Evolution of the JULGI-SMXL4/5 module for phloem development in angiosperms

Area of Science:

• Plant Molecular Biology
• Developmental Biology
• Plant Physiology

Background:

• Seed plants (angiosperms and gymnosperms) possess bifacial cambium for xylem/phloem production and monopodial architecture for growth.
• SUPPRESSOR OF MAX2-LIKE (SMXL) genes are crucial for phloem development and regulating axillary bud outgrowth via strigolactone signaling.
• JULGI, an RNA-binding protein, is the sole known regulator of SMXL4 and SMXL5, but its interaction with strigolactone signaling was unclear.

Purpose of the Study:

• To investigate the regulatory role of JULGI in SMXL4/5 expression and its relationship with strigolactone signaling in angiosperms.
• To understand the evolutionary conservation and functional divergence of JULGI and SMXL genes across vascular plants.

Main Methods:

• Comparative analysis of JULGI and SMXL gene expression in vascular plant tissues.
• Functional characterization using heterologous expression in Arabidopsis mutants (jul1 jul2, smxl4 smxl5).
• Analysis of SMXL protein stability in response to strigolactone analog treatment.

Main Results:

• JULGI directly regulates SMXL4/5 expression in angiosperms, decoupling it from strigolactone signaling.
• A conserved G-rich element in the 5' UTR of angiosperm SMXL4/5 mRNA acts as a JULGI target site for negative regulation.
• Angiosperm SMXL4/5 proteins lack a degradation motif, unlike ancestral SMXLs, conferring resistance to strigolactone-induced proteasomal degradation.

Conclusions:

• The JULGI-SMXL4/5 regulatory module in angiosperms allows for independent control of phloem development.
• Evolution of the 5' UTR in SMXL4/5 likely facilitated the divergence of strigolactone signaling pathways in angiosperms.
• This regulatory innovation contributed to the adaptive success of angiosperms.