INHIBITION OF GRAVITROPISM IN PRIMARY ROOTS OF ZEA MAYS BY CHLORAMPHENICOL
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Abstract:
Primary roots of Zea mays seedlings germinated and grown in 0.1 mm chloramphenicol (CMP) were significantly less graviresponsive than primary roots of seedlings germinated and grown in distilled water. Elongation rates of roots treated with CMP were significantly greater than those grown in distilled water. Caps of control and CMP‐treated roots possessed extensive columella tissues comprised of cells containing numerous sedimented amyloplasts. These results indicate that the reduced graviresponsiveness of CMP‐treated roots is not due to reduced rates of elongation, the absence of the presumed gravireceptors (i.e., amyloplasts in columella cells), or reduced amounts of columella tissue. These results are consistent with CMP altering the production and/or transport of effectors that mediate gravitropism.Keywords:
Amyloplast
Gravitropism
Columella
Elongation
Coleoptile
Distilled water
Root cap
Correlations between regeneration of the root cap and recovery of a gravitropic response were studied using primary roots of Phaseolus vulgaris. After removal of various lengths of the root tip a gravistimulus was continuously given to the root. The statistical analysis of data showed that recovery of the gravitropic response was gradually delayed as the length of the tips removed increased. This suggested that the columella cells of the root cap were involved in gravitropism. When the root cap was completely removed, the roots did not respond to gravistimuli for the first 15 h and began to reorient their growth direction at 20 h. At this time, the columella cells had just begun to regenerate and had immature amyloplasts which did not sufficiently form a sediment. These results suggest that other systems of perception exist in plant cells in addition to the amyloplast-based model of graviperception.
Amyloplast
Gravitropism
Columella
Root cap
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Removal of the cap from the primary roots of Zea mays and Triticum aestivum renders the roots unresponsive to gravity. In both species a geotropic response is recovered before a new cap has started to regenerate. Immediately after decapping amyloplasts start to develop in cells of the root apex and it is proposed that as the development of amyloplasts continues so they become functional as gravity sensors. It is also suggested that the amyloplasts may be the source of an inhibitor that has been postulated to be the intermediary between the perception of gravity and the geotropic response.
Amyloplast
Gravitropism
Root cap
Root tip
Plant roots
Plant Roots
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Primary roots of Zea mays seedlings germinated and grown in 0.1 mm chloramphenicol (CMP) were significantly less graviresponsive than primary roots of seedlings germinated and grown in distilled water. Elongation rates of roots treated with CMP were significantly greater than those grown in distilled water. Caps of control and CMP‐treated roots possessed extensive columella tissues comprised of cells containing numerous sedimented amyloplasts. These results indicate that the reduced graviresponsiveness of CMP‐treated roots is not due to reduced rates of elongation, the absence of the presumed gravireceptors (i.e., amyloplasts in columella cells), or reduced amounts of columella tissue. These results are consistent with CMP altering the production and/or transport of effectors that mediate gravitropism.
Amyloplast
Gravitropism
Columella
Elongation
Coleoptile
Distilled water
Root cap
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Citations (1)
Primary roots of a starchless mutant of Arabidopsis thaliana L. are strongly graviresponsive despite lacking amyloplasts in their columella cells. The ultrastructures of calyptrogen and peripheral cells in wild-type as compared to mutant seedlings are not significantly different. The largest difference in cellular differentiation in caps of mutant and wild-type roots is the relative volume of plastids in columella cells. Plastids occupy 12.3% of the volume of columella cells in wild-type seedlings, but only 3.69% of columella cells in mutant seedlings. These results indicate that: (1) amyloplasts and starch are not necessary for root graviresponsiveness; (2) the increase in relative volume of plastids that usually accompanies differentiation of columella cells is not necessary for root graviresponsiveness; and (3) the absence of starch and amyloplasts does not affect the structure of calyptrogen (i.e. meristematic) and secretory (i.e. peripheral) cells in root caps. These results are discussed relative to proposed models for root gravitropism.
