The present invention relates to a Lactuca sativa seed designated 45-89 RZ, which exhibits a combination of traits including resistance to downy mildew (Bremia lactucae Regel.) races NL:1-16 and Bl:18-28, and resistance to currant-lettuce aphid (Nasonovia ribisnigri) biotype Nr:0. The present invention also relates to a Lactuca sativa plant produced by growing the 45-89 RZ seed. The invention further relates to methods for producing the lettuce cultivar, represented by lettuce variety 45-89 RZ.