Hydrotropism in Plants

COMMENTARY Where’s the water? Hydrotropism in plants John Z. Kiss* Department of Botany, Miami University, Oxford, OH 45056 lants are constantly bombarded gravity, and the microgravity environ- with ground conditions (20, 21). In fact, with sensory inputs and receive ment of spaceflight. One of the first in the only available microgravity study, numerous biotic and abiotic sig- modern papers to convincingly confirm Takahashi and coworkers (19) found that nals from their environment. the existence of hydrotropism used a in their space experiments with cucumber PAbiotic signals include gravity, light, pea mutant lacking both gravitropism seedlings, lateral roots exhibited a positive water, temperature, oxygen, and carbon and phototropism in roots (12). These hydrotropic response (not apparent on the dioxide as well as other gases. One way authors also suggested that the early- ground) that was stronger in those roots in which plants deal with these inputs is perception phase of hydrotropism occurs closer to the wet substrate. by tropistic growth (or tropism), which in the root cap, because roots grow nor- Another obstacle to studying root hy- is directed growth in response to a stim- mally but do not respond to moisture drotropism is the difficulty of setting up ulus. A tropism is generally termed gradients after removal of the cap. a system in which there is a reproduc- ‘‘positive’’ if growth is toward the signal Later, Takahashi et al. (13) used a ible moisture gradient. The classical and ‘‘negative’’ if it is away from the starchless mutant of Arabidopsis, which methods of the German botanists (re- signal. For example, stems usually ex- has a greatly reduced sensitivity to grav- viewed in refs. 1 and 12), also used by hibit positive phototropism, because ity compared with WT (14, 15), to show the Darwins (3) included placing seeds they grow toward light. One of the less- it is enhanced in root hydrotropism. in a hanging cylinder of wet sawdust, er-known tropisms is hydrotropism, di- Another approach to studying hydro- which resulted in roots first growing rected growth in response to water or tropism is to use instruments to alter downward (gravitropism) but then grow- moisture gradients. Even though hydrot- ing back up toward the wet substrate ropism had been studied in plant roots (hydrotropism). Several other approaches by 19th century German botanists (re- Orientation of root were used, but the first to measure mois- viewed in refs. 1 and 2) and by the Dar- ture gradients was Hooker in 1915 (22), wins (3), the existence of this tropism growth is shaped by who made a hygrometer to measure rela- has been questioned until recent years. tive humidity at two points. The paper by Kobayashi et al. (4) in this the interaction between Modern assays to study hydrotropism issue of PNAS uses an interesting sys- have been developed by two groups, tem to study hydrotropism and identifies positive gravitropism who also used these methods as screens a novel gene in the hydrotropism path- for isolating novel mutants of hydrotro- way in roots. and negative pism in Arabidopsis roots (23, 24). Tropisms frequently interact between Eapen et al. (24) used a Petri dish with and among each other, and the final phototropism. a normal nutrient agar medium at the growth form of the plant is influenced top of the dish and an agar with a by such interactions. A good example is water-stressed medium in the bottom of the interplay between light and gravity the direction of the gravity vector re- the dish. The water-stressed medium in determining the directional growth of ceived by the plants. Although it is not was made by adding glycerol and alginic a stem (5). Thus, in shoots, interactions possible to eliminate the effect of grav- acid as osmolytes to the agar. WT seed- between positive phototropism and neg- ity on Earth, clinostats rotate plants lings would grow downward (gravitro- ative gravitropism determine the direc- around an axis or, in some cases, three tion of growth in young seedlings (6). dimensions in an attempt to neutralize pism) for several days until the roots Less known is the observation that the gravity’s effects. The most commonly neared the agar with the lower water orientation of root growth in many plant used varieties are the one-axis slow- potential, and then the roots would species is shaped by the interaction be- rotating (1–4 rpm) clinostat and the grow upward toward the higher water tween positive gravitropism and negative two-axis or 3D clinostat, which also is potential. Using this screen, the authors phototropism (7, 8). In fact, it has been termed the random positioning machine identified a semidominant mutant difficult to study phototropism in roots (16, 17). In fact, hydrotropism in roots termed no hydrotropic response (NHR). because of the overpowering effects