Elucidating the Gene Networks Controlling Branch Angle and the Directional Growth of Lateral Meristems in Trees

Trees can adopt a wide variety of architectural forms. Architectural plasticity plays important roles in forest ecosystems, agriculture, and landscape aesthetics. Tree architecture is a consequence of numerous developmental traits that include branching pattern, branch number, branch length, and branch angle. These traits are largely a function of two key developmental processes: apical dominance and apical control. Apical dominance is a well understood process that inhibits lateral bud outgrowth through signals emanating from the shoot apex. Intensive studies have revealed the signals and the underlying molecular mechanisms that operate in herbaceous plants such as Arabidopsis and pea. In contrast, apical control is the process by which the apex influences the overall tree structure upon successive years of growth and development in woody species. Although some progress has been made from a physiological perspective, the genetic and molecular mechanisms of apical control are largely unknown. The overarching goal of the project is to develop detailed knowledge about how trees adopt specific architectural forms, specifically with regard to apical control of the lateral branch angle and directional growth. Using a combination of genome-scale studies to elucidate key gene networks and molecular pathways coupled with innovative whole tree imaging technologies that will enable non-destructive structural phenotyping, the project will address the following questions: 1) What gene expression networks differentiate the shoot apical meristem from lateral meristems? What changes take place when a lateral shoot meristem transitions to becoming the apical meristem? How are these expression networks altered in branch angle mutants including pillar/columnar tree forms and weeping types? 2) What is the genetic and molecular basis for these mutant tree forms in peach and apple? What are the identities of the mutated genes? 3) With regard to the identified genes, what protein-protein interaction networks are they associated with? Through which pathways do they exert their effects on tree form?