Highlights d GEMs enable high-throughput microrheology in unperturbed living cells d mTORC1 controls diffusion by tuning ribosome concentration d Diffusion can be accurately predicted as a function of ribosome concentration d Crowding of the cytoplasm by ribosomes increases phase separation

Tiller angle, a key agronomic trait for achieving ideal plant architecture and increasing grain yield, is regulated mainly by shoot gravitropism. Strigolactones (SLs) are a group of newly identified plant hormones that are essential for shoot branching/ rice tillering and have further biological functions as yet undetermined. Through screening for suppressors of lazy1 (sols), a classic rice mutant exhibiting large tiller angle and defective shoot gravitropism, we identified multiple SOLS that are involved in the SL biosynthetic or signaling pathway. We show that SL biosynthetic or signaling mutants can rescue the spreading phenotype of lazy1 (la1) and that SLs can inhibit auxin biosynthesis and attenuate rice shoot gravitropism, mainly by decreasing the local indoleacetic acid content. Although both SLs and LA1 are negative regulators of polar auxin transport, SLs do not alter the lateral auxin transport of shoot base, unlike LA1, which is a positive regulator of lateral auxin transport in rice. Genetic evidence demonstrates that SLs and LA1 participate in regulating shoot gravitropism and tiller angle in distinct genetic pathways. In addition, the SL-mediated shoot gravitropism is conserved in Arabidopsis. Our results disclose a new role of SLs and shed light on a previously unidentified mechanism underlying shoot gravitropism. Our study indicates that SLs could be considered as an important tool to achieve ideal plant architecture in the future. P lant shoot gravitropism is a process in which plants perceive gravity stimuli and reorient the direction of growth during the plant growth and development. Gravitropism is a dynamic process, including the perception of gravity, transduction of the corresponding information into a biochemical signal, transmission of the biochemical signal to a response site, and organ curvature (1). Genetic studies have disclosed that shoot endodermal cells act as statocytes for shoot gravitropism (2). Amyloplast sedimentation transduces the gravitropic signal and leads to auxin redistribution, resulting in a higher auxin level on the lower side than on the upper side (1, 3). Although auxin redistribution upon gravistimulation plays an important role in shoot gravitropism, the mechanism by which auxin regulates shoot gravitropism is not yet understood.In rice the tiller angle, which is defined as the angle between the tiller and main culm, plays a key role in determining rice plant architecture and thus grain yield (4). Several genes controlling the tiller angle have been identified previously in rice, including LAZY1 (LA1), TILLER ANGLE CONTROL1 (TAC1), PROSTRATE GROWTH1 (PROG1), and LOOSE PLANT ARCHITECTURE1 (LPA1) (5-9). Among these genes, LA1 and LPA1 are reported to regulate tiller angle through shoot gravitropism. In the la1 mutant, the polar auxin transport (PAT) is enhanced, and the lateral auxin transport is decreased, resulting in a disturbance of auxin asymmetric distribution in the shoot base and therefore the tiller-spreading phenotype of rice plants (5).Strigolactones (S...

Summary (Abstract): (less than 150 words)Macromolecular crowding has a profound impact on reaction rates and the physical properties of the cell interior, but the mechanisms that regulate crowding are poorly understood. We developed Genetically Encoded Multimeric nanoparticles (GEMs) to dissect these mechanisms. GEMs are homomultimeric scaffolds fused to a fluorescent protein. GEMs self-assemble into bright, stable fluorescent particles of defined size and shape. By combining tracking of GEMs with genetic and pharmacological approaches, we discovered that the mTORC1 pathway can tune the effective diffusion coefficient of macromolecules ≥15 nm in diameter more than 2-fold without any discernable effect on the motion of molecules ≤5 nm. These mTORC1-dependent changes in crowding and rheology affect phase-separation both in vitro and in vivo. Together, these results establish a role for mTORC1 in controlling both the biophysical properties of the cytoplasm and the phase-separation of biopolymers.