The primary root procambium contributes to lateral root formation through its impact on xylem connection

Area of Science:

• Plant Biology
• Developmental Biology
• Molecular Genetics

Background:

• Lateral root (LR) formation originates in the pericycle, a root tissue.
• Establishing vascular continuity between the primary root (PR) and LRs is critical for plant development.
• The roles of the pericycle and other cell types in vascular connection remain unclear.

Purpose of the Study:

• To investigate how vascular connections are established between the PR and emerging LRs.
• To identify the cell types and genetic factors involved in forming the xylem bridge (XB).
• To understand the plasticity and conservation of this vascular connection mechanism.

Main Methods:

• Clonal analysis and time-lapse imaging to track cell behavior during LR development.
• Mutant analyses to identify key genes and transcription factors (TFs).
• Comparative observations in *Solanum lycopersicum* to assess conservation.

Main Results:

• Both PR procambium and pericycle coordinate to form the xylem bridge (XB).
• Procambial derivatives and pericycle cells contribute to the XB, establishing xylem continuity.
• CLASS III HOMEODOMAIN-LEUCINE ZIPPER (HD-ZIP III) TFs regulate early XB cell specification, while VASCULAR-RELATED NAC-DOMAIN (VND) TFs control secondary cell wall deposition.

Conclusions:

• Plants maintain vascular procambium activity to ensure xylem strand continuity in the root system.
• The XB formation mechanism, involving procambium and pericycle, is essential for lateral organ functionality.
• The observed plasticity and conservation in *Solanum lycopersicum* suggest a fundamental role for XB in plant vascular development.