Peter K. Hepler Explained

Peter K. Hepler
Birth Date:1936 10, mf=yes
Birth Place:Dover, New Hampshire, US
Field:Cell biology, plant physiology, microscopy
Work Institutions:Stanford University
University of Massachusetts at Amherst
Alma Mater:University of New Hampshire, B.S. Chemistry 1958
University of Wisconsin, Ph.D. Plant Cell Biology 1964
Known For:Cell biology, plant physiology, microscopy
Website:Peter K. Hepler
Molecular & Cellular Biology

Peter Klock Hepler HonFRMS (born 1936) is the Constantine J. Gilgut and Ray Ethan Torrey Professor Emeritus in the Biology Department of the University of Massachusetts at Amherst who is notable for his work on elucidating the roles of calcium,[1] membranes[2] and the cytoskeleton[3] [4] in plant cell development and cell motility.

Personal life

Peter Klock Hepler was born on October 29, 1936, in Dover, New Hampshire, to Jesse Raymond Hepler[5] [6] [7] and Rebecca Orpha Peterson Hepler. He married Margaret (Peggy) Dennison Hunt on March 7, 1964. They have three children: Sarah, Anna[8] and Lukas. Peter and Peggy have six grandchildren: Finn, Leif, Louisa (Lulu), Jesse, Marit, and Haakon. In an interview published in the Newsletter of the American Society of Plant Biologists, Hepler was asked, "What is your most treasured possession?" He answered, "My family; but I don't possess them."[9] Peter and Peggy Hepler live on a farm in Pelham, Massachusetts that was established in 1740[10] and is now a part of the Kestrel Land Trust.[11]

University life

Peter Hepler graduated from Dover High School in 1954. He received his B.S. in chemistry from the University of New Hampshire in 1958 and earned his Ph.D. in plant cell biology from University of Wisconsin in 1964, studying the role of cortical microtubules in plant cell development with Eldon H. Newcomb. After receiving his Ph.D., Hepler served at the Walter Reed Army Institute of Research until 1966, studying malarial parasites. Hepler then returned to the University of Wisconsin for a postdoctoral fellowship[12] and then became a postdoctoral fellow with Keith Porter[13] at Harvard University from 1966 to 1967, where he continued his investigation of microtubules, focusing on their role in the mitotic apparatus and the phragmoplast of the endosperm cells of Haemanthus Katharinae. After being an assistant professor at Stanford University, Hepler joined the faculty in the Botany Department at the University of Massachusetts at Amherst. He was an associate professor from 1977 to 1980, a professor from 1980 to 1989, and became the Ray Ethan Torrey Professor in 1989 and the Constantine J. Gilgut Professor in 1998. Hepler retired from the Biology Department as the Constantine J. Gilgut and Ray Ethan Torrey Professor Emeritus, although he continues to do research.[14] Hepler spent many summers teaching and doing research at the Marine Biological Laboratory[15] [16] at Woods Hole, Massachusetts. Hepler also participated in a multiyear international collaboration with Brian E. S. Gunning.[17]

Hepler was an Associate Editor of Protoplasma from 1994 to 2001 and Associate Editor of Plant Physiology from 1998 to 2000. He has been on the editorial boards of the Annual Review Plant Physiology, Plant and Cell Physiology, the Journal of Submicroscopic Cytology, Cell Motility and the Cytoskeleton, and BioEssays.

Research

Hepler's scientific method is to know thoroughly the classical botanical literature and then develop or apply modern physico-chemical techniques to answer salient and extensive biological questions using plants that are well-suited to answer those questions. In so doing, Hepler opened whole areas of research.[18] [19] Hepler did pioneering work in showing the relationship of the microscopic elements of the cytoskeleton to the macroscopic properties of plant growth, development and function. He also did pioneering work on plasmodesmata,[20] [21] [22] stomatal function,[23] [24] [25] [26] the role of calcium in plant development[27] and in the development of techniques useful for answering questions using light[28] [29] [30] [31] [32] and electron microscopy.[33] Hepler's scientific publications with Barry A. Palevitz are notable for quoting Woody Allen and Yogi Berra.[34]

Hepler described his realization of the influence a review he and Palevitz wrote on microtubules and microfilaments "to introduce new thoughts and promising avenues for future research" had with his characteristic self-deprecating sense of humor: "I became aware that the review was being read widely one summer (1979) while working in the library at the Marine Biological Laboratory. I turned to the library's volume of the Annual Review of Plant Physiology that contained our paper and when I put the volume down, it literally fell open at our article; worn edges on the pages and the penciled corrections of all the misspellings and punctuation errors indicated that the chapter had been thoroughly perused."

