Orientation of wheat seedling organs in relation to gravity

LAZY Genes Mediate the Effects of Gravity on Auxin Gradients and Plant Architecture

A rice (Oryza sativa) mutant led to the discovery of a plant-specific LAZY1 protein that controls the orientation of shoots. Arabidopsis (Arabidopsis thaliana) possesses six LAZY genes having spatially distinct expression patterns. Branch angle phenotypes previously associated with single LAZY genes were here studied in roots and shoots of single and higher-order atlazy mutants. The results identify the major contributors to root and shoot branch angles and gravitropic behavior of seedling hypocotyls and primary roots. AtLAZY1 is the principal determinant of inflorescence branch angle. The weeping inflorescence phenotype of atlazy1,2,4 mutants may be due at least in part to a reversal in the gravitropism mechanism. AtLAZY2 and AtLAZY4 determined lateral root branch angle. Lateral roots of the atlazy2,4 double mutant emerged slightly upward, approximately 10° greater than perpendicular to the primary root axis, and they were agravitropic. Etiolated hypocotyls of the quadruple atlazy1,2,3,4 mutant were essentially agravitropic, but their phototropic response was robust. In light-grown seedlings, the root of the atlazy2,3,4 mutant was also agravitropic but when adapted to dim red light it displayed a reversed gravitropic response. A reversed auxin gradient across the root visualized by a fluorescent signaling reporter explained the reversed, upward bending response. We propose that AtLAZY proteins control plant architecture by coupling gravity sensing to the formation of auxin gradients that override a LAZY-independent mechanism that creates an opposing gravity-induced auxin gradient.

Auxin controls gravitropic setpoint angle in higher plant lateral branches

Lateral branches in higher plants are often maintained at specific angles with respect to gravity, a quantity known as the gravitropic setpoint angle (GSA) [1]. Despite the importance of GSA control as a fundamental determinant of plant form, the mechanisms underlying gravity-dependent angled growth are not known. Here we address the central questions of how stable isotropic growth of a branch at a nonvertical angle is maintained and of how the value of that angle is set. We show that nonvertical lateral root and shoot branches are distinguished from the primary axis by the existence of an auxin-dependent antigravitropic offset mechanism that operates in tension with gravitropic response to generate angled isotropic growth. Further, we show that the GSA of lateral roots and shoots is dependent upon the magnitude of the antigravitropic offset component. Finally, we show that auxin specifies GSA values dynamically throughout development by regulating the magnitude of the antigravitropic offset component via TIR1/AFB-Aux/IAA-ARF-dependent auxin signaling within the gravity-sensing cells of the root and shoot. The involvement of auxin in controlling GSA is yet another example of auxin's remarkable capacity to self-organize in development [2] and provides a conceptual framework for understanding the specification of GSA throughout nature.

Growth and epinasty of marigold plants maintained from emergence on horizontal clinostats

Dry weight, leaf number, and leaf size of marigold plants (Tagetes patula) grown from emergence for 18 days on horizontal clinostats rotating at 15 revolutions per hour (rph), were similar to those of plants grown for the same period on vertically oriented clinostats rotating at 15 rph. The horizontally grown plants exhibited some epinasty which disappeared when plants were placed upright for 24 hours. Vertically grown plants when placed on horizontal clinostats for 24 hours exhibited more epinasty than plants grown from emergence on horizontal clinostats.Data are provided to demonstrate that leaves undergo movement (bending) during each rotation cycle that leads to the development of a leaf curvature that is oriented away from the direction of rotation. The results of this study suggest that epinasty of plants placed on horizontal clinostats could be due to uncontrolled movement of plants during rotation rather than controlled by gravity nullification. The usefulness of horizontal clinostats for gravity nullification or simulating weightlessness on plants is questioned.

