Evidence for a Gravity-Regulated Level of Endogenous Auxin Controlling Cell Elongation and Ethylene Production during Geotropic Bending in Grass Nodes

Pulvini of excised stem segments from barley (Hordeum vulgare cv 'Larker') were pretreated with 1 millimolar coumarin before gravistimulation to reduce longitudinal cell expansion and exaggerate radial cell enlargement. The cellular localization and pattern of graviresponse across individual pulvini were then evaluated by cuffing the organ in cross-section, photographing the cross-section, and then measuring pulvinus thickness and the radial width of cortical and epidermal cells in enlargements of the photomicrographs. With respect to orientation during gravistimulation, we designated the uppermost point of the crosssection 0°and the lowermost point 1800. A gravity-induced increase in pulvinus thickness was observable within 400 of the vertical in coumann-treated pulvini. In upper halves of coumarintreated gravistimulated pulvini, cells in the inner cortex and inner epidermis had increased radial widths, relative to untreated gravistimulated pulvini. In lower halves of coumarin-treated pulvini, cells in the central and outer cortex and in the outer epidermis showed the greatest increase in radial width. Cells comprising the vascular bundles also increased in radial width, with this pattem following that of the central cortex. These results indicate (a) that all cell types are capable of showing a graviresponse, (b) that the graviresponse occurs in both the top and the bottom of the responding organ, and (c) that the magnitude of the response increases approximately linearly from the uppermost point to the lowermost. These results are also consistent with models of gravitropism that link the pattern and magnitude of the graviresponse to graviperception via statolith sedimentation.

Section: Discussionsupporting

confidence: 83%

Localization and Pattern of Graviresponse across the Pulvinus of Barley Hordeum vulgare

Brock¹,

Lu²,

Ghosheh³

et al. 1989

“…For the most part, growth induced by gravistimulation has been assumed to be caused by an increase in auxin levels in the lower side of responding tissues. The mechanism by which such an asymmetrical distribution ofauxin would occur is not clear, although involvements of differential auxin biosynthesis (18), differential metabolism (i.e. conjugation) (5) or lateral auxin movement (16) …”

Section: Discussionmentioning

confidence: 99%

Investigations on the Mechanism of the Brassinosteroid Response

Meudt¹

1987

Brassinosteroids are steroidal lactones of plant origin that promote growth of a number of plant systems, and particularly the growth induced by auxins. Biologically active brassinosteroids (BR) also promote the growth of gravisensitive hypocotyls of 7-day-old light grown Phaseolus vulgaris when gravistimulated. Brassinolide-mediated promotion of curvature of gravistimulated internodes occurs in the absence of exogenously supplied indole-3-acetic acid (IAA). This is in contrast to the BRpromoted bending of vertically positioned bean hypocotyls, which is dependent upon exogenous IAA. Brassinosteroid treatment increased the graviperception of young internode tissues and the bending of the gravistimulated sections as well as the subsequent reversal of bending after the sections were placed vertically. These results indicate that BR sensitizes bean hypocotyls to gravistimulation and potentiates the action of a growth factor that induces gravitropic growth.Brassinolide is a steroidal lactone first isolated from rape (Brassica nupus L.) pollen in 1970 (14) and chemically identified in 1979 (7). A total of 11 biologically active analogs of brassinolide have since been isolated primarily from immature parts of a variety of plants (1-4, 8, 11, 15, 19-22). Although a physiological function for BR' has yet to be assigned, they stimulate growth when applied exogenously to green plant tissues (13,14,17 Chemicals Used. Indole-3-acetic acid (IAA) was from Sigma. The biologically active 22i3,23j3-dihydroxy-24a-methyl isomer, BR-1 105, and the biologically inactive 2(3,3#3-dihydroxy isomer, BR-940, of brassinolide (17) were a gift of Mr. Malcolm Thompson of the Insect Physiology Laboratory, Plant Protection Institute, Beltsville Agricultural Research Center. Brassinosteroids and IAA were first dissolved in ethanol, and 10 ml were blotted onto filter paper disks, and, after drying, the disks were either transferred to 10 mm phosphate buffer and used as a dip or the disks were applied directly to one side of hypocotyls (12).Bioassay. Preliminary experiments showed that hypocotyls of young bean plants respond well to gravitropic stimulation, as long as the length of the second internode was no longer than 5 mm. Increasing the length of second internodes seemed to decrease the sensitivity of the first internode and concomitantly shift the sensitivity from the lower portion of the section to the apical portion of the internode. Removal of the apical meristem, however, does not change the gravitropic response, suggesting that the gravity-perceiving region and subsequent gravitationally related growth response of the bean hypocotyl is independent of the apices of the young bean seedlings. Based on these findings, isolated hypocotyls were used throughout the experiments described. Four-cm-long bean hypocotyls were cut from below the second node and above the first node and placed vertically, apical portion up, into scintillation vials and held in place by a piece of sponge saturated with 10 mm phosphate buffer. The apical porti...

