We analyzed the abnormal shoot in youth (asy) mutant to understand the phase-specific regulation of shoot development. asy showed various shoot abnormalities, including small leaves due to the precocious termination of cell division, defects in leaf blade-sheath boundary formation, and abnormal shoot apical meristem maintenance at the early vegetative stage. These defects recovered with advanced development. ASY encodes a DUF791 domain protein, which is part of the major facilitator superfamily. Despite stage-specific phenotypes, the ASY expression level was roughly constant throughout development. A paralog of ASY, ASL, exists in the rice genome and is supposed to have redundant functions. ASL expression was relatively low in early-stage embryos but increased at later stages. Thus, asy phenotypes were limited to the stage when ASL expression was suppressed. A homology search revealed that ASY is a homolog of the Chlamydomonas CrMoT2 gene, which encodes a molybdate transporter. ASY was suggested to encode a molybdate transporter based on its sequence similarity with CrMoT2 and predicted transmembrane topology. This is the first report of a CrMOT2-type molybdate transporter in higher plants.
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