Department of Plant Science, McGill University, Macdonald Campus, 21111 Lakeshore, Ste-Anne-de-Bellevue, Québec, H9X 3V9, Canada.
Plant Reprod. 2023 Jun;36(2):157-171. doi: 10.1007/s00497-023-00458-7. Epub 2023 Jan 31.
Callose, a β-1,3-glucan, lines the pollen tube cell wall except for the apical growing region, and it constitutes the main polysaccharide in pollen tube plugs. These regularly deposited plugs separate the active portion of the pollen tube cytoplasm from the degenerating cell segments. They have been hypothesized to reduce the total amount of cell volume requiring turgor regulation, thus aiding the invasive growth mechanism. To test this, we characterized the growth pattern of Arabidopsis callose synthase mutants with altered callose deposition patterns. Mutant pollen tubes without callose wall lining or plugs had a wider diameter but grew slower compared to their respective wildtype. To probe the pollen tube's ability to perform durotropism in the absence of callose, we performed mechanical assays such as growth in stiffened media and assessed turgor through incipient plasmolysis. We found that mutants lacking plugs had lower invading capacity and higher turgor pressure when faced with a mechanically challenging substrate. To explain this unexpected elevation in turgor pressure in the callose synthase mutants we suspected that it is enabled by feedback-driven increased levels of de-esterified pectin and/or cellulose in the tube cell wall. Through immunolabeling we tested this hypothesis and found that the content and spatial distribution of these cell wall polysaccharides was altered in callose-deficient mutant pollen tubes. Combined, the results reveal how callose contributes to the pollen tube's invasive capacity and thus plays an important role in fertilization. In order to understand, how the pollen tube deposits callose, we examined the involvement of the actin cytoskeleton in the spatial targeting of callose synthases to the cell surface. The spatial proximity of actin with locations of callose deposition and the dramatic effect of pharmacological interference with actin polymerization suggest a potential role for the cytoskeleton in the spatial control of the characteristic wall assembly process in pollen tubes.
胼胝质是一种β-1,3-葡聚糖，排列在花粉管细胞壁上，除了顶端生长区域外，它构成花粉管塞的主要多糖。这些定期沉积的塞子将花粉管细胞质的活跃部分与退化的细胞片段隔开。它们被假设可以减少需要膨压调节的细胞体积总量，从而有助于侵袭性生长机制。为了验证这一点，我们对具有改变的胼胝质沉积模式的拟南芥几丁质合酶突变体的生长模式进行了表征。没有细胞壁 lining 或塞子的突变体花粉管直径较宽，但与相应的野生型相比生长速度较慢。为了探究在没有胼胝质的情况下花粉管进行坚韧生长的能力，我们进行了机械测定，例如在刚性培养基中生长，并通过初始质壁分离评估膨压。我们发现，缺乏塞子的突变体在面临机械挑战的基质时，入侵能力较低，膨压较高。为了解释几丁质合酶突变体中膨压升高的这种意外情况，我们怀疑这是由于细胞壁中去酯化果胶和/或纤维素的反馈驱动水平增加所导致的。通过免疫标记，我们验证了这一假设，并发现这些细胞壁多糖的含量和空间分布在缺乏胼胝质的突变体花粉管中发生了改变。综合起来，这些结果揭示了胼胝质如何有助于花粉管的侵袭能力，因此在受精过程中起着重要作用。为了了解花粉管如何沉积胼胝质，我们研究了肌动蛋白细胞骨架在几丁质合酶空间靶向到细胞表面的过程中的参与情况。肌动蛋白与胼胝质沉积位置的空间接近性以及用药理学方法干扰肌动蛋白聚合的剧烈影响表明，细胞骨架在花粉管中特征性细胞壁组装过程的空间控制中可能发挥作用。