Hepler, along with Ledbetter and Porter, is considered to be a co-discoverer of microtubules.

Microtubules and cell shape

In late 1962 and early 1963, Hepler tested the newly developed procedure using a glutaraldehyde pre-fix followed by an osmium post-fix to study plant cell structure using an electron microscope.[35] Building on the earlier work by Sinnott and Bloch,[36] who had shown that wounding the existing tracheary elements in a Coleus stem induced neighboring parenchyma cells to differentiate into new tracheary elements, Hepler showed that cytoplasmic microtubules were localized specifically in the cortical cytoplasm immediately over the bands of new secondary wall thickenings.[37] Moreover, Hepler discovered that the microtubules were oriented parallel to the cellulose microfibrils of the newly formed secondary wall thickenings. This work, along with the studies of Ledbetter and Porter[38] and Green[39] established the importance of cortical microtubules in controlling the alignment of cellulose microfibrils in the cell wall.[40] [41] Further work with Barry Palevitz showed that microtubules were involved in orienting the cellulose microfibrils in the walls of guard cells in a pattern of radial micellation that is necessary for stomatal function.[42] Hepler, along with the husband and wife team of Dale Callaham and Sue Lancelle, developed a method to achieve rapid freeze fixation of particularly small plant cells that showed that cortical microtubules are closely associated with one another, actin microfilaments, the endoplasmic reticulum and the plasma membrane.[43]

Microtubules and cell motility

Building on the work of Shinya Inoué and Andrew Bajer using polarized light microscopy,[44] Hepler used electron microscopy to elucidate the nature of the microtubule/chromosome attachments at the kinetochore as well as the arrangement of the microtubules in the phragmoplast during the development of the new cell wall, where microtubules from both sides of the phragmoplast were seen to overlap with one another in the plane of the cell plate.[45]

Hepler realized that microtubules were dynamic structures that were deployed in various locations throughout the cell, and became interested in the mechanisms involved in microtubule organization in cells that lacked a microtubule-organizing center known as the centrosome. In order to understand how microtubule-organizing centers were generated, Hepler examined the de novo formation of the blepharoplast in the spermatogenous cells of Marsilea vestita. The blepharoplast in each spermatid generates 100–150 basal bodies, each of which gives rise to the 9+2 arrangement of microtubules in a cilium. During telophase of the penultimate division, flocculent material appears near clefts on the distal surfaces of the daughter nuclei. During prophase of the final division which gives rise to the spermatids, the flocculent material near each nucleus condenses to give rise to two blepharoplasts, which then separate, one going to each spermatid.[46]

While Hepler was successful in identifying an aggregation of material that possessed microtubule-organizing capacity, he was not able to specify the biophysical mechanisms involved in organization. After Richard Weisenberg[47] discovered that microtubule polymerization was sensitive to calcium concentration, Hepler realized that he had already seen a close association between elements of the endoplasmic reticulum and microtubules in the mitotic apparatus and in the phragmoplast and suggested that these membranes may function in controlling the concentration of free calcium in the mitotic apparatus.[48] Along with Susan Wick and Steve Wolniak, Hepler showed that the endoplasmic reticulum contained stores of calcium and suggested that the endoplasmic reticulum may locally control the calcium concentration and thus the polymerization/depolymerization of microtubules. Subsequently,[49] [50] Hepler, along with Dale Callaham, Dahong Zhang, and Patricia Wadsworth, observed calcium ion transients during mitosis[51] [52] and showed that the microinjection of calcium ions into the mitotic spindle does regulate the depolymerization of microtubules and the movement of chromosomes to the poles during mitosis.[53] [54] [55]

Microfilaments and cytoplasmic streaming

Hepler identified actin microfilaments in bundles at the ectoplasm-endoplasm interface of Nitella internodal cells by showing that the bundles bound heavy meromyosin, giving the characteristic arrowhead arrangement.[56] [57] The actin microfilaments had the correct polarity to be part of the actomyosin motor that provides the motive force for cytoplasmic streaming in these giant algal cells.[58]

Calcium and plant development

Hepler has shown that calcium ions are a central regulator of plant growth and development[59] specifically demonstrating that calcium is important for tip growth[60] [61] [62] and in phytochrome.[63] [64] and cytokinin[65] [66] [67] action.

