Photostable Abscisic Acid Agonists with a Geometrically Rigid Cyclized Side Chain

The plant hormone abscisic acid (ABA) plays a central role in adaptive responses to abiotic stresses that adversely affect crop growth and productivity. However, ABA photoinstability limits its use in agriculture. To overcome this drawback, in this study, we developed photostable ABA analogues, the (+)-BP2A compound series (compounds 5-9), in which the dienoic acid side chain of ABA was replaced with phenylacetic acid. All BP2A analogues showed higher stability against UV-B irradiation at 302 nm than ABA, and compounds 6 and 7 barely decomposed even under sunlight. In physiological assays, (+)-BP2A and (+)-compound 7, in which the α,β-unsaturated carbonyl group of BP2A was reduced, exhibited ABA-like activities, including inhibition of seed germination and induced drought tolerance in Arabidopsis. Biochemical studies revealed that (+)-compound 7, unlike (+)-BP2A, did not activate pyrabactin resistance-like (PYL) receptors in vitro and was converted to (+)-BP2A in plants, suggesting that it functions as a prodrug PYL agonist. Furthermore, (+)-compound 7 inhibited seed germination of tomato, lettuce, and rice. Thus, this compound represents a potential plant growth regulator that induces ABA-type responses in agricultural fields.