In cooperation with other chaperones, Hsp70s are key components that facilitate folding of de novo synthesized proteins, assist translocation of precursor proteins into organelles, and are responsible for degradation of damaged protein under stress conditions (Probable). Involved in polar nuclei fusion during female gametophyte development and is essential for the regulation of endosperm nuclei proliferation (PubMed:20080634). Involved in sperm nuclear fusion with central cell polar nuclei at fertilization, which is critical for normal endosperm nuclear proliferation (PubMed:31410484). Required for pollen development and pollen tube growth (PubMed:24486762). Interacts with BAG7. A number of isoforms are produced. According to EST sequences. Expressed in mature pollen grains, and pollen tubes. Down-regulated during seed maturation. Up-regulated during germination. Induced by heat shock and sugar. Up-regulated by light, DTT and tunicamycin treatment. Bip1 and bip2 double mutation affects the fusion of polar nuclei during female gametophyte development (PubMed:20080634). Bip1 and bip2 double mutation affects pollen tube growth and length (PubMed:24486762). Bip1, bip2 and bip3 triple mutation is pollen lethal (PubMed:24486762). Bip1, bip2 and bip3 triple mutation affects female gametophyte development during the early stages (PubMed:26251880). Belongs to the heat shock protein 70 (TC 1.A.33) family. DnaK subfamily.