of was more readily apparent when pea Although the exact nature of NHR is gravity in modulating the growth of this and cucumber seedlings were grown on not known, the results suggest that the organ (9, 10). In a similar way, one of a rotating clinostat (18, 19), and the plant hormone abscisic acid is involved the main difficulties of studying hydro- study by Kobayashi et al. (4) used a cli- in water sensing by the root cap. tropism in roots has been that roots are nostat to optimize the hydrotropic re- In another approach, the Takahashi strongly gravitropic, which seems to sponse in Arabidopsis seedlings. group (13, 23) used a closed acrylic overwhelm any hydrotropic response (1, An even more interesting approach to chamber (Fig. 1) with seedlings growing 11). Thigmotropism, directed growth in studying root hydrotropism is to use the on agar (at the top of the container) response to touch, also may interfere with microgravity conditions present during and with an open container with a satu- the expression of hydrotropism (12). spaceflight. The idea is that, in the ab- rated solution of salts (on the bottom). There are several ways to overcome sence of significant gravitational forces, the challenges of studying root hydrotro- the overriding gravitropic responses of pism, and these methods involve overrid- roots are effectively negated, so that other Author contributions: J.Z.K. wrote the paper. ing the competing gravitropic response. root tropisms become more apparent and The author declares no conﬂict of interest. Some of these techniques include the easier to study. This concept has worked See companion article on page 4724. use of agravitropic mutants, ground- well in terms of phototropism, which can *E-mail: [email protected]. based instruments to mitigate unilateral be stronger in microgravity compared © 2007 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0700846104 PNAS ͉ March 13, 2007 ͉ vol. 104 ͉ no. 11 ͉ 4247–4248 Downloaded by guest on September 25, 2021 Thus, a moisture gradient was formed TW PW conserved among terrestrial plants from between the agar and the saturated salt mosses to monocots to dicots. Interest- solution in the container. The gradient A A rag ingly, the MIZ domain is absent in cya- was measured with a hygrometer, and nobacteria, algae, and animals. Thus, the greater the distance from the con- although they do not know the exact tainer, the greater the relative humidity g function of this gene, the authors (4) (i.e., moisture). At the beginning of the EGDE propose that MIZ1 played an important experiment, roots of seedlings on the role in the adaptation to terrestrial life agar grew toward the bottom (gravitro- riA for plants. pism), but then, after they reached the emiT The Darwins first hypothesized that edge of the agar, grew upward toward the root cap was the brain of the grow- the higher level of moisture (hydrotro- ing root (3). More modern studies have atuM nt PW pism; Fig. 1A). An explicit discussion of confirmed that the root cap is specifi- the differences between the two meth- B ragA cally involved in the sensing of gravity ods to assay for hydrotropism and the (26), touch (27), and light (10, 28). In- potential advantages/disadvantages of terestingly, Kobayashi et al. (4) report each system is provided in the review by g that the ultrastructure of root caps of Eapen et al. (2). EDGE miz1 mutants is identical to those of In the latest paper by the Takahashi WT plants and that the MIZ1-GUS fu- group (4), the authors describe a novel riA sion gene was strongly expressed in the mutant miz1, which is impaired in hy- columella cells of the root cap. These drotropism (Fig. 1B). The term miz T emi results suggest that the root cap plays a comes from ‘‘mizu-kussei,’’ which means key role in sorting out the signals re- ‘‘water tropism’’ in Japanese. The miz1 Fig. 1. Illustration of the vertically oriented (g, gravity vector) experimental apparatus used by ceived from the environment (gravity, mutant has normal gravitropism and a Kobayashi et al. (4) to study hydrotropism in roots touch, light, and water) and in deter- normal growth rate but reduced root of (A) WT and (B) mutant Arabidopsis seedlings. phototropism and a modified wavy mining the final direction of root When seedlings are grown on agar, the water po- growth. growth response (described in ref. 25). tential (WP) is high. Roots then grow beyond the Thus, in addition to its role in hydrotro- edge of the agar into humid air with a moisture This research has improved our under- pism, MIZ1 could have a role in differ- gradient as shown. Roots of WT seedlings (A) grow standing of hydrotropism, a phenomenon ent steps in the phototropism and wavy downward to the edge and then upward toward whose existence was doubted by some growth response pathways.

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