A transcription factor ensemble orchestrates bundle sheath expression in rice.

Area of Science:

• Plant Biology
• Molecular Genetics
• Photosynthesis Research

Background:

• C4 photosynthesis enhances efficiency by ~50% across diverse plant lineages.
• Bundle sheath cell-specific gene expression is a hallmark of C4 plants.
• Regulatory networks controlling bundle sheath gene expression remain poorly understood, especially in grasses.

Purpose of the Study:

• To define the regulatory logic governing bundle sheath gene expression in rice.
• To identify cis-regulatory elements responsible for cell-type-specific gene patterning.
• To explore the conservation and potential engineering applications of these regulatory elements.

Main Methods:

• Utilized the SULFITE REDUCTASE promoter in rice to investigate bundle sheath expression.
• Identified and characterized a distal cis-regulatory module (CRM) through promoter analysis.
• Tested the CRM's function in both rice and Arabidopsis to assess cross-species conservation.

Main Results:

• The SULFITE REDUCTASE promoter contains complex cis-regulatory logic for bundle sheath expression.
• A specific distal CRM, activated by multiple transcription factors, was found to be necessary and sufficient for bundle sheath patterning.
• CRM oligomerization and fusion to Y-patch promoters increased activity 220-fold.
• The identified CRM drives bundle sheath-specific expression in both rice and Arabidopsis, indicating deep functional conservation.

Conclusions:

• An ancient, short, and tunable CRM has been identified that patterns gene expression to the bundle sheath.
• This CRM is conserved across monocots and dicots, highlighting fundamental regulatory mechanisms.
• The findings offer a valuable tool for engineering bundle sheath cell identity in various crop species to improve photosynthesis.