Assembling the thickest plant cell wall: exine development in Echinops (Asteraceae, Cynareae)

Area of Science:

• Plant Biology
• Developmental Biology
• Biochemistry

Background:

• The exine, or pollen wall, is crucial for plant reproduction and species-specific identification.
• Echinops (Asteraceae) possesses a remarkably complex exine, suggesting significant developmental investment.

Purpose of the Study:

• To investigate the detailed organization and developmental process of the massive sporoderm in two Echinops species.
• To elucidate the self-assembly mechanisms and sporopollenin polymerization involved in exine formation.

Main Methods:

• Transmission electron microscopy (TEM) of pollen development.
• Scanning electron microscopy (SEM) of pollen structure.

Main Results:

• Detailed sequential stages of exine deposition were observed, involving micellar mesophases and sporopollenin incorporation.
• Specific structures like spherical units, dendritic networks, and alveoli were identified during sporoderm development.
• The formation of the foot layer and endexine lamellae was documented in the free microspore stage.

Conclusions:

• Pollen wall morphogenesis in Echinops involves a sophisticated interplay between biological processes and physicochemical factors.
• The elaborate glycocalyx self-assembles into a multi-layered system facilitating sporopollenin polymerization for complex exine patterns.