Pollen tube growth

Hepler's research is currently aimed at finding the ionic and molecular components that make up the pacemaker that regulates the oscillatory growth of pollen tubes. He has shown that calcium ions and protons are essential for growth.[68] The intracellular free calcium ions exist in a gradient dropping from 3000 nM at the tip to 200 nM 20 μm from the tip [69] and the intracellular H+ gradient falls from pH 6.8 at the tip to pH 7.5 10–30 μm from the tip.[70] The higher concentrations of intracellular Ca2+ and H+ at the tip result from the localization of the influx of these ions at the tip. The protons are effluxed at a region on the sides of the tube that corresponds to the location of the intracellular alkaline band.[71] Energy is required for pollen tube growth[72] and an H+-ATPase may mediate the efflux.Hepler has shown that the magnitude of the intracellular calcium and proton gradients and the extracellular fluxes of these ions oscillate with a period of 15-50 s. This period is identical to the period of oscillation in the rate of pollen tube growth, however, the intracellular calcium peak follows the growth rate peak by 1–4 seconds, and the extracellular calcium peak follows the growth rate peak by 11–15 seconds.[73] The delay between the extracellular and intracellular calcium peaks indicates that calcium ions do not immediately enter the cytoplasmic pool. Hepler postulates that the extracellular influx of calcium is not governed by the plasma membrane but by changes in the ion-binding properties of the pectin within the cell wall. The pectin is secreted in its uncharged methylester form. Subsequently, a pectin methylesterase in the wall results in the de-esterification of the methyl groups that yields carboxyl residues that bind calcium and form calcium-pectate cross-bridges. This calcium binding may account for the bulk of the observed extracellular current. The intracellular calcium gradient may direct the location of secretion of cell wall components that define the direction of pollen tube growth.

The intracellular components that contribute to pollen tube growth include the actin-mediated transfer of Golgi-derived secretory vesicles filled with methylesterified homogalacturonans and pectin methylesterase synthesized on the ER to the growing tip.[74] The secretion of the vesicles at the growing tip anticipates the increase in growth rate,[75] indicating that the turgor pressure driven intussusception of the methylesterified pectin into the cell wall at the growing tip and its subsequent demethylesterification by pectin methylesterase may relax the cell wall by robbing the load-bearing calcium pectate bonds of its Ca2+.[76] This would result in a slightly delayed yet increased growth rate. The removal of the methoxy groups in the pectins at the flanks of the apical dome unmasks their negatively charged carboxylate groups. The anionic homogalacturonans then bind Ca2+ and become stiffer as the new apical dome, which will incorporate more methylesterified pectins and pectin methylesterase, grows away from the stiffened flanks composed of calcium pectate. The external Ca2+ concentration is critical. When the external Ca2+ concentration is below 10 μM, the amount of calcium pectate is so low that the cell wall is too weak and the pollen tube bursts. When the external Ca2+ concentration is above 10 mM, the amount of calcium pectate is so high that the cell wall is too stiff and the pollen tube will not grow.

