A Structurally Specialized Uniform Wall Layer is Essential for Constructing Wall Ingrowth Papillae in Transfer Cells

Area of Science:

• Plant Cell Biology
• Plant Anatomy
• Plant Physiology

Background:

• Transfer cells facilitate nutrient transport in plants and exhibit distinct wall labyrinth architectures (flange or reticulate).
• Reticulate transfer cells possess a uniform wall layer, a structure absent in flange transfer cells, which is critical for papillae formation.

Purpose of the Study:

• To investigate the deposition characteristics and functional role of the uniform wall layer in reticulate transfer cells.
• To elucidate the relationship between the uniform wall layer, cellulose microfibrils, and wall ingrowth papillae formation.

Main Methods:

• Utilized a *Vicia faba* cotyledon culture system to induce *trans*-differentiation of adaxial epidermal cells into reticulate transfer cells.
• Analyzed the deposition of the uniform wall layer and its cellulose microfibril organization using microscopy.
• Examined gene expression patterns related to cellulose biosynthesis during uniform wall layer and papillae formation.

Main Results:

• Uniform wall layer deposition begins after cell expansion ceases, featuring sparsely dispersed linear cellulose microfibrils, unlike the dense lattice of the primary wall.
• Uniform wall layer formation is independent of a cellulose scaffold, but its microfibrils are essential precursors for wall ingrowth papillae.
• Papillae formation involves a 3-fold increase in cellulose microfibril diameter, correlated with specific upregulation of *VfCesA3B* and downregulation of other cellulose biosynthetic genes.

Conclusions:

• The uniform wall layer is a specialized structure in reticulate transfer cells, essential for developing wall ingrowth papillae.
• Cellulose microfibrils within the uniform wall layer play a critical role in papillae construction, with specific gene regulation driving this process.