Phosphorus Remobilization During Developmental and Nutrient Stress-Induced Senescence In Petunias

The programmed degradation of macromolecules during senescence allows the plant to remobilize nutrients from dying to developing tissues.  Senescence is the last stage of leaf and petal development and this process can be accelerated by a number of biotic and abiotic stresses.  The plant hormone ethylene is the primary regulator of senescence in many plant species.  These studies investigated ethylene’s role in nutrient remobilization during developmental and nutrient stress-induced senescence in petunia.  Only nitrogen and phosphorus levels were found to change significantly from petal opening to the advanced stages of senescence in both pollinated and unpollinated Petunia x hybrida ‘Mitchell Diploid’ (MD) flowers.  The largest senescence-related changes in the nutrient content of petals were consistently observed with phosphorus.  To further investigate the mechanisms of P remobilization during petal senescence the expression of five high-affinity phosphate transporters was investigated.  Only one phosphate transporter (PhPT1) was found to be induced during petal senescence.  Relative abundance of PhPT1 in petals increased following treatment with 0.1 µl l^-1 ethylene for 2 hours.  The P and N content of petals was determined in ethylene-sensitive MD petunias and transgenic petunias with reduced sensitivity to ethylene (35S::etr1-1).   When compared to the total P content of corollas on the day of flower opening, P in MD corollas had decreased 74% by the late stage of senescence.  In contrast, P levels were only reduced by an average of 32% during etr1-1 corolla senescence.  The N content decreased by 60% in MD and 45% in etr1-1 corollas.  PhPT1 transcript abundance increased in senescing MD corollas and much smaller increases were detected in etr1-1 corollas.  PhPT1 transcripts were also detected in petunia roots and leaves and mRNA abundance increased in both organs following P deprivation.  Phosphorus deprivation resulted in the induction of PhPT1 gene expression in the leaves of MD but not etr1-1 petunias.  These experiments indicate that expression of the high-affinity phosphate transporter PhPT1 is regulated by ethylene in both petals and leaves and PhPT1 appears to be involved in Pi reallocation during senescence.