Drought-induced Leaf Senescence and Horticultural Performance of Transgenic PSAG12-IPT Petunias

Cytokinins have been shown to delay the onset of leaf senescence. The focus of this project was to produce transgenic petunia (Petunia ×hybrida) plants that over-produced endogenous cytokinins in a senescence specifi c manner. This was achieved by transforming plants with the IPT (isopentenyl transferase) gene driven by the senescence-associated transcriptional promoter, PSAG12. Two independent transgenic events produced T1 and T2 generation seedling lines that demonstrated the desired nonsenescent phenotype in progeny trials. These lines were used to evaluate the horticultural performance of PSAG12-IPT petunia plants in terms of delayed senescence, rooting of vegetative cuttings, lateral branch growth, fl ower number, fl oral timing, and fruit set. Although both lines displayed a delayed senescence phenotype the two PSAG12-IPT transgenic lines differed from each other in regard to other horticultural traits. In addition to delayed leaf senescence, line I-1-7 also demonstrated a decrease in adventitious rooting and an increased number of branches during plant production. Line I-3-18 also demonstrated a delayed leaf senescence phenotype; however, plants of this line were not greatly altered in any other horticultural performance traits in comparison to wild-type 'V26ʼ. IPT transcript was detected in young fully expanded leaves of both lines, although mRNA levels were higher in I-1-7 plants. A greater than 50-fold increase in IPT transcript abundance was detected in leaves of transgenic plants following drought stress. These results demonstrate that it is possible to use PSAG12-IPT to produce transgenic plants with delayed leaf senescence but differences in plant morphology between transgenic lines exist, which may alter horticultural performance characteristics. Senescence is the fi nal developmental process in the lifecycle of a leaf. It is a process through which the nutrients contained within the macromolecules (e.g., chlorophyll, proteins, nucleic acids) of leaf cells are metabolized to basic components and trans- ported to the growing shoot and reproductive organs of the plant. This strategy imparts a survival advantage for plants that grow under adverse conditions where nutrients are limiting (Nooden, 1988). Natural leaf senescence in many plants is characterized by yellowing or chlorosis of the lower leaves as nutrients and other components of the cells are degraded (esp. chlorophyll). In horticultural terms, chlorosis can decrease the aesthetic appear- ance and thereby decrease the salability of that plant. With so much at risk, it is no wonder that growers and researchers alike have tried to understand the components of leaf senescence in order to prevent it. One way to prevent leaf senescence is through the manipulation of cytokinins. Cytokinins are an important class of plant hormones that infl uence numerous aspects of plant growth and development. They have been shown to delay and, in some cases, reverse the leaf senescence process (Gan and Amasino, 1996). There is an inverse correlation between the concentration of cytokinins in leaves and the magnitude of leaf senescence. This can be dramati- cally seen as the formation of green islands on leaves infected by cytokinin-producing pathogens, such as Helminthosporium teres (Angra and Mandahar, 1991). The use of applied cytokinins to prevent senescence is limited because it is diffi cult to supply them