Appropriate pollen germination is crucial for plant reproduction. Previous studies have revealed the importance of dehydration in maintaining pollen dormancy; here, we show that phosphatidylinositol pathway-controlled Ins(1,4,5)P(3)/Ca(2+) levels are crucial for maintaining pollen dormancy in Arabidopsis thaliana. An interesting phenotype, precocious pollen germination within anthers, results from a disruption of inositol polyphosphate 5-phosphatase 12 (5PT12). The knockout mutant 5pt12 has normal early pollen development and pollen dehydration, and exhibits hypersensitive ABA responses, indicating that precocious pollen germination is not caused either by abnormal dehydration or by suppressed ABA signaling. Deficiency of 5PT13 (a close paralog of 5PT12) synergistically enhances precocious pollen germination. Both basal Ins(1,4,5)P(3) levels and endogenous Ca(2+) levels are elevated in pollen from 5pt12 mutants, and 5pt12 5pt13 double mutants show an even higher precocious germination rate along with much higher levels of Ins(1,4,5)P(3)/Ca(2+). Strikingly, exogenous Ca(2+) stimulates the germination of wild-type pollen at floral stage 12, even in very low humidity, both in vitro and in vivo, and treatment with BAPTA, a [Ca(2+)](cyt) inhibitor, reduces the precocious pollen germination rates of 5pt12, 5pt13 and 5pt12 5pt13 mutants. These results indicate that the increase in the levels of Ins(1,4,5)P(3)/Ca(2+) caused by deficiency of inositol polyphosphate 5-phosphatases is sufficient to break pollen dormancy and to trigger early germination. The study reveals that independent of dehydration, the control of Ins(1,4,5)P(3)/Ca(2+) levels by Inositol polyphosphate 5-phosphatases is crucial for maintaining pollen dormancy.