Chang Hua-Chen, Chen I-Chian, Chen Jhun-Chen, Hou Yueh-Ju, Fang Su-Chiung
Biotechnology Center in Southern Taiwan, Academia Sinica, Tainan, Taiwan.
Department of Life Sciences, National University of Kaohsiung, Kaohsiung, Taiwan.
Physiol Plant. 2025 Sep-Oct;177(5):e70505. doi: 10.1111/ppl.70505.
Epiphytic orchids have evolved specialized adaptive strategies, such as aerial roots with water-absorbing velamen tissues, to cope with water-scarce and nutrient-deficient habitats. Our previous study revealed that the aerial roots of the epiphytic orchid Phalaenopsis aphrodite lack a gravitropic response, raising the possibility that alternative tropic mechanisms may contribute to their adaptation. In this study, we examined the effects of light and moisture on aerial root growth in P. aphrodite. Surprisingly, we found that light had no effect on root growth orientation. In contrast, localized moisture gradients consistently directed the growth of young aerial roots, indicating a hydrotropic response. To explore the underlying regulatory mechanisms, we performed hormone profiling of hydrostimulated root tissues. Our data showed that indole-3-acetic acid (IAA), salicylic acid (SA), jasmonic acid (JA), and abscisic acid (ABA) levels did not differ significantly between the water-facing and air-facing sides of the roots, suggesting other mechanisms may regulate this hydrotropism. In summary, our findings demonstrate that hydrotropism, rather than phototropism or gravitropism, guides aerial root growth in P. aphrodite. This hydrotropic response may represent a key adaptation that enables epiphytic orchids to effectively acquire water in the forest canopy.
附生兰花已经进化出专门的适应性策略，比如具有能吸收水分的根被组织的气生根，以应对缺水和养分缺乏的栖息地。我们之前的研究表明，附生兰花蝴蝶兰的气生根缺乏向地性反应，这增加了其他向性机制可能有助于其适应环境的可能性。在本研究中，我们研究了光照和湿度对蝴蝶兰气生根生长的影响。令人惊讶的是，我们发现光照对根的生长方向没有影响。相反，局部湿度梯度始终引导着幼嫩气生根的生长，表明存在向水性反应。为了探究潜在的调控机制，我们对受水刺激的根组织进行了激素分析。我们的数据显示，根的向水侧和向空气侧的吲哚 - 3 - 乙酸（IAA）、水杨酸（SA）、茉莉酸（JA）和脱落酸（ABA）水平没有显著差异，这表明可能有其他机制调节这种向水性。总之，我们的研究结果表明，在蝴蝶兰中，引导气生根生长的是向水性，而非向光性或向地性。这种向水性反应可能是一项关键的适应性特征，使附生兰花能够在树冠层有效地获取水分。
