IGT Genes Shape Plant Shoot and Root Architecture

The architecture of the shoot and root system influences a plant’s access to sunlight, water, and nutrients and thus plays a critical role in agricultural productivity. Shoot and root architecture is shaped by genetic as well as developmental and environmental factors, and mechanical manipulation. Here we focus on the genetic contribution of a recently discovered IGT gene family, which regulates the orientation of lateral organs. TILLER ANGLE CONTROL 1 (TAC1) and LAZY1 antagonistically affect the trajectory of lateral branches and leaves. Loss of TAC1 results in upright, narrow tiller and leaf angles in monocots, and narrow branch angles in dicots. In contrast, loss of LAZY1 leads to wide or prostrate tiller, leaf and branch angles. In roots, a third IGT family member, DEEP ROOTING 1 (DRO1), promotes downward root growth in rice. Here we studied the functions of these three genes in Arabidopsis thaliana and Prunus species, as well as relationships among them. Expression studies indicated that TAC1, LAZY1, and DRO1 were expressed in a tissue specific fashion. LAZY1 was primarily expressed in shoots, TAC1 was expressed in both shoots and roots, while DRO1 was root specific. Despite high levels of root expression, a root phenotype was not observed in tac1 Arabidopsis mutants, however peach tac1 mutants have a narrower root system. A tac1;lazy1 double mutant mimics the lazy1 mutant in Arabidopsis, suggesting the two genes work in the same pathway. Light exaggerated the lazy1 phenotype, but surprisingly only the TAC1 expression was dependent on light. In trees, silencing of LAZY1 often led to a weeping habit, which increased with age. Loss of DRO1 in Arabidopsis led to horizontally oriented lateral roots and wide root systems. Collectively, the results show a complex relationship among IGT proteins regulating the branch angles of lateral shoots and roots.