Where's the water? Hydrotropism in plants

Plants are constantly bombarded with sensory inputs and receive numerous biotic and abiotic signals from their environment. Abiotic signals include gravity, light, water, temperature, oxygen, and carbon dioxide as well as other gases. One way in which plants deal with these inputs is by tropistic growth (or tropism), which is directed growth in response to a stimulus. A tropism is generally termed “positive” if growth is toward the signal and “negative” if it is away from the signal. For example, stems usually exhibit positive phototropism, because they grow toward light. One of the lesser-known tropisms is hydrotropism, directed growth in response to water or moisture gradients. Even though hydrotropism had been studied in plant roots by 19th century German botanists (reviewed in refs. 1 and 2) and by the Darwins (3), the existence of this tropism has been questioned until recent years. The paper by Kobayashi et al. (4) in this issue of PNAS uses an interesting system to study hydrotropism and identifies a novel gene in the hydrotropism pathway in roots.

Tropisms frequently interact between and among each other, and the final growth form of the plant is influenced by such interactions. A good example is the interplay between light and gravity in determining the directional growth of a stem (5). Thus, in shoots, interactions between positive phototropism and negative gravitropism determine the direction of growth in young seedlings (6). Less known is the observation that the orientation of root growth in many plant species is shaped by the interaction between positive gravitropism and negative phototropism (7, 8). In fact, it has been difficult to study phototropism in roots because of the overpowering effects of gravity in modulating the growth of this organ (9, 10). In a similar way, one of the main difficulties of studying hydro …

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