# Auxin Biosynthesis, Transport, Signaling, and Its Roles in Plant Leaf Morphogenesis

## TL;DR

This review explains how auxin regulates leaf shape and size through biosynthesis, transport, and signaling, with recent discoveries offering new targets for crop improvement.

## Contribution

The paper highlights recent breakthroughs, such as cAMP as a second messenger and re-evaluation of auxin pathways, offering new insights into leaf morphogenesis.

## Key findings

- cAMP functions as a second messenger produced by TIR1/AFB receptors, directly activating ARF-mediated transcription.
- Key network nodes like PIN and YUC are validated targets for crop improvement.
- The IAOx pathway's role in auxin pools is under scrutiny, suggesting undiscovered enzymes or alternative metabolic routes.

## Abstract

Leaf morphogenesis is governed by a tightly integrated regulatory network centered on auxin, which operates through a sequential axis of synthesis, transport, and signal transduction. This review elucidates how pivotal molecular hubs previously identified in this regulatory network, including biosynthetic enzymes, polar transporters, and auxin response factors, interconnect through dynamic feedback mechanisms to orchestrate leaf initiation, polarity establishment, and the determination of its final size and shape. Notably, recent breakthroughs are transforming the field: the re-evaluation of established pathways like indole-3-acetaldoxime (IAOx), whose direct contribution to auxin pools is under scrutiny, hinting at the existence of undiscovered enzymes or alternative metabolic branches and the paradigm-shifting discovery that cAMP functions as a second messenger produced by Transport Inhibitor Resistant 1/Auxin signaling F-box (TIR1/AFB) receptors, which directly activates Auxin Response Factor (ARF)-mediated transcription. These foundational mechanistic insights provide the critical groundwork for application. Key network nodes—such as PIN-FORMED (PIN) transporters and YUCCA (YUC) flavin-containing monooxygenases—are now validated targets for crop improvement. Consequently, the elucidated network serves as a blueprint for rationally designing crop architecture.