Pollen development of Mitrephora tomentosa (Annonaceae) with coexistence of bicellular and tricellular pollen during anther dehiscence

Various organelles change their morphological structure and function during development as male gametophytes. The coexistence of bicellular and tricellular pollen types, a rare feature among angiosperms, positions Mitrephora tomentosa as an intriguing model for studying organelle dynamics during the final stages of pollen development. Using transmission electron microscopy (TEM), we observed the dynamic changes of ultrastructure and number in endoplasmic reticulum, plastids, vacuoles and mitochondria during M. tomentosa pollen development. Striking ultrastructural differences in these organelles, e.g. endoplasmic reticulum, plastids, vacuoles and mitochondria between binucleate and trinucleate pollens suggested that the binucleate pollen in M. tomentosa is shed in a metabolically active and highly hydrated state, representing an intermediate developmental stage during pollen maturation. Asynchrony in pollen development within the anther likely caused the coexistence of binucleate and trinucleate pollen in M. tomentosa. We propose that the year-round stable tropical climate – characterised by high temperature and humidity – provides an accommodative environment for incompletely dehydrated pollen, i.e. bicellular pollen, to undergo a second mitosis post anther dehiscence in M. tomentosa.