ABA-IMPORTING TRANSPORTER 1.1 activity in the endosperm of tomato seeds restrains germination under salinity stress

The plant hormone abscisic acid (ABA) has a central role in the regulation of seed maturation and dormancy. ABA also restrains germination under abiotic-stress conditions. Here we show in tomato ( Solanum lycopersicum ) that the ABA importer ABA-IMPORTING TRANSPORTER 1 . 1 (AIT1.1) has a role in radicle emergence following imbibition. AIT1 . 1 expression was upregulated during imbibition, and CRISPR/Cas9-derived ait1 . 1 mutant seeds exhibited faster radicle emergence, increased germination and partial resistance to ABA. AIT1 . 1 was highly expressed in the endosperm, but not in the embryo, and ait1 . 1 isolated embryos did not show resistance to ABA. On the other hand, loss of AIT1.1 activity promoted the expression of endosperm-weakening related genes, and seed-coat scarification eliminated the promoting effect of ait1 . 1 on radicle emergence. Therefore, we propose that imbibition-induced AIT1 . 1 expression in the endosperm, mediates ABA-uptake into endosperm cells to restrain endosperm weakening. While salinity conditions strongly inhibited the wild-type (WT) M82 seed germination, it had much weaker effect on ait1 . 1 germination. We suggest that this AIT1.1 function was evolved to inhibit germination under unfavorable conditions, such as salinity. Unlike other ABA mutants, ait1 . 1 exhibited normal seed longevity, and therefore, the ait1 . 1 allele may have a potential for improving seed germination in crops.