“…Since the dry weight was also low, the 80%o methanolic extract of grouped plant part halves was subjected directly to sio2 partition column chromatography (9) without further purification. Acidic, ethyl acetate soluble dpm were eluted in the first 26 fractions and the "highly water-soluble" dpm (probably glucosyl esters and glucosides of the [3HJGA4 precursor and/or its metabolites) were eluted by washing the column with absolute methanol. Estimates of dpm were made by liquid scintilPlant Physiol.…”

Section: Methodsmentioning

confidence: 99%

“…10), including peanut shoots (10), sunflower buds (21), maize coleoptiles (22), and for conifer shoots (3,18,19), which indicate that increased levels of GAs, present endogenously, or applied exogenously, are associated with, and indeed appear to control, the differential growth that allows plant shoots to respond to geostimulation by growing upright. Auxins are also implicated in many of these systems, including the Avena shoot system (12,13), as is ethylene (3,17,26), the latter perhaps being the first growth regulator involved in sensing the environmental stimulus and/or signaling the receptor tissue [secondary cambial meristem (3) and p-l node-pulvinus/internode (Kaufman, Reid, and Pharis, unpublished)].…”

Section: Effects Of Geostimulation On the Distribution Of Dry Mattermentioning

confidence: 99%

Changes in Endogenous Gibberellins and the Metabolism of [³H]GA₄ after Geostimulation in Shoots of the Oat Plant (Avena sativa)

Pharis¹,

Legge²,

Noma³

et al. 1981

The recovery from "lodging," or bending over, by shoots of 42-day-old Avena sativa plants is controlled primarily by a negatively geotropic differential growth of the lower halves of the p-I node-pulvinus and the base of the p-I internode, relative to the upper halves. Although geostimulation causes a significant reduction in p-1 internode length, dry matter accumulation in the p-i node-pulvinus is increased, apparently at the expense of the sheath. Recovery to an angle of 300 is associated with changes in endogenous gibberellin-like substances (GAs), and in differential metabolism of applied I3HIGA4 (1.4 Curie per millimole). Although geostimulation depressed total GAs (relative to upright plant parts) to 0.40 and 0.13 for node-pulvini and sheaths, respectively, it increased them 2-fold for internodes. Within the plant part geostimulation increased GAs (relative to upper halves) 29-and 7-fold in lower halves of node-pulvini and internodes, respectively, but reduced GAs to 0.3 in lower halves of sheaths. At age 42 days a GA4/7-like (nonpolar) substance predominates, with lesser amounts of a GA3-llke (polar) substance. Native GAs of Avena include GA3, GA4, and GA7. Geostimulation enhanced the ratio of nonpolar to polar GAs for both halves of internodes, but tended to depress it for sheaths and nodepulvini.The disposition and metabolism of applied 13HIGA4 confirmed the above trends for endogenous GAs regarding localization (e.g. up to 2-fold increases in 13HIGA4 and acidic 3H-metabolites in the lower halves, relative to upper halves). Also, metabolism into highly water-soluble 3H-metabolites (biologically jnactive conjugates?) was greater (up to 1.8-fold) in upper than in lower halves. The end result of such metabolic trends would be to reduce acidic (biologically active?) GAs in the upper half, while retaining them for a longer time in the lower half.Geotropically stimulated Avena shoots thus increase, within 24 hours, the levels of acidic GAs in the lower halves of the p-1 node-pulvinus and p-I internode, the two plant parts responsible for the geostimulated growth.