Prunus persica (peach) trees carrying the “Pillar” or “Broomy” trait (br) have vertically oriented branches caused by loss-of-function mutations in a gene called TILLER ANGLE CONTROL 1 (TAC1). TAC1 encodes a protein in the IGT gene family that includes LAZY1 and DEEPER ROOTING 1 (DRO1), which regulate lateral branch and root orientations, respectively. Here we found that some of the native TAC1 alleles in the hexaploid plum species Prunus domestica, which has a naturally more upright stature, contained a variable length trinucleotide repeat within the same exon 3 region previously found to be disrupted in pillar peach trees. RNAi silencing of TAC1 in plum resulted in trees with severely vertical branch orientations similar to those in pillar peaches but with an even narrower profile. In contrast, PpeTAC1 overexpression in plum led to trees with wider branch angles and more horizontal branch orientations. Pillar peach trees and transgenic plum lines exhibited pleiotropic phenotypes, including differences in trunk and branch diameter, stem growth, and twisting branch phenotypes. Expression profiling of pillar peach trees revealed differential expression of numerous genes associated with biotic and abiotic stress, hormone responses, plastids, reactive oxygen, secondary, and cell wall metabolism. Collectively, the data provide important clues for understanding TAC1 function and show that alteration of TAC1 expression may have broad applicability to agricultural and ornamental tree industries.

Light serves as an important environmental cue in regulating plant architecture. Previous work had demonstrated that both photoreceptor-mediated signaling and photosynthesis play a role in determining the orientation of plant organs. TILLER ANGLE CONTROL 1 (TAC1) was recently shown to function in setting the orientation of lateral branches in diverse plant species, but the degree to which it plays a role in light-mediated phenotypes is unknown. Here, we demonstrated that TAC1 expression was light dependent, as expression was lost under continuous dark or far-red growth conditions, but did not drop to these low levels during a diurnal time course. Loss of TAC1 in the dark was gradual, and experiments with photoreceptor mutants indicated this was not dependent upon red/far-red or blue light signaling, but partially required the signaling integrator CONSTITUTIVE PHOTOMORPHOGENESIS 1 (COP1). Overexpression of TAC1 partially prevented the narrowing of branch angles in the dark or under far-red light. Treatment with the carotenoid biosynthesis inhibitor norflurazon or the PSII inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) led to loss of TAC1 expression similar to dark or far-red conditions, but expression increased in response to the PSI inhibitor paraquat. Treatment of adult plants with norflurazon resulted in upward growth angle of branch tips. Our results indicate that TAC1 plays an important role in modulating plant architecture in response to photosynthetic signals.

TAC1 and LAZY1 are members of a gene family that regulates lateral shoot orientation in plants. TAC1 promotes outward orientations in response to light, while LAZY1 promotes upward shoot orientations in response to gravity via altered auxin transport. We performed genetic, molecular, and biochemical assays to investigate possible interactions between these genes. In Arabidopsis they were expressed in similar tissues and double mutants revealed the wide-angled lazy1 branch phenotype, indicating it is epistatic to the tac1 shoot phenotype. Surprisingly, the lack of TAC1 did not influence gravitropic shoot curvature responses. Combined, these results suggest TAC1 might negatively regulate LAZY1 to promote outward shoot orientations. However, additional results revealed that TAC1-and LAZY1 influence on shoot orientation is more complex than a simple direct negative regulatory pathway. Transcriptomes of Arabidopsis tac1 and lazy1 mutants compared to wild type under normal and gravistimulated conditions revealed few overlapping differentially expressed genes. Overexpression of each gene did not result in major branch angle differences. Shoot tip hormone levels were similar between tac1, lazy1, and Col, apart from exceptionally elevated levels of salicylic acid in lazy1. The data presented here provide a foundation for future study of TAC1 and LAZY1 regulation of shoot architecture.Lateral organ orientation in both shoots and roots plays a key role in a plant's interaction with the environment and its ability to access resources such as light and water. The IGT gene family members TILLER ANGLE CONTROL 1 (TAC1) and the related set of LAZY genes are important regulators of lateral organ orientation 1,2 . These genes share four conserved amino acid regions or domains, and LAZY genes share an additional C-terminal domain 3,4 . TAC1 generally occurs as a single copy gene, but many species possess multiple LAZY genes, with six identified in Arabidopsis 2-8 . While the LAZY1 gene of Arabidopsis thaliana (Arabidopsis) contributes almost exclusively to shoot architecture, the remaining Arabidopsis LAZY genes primarily control root architecture 7,8 . However, GUS reporter activity and additive shoot phenotypes in plants with combinatorial lazy1, lazy2, and lazy4 mutations suggest LAZY2 and LAZY4 also have a role in regulating shoot orientation 7,8 . LAZY2, 3, and 4 are also known as DEEPER ROOTING (DRO3, 2, & 1) and NEGATIVE GRAVITROPIC RESPONSE OF ROOTS (NGR1, 3, & 2), respectively, as they were separately identified as regulators of lateral root orientation 6,9-12 . Further, an alternate LAZY gene nomenclature denotes LAZY3 as LZY2 and LAZY4 as LZ3Y 8 .Lateral branches, tillers, leaves, and flower buds of plants with loss-of-function tac1 mutations or reduced TAC1 expression exhibit upright orientations 3,13-19 . In contrast, lazy1 mutants have wider branch or tiller angles and lazy4/dro1 mutants display prostrate lateral root orientations 4-9,20-24 . Plants with multiple lazy/dro mutations exhibit even wider lateral shoot/root...