RESUMEN
Key message: Pollen maturation in Poaceae. Another development has been extensively examined by various imaging tools, including transmission electron microscopy, scanning electron microscopy, and light microscopy, but none is capable of identifying liquid water. Cryo-scanning electron microscopy with high-pressure rapid freeze fixation is excellent in preserving structures at cellular level and differentiating gas- versus liquid-filled space, but rarely used in anther study. We applied this technique to examine anther development of Poaceae because of its economic importance and unusual peripheral arrangement of pollen. Maize and longstamen rice were focused on. Here, we report for the first time that anthers of Poaceae lose the locular free liquid during late-microspore to early pollen stages; the majority of pollen grains arranged in a tight peripheral whorl develops normally and reaches maturity in the gas-filled loculus. Occasionally, pollen grains are found situated in the locular cavity, but they remain immature or become shrunk at anthesis. At pollen stage, microchannels and cytoplasmic strands are densely distributed in the entire pollen exine and intine, respectively, suggesting that nutrients are transported into the pollen from the entire surface. We propose that in Poaceae, the specialized peripheral arrangement of pollen grains is crucial for pollen maturation in the gas-filled loculus, which enables pollen achieving large surface contact area with the tapetum and neighboring grains to maintain sufficient nutrient flow. This report also shows that the single aperture of pollen in Poaceae usually faces the tapetum, but other orientation is also common; pollen grains with different aperture orientations show no morphological differences.