Branch orientation is a crucial factor in the life of a plant, determining aspects as diverse as light interception, ability to compete with surrounding plants, and capacity to support a fruit load. Perhaps nowhere is this more true than in trees and other woody perennials, where a branch represents a long-term investment of metabolic resources which must be compensated for by new photosynthetic opportunities. To determine branch orientation, plants integrate signals from gravity and light, but each species uniquely responds to those signals in determining crotch angle and trajectory. The underlying genetics which control these responses are still largely unknown but are crucial both to plant physiology and to manipulating branch orientation in ways desirable for cultivation. In this work, I examine three genes involved in integration of gravitropic and phototropic signals for branch angle control—WEEP, LAZY1, and TILLER ANGLE CONTROL 1 (TAC1)—in two commercially important tree fruit crops: peach (Prunus persica) and European plum (Prunus domestica). These three genes represent different control points in the determination of branch angle, but likely all function in the same pathway, as LAZY1 is epistatic to TAC1, and TAC1 is epistatic to WEEP. In the first chapter, I provide an overview of the genetic and hormonal mechanisms known to control plant architecture. In the second chapter, I investigate the function of WEEP, a Sterile Alpha Motif domain gene previously identified as involved in branch trajectory, as a homozygous mutation in WEEP causes a pendulous, downward arching branch trajectory in peach. Here, I present data that WEEP is crucial to the formation of an auxin gradient during gravitropism, and weeping peach branches have an inversion of that gradient in the shoot, perceiving the world “upside-down”. I also present data connecting WEEP to set-point angle in roots. In the third chapter, I characterize the phenotype of LAZY1-antisense in transgenic plum. LAZY1 is also essential to formation of the auxin gradient during gravitropism, directing the polarization of auxin efflux carriers and promoting upward branch orientation. Here, I describe phenotypes of LAZY1-antisense in plum, including impacts on branch angle and photosynthesis, note reproductive phenotypes observed in LAZY1-antisense lines, and discuss use of LAZY1- antisense in two planar training systems—super spindle axe and espalier. Finally, in the fourth chapter, I investigate how dosage of TAC1 affects novel planar training in peach. While in the same gene family (IGT) as LAZY1, TAC1 functions in light response, and promotes the opposite phenotype, directing branches outward. Using peach varieties which are homozygous wild type iii (Bounty, spreading habit), heterozygous (Sweet-N-UP, upright habit) or homozygous mutant (Crimson Rocket, pillar habit) for TAC1, I look at implications of planar training systems for fruit quality and yield.
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