Honors and awards

Notes and References

  1. Hepler, P. K. . R. O. Wayne . This Week's Citation Classic . Current Contents . July 26, 1993 . 30 . 8 . October 6, 2016.
  2. Book: Hepler, P. K., S. M. Wick and S. M. Wolniak . The structure and role of membranes in the mitotic apparatus. in: International Cell Biology 1980–1981, H.G. Schweiger, ed. . 1981 . Springer-Verlag . Berlin . 673–686.
  3. Hepler, P. K. . B. A. Palevitz . Microtubules and microfilaments . Annual Review of Plant Physiology . 1974 . 25 . 309–362 . 10.1146/annurev.pp.25.060174.001521.
  4. Hepler, P. K. . B. A. Palevitz . Microtubules and microfilaments . Current Contents . August 11, 1986 . 32 . 20 . October 7, 2016.
  5. Book: Hepler, J. R. . Methods in Forcing Rhubarb: M.S. Thesis . 1922 . University of Wisconsin . 978-1273396984.
  6. Hepler . Billy . America's Youngest Seed Grower . Heritage Farm Companion . 2012 . Summer . 6–9.
  7. Web site: A Bean Collector's Window . October 18, 2016.
  8. Web site: Hepler . Anna . Anna Hepler Intricate Universe . October 7, 2016.
  9. News: Membership Corner . APBS News September/October 2004 . 31(5), 22. 2016-10-07. https://web.archive.org/web/20160404095533/http://newsletter.aspb.org/2004/septoct04.pdf#page=22. 2016-04-04. dead.
  10. Web site: Hepler Family (Pelham, MA) . UmassAmherst: MassWoods . October 6, 2016.
  11. Web site: Kestrel Land Trust: Conserve the Valley You Love . Kestrel Land Trust . October 6, 2016.
  12. VandenBosch, K. A., W. Becker and B. A Palevitz . The natural history of a scholar and gentleman: A biography of Eldon H. Newcomb . Protoplasma . 1996 . 195 . 1–4 . 4–11 . 10.1007/bf01279181 . 32568416.
  13. Hepler, P. K., J. D. Pickett-Heaps and B. E. S. Gunning . Some retrospectives on early studies of plant microtubules . The Plant Journal . 2013 . 75 . 2 . 189–201 . 10.1111/tpj.12176 . 23496242. free.
  14. Hepler . Peter K. . Founders' Review: The Cytoskeleton and Its Regulation by Calcium and Protons . Plant Physiology . 2016 . 170 . 1 . 3–22 . 10.1104/pp.15.01506 . 26722019 . 4704593.
  15. Web site: MBL Society Members . Marine Biological Laboratory . October 6, 2016.
  16. Web site: Physiology 1981 . History of the Marine Biological Laboratory . October 6, 2016.
  17. Hepler, P. K. . B. E. S. Gunning . Confocal fluorescence microscopy of plant cells . Protoplasma . 1998 . 201 . 3 . 121–157 . 10.1007/bf01287411 . 1258312.
  18. Web site: AAAS Members Elected as Fellows . AAAS . October 6, 2016.
  19. News: Members in the News . ASPB Newsletter 33(3), 26 . October 6, 2016 . April 2010. https://web.archive.org/web/20161009171201/http://www.ascb.org/newsletter/april-2010-newsletter. October 9, 2016. dead.
  20. Hepler, P. K. . E. H. Newcomb . Fine structure of cell plate formation in the apical meristem of Phaseolus roots . Journal of Ultrastructure Research . 1967 . 19 . 5–6 . 498–513 . 10.1016/s0022-5320(67)80076-5 . 6055780.
  21. Palevitz, B. A. . P. K. Hepler . Changes in dye coupling of stomatal cells of Allium and Commelina demonstrated by microinjection of Lucifer yellow . Planta . 185 . 164 . 4 . 473–479 . 10.1007/bf00395962 . 24248219 . 30377452.
  22. Turgeon, R. . P. K. Hepler . Symplastic continuity between mesophyll and companion cells in minor veins of mature Cucurbita pepo L. leaves . Planta . 1989 . 179 . 1 . 24–31 . 10.1007/bf00395767 . 24201418 . 21975131.
  23. Zeiger, E. . P. K. Hepler . Production of Guard Cell Protoplasts from Onion and Tobacco . Plant Physiology . 1976 . 58 . 4 . 492–498 . 10.1104/pp.58.4.492 . 16659703 . 543252.
  24. Zeiger, E., W. Moody, P. Hepler and F. Varela . Light-sensitive membrane potentials in onion guard cells . Nature . 1977 . 270 . 5634 . 270–271 . 10.1038/270270a0 . 1977Natur.270..270Z . 4162345.
