Researchers have uncovered a surprising mechanism that allows plants to carefully coordinate the formation and growth of new leaf parts. The study shows that the hormone auxin regulates different phases in organ formation by oppositely affecting the activity of another hormone, gibberellin, to trigger the formation of new leaf structures before reversing course and boosting gibberellin to drive their expansion. The findings offer new insight into how plants build complex organs and could eventually help scientists develop crops with improved growth and architecture.

A plant's ability to produce leaves, flowers and other organs depends on precise location and timing. It must first determine where a new structure will form before allowing it to expand. Now, researchers have uncovered the molecular switch that coordinates these two steps, revealing how plants carefully alternate between putting on the brakes and stepping on the accelerator during organ development.

The study, led by Dr. Alon Israeli and Prof. Naomi Ori from the Robert H. Smith Faculty of Agriculture, Food and Environment at the Hebrew University of Jerusalem and published in Development, identifies a surprising two-step communication system between two major plant hormones. The findings explain how plants use the hormone auxin to first repress, and later promote, the activity of gibberellin (GA), allowing new leaf structures to form before stimulating their growth.

Plant organs such as leaves develop in two distinct stages. First, new structures are initiated at specific locations. Only afterward do these structures expand into mature organs. While scientists have long known that auxin plays a central role throughout both phases, it has remained unclear how the same hormone can coordinate two very different developmental processes.

Using tomato plants as a model, the research team discovered that auxin changes its relationship with gibberellin over time. Immediately after auxin signaling begins, it activates genes that break down gibberellin while suppressing genes responsible for producing it. This temporary reduction in gibberellin creates the conditions necessary for new leaflets to emerge. As development progresses, however, auxin reverses course, activating gibberellin production and triggering the rapid tissue expansion required for leaf growth.

To confirm the mechanism, the researchers combined genetic engineering, hormone treatments and gene-expression analyses. Plants engineered to maintain higher gibberellin activity produced fewer leaflets, while locally reducing gibberellin levels generated extra leaflets in places where they would not normally develop. Conversely, plants unable to produce sufficient gibberellin failed to undergo the robust blade expansion normally driven by auxin.
Together, the experiments demonstrate that the same hormone can orchestrate two opposite developmental outcomes simply by altering when and how it regulates its hormonal partner.

Beyond explaining leaf formation, the findings suggest that this sequential hormonal strategy may represent a broader developmental principle used throughout the plant kingdom. Similar patterns of hormone activity have been observed during the formation of flowers, roots and other plant organs, raising the possibility that the newly identified mechanism serves as a universal blueprint for plant growth.

"The same hormone has to accomplish two very different tasks during organ formation," said Prof. Naomi Ori, who led the study. "Our work shows that auxin achieves this by changing its conversation with gibberellin over time - first lowering gibberellin activity to allow a new organ to form and then increasing it to drive growth. This is done by affecting different target genes. This elegant two-step mechanism helps explain how plants build complex organs with remarkable precision."

Beyond advancing fundamental understanding of plant biology, the discovery could eventually help researchers develop crops with improved architecture, productivity and resilience. By manipulating the balance between organ initiation and growth, breeders may one day be able to fine-tune leaf shape, branching patterns and other traits that influence agricultural performance.