by the pulvinus, which is a specialized motor organ located at the base of leaf and leaflet. The molecular basis and functional reason behind nyctinasty are unknown., In a forward screening of a retrotransposon-tagged mutant population of Medicago truncatula, four petiolule-like pulvinus ( plp) mutant lines with defects in leaf movement were identified and characterized., Loss of function of PLP results in the change of pulvini to petiolules. PLP is specifically expressed in the pulvinus, as demonstrated by quantitative reverse-transcription polymerase chain reaction analysis, expression analysis of a PLP promoter- β-glucuronidase construct in transgenic plants and in situ hybridization. Microarray analysis revealed that the expression levels of many genes were altered in the mutant during the day and at night. Crosses between the plp mutant and several leaf pattern mutants showed that the developmental mechanisms of pulvini and leaf patterns are likely independent., Our results demonstrated that PLP plays a crucial role in the determination of pulvinus development. Leaf movement generated by pulvini may have an impact on plant vegetative growth. [ABSTRACT FROM AUTHOR]