  25. Zeiger, E. . P. K. Hepler . Light and stomatal function: blue light stimulates swelling of guard cell protoplasts . Science . 1977 . 196 . 4292 . 887–889 . 10.1126/science.196.4292.887 . 17821809 . 1977Sci...196..887Z . 13433483.
  26. Zeiger, E. . P. K. Hepler . Blue light-induced, intrinsic vacuolar fluorescence in onion guard cells . Journal of Cell Science . 1979 . 37 . 1–10 . 10.1242/jcs.37.1.1 . October 6, 2016 . 479318.
  27. Hepler . Peter . Calcium: An essential regulator of plant growth and development . The Plant Cell . 2005 . 17 . 8 . 2142–2155 . 10.1105/tpc.105.032508 . 16061961 . 1182479.
  28. Zhang, D., P. Wadsworth, and P. K. Hepler . Microtubule dynamics in living dividing cells: Confocal imaging of microinjected fluorescent brain tubulin . Proc. Natl. Acad. Sci. USA . 1990 . 87 . 22 . 8820–8824 . 55051 . 10.1073/pnas.87.22.8820 . 11607116 . 1990PNAS...87.8820Z. free.
  29. Zhang, D., P. Wadsworth and P. K. Hepler . Dynamics of microfilaments are similar, but distinct from microtubules during cytokinesis in living, dividing plant cells . Cell Motility and the Cytoskeleton . 1993 . 24 . 3 . 151–155 . 10.1002/cm.970240302.
  30. Valster, A. H., E. S. Pierson, Valenta, P. K. Hepler and A. M. C. Emons . Probing the Plant Actin Cytoskeleton during Cytokinesis and Interphase by Profilin Microinjection . The Plant Cell . 1997 . 9 . 10 . 1815–1824 . 10.1105/tpc.9.10.1815 . 12237348 . 157024.
  31. Book: Vos, J. W., A. H. Valster and P. K. Hepler . Methods for Studying Cell Division in Higher Plants . Methods in Cell Biology . 1988 . 61 . 413–437 . 10.1016/S0091-679X(08)61992-5 . 9891326 . 9780125441636.
  32. Hepler, P. K. . J. Hush . Behavior of Microtubules in Living Plant Cells . Plant Physiology . 1996 . 112 . 2 . 455–461 . 157968 . 10.1104/pp.112.2.455 . 12226402.
  33. Lancelle, S. A., D. A. Callaham and P. K. Hepler . A method for rapid freeze fixation of plant cells . Protoplasma . 1986 . 131 . 2 . 153–165 . 10.1007/bf01285037 . 19236616.
  34. Web site: Poems and Quotations About the MicroWorld . Microscopy Society of America . October 6, 2016.
  35. Newcomb, E. H. . A career in science: Fulfillment of a dream . Protoplasma . 1996 . 195 . 1–4 . 1–3 . 10.1007/bf01279180 . 12850336.
  36. Sinnott, E. W. . R. Bloch . The cytoplasmic basis of intercellular patterns in vascular differentiation . American Journal of Botany . 1945 . 32 . 3 . 151–156 . 10.2307/2437535 . 2437535.
  37. Hepler, P. K. . E. H. Newcomb . The Fine Structure of Young Tracheary Xylem Elements Arising by Redifferentiation of Parenchyma in Wounded Coleus Stem . Journal of Experimental Botany . 1964 . 14 . 3 . 496–503 . 10.1093/jxb/14.3.496.
  38. Ledbetter, M. C. . K. R. Porter . A 'microtubule' in plant cell fine structure . Journal of Cell Biology . 1963 . 19 . 1 . 239–250 . 10.1083/jcb.19.1.239 . 19866635 . 2106853.
  39. Green, P. B. . Mechanism for plant cellular morphogenesis . Science . 1962 . 138 . 3548 . 1404–1405 . 10.1126/science.138.3548.1404 . 17753861 . 1962Sci...138.1404G . 39081841.
  40. Torrey, J. G., D. E. Fosket and P. K. Hepler . Xylem Formation: A Paradigm of Cytodifferentiation in Higher Plants: Plant cells divide and differentiate under the control of changing hormone levels. Xylem offers a model tissue for the study of these cellular events . American Scientist . 1971 . 59 . 3 . 338–352 . 27829621.
  41. Wasteneys, G. O. . F. Brandizzi . A Glorious Half-Century of Microtubules . The Plant Journal . 2013 . 75 . 2 . 185–188 . 10.1111/tpj.12260 . 23834223. free.
