Loss of the phosphate sensor CsSPX2 impairs lateral root initiation and development in cucumber

Area of Science:

• Plant Biology
• Molecular Plant Physiology
• Agricultural Science

Background:

• Lateral root (LR) development is crucial for plant nutrient uptake, especially under phosphorus deficiency.
• The molecular mechanisms connecting phosphate homeostasis to root development, specifically xylem-pole-pericycle (XPP) cell competence, are not fully understood.
• Auxin is known to trigger LR formation, but its regulation by phosphate status is unclear.

Purpose of the Study:

• To identify and characterize the role of phosphate-responsive genes in regulating lateral root development in cucumber.
• To elucidate the molecular link between phosphate (Pi) homeostasis and the developmental competence of root pericycle cells.
• To investigate the function of CsSPX2 in mediating plant responses to phosphate availability.

Main Methods:

• CRISPR/Cas9 gene editing was used to generate CsSPX2 knockout cucumber lines.
• Phenotypic analysis of lateral root density, length, and root meristem structure was performed.
• Phosphate content, lignin deposition, and cell wall thickness in XPP cells were quantified.
• Auxin responsiveness was assessed using DR5::GUS reporter assays.
• Effects of lignin biosynthesis inhibitors and synthetic auxin on mutant phenotypes were evaluated.

Main Results:

• CsSPX2, a phosphate sensor gene, is induced by Pi starvation in cucumber.
• CsSPX2 knockout mutants exhibit reduced lateral root density, shorter roots, and abnormal root meristems.
• Loss of CsSPX2 leads to phosphate overaccumulation and excessive lignin deposition under Pi-sufficient conditions, with thickened XPP cell walls and impaired auxin response.
• Under Pi-deficient conditions, spx2 mutants show reduced lignin and thinner XPP cell walls.
• Exogenous application of lignin inhibitors or auxin partially rescued the LR developmental defects in spx2 mutants.
• CsSPX2 is essential for maintaining root stem cell niche organization and meristematic activity.

Conclusions:

• CsSPX2 acts as a critical phosphate sensor linking phosphate homeostasis to lateral root development.
• The study reveals CsSPX2's role in regulating XPP cell wall properties and auxin response, impacting LR initiation.
• CsSPX2 is vital for root meristem organization and function.
• This research provides a genetic target for developing phosphate-efficient crops, enhancing agricultural sustainability.