|Samac, Deborah - Debby|
Submitted to: Alfalfa Improvement Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 7/17/1997
Publication Date: N/A
Technical Abstract: Significant yield losses can occur in alfalfa (Medicago sativa) stands due to defoliation caused by leaf senescence and foliar diseases. The cytokinin class of plant growth regulators are thought to play a key role in controlling leaf senescence. We tested the effect of controlled expression of isopentenyl transferase (IPT), a key enzyme in cytokinin biosynthesis, on leaf senescence and disease resistance in alfalfa. Detached wild type alfalfa leaves floated on water in darkness rapidly lost chlorophyll and total protein compared to control leaves held in the light. Application of 10 uM kinetin to dark-treated leaves delayed chlorophyll and protein loss. Transgenic alfalfa plants containing a chimeric gene composed of an Arabidopsis senescence-specific promoter (SAG12) fused to the IPT coding region or the SAG12 promoter fused to the marker gene beta-glucuronidase (GUS) were generated by Agrobacterium-mediated transformation. Little to no GUS activity was observed in 17 transgenic alfalfa plants with the SAG12-GUS construct. However, 2 of 34 plants containing the SAG12-IPT construct displayed a delay in leaf senescence. Detached leaves held in darkness for 6 d retained twice as much chlorophyll as leaves from control plants. Leaves from one plant were less diseased 8 d after inoculation with Phoma medicaginis than leaves from control plants. Research has been initiated to isolate alfalfa leaf senescence-specific gene promoters. cDNA libraries from senescent leaves of greenhouse-grown plants and from leaves held in darkness for 48 h have been produced and clones preferentially expressed in senescent leaves were isolated and are currently being characterized.