  42. Palevitz, B. A. . P. K. Hepler . Cellulose microfibril orientation and cell shaping in developing guard cells of Allium: The role of microtubules and ion accumulation . Planta . 1976 . 132 . 1 . 71–93 . 10.1007/BF00390333 . 24424910 . 2744599.
  43. Lancelle, S. A., M. Cresti and P. K. Hepler . Ultrastructure of the cytoskeleton in freeze-substituted pollen tubes of Nicotiana alata . Protoplasma . 1987 . 140 . 2–3 . 141–150 . 10.1007/bf01273723 . 6452268.
  44. Inoué, S. . A. Bajer . Birefringence in endosperm mitosis . Chromosoma . 1961 . 12 . 48–63 . 10.1007/bf00328913 . 13717778 . 5069716.
  45. Hepler, P. K. . W. T. Jackson . Microtubules and early stages of cell plate formation in the endosperm of Haemanthus katherinae Baker . Journal of Cell Biology . 1968 . 38 . 2 . 437–446 . 10.1083/jcb.38.2.437 . 5664211 . 2107485.
  46. Hepler, P. K. . The blepharoplast of Marsilea: Its de novo formation and spindle association . Journal of Cell Science . 1976 . 21 . 2 . 361–390 . 10.1242/jcs.21.2.361 . 972175 . October 6, 2016.
  47. Weisenberg, R. C. . Microtubule formation in vitro in solutions containing low calcium concentration . Science . 1972 . 177 . 4054 . 1104–1105 . 10.1126/science.177.4054.1104 . 4626639 . 1972Sci...177.1104W . 34875893.
  48. Hepler, P. K. . Membranes in the mitotic apparatus of barley cells . Journal of Cell Biology . 1980 . 86 . 2 . 490–499 . 10.1083/jcb.86.2.490 . 7400216 . 2111505.
  49. Wick, S. M. . P. K. Hepler . Localization of Ca++-containing antimonate precipitates during mitosis . Journal of Cell Biology . 86 . 2 . 1980 . 500–513 . 10.1083/jcb.86.2.500 . 7400217 . 2111497.
  50. Wolniak, S. M., P. K. Hepler, and W. T. Jackson . Detection of the membrane-calcium distribution during mitosis in Haemanthus endosperm with chlorotetracycline . Journal of Cell Biology . 1980 . 87 . 1 . 23–32 . 10.1083/jcb.87.1.23 . 7419592 . 2110715.
  51. Hepler, P. K. . D. A. Callaham . Free calcium increases during anaphase in stamen hair cells of Tradescantia . Journal of Cell Biology . 1987 . 105 . 5 . 2137–2143 . 10.1083/jcb.105.5.2137 . 3680374 . 2114859.
  52. Hepler, P. K. . Calcium transients during mitosis: Observations in flux . Journal of Cell Biology . 1989 . 109 . 6 . 2567–2573 . 10.1083/jcb.109.6.2567 . 2687283 . 2115931.
  53. Zhang, D. H. . Regulation of anaphase chromosome motion in Tradescantia stamen hair cells by calcium and related signaling agents . Journal of Cell Biology . 1990 . 111 . 1 . 171–182 . 10.1083/jcb.111.1.171 . 2114409 . 2116166. free.
  54. Zhang, D. H., P. Wadsworth, and P. K. Hepler . Microtubule dynamics in living dividing plant cells: Confocal imaging of microinjected fluorescent brain tubulin . Proc. Natl. Acad. Sci. USA . 1990 . 87 . 22 . 8820–8824 . 10.1073/pnas.87.22.8820 . 11607116 . 55051 . 1990PNAS...87.8820Z. free.
  55. Zhang, D. H., P. Wadsworth and P. K. Hepler . Modulation of anaphase spindle microtubule structure in stamen hair cells of Tradescantia by calcium and related agents . Journal of Cell Science . 1992 . 102 . 1 . 79–89 . 10.1242/jcs.102.1.79 . October 6, 2016.
  56. Palevitz, B. A., J. F. Ash, and P. K. Hepler . Actin in the green alga, Nitella . Proc. Natl. Acad. Sci. USA . 1974 . 71 . 2 . 363–366 . 388005 . 4592689 . 10.1073/pnas.71.2.363 . 1974PNAS...71..363P. free.
  57. Palevitz, B. A. . P. K. Hepler . Identification of actin in situ at the ectoplasm-endoplasm interface of Nitella. Microfilament-chloroplast association . Journal of Cell Biology . 1975 . 65 . 1 . 29–38 . 10.1083/jcb.65.1.29 . 1127014 . 2111164.
  58. Kersey, Y. M., P. K. Hepler, B. A. Palevitz, and N. K. Wessells . Polarity of actin filaments in Characean algae . Proc. Natl. Acad. Sci