Limitation on the use of the horizontal clinostat as a gravity compensator

If the horizontal clinostat effectively compensates for the influence of the gravity vector on the rotating plant, it should make the plant unresponsive to whatever chronic acceleration may be applied transverse to the axis of clinostat rotation. This was tested by centrifuging plants while they were growing on clinostats. For a number of morphological end-points of development the results depended on the magnitude of the applied g-force. Therefore, gravity compensation by the clinostat was incomplete. This conclusion is in agreement with results of satellite experiments which are reviewed.

Auxin control for orientation of pea roots grown on a clinostat or exposed to ethylene

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Choice of rotation rate for the horizontal clinostat

A series of nine rates of rotation of the clinostat were tested to determine optimal and acceptable conditions for simulating weightlessness in plants. Young seedlings of wheat (Triticum aestivum L.) developed roots and coleoptiles of equal lengths and with the same orientation angles over a range of rotation rates from 0.25 to 480 minutes per revolution. Rates from 0.25 to 3 minutes per revolution provided for maximal epinastic curvatures of leaves and branches of Coleus blumei Benth. except for a reduced mean curvature of branches at 0.25 minute per revolution, due probably to physical disturbances in their growth. Smaller epinastic curvatures developed in both leaves and branches rotated at 15 minutes per revolution or more slowly. Indoleacetic acid-2-(14)C was used for measurements of extractable radioactivity in determining the reason for smaller curvatures of Coleus branches and tomato (Lycopersicon esculentum Mill.) leaves at the rotation rate of 60 minutes per revolution than at 1 minute per revolution. The cause was determined to be movement of some auxin from the upper into the lower side of a plagiotropic leaf or branch under the transient influence of gravity during rotation on the slower clinostat. A rotation period of 1 to 3 minutes was found to be acceptable for most plants.

Wheat seedling growth in the absence of gravitational force

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[Experiments in the analysis of the positive and the negative geotropic reactions of primary roots]

In the present work the influence of moist air, of sand and of several solutions on the geotropic behaviour of primary roots is studied. The course of the geotropic movement is the result of a concerted action of positive and negative reactions the intensity and duration of which differ in roots of various species.In pea roots the negative movement appears only during a short stage of development. No direct relation exists between the speed of elongation and the appearance of the negative reaction.Primary roots of Zea mays and of Pisum arvense are indifferent to thigmotropic stimuli. The negative movement has, at least in pea roots a smaller mechanically effective force than the positive movement. Therefore the negative reaction does not appear in sand because it cannot overcome the mechanical resistance of the granular medium.In liquid media pea roots react in another way than in air: here the negative reaction begins later, but it has then more influence on the course of the geotropic curvature.The influence of different cations on the geotropic behaviour and on the elongation of the roots can be understood as a combined action of the osmotic effect and of the specific ionic permeability.In pea roots the negative reaction, which appears during the time from 3 to 6 hours after the induction also depends on a definite level of turgor.Primary roots of Zea mays, which grow in a relatively large angle to the vertical line do not lose their geotropic sensibility. They react like plagiotropic organs.In pea roots relations exist between the development of the positive and the negative reaction and the presence of the cotyledons and the tip of the root.Both reactions are induced at the same time by the gravitional stimulus. Their reaction times, however, are different.The root tip is necessary for the induction of both reactions. The negative curvature also appears when the tip is cut off before the end of the reaction time.The course of the geotropic movement of primary roots is compared with the geotropic behaviour of rhizomes. As a possible explanation of both kinds of reactions a two-hormone-hypothesis is discussed.

Rotation axes for analysis of gravity effects on plant organs

Epinastic curvatures of branches of Coleus blumei Benth. and the growth pattern of wheat (Triticum aestivum L.) seedlings on clinostats were used for bioassay of rotation methods for preventing growth responses to gravity. Tumbling a plant end over end was found to be just as effective as rotation about its horizontal axis. The results support the reliability of data from experiments in which an entire plant is rotated about a single horizontal line with only part or none of its immature tissues in horizontal orientation to gravity.