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Vacuolar sugar transporters transport sugar across the tonoplast, are major players in maintaining sugar homeostasis, and therefore play vital roles in plant growth, development, and biomass yield. In this study, we analyzed the physiological roles of the tonoplast monosaccharide transporter 2 (TMT2) in Arabidopsis. In contrast to the wild type (WT) that produced uniform seedlings, the <i>tmt2</i> mutant produced three types of offspring: un-germinated seeds (UnG), seedlings that cannot form true leaves (<i>tmt2-S</i>), and seedlings that develop normally (<i>tmt2-L</i>). Sucrose, glucose, and fructose can substantially, but not completely, rescue the abnormal phenotypes of the <i>tmt2</i> mutant. Abnormal cotyledon development, arrested true leaf development, and abnormal development of shoot apical meristem (SAM) were observed in <i>tmt2-S</i> seedlings. Cotyledons from the WT and <i>tmt2-L</i> seedlings restored the growth of <i>tmt2-S</i> seedlings through micrografting. Moreover, exogenous sugar sustained normal growth of <i>tmt2-S</i> seedlings with cotyledon removed. Finally, we found that the TMT2 deficiency resulted in growth defects, most likely via changing auxin signaling, target of rapamycin (TOR) pathways, and cellular nutrients. This study unveiled the essential functions of TMT2 for seed germination and initial seedling development, ensuring cotyledon function and mobilizing sugars from cotyledons to seedlings. It also expanded the current knowledge on sugar metabolism and signaling. These findings have fundamental implications for enhancing plant biomass production or seed yield in future agriculture.