Do Starch Statoliths Act as the Gravisensors in Cereal Grass Pulvini?

Abstract

To determine if starch statoliths do, in fact, act as gravisensors in cereal grass shoots, starch was removed from the starch statoliths by placing 45-day-old intact barley plants (Hordeum vulgare cv 'Larker') in the dark at 25°C for 5 days. Evidence from staining with I2-KI, scanning electron microscopy, and transmission electron microscopy indicated that starch grains were no longer present in plastids in the pulvini of plants placed in the dark for 5 days. Furthermore, gravitropic curvature response in these pulvini was reduced to zero, even though pulvini from vertically oriented plants were still capable of elongating in response to applied auxin plus gibberellic acid. However, when 0.1 molar sucrose was fed to the dark pretreated, starch statolith-free pulvini during gravistimulation in the dark, they not only reformed starch grains in the starch-depleted plastids in the pulvini, but they also showed an upward bending response. Starch grain reformation appeared to precede reappearance of the graviresponse in these sucrose-fed pulvini. These results strongly support the view that starch statoliths do indeed serve as the gravisensors in cereal grass shoots.Recent studies in our laboratory (19-21) have been concerned with the cereal grass leaf-sheath pulvinus as a graviresponsive organ system and whether or not the starch-containing plastids in pulvinus cells of this system are indeed the gravisensors (statoliths). The young, undifferentiated grass shoot pulvinus does not possess any starch-containing statoliths, and it shows no upward bending curvature response when oriented horizontally (9,21,29). However, when the pulvinus is mature, starch statoliths are present in statenchyma cells along the inside of each of the longitudinally oriented vascular bundles, as seen in transverse sections of the pulvinus (Fig. 1). The structure of starch statoliths in grass shoots as examined by means of LM,2 SEM, and TEM (9, 19-21, 23, 26), as well as their purported function as gravisensor organelles (1,14,17,22,25,27), have been studied extensively. Furthermore, it has been shown that these organelles descend within the presentation time (1-2 min) for initiation of an upward bending graviresponse in the grass shoot pulvinus (21). Even so, and in spite of all previous work, we still have no definitive proof that starch statoliths are the gravisensors that trigger an upward bending response in prostrated cereal grass shoots. In other graviresponsive systems, such as in rootcaps of starch-free Arabidopsis mutants, they are not required (7,25); that is, the roots respond to gravistimulation to the same extent as do nonmutant plants, with starch statoliths present in the root ' Supported by National Aeronautics and Space Administration grant NAGW-34.2 Abbreviations: LM, light-microscopy; SEM, scanning electron microscopy; TEM, transmission electron microscopy.caps.In light of the above, we set out to determine whether or not gravitropic curvature could occur in barley shoots in the absence of starch in the pulvinus pla...

Effect of Dark Pretreatment on the Kinetics of Response of Barley Pulvini to Gravistimulation and Hormones

“…1), from a mean lag time of 1.2 h for pulvini with no dark pretreatment to 9.0 h for pulvini given a five day dark pretreatment. As noted previously ( 15) pulvinus amyloplasts decreases until none is present after five days pretreatment. As indicated by the calculations of Audus (1) and the observations of Bouck (3), decreasing the mass of amyloplasts by loss of starch would decrease amyloplast sedimentation rate.…”

“…The addition of sucrose simultaneous with stimulus presentation causes the reformation of starch in amyloplasts and allows a significant graviresponse (15). The unilateral application of hormones to vertical pulvini from similarly pretreated plants produces a significant bending response with or without sucrose.…”

“…In contrast, removal of starch from amyloplasts of barley pulvini, by enzymic degradation ( 11) (15). Stem sections containing a single pulvinus were excised, on a daily basis, 10 cm below the pulvinus and 6 cm above, abraded at the pulvinus on all sides with jeweler's rouge, and mounted in vials containing 0.1 M sucrose (5), unless otherwise noted.…”

Inhibition of the Gravitropic Response of Snapdragon Spikes by the Calcium-Channel Blocker Lanthanum Chloride

“…Additional support for the view that LaCl 3 affects specific processes induced by gravistimulation is provided by studies of amyloplast/chloroplast sedimentation, which is believed to be one of the susceptors that trigger gravitropic sensing in roots and shoots of higher plants (Song et al, 1988;Sack, 1997;Vitha et al, 1998). As was previously found for columella cells of roots or for endodermis of stem seedlings (Sack, 1997), the bending zone of the snapdragon stem was found to be very rich in starch-containing chloroplasts (Figs.…”

“…Hand-cut crosssections were prepared from the middle part of the stembending zone (section II) taken from LaCl 3 -treated or nontreated spikes at the indicated intervals during gravistimulation. The sections were stained with iodine solution (10% KI/5% I 2 in distilled water), as described previously (Song et al, 1988) and immediately examined with a light microscope (Vanox, Olympus), and photographs were taken with Kodak 100-ASA film.…”