Photobiology of Diagravitropic Maize Roots

Abstract

Light-induced modification of gravitropism in etiolated roots of Zea nays cv Bear x W38 is a low fluence response mediated by phytochrome. This cultivar has a threshold of 10-' mol m2 and becomes saturated with 10-2 mol m-2 of red light. The maximum light-mediated response of 32 degrees downward from horizontal occurs in roots 10 to 30 millimeters in length, 120 to 165 minutes after irradiation. Reciprocity is valid from 2 to at least 9,000 seconds and the response can be about 90% reversed by far red light. Photoreversibility is lost ('escape' occurs) about 20 minutes after red irradiation but appears to be regained 60 to 80 minutes later. A red light-induced (or synchronized) nutation in the apparent curvature rather than unusual escape characteristics may explain these results.Many roots are diagravitropic in darkness and change their response to gravity, becoming more orthogravitropic when exposed to light (2; review 24). The root cap is apparently the site of photoperception both for this change in gravitropism (14,15,25,26) and a general growth inhibition (16,26 broad band R,4 and determined the reciprocity characteristics and FR reversibility for this phenomenon. The ultimate aim is to elucidate the photobiology that governs directional growth of roots underground.MATERIAIS AND METHODS Zea mays (Bear x W38) caryopses were planted, imbibed, and grown in rows. Two rolls of modeling clay (Little Sculptor, 3M Co.) were positioned along the edges of a 1.5 x 17 cm black Plexiglas strip and about 20 maize caryopses were pressed into the clay so that all the embryos faced upward and in one direction. Each row was partially covered with water and imbibed in absolute darkness for at least 4 h. Styrofoam blocks, covered with absorbent paper (Kimpac, Kimberley-Clark) were placed inside opaque plastic boxes and the bottom of the boxes filled with deionized H20. The paper acted as a water wick for the duration of growth and treatment of the roots. Individual rows of imbibed seeds were placed into a rectangular groove which had been precut from a long edge of the styrofoam block, and the seedlings allowed to grow in the dark (95% humidity, 26C) for 2 d so that the roots grew out over the edge of the styrofoam block and above the water soaked paper. All manipulations were done in the absence of any light.Two-d-old roots were irradiated from above with broad band R or FR according to Mandoli and Briggs (10). After irradiation, seeds were grown for an additional time in the dark and then harvested in white light. Excised roots were arranged on lucite sheets and photocopied. Root lengths and angles were measured with a computerized digitizer (10). All experiments were done three times with 20 to 40 roots per datum point. SE per datum point rarely exceeded ±4°(e.g. Fig.

Interactions between Red Light, Abscisic Acid, and Calcium in Gravitropism

“…In some lines of corn, red light can shift the gravitropic responses ofroots from diagravitropic (seeking the horizontal) to orthogravitropic mode (seeking the direction of the earth) (6,7). The ability of added ABA to substitute for the red light (15), combined with the evidence that red light results in an increase in ABA in the roots (3) suggests that these two factors may influence similar components of the gravitropic system.…”

Light-Regulated Gravitropism in Seedling Roots of Maize

“…Suzuki and Fujii (8) were unable to reverse the promotive effects of R with a subsequent FR (730 nm) light treatment and therefore concluded that the gravitropic response in Zea roots ".may not be controlled by phytochrome." On the other hand, Shen-Miller (7) and later Mandoli et al (3) were able to prevent R-induced curving by following the R with FR. These workers concluded that light-induced gravitropism in maize roots was mediated by phytochrome.…”

“…grow parallel or nearly parallel to the surface) (1,3,6,8). Illuminating these roots induces gravitropic curving, with R2 (660-670 nm) and to a lesser degree B (410-640 nm) particularly effective in promoting downward curving (1,3,(5)(6)(7)(8)(9).…”

Regulation of Phytochrome Message Abundance in Root Caps of Maize

“…6). In a variety of plants, including a number of maize cultivars, root gravitropism is a light-regulated process wherein the primary roots of young, dark-grown seedlings grow plagiotropically (in a horizontal orientation) unless given a light pulse, whereupon they grow orthotropically (in a positively gravitropic fashion) (1 1, 16; earlier references reviewed in ref. 3).…”