While mammals and fungi possess nicotinate/nicotinamide mononucleotide adenyltransferase (NMNAT) isoforms,Arabidopsis thalianaonly contains a single NMNAT gene,AtNMNAT(At5g55810). We analyzed the enzymatic activity of theAtNMNAT-encoded protein to determine the role of AtNMNAT in plant development. AtNMNAT catalyzed the synthesis of nicotinate adenine dinucleotide (NaAD) from nicotinate mononucleotide (NaMN) in the Preiss–Handler-dependent pathway, and of nicotinamide adenine dinucleotide (NAD) from nicotiamide mononucleotide (NMN) in the Preiss–Handler-independent pathway. ProminentAtNMNATexpression was detected in the male gametophyte. Moreover,AtNMNATexpression was spatio-temporally regulated during microspore development and pollen tube growth. Disruption of theAtNMNATgene (atnmnatmutant) was characterized by a decrease in NAD content in pollen. Cytological examinations revealed that theatnmnatmutant was gametophytically impaired inin vivoandin vitropollen tube growth. Our results suggest that metabolic fulfillment via the NAD pathway is indispensable for normal pollen growth and subsequent normal seed production.