Auxin and cytokinin related gene expression during active shoot growth and latent bud paradormancy in Vitis riparia grapevine.

Cultural practices for canopy management in grapevines rely on intensive manipulation of shoot architecture to maintain canopy light levels. In contrast to common model plant systems used to study regulation of branch outgrowth, the grapevine has a more complex architecture. The node contains first, second and third order axillary meristems. The prompt bud (N+1) develops into a summer lateral and a latent compound bud develops in the basal node of the summer lateral (N+2, N+3(1,2)). The outgrowth potential of latent buds was determined using common canopy management treatments (shoot tip decapitation and removal of summer laterals and leaves) and monitoring the rate of latent bud outgrowth. Two shoot node regions (apical and basal) with differential outgrowth potential were characterized and it was noted that the shoot tip, summer laterals and leaves in addition to node position contributed to the inhibition of latent bud outgrowth. To advance the understanding of the molecular regulation of bud outgrowth and paradormancy in the complex shoot architecture of grapevines, the expression of auxin and cytokinin genes involved in branching (amidase (VrAMI1), PINFORMED-3 (VrPIN3) and isopentenyl transferase (VrIPT)) were monitored in shoot tips and differentially aged buds of Vitis riparia grapevine shoots. In addition, Histone 3 (VrH3) and a hexose transporter (VrHT1) expression were monitored as a measure of tissue activity. The expression of VrAMI1 and VrPIN3 remained constant in actively growing shoot tips and decreased significantly with increasing bud maturation in paradormant buds. VrHT1 expression was greater in buds than in any other plant tissue tested. VrHT1 may have the potential to be used as an indicator of paradormancy status in grapevines. These characterizations in the complex architecture of the grapevine provide an excellent model system for molecular analysis of bud outgrowth and shoot architecture development.