Water as a Compass: Hydrostimulation-Triggered Aerial Root Growth in Phalaenopsis aphrodite

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.