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Columella
Gravitropism
Root cap
Wild type
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Gravitropism
Root cap
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Primary roots of Phaseolus vulgaris (Fabaceae) are positively geotropic, while lateral roots are not responsive to gravity In order to elucidate the structural basis for this differential georesponse, we have performed a qualitative and quantitative analysis of the ultrastructure of columella cells of primary and lateral roots of P. vulgaris. Root systems were fixed in situ so as not to disturb the ultrastructure of the columella cells. The columellas of primary roots are more extensive than those of lateral roots. The volumes of columella cells of primary roots are approximately twice those of columella cells of lateral roots. However, columella cells of primary roots contain greater absolute volumes and numbers of all cellular components examined than do columella cells of lateral roots. Also, the relative volumes of cellular components in columella cells of primary and lateral roots are statistically indistinguishable. The endoplasmic reticulum is sparse and distributed randomly in both types of columella cells. Both types of columella cells contain numerous sedimented amyloplasts, none of which contact the cell wall or form complexes with other cellular organelles. Therefore, positive geotropism by roots must be due to a factor(s) other than the presence of sedimented amyloplasts alone. Furthermore, it is unlikely that amyloplasts and plasmodesmata form a multi-valve system that controls the movement of growth regulating substances through the root cap.
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Gravitropism
Root cap
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Starch occupies 4.2 per cent of the volume of plastids in calyptrogen cells in primary roots of Zea mays L. cv. vp-7 wild type. Plastids in calyptrogen cells are distributed randomly around large, centrally located nuclei. The differentiation of calyptrogen cells into columella cells is characterized by cellular enlargement and the sedimentation of plastids to the bottom of the cells. Although sedimented plastids in columella cells do not contain significantly more starch than those in calyptrogen cells, primary roots are graviresponsive. The onset of root gravicurvature is not associated with a significant change in the distribution of plastids in columella cells. These results indicate that in this cultivar of Z. mays (1) the sedimentation of plastids in columella cells is not based upon their increased density resulting from increased starch content alone, (2) starch-laden amyloplasts need not be present in columella cells for roots to be graviresponsive, and (3) the onset of root gravicurvature does not require a major redistribution of plastids in columella cells.
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Gravitropism
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The root cap of primary roots of Zea mays is composed of five distinctive types of cells: (i) the calyptrogen, or cap meristem, (ii) columella statocytes, located in the center of the root cap and characterized by the presence of sedimented amyloplasts, (iii) peripheral cells (PC) I, cells adjacent to the columella statocytes characterized by a more random distribution of amyloplasts and the absence of mucilage, (iv) PCII, cells containing mucilage between the cell wall and plasmalemma, and (v) PCIII, highly vacuolate cells typically detached from the root cap proper. As cells of the calyptrogen differentiate into columella statocytes, proplastids differentiate into amyloplasts which sediment to the bottom of the cell. Sedimented amyloplasts do not contact plasmodesmata in the bottom of columella statocytes. The starch content of amyloplasts decreases as columella statocytes differentiate into PCI. As PCI differentiate into PCII, mucilage is secreted into the space between the outer tangential cell wall and plasmalemma. As PCII differentiate into PCIII, numerous small vacuoles fuse into a large central vacuole. This vacuolation may be at least partially responsible for the movement of mucilage through the cell wall.
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Gravitropism
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Root-cap columella with movable amyloplasts may cause gravitropism of primary roots of Brassica rapa
Complete absence of amyloplasts in columella cells meant there was no gravitropic response in the primary roots of Brassica rapa; and abnormal development of amyloplasts reduced the response. These facts suggest that the amyloplasts need tomove freelyinthe cytoplasm as partof gravisensing. The seedlings were turned upside down and the columella cells with amyloplasts that aligned signicantly towards the gravity vector were mapped. It was found that movable amyloplasts are localized in young and central columella cells. It therefore appears that young and central columella cells generate the largest gravisensing signal.
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Gravitropism
Brassica rapa
Root cap
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Columella
Amyloplast
Gravitropism
Root cap
Plasmodesma
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