The pollination drop (PD), also known as an ovular secretion, is a critical feature of most wind-pollinated gymnosperms and function as an essential component of pollination systems. However, the metabolome and small RNAs of gymnosperm PDs are largely unknown. We employed gas chromatography–mass spectrometry to identify a total of 101 metabolites in Ginkgo biloba L. PDs. The most abundant metabolites were sugars (45.70%), followed by organic acids (15.94%) and alcohols (15.39%) involved in carbohydrate metabolism, glycine, serine and threonine metabolism. Through pollen culture of the PDs, we further demonstrated that the metabolic components of PDs are indispensable for pollen germination and growth; in particular, organic acids and fatty acids play defensive roles against microbial activity. In addition, we successfully constructed a small RNA library and detected 45 known and 550 novel miRNAs in G. biloba PDs. Interestingly, in a comparative analysis of miRNA expression between PDs and ovules, we found that most of the known miRNAs identified in PDs were also expressed in the ovules, implying that miRNAs in PDs may originate from ovules. Further, combining with potential target prediction, degradome validation and transcriptome sequencing, we identified that the interactions of several known miRNAs and their targets in PDs are involved in carbohydrate metabolism, hormone signaling and defense response pathways, consistent with the metabolomics results. Our results broaden the knowledge of metabolite profiling and potential functional roles in gymnosperm PDs and provide the first evidence of extracellular miRNA functions in ovular secretions from gymnosperms.

中文翻译：

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植物中的液体概况分析：银杏授粉滴中细胞外代谢物和miRNA的鉴定和分析。

授粉滴（PD），也称为卵子分泌物，是大多数风授粉裸子植物的关键特征，并且是授粉系统的重要组成部分。然而，裸子植物PD的代谢组和小RNA在很大程度上是未知的。我们采用气相色谱-质谱法鉴定了银杏叶中总共101种代谢物L. PD。代谢最丰富的是糖（45.70％），其次是参与碳水化合物代谢，甘氨酸，丝氨酸和苏氨酸代谢的有机酸（15.94％）和醇（15.39％）。通过PD的花粉培养，我们进一步证明PD的代谢成分对于花粉萌发和生长是必不可少的。尤其是有机酸和脂肪酸对微生物的活性起防御作用。此外，我们成功构建了一个小的RNA文库，并在银杏假单胞菌中检测到45种已知和550种新颖的miRNA 。PD。有趣的是，在PD和胚珠之间的miRNA表达的比较分析中，我们发现PD中鉴定的大多数已知miRNA也在胚珠中表达，这暗示PD中的miRNA可能源自胚珠。此外，结合潜在的靶标预测，降解组验证和转录组测序，我们确定了几种已知的miRNA及其在PD中的靶标之间的相互作用涉及糖代谢，激素信号传导和防御反应途径，与代谢组学结果一致。我们的结果拓宽了裸子植物PD中代谢物谱分析和潜在功能作用的知识，并提供了裸子植物卵子分泌物中细胞外miRNA功能的第一个证据。