Study Reveals How Plants Detect When to Grow or Pause

Plants build organs by coordinating “initiation” and “growth,” but how one signal can drive two opposite outcomes has been unclear. A new study reveals a timed hormone handoff that allows plants to first suppress an architecture-promoting pathway and then activate it to expand new tissue.

Researchers led by Dr. Alon Israeli and Prof. Naomi Ori from the Hebrew University of Jerusalem focused on tomato as a model system. Writing in Development, they show that the plant hormone auxin orchestrates organ formation through two sequential phases of cross talk with gibberellin (GA).

In the earliest window after auxin signaling starts, auxin shifts gene regulation to lower GA activity. It activates pathways that break down gibberellin while simultaneously suppressing genes required for GA biosynthesis. This transient reduction creates permissive conditions for leaflets and other new structures to emerge at precise locations.

As development proceeds, auxin reverses its influence. Instead of maintaining GA suppression, auxin transitions to a mode that enhances gibberellin production and/or signaling. The same hormone therefore triggers a later growth program rather than initiation.

To test causality, the team used genetic engineering, hormone treatments, and gene-expression analyses. When GA activity was experimentally sustained at higher levels, plants produced fewer leaflets, consistent with a failure to properly initiate new structures.

Conversely, local reduction of gibberellin caused extra leaflet formation in areas where leaflets would not normally develop. This demonstrates that proper GA downshifting is not merely correlated with initiation—it is functionally required.

For the expansion phase, plants engineered to fail to mount sufficient GA responses were unable to execute the robust blade growth normally induced downstream of auxin. Together, these results argue that the auxin–GA relationship is dynamic, not static.

The work also highlights an elegant logic: auxin accomplishes two distinct developmental tasks by altering which GA-regulating target genes it controls over time. This provides a mechanistic explanation for how plants “alternate between brakes and accelerator” during organ assembly.

Beyond leaf development, the sequential hormonal strategy may generalize to other organs such as flowers and roots. If so, manipulating this timing could become a new lever for engineering crop architecture with improved growth and resilience.