How Cereal Grass Shoots Perceive and Respond To Gravity

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Starch in pulvinus amyloplasts of barley (Hordeum vulgare cv Larker) disappears when 45-day-old, light-grown plants are given 5 days of continuous darkness. The effect of this loss on the pulvinus graviresponse was evaluated by following changes in the kinetics of response during the 5-day dark period. Over 5 days of dark pretreatment, the lag to initial graviresponse and the subsequent half-time to maximum steady state bending rate increased significantly while the maximum bending rate did not change. The change in response to applied indoleacetic acid (100 micromolar) plus gibberellic acid (10 micromolar) without gravistimulation, under identical dark pretreatments, was used as a model system for the response component of gravitropism. Dark pretreatment did not change the lag to initial response following hormone application to vertical pulvini, but both the maximum bending rate and the half-time to the maximum rate were significantly reduced. Also, after dark pretreatment, significant bending responses following hormone application were observed in vertical segments with or without added sucrose, while gravistimulation produced a response only if segments were given sucrose. These results indicate that starch-filled amyloplasts are required for the graviresponse of barley pulvini and suggest that they function in the stimulus perception and signal transduction components of gravitropism.

Section: Resultsmentioning

confidence: 99%

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

Brock

Kaufman²

1988

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“…This altered physical state could make possible the lateral transport of hormones, such as IAA or GAs, or allow for intake or outflow of ions, such as K+ and Ca2+ (8,10,11,13,15,16,25,28). While processes (a) and/or (b) occur, a third possible role (c) is that starch in the statoliths may also serve as a source of substrate (D-glucose) that is used in cell growth metabolism, particularly in the the synthesis of new cell wall polysaccharides (4,5,19,20).…”

Section: Starch Statoliths and Cereal Grass Pulvinimentioning

confidence: 99%

“…Recent studies in our laboratory (19)(20)(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).…”

mentioning

confidence: 99%

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

Song

Lu²,

Brock³

et al. 1988

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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...

“…Since ,-D-glucan was the only wall component to change, it is hypothesized that this change is the basis for the change in wall extensibility. Since wall extensibility changed too slowly to account for growth initiation, it is postulated that asymmetric changes in osmotic solutes act as the driving factor for growth promotion in the graviresponse, while wall extensibility acts as a limiting factor during growth.The graviresponse of the leaf-sheath pulvinus of oat is initiated by the sedimentation of starch grains, which culminates in a hormone-mediated growth response (7,22 …”

mentioning

confidence: 99%

Cell wall and enzyme changes during the graviresponse of the leaf-sheath pulvinus of oat (Avena sativa).

Gibeaut

Karuppiah²,

S-R³

et al. 1990

The graviresponse of the leaf-sheath pulvinus of oat (Avena sativa) involves an asymmetric growth response accompanied by several asymmetric processes, including degradation of starch and cell wall synthesis. To understand further the cellular and biochemical events associated with the graviresponse, changes in cell walls and their constituents and the activities of related enzymes were investigated in excised pulvini. Asymmetric increases in dry weight with relatively symmetric increases in wall weight accompanied the graviresponse. Starch degradation could not account for increases in wall weight. However, a strong asymmetry in invertase activity indicated that hydrolysis of exogenous sucrose could contribute significantly to the increases in wall and dry weights. Most cell wall components increased proportionately during the graviresponse. However, fl-D-glucan did not increase symmetrically, but rather increased in proportion in lower halves of gravistimulated pulvini. This change resulted from an increase in glucan synthase activity in lower halves. The asymmetry of ft-D-glucan content arose too slowly to account for initiation of the graviresponse. A similar pattem in change in wall extensibility was also observed. Since ,-D-glucan was the only wall component to change, it is hypothesized that this change is the basis for the change in wall extensibility. Since wall extensibility changed too slowly to account for growth initiation, it is postulated that asymmetric changes in osmotic solutes act as the driving factor for growth promotion in the graviresponse, while wall extensibility acts as a limiting factor during growth.The graviresponse of the leaf-sheath pulvinus of oat is initiated by the sedimentation of starch grains, which culminates in a hormone-mediated growth response (7,22