The review centers on a deceptively simple concept: the gravitropic setpoint angle, or GSA, the angle at which a plant organ grows relative to the gravity vector. Primary roots and stems grow vertically, at GSAs of zero and 180 degrees respectively, a behavior called ortho-gravitropism. Lateral organs, however, grow at intermediate angles through plagio-gravitropism, a balance between gravity and what the authors call the anti-gravitropic offset. That balance is precisely what breeders manipulate. Compact modern crops with small tiller and leaf angles tolerate dense planting because their aboveground lateral organs adopt large GSAs, while large lateral root angles anchor plants more firmly and resist lodging. Both deep roots with large GSAs and shallow roots with small GSAs can improve uptake of water and minerals, depending on soil conditions.

At the cellular level, the story begins with gravity perception. In roots, gravity is sensed by columella cells in the root cap; in shoots, by a single layer of endodermal cells. Both contain dense, starch-filled amyloplasts that sediment toward gravity like tiny stones, acting as statoliths. Three theoretical models have competed to explain how this sedimentation becomes a biochemical signal. A mechanosensitive channel model proposes that falling amyloplasts press on ion channels, but the plant mechanosensitive channels identified so far appear irrelevant to gravitropism, and no evidence links amyloplasts to the observed calcium changes. A ligand-receptor model, inspired by green algae, remains largely unexplored. The favored explanation is now the position-sensor model: experiments combining centrifugal hyper-gravity with clinostats show that shoot gravitropism tracks the angle of inclination rather than the magnitude of gravitational force, meaning amyloplast position itself, functioning like a liquid clinometer, is the primary signal.

Amyloplast behavior is fine-tuned by starch content, the actin cytoskeleton and the vacuolar membrane. Genes such as AtSGR5 promote starch accumulation and amyloplast displacement, while the auxin signaling complex AtTIR1/AtAFB-AtAXR3 links the hormone to statolith production. The E3 ubiquitin ligase AtSGR9 and actin-related proteins modulate how amyloplasts interact with actin filaments, and vacuolar membrane dynamics, governed by SNARE proteins like AtZIG/SGR4 and the membrane protein AtSGR6, determine whether amyloplasts can move freely when the plant is reoriented. In root tips, the membrane protein AtMLO4 and its partner AtCML12 implicate vesicle trafficking in root gravisensing as well.

The most striking recent breakthrough concerns signal conversion, the moment when organelle movement becomes molecular polarity. Gravity stimulation activates the kinase AtMPK3, which phosphorylates the LAZY family proteins AtLZY3 and AtLZY4. Phosphorylated LZYs are recruited by components of the TOC complex on the amyloplast surface and, as the amyloplasts sediment, are shuttled to the new lower side of the plasma membrane. There, polarized LZYs recruit RLD proteins and the auxin exporter PIN3, establishing PIN polarization on the lower membrane. This chain, from amyloplast position through LZY translocation to PIN repolarization, is largely compatible with the position-sensor hypothesis. A parallel pathway involves clathrin-mediated endocytosis and GNOM-dependent recycling of PIN3, while in stems the BRXL4-LZY1 interaction provides a negative feedback that damps the response.

Once PINs are polarized, asymmetric auxin flow carries the signal from sensing sites to responsive zones. In roots, PIN3 and PIN7 redirect auxin downward, PIN2 redistributes across the epidermis, and AUX1 supports a phloem-based delivery route to the root apex. The resulting auxin gradient suppresses cell elongation on the lower side, bending the organ until amyloplasts restore their original position. In lateral roots, auxin is balanced between gravity-dependent flow and the anti-gravitropic offset, which is how a stable GSA is maintained. Other hormones modulate this system: ethylene, cytokinin, jasmonate, brassinolide, gibberellin, abscisic acid and strigolactone all influence gravitropic bending, mostly through auxin synthesis, transport or LZY regulation.

The review then connects this molecular network to agronomic traits. In rice, LAZY1 homologs, TAC1, TAC4, PROG1 and a cascade involving HSFA2D control tiller angle, and several of these genes were selected during domestication. In maize, only three gravitropic genes have been firmly identified: ZmCLA4/ZmLA1, an AtLZY1 ortholog that regulates leaf angle and inflorescence development, and ZmYUC2 and ZmYUC4, auxin biosynthesis genes that control brace root angle and lodging resistance. Root-angle genes are especially valuable: the rice QTL DRO1 confers deep rooting without reducing biomass, and Dro1-NIL lines show better pushing resistance, drought avoidance, nitrogen acquisition from deep soil and reduced cadmium uptake, while the DRO1 homolog SOR1 favors shallow roots that escape salt accumulation and capture topsoil phosphorus. The OsGLS1-OsPIN2 pathway shapes root angle and nutrient uptake efficiency.

Because so few maize gravitropic genes are known, the authors performed a systematic prediction. By aligning known Arabidopsis and rice gravitropic genes against maize candidate genes drawn from prior studies of leaf angle, stalk lodging and root traits, and filtering by expression in relevant organs, they identified 69 maize loci that overlap with gravitropic genes, distributed across all nine maize chromosomes. Among 23 highlighted candidates, ZmSGR5 and ZmPIN1-1 are leaf-angle candidates, ZmSGR9-1 and ZmSHR relate to stalk strength traits such as rind penetrometer resistance and stem bending strength, and ZmMPK3-3, ZmMPK3-5, ZmABCB19 and ZmMYB88-4 are expressed in roots and linked to brace root traits. The authors caution that these predictions lack CRISPR mutants or natural-variant validation, and that forward genetics is difficult because gravitropic phenotyping requires physically reorienting large mapping populations, especially for roots.

The breeding implications are considerable. Root lodging alone can cut maize grain yield by roughly 48 percent while raising harvest costs, and dense planting, the backbone of modern yield gains, depends on compact architecture that gravitropic genes directly control. The authors outline a practical agenda: functional validation of the 69 candidate genes, prioritizing the four root-expressed candidates via CRISPR editing or natural-variant screening; dissection of multi-gene networks underlying leaf angle and lodging resistance; and field testing of candidate stability under variable soil moisture and density stress. Tools such as haploid-inducer-mediated genome editing and marker-assisted pyramiding of favorable alleles could accelerate translation. They also point toward an ideal canopy structure of upper tight and lower loose leaves, and toward open questions that include how amyloplast positioning initiates signaling, how actin turnover enables a second, root-cap-independent gravisensing pathway in maize, and how crops adapt to microgravity aboard space stations. If even a fraction of the predicted genes proves functional, gravity-shaped architecture could become one of the most powerful design principles in crop improvement.

Subject of Research: Molecular mechanisms of plant gravitropism and their application to maize and crop breeding for dense planting, lodging resistance and nutrient uptake

Article Title: Gravity-shaped plant architecture to enhance dense planting, lodging resistance and substance uptake and their applications in maize and other crop breeding

Article References: Zhu, L., Long, Y., Han, J., Ji, K., Wu, J., & Wan, X. (2026). Gravity-shaped plant architecture to enhance dense planting, lodging resistance and substance uptake and their applications in maize and other crop breeding. Journal of Advanced Research. https://doi.org/10.1016/j.jare.2026.10.004

Image Credits: AI Generated

DOI: 10.1016/j.jare.2026.10.004

Keywords: gravitropism, amyloplasts, auxin transport, PIN proteins, LAZY genes, maize breeding, leaf angle, root angle, lodging resistance, dense planting, rice tiller angle, nutrient uptake