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Fred Sack
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Research Interests
Major foci are stomatal development and gravity sensing using microscopic, physiological, molecular and genetic techniques.
Mutational analysis of stomatal development in Arabidopsis.
Stomata regulate gas exchange by controlling the width of a pore bordered by two guard cells. Our long term goals are to identify the genes and processes that determine how stomatal cell fate is chosen, how stomata are patterned (mature stomata do not contact each other) and how guard cells differentiate. To identify relevant genes, we screened for Arabidopsis mutants that alter stomatal morphology and distribution. We found two sets of mutants, too many mouths ( tmm ) and four lips ( flp ) that have adjacent stomata and that disrupt patterning. Phenotypic analysis shows that TOO MANY MOUTHS is necessary for the correct placement and polarity of asymmetric divisions of stomatal precursors. Moreover, TMM appears to act in a pathway that normally involves cell-cell signaling. Although tmm has stomatal clusters in some organs, such as cotyledons and leaves, in other organs it has no stomata at all. This suggests that TMM is an important regulator of stomatal formation that is positive or negative in effect depending upon organ. The FOUR LIPS gene product appears to act later in development than TMM. It limits the number of divisions of the guard mother cell to one, but when it is defective, cell duplication results. We are also characterizing other mutations that affect the morphogenesis of guard cells ( mustaches ) and the cytokinesis of their parent cell ( cytokinesis defective1 ).
Gravitropic sensing in higher plants.
Our main focus is on understanding how plants sense gravity and especially on the possible role of heavy starch-filled plastids (amyloplasts). Because starchless or starch-deficient mutants of Arabidopsis and Nicotiana are less sensitive to threshold g stimuli, it is clear that significant amounts of starch are necessary for normal levels of g-sensing. The presence of residual gravitropism in starchless mutants might be explained if the starchless plastids had sufficient mass to provide enough signal above cellular noise.
Gravitropism and plastid distribution in tip-growing moss cells.
Moss cells provide a rich system for gravitational and spaceflight research. The early haploid phase of moss consists of filaments (protonemata) of cells that grow only at their tips. Gravity controls the direction of their growth. Protonemata of the moss Ceratodon grow away from gravity in the dark (negative gravitropism. The direction of gravity is probably sensed by amyloplasts that sediment in the tip half of the apical cell. These cells exhibit a complex plastid zonation in that only some plastids sediment and only part way to the lower wall. Microtubules appear to restrict the amount of sedimentation and microfilaments are required for the existence of some plastid zones. We conducted an experiment with this moss on a NASA Space Shuttle in 1997. Unexpectedly, the plastids were non-randomly oriented in space in that they were clustered towards the tip of the cell. We hope to analyze the contribution of the cytoskeleton to maintaining this zonation in an upcoming Shuttle experiment.
Selected References
Stomatal Development
- Yang M, FD Sack 1995 The too many mouths and four lips mutations affect stomatal production in Arabidopsis . Plant Cell 7: 2227-39.
- Larkin J, MD Marks, J Nadeau, FD Sack 1997 Epidermal cell fate and patterning in leaves. Plant Cell 9: 1109-1120.
- Geisler M, M Yang, FD Sack 1998 Divergent regulation of stomatal initiation and patterning in organ and suborgan regions of the Arabidopsis mutants too many mouths and four lips . Planta 205: 522-530.
- Zhao L, FD Sack 1999 Ultrastructure of stomatal development in Arabidopsis (Brassicaceae) leaves. Amer J Botany 86: 929-39.
- Yang M, JA Nadeau, L Zhao, FD Sack 1999 Characterization of a cytokinesis defective mutant ( cyd1 ) of Arabidopsis . J Experimental Botany 50: 1437-1446.
Gravitropism in Higher Plants
- Sack FD 1991 Plant Gravity Sensing. Intern Rev Cytol 127: 193-252.
- Sack FD 1997 Plastids and gravitropic sensing. Planta 203: S63-S68.
- Vitha S, M Yang, FD Sack 1998 Light promotion of hypocotyl gravitropism of a starch-deficient tobacco mutant correlates with plastid enlargement and sedimentation. Plant Physiol. 116: 495-502.
- Stankovic B, FD Sack 1998 Autotropism, automorphogenesis and gravity. Minireview in Physiologia Plantarum 102: 328-335.
- Stankovic B, D Volkmann, FD Sack 1998 Autonomic straightening after gravitropic curvature of Lepidium roots. Plant Physiol. 117: 893-900.
- Vitha S, L Zhao, FD Sack 2000 Interaction of root gravitropism and phototropism in Arabidopsis wild type and starchless mutants. Plant Physiol. Feb. issue: In press.
Gravitropism in Mosses
- Schwuchow JM, FD Sack 1994 Microtubules restrict plastid sedimentation in protonemata of the moss Ceratodon . Cell Motility Cytoskeleton 29: 366-74.
- Walker LM, FD Sack 1995 Microfilament distribution in protonemata of the moss Ceratodon . Protoplasma, 189: 229-37.
- Wagner TA, DJ Cove, FD Sack 1997 A positively gravitropic mutant mirrors the wild-type protonemal response in the moss Ceratodon purpureus . Planta 202: 149-54.
- Wagner TA, FD Sack 1998 Gravitropism and gravimorphism during regeneration from protoplasts of the moss Ceratodon purpureus (Hedw.) Brid. Planta 205: 352-358.
- Sack FD, VD Kern, TA Wagner 1998 Gravitropism in moss protonemata. Bryology for the Twenty-First Century. JW Bates, NW Ashton, JG Duckett, eds. Maney Publishing and The British Bryological Society: Leeds. pp. 247-260.
- Kuznetsov O, JM Schwuchow, FD Sack, KH Hasenstein 1999 Curvature induced by amyloplast magnetophoresis in protonemata of the moss Ceratodon purpureus . Plant Physiology 119: 645-50.
- Kern VD, FD Sack 1999 Irradiance-dependent regulation of gravitropism by red light in protonemata of the moss Ceratodon purpureus . Planta 209: 299-307.
- Schwuchow JM, VD Kern, TA Wagner, FD Sack 2000 The density of apical cells of dark-grown protonemata of the moss Ceratodon . Protoplasma, in press.
Dr. Sack's
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Plant Biology
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