Tapetum uncommon behavior, orbicule development, and pollenkitt: mini-review, with new data on orbicule simulations

Area of Science:

• Plant reproductive biology
• Palynology
• Cellular biology

Background:

• The tapetum, a nutritive layer in anthers, and orbicules (pollenkitt precursors) play crucial roles in pollen development.
• Understanding their structure, function, and formation is key to comprehending pollen fertility and evolutionary adaptations.
• Existing knowledge presents diverse tapetum types and orbicule morphologies, necessitating a consolidated review.

Purpose of the Study:

• To synthesize current knowledge on the tapetum, orbicules, and pollenkitt.
• To trace the historical development of ideas regarding these structures.
• To present novel experimental data on orbicule formation and its implications.

Main Methods:

• Comprehensive literature review of tapetum and orbicule research.
• Analysis of diverse tapetum types and orbicule development.
• In vitro simulation experiments mimicking microspore exine formation.

Main Results:

• Tapetum types exhibit significant variability, challenging the concept of a 'norm'.
• Cyclic-invasive tapeta, exine-like tapetal surfaces, and tapetum-microspore connections appear common.
• Experimental simulations produced orbicule-like structures, supporting their by-product nature via self-assembly.

Conclusions:

• The formation of orbicules may be a self-assembly process, not solely a directed developmental pathway.
• Tapetum and orbicule functions, particularly in pollenkitt and sporopollenin production, are complex and interconnected.
• Further molecular and genetic studies are needed to fully elucidate these processes.