A Spaceflight Experiment for the Study of Gravimorphogenesis and Hydrotropism in Cucumber Seedlings

peg , on the transition zone between hypocotyl and root. Our spaceflight experiment verified that the lateral positioning of a peg in cucumber seedlings is modified by gravity. It has been suggested that auxin plays an important role in the gravity-controlled positioning of a peg on the ground. Furthermore, cucumber seedlings grown in microgravity developed a number of the lateral roots that grew towards the water-containing substrate in the culture vessel, whereas on the ground they oriented perpendicular to the primary root growing down. The response of the lateral roots in microgravity was successfully mimicked by clinorotation of cucumber seedlings on the three dimensional clinostat. However, this bending response of the lateral roots was observed only in an aeroponic culture of the seedlings but not in solid medium. We considered the response of the lateral roots in microgravity and on clinostat as positive hydrotropism that could easily be interfered by gravitropism on the ground. This system with cucumber seedlings is thus a useful model of spaceflight experiment for the study of the gravimorphogenesis, root hydrotropism and their interaction. Received 13 September 1999/ Accepted in revised form 12 October 1999     

Hormonal interactions during root tropic growth: hydrotropism versus gravitropism

Terrestrial plants have evolved remarkable morphological plasticity that enables them to adapt to their surroundings. One of the most important traits that plants have acquired is the ability to sense environmental cues and use them as a basis for governing their growth orientation. The directional growth of plant organs relative to the direction of environmental stimuli is a tropism. The Cholodny–Went theory proposes that auxin plays a key role in several tropisms. Recent molecular genetic studies have strongly supported this hypothesis for gravitropism. However, the molecular mechanisms of other tropisms are far less clear. Hydrotropism is the response of roots to a moisture gradient. Since its re-discovery in 1985, root hydrotropism has been shown to be common among higher plant species. Additionally, in some species, gravitropism interferes with hydrotropism, suggesting that both shared and divergent mechanisms mediating the two tropisms exist. This hypothesis has been supported by recent studies, which provide an understanding of how roots sense multiple environmental cues and exhibit different tropic responses. In this review, we focus on the overlapping and unique mechanisms of the hormonal regulation underlying gravitropism and hydrotropism in roots.     

Roots of Pisum sativum L. exhibit hydrotropism in response to a water potential gradient in vermiculite

In the present study, root hydrotropism in an agravitropic mutant of Pisum sativum L. grown in vermiculite with a steep water potential gradient was examined. When wet and dry vermiculite were placed side by side, water diffused from the wet (-0.04 MPa) to the dry (-1.2 MPa) and a steep water potential gradient became apparent in the dry vermiculite close to the boundary between the two. The extent and location of the gradient remained stable between the fourth and sixth day after filling a box with vermiculite, and the steepest gradient (approx. 0.02 MPa mm-1) was found in the initially dry vermiculite between 60 and 80 mm from the boundary. When seedlings with 25-35 mm long roots were planted in the initially dry vermiculite near where the gradient had been established, each of the main roots elongated toward the wet vermiculite, i.e. toward the high water potential. Control roots elongated without curvature in both the wet and the dry vermiculite, in which no water potential gradient was detectable. These results show that pea roots respond to the water potential gradient around them and elongate towards the higher water potential. Therefore, positive hydrotropism occurs in vermiculite just as it does in air. Hydrotropism in soil may be significant when a steep water potential gradient is apparent, such as when drip irrigation is applied.     

Hydrotropism: The current state of our knowledge

The response of roots to a moisture gradient has been reexamined, and positive hydrotropism has been demonstrated in recent years. Agravitropic roots of a pea mutant have contributed to the studies on hydrotropism. The kinetics of hydrotropic curvature, interactions between hydrotropism and gravitropism, moisture gradients required for the induction of hydrotropism, the sensing site for moisture gradients, characteristics of hydrotropic signal and differential growth, and calcium involvement in signal transduction have been subjects of these studies. This review summarizes the current state of our knowledge on hydrotropism in roots.     

Simulation of Growth and Hydrotropism of Maize Roots

A three-dimensional model simulating the formation of root system architecture of maize was designed using object oriented programming (OOP) techniques. The model has been used to simulate the growth of roots in contrasting water profiles with or without gravitropism, and the mechanism of hydrotropism of root system and its relationship with gravitropism has been studied. In this model, the frontier of root system was treated as a population of root tips, each member of which responded individually to its local environment, and only a few of them could branch. The results of simulation showed that hydrotropism of maize roots could arise through the control of the elongation rate of single root by its local soil water potential. The difference in growth rate caused by the gradient of water potential along the soil profile alone could cause the root system as a whole to grow predominantly downwards, resulting in a shift of root distribution towards deeper layers. Gravitropism enhanced the downward predominance of the growth of root system, but the mechanism was different from that of hydrotropism.     

