Characterization of ipt Gene Expression and Leaf Senescence in Creeping Bentgrass Subjected to Nitrogen Deficiency.

Cytokinins (CKs) have been well known for delaying leaf senescence. Creeping bentgrass (Agrostis stolonifera L.) with increased endogenous levels of CK have been generated by transformation of the Agrobacterium tumefaciens (Smith and Townsend 1907) CK biosynthetic gene, isopentenyl transferase (ipt) with an autoregulated senescence-activated promoter, SAG12. The transgenic plants with the SAG12:ipt construct display a normal growth habit except for delayed leaf senescence. To characterize the impact of ipt gene on leaf senescence in creeping bentgrass subjected to nitrogen deficiency, two SAG12:ipt transgenic lines (S41 and S43) and an empty vector transgenic line (EV) were evaluated using plants grown in a complete nutrient solution or nitrogen starvation at 0, 7 and 21 days after treatment. Significant decline in photochemical efficiency (Fv/Fm ratio), mature leaf chlorophyll content, leaf zeatin riboside (ZR), and root isopentenyl adenine (IPA) content were detected in the control line and SAG12:ipt plants exposed to nitrogen deficiency. Compared to the control line, SAG12:ipt plants had higher levels of Fv/Fm ratio, content of chlorophyll and cytokinins (ZR, DHZR, and IPA) in youngest and mature leaves, and root IPA content under limited nitrogen supply. These differences between the control line and SAG12:ipt lines were increased with prolonged treatment (21 days vs. 7 days). Among all the measurements, the most significant difference between the control line and SAG12:ipt lines was observed in Fv/Fm ratio, followed by leaf DHZR, and ZR content at 21 days of treatment. The differential expression of ipt gene in the transgenic lines indicated that CK production was related to ipt expression. Our results suggested that over-expression of ipt in creeping bentgrass suppressed leaf senescence induced by low nitrogen supply by maintaining photochemical efficiency with high level of cytokinins due to the stress-autoregulated ipt expression driven by a senescence-specific promoter.