Hydrotropism in roots: sensing of a gradient in water potential by the root cap

Roots of the agravitropic pea (Pisum sativum L.) mutant, ageotropum, responded to a gradient in water potential as small as 0.5 MPa by growing toward the higher water potential. This positive response occurred when a sorbitol-containing agar block was unilaterally applied to the root cap but not when applied to the elongation region. Unilateral application of higher concentrations of sorbitol to the elongation region caused root curvature toward the sorbitol source, presumably because of growth reduction on the water-stressed side. The control blocks of plain agar applied to either the root cap or the elongation region did not cause significant curvature of the roots. These results demonstrate that hydrotropism in roots occurs following perception of a gradient in water potential by the root cap.     

玉米根系生长及向水性的模拟

用面向对象的程序设计（ＯＯＰ）技术组建了玉米（Ｚｅａ ｍ ａｙｓ）根系生长的三维模型,并用该模型模拟了根系在不同土壤水分剖面以及有、无向地性响应时的生长过程,探讨了根系向水性产生的机制以及向水性与向地性之间的关系．模型中将根系前沿看作由根尖构成的群体,每个根尖都对其周围环境独自作出响应,其中只有少数能继续分支． 模拟结果显示,玉米根系各单根受局部水势的影响而以不同速率伸长,即可造成总体的向水性． 如果土壤剖面上的水势由下到上递减,由此引起的单根伸长速率的不均匀分布将使整个根系在总体上表现出收拢和一致向下的生长趋势,并导致下层的根量相对增加．根系的向地性虽能使上述趋势增强,但它与向水性的机制是完全不同的     

Induction of hydrotropism in clinorotated seedling roots of Alaska pea,Pisum sativum L.

Roots of the agravitropic pea (Pisum sativum L.) mutantageotropum show positive hydrotropism, whereas roots of Alaska peas are hydrotropically almost non-responsive. When the gravitropic response was nullified by rotation on clinostats, however, roots of Alaska peas showed unequivocal positive hydrotropism in response to a water potential gradlent. These results suggest that roots of Alaska peas possess normal ability to respond hydrotropically and their weak hydrotropic response results from a counteracting effect of gravitropism.     

A molecular mechanism unique to hydrotropism in roots

Plants are sessile in nature and must respond to various environmental cues to regulate their growth orientation. Root hydrotropism, a response to moisture gradients, has been considered to play an important role in drought avoidance. Nonetheless, the processes underlying hydrotropism in roots have remained obscure until recently because of the interfering effect of gravitropism. To shed light on root hydrotropism, we isolated and analyzed two Arabidopsis mutants, mizu-kussei (miz) 1 and 2, that have abnormal hydrotropic responses but normal responses to gravity. MIZ1 encodes a protein of unknown function with a conserved domain at its C-terminus. MIZ2 encodes a guanine-nucleotide exchange factor for ADP-ribosylation factor-type G proteins, which has been identified as GNOM. These findings suggest that roots possess molecular mechanisms essential for hydrotropism but independent of gravitropism. One of such mechanisms involves vesicle transport unique to hydrotropism in roots. Here we summarize recent progress on the molecular mechanism of root hydrotropism and the roles of MIZ1 and MIZ2.     

Effect of Hydrotropism on Root System Development in Soybean ( Glycine max): Growth Experiments and a Model Simulation

To observe root system development, soybean plants (Glycine max) were grown in root boxes that were set horizontally to reduce the effect of gravity. Along with the root system development, the two-dimensional distribution of soil water content in the root boxes was measured continuously by the time domain reflectometry (TDR) method. Root system development and its morphological architecture were strongly affected by the positions of the water supply. It is suggested that root hydrotropism plays the dominant role in root system development. In addition to root hydrotropism, the importance of root compensatory growth is suggested. A combined model of root system development and soil water flow considering root hydrotropism and compensatory growth was used to simulate root system development and soil water flow. The morphological architecture of root systems and the distribution of soil water content obtained in the experiment were successfully explained by the model simulation. These results confirmed that root hydrotropism and compensatory growth are dominant factors in root system development under a reduced effect of gravity. The validity of the model was confirmed, and its applications for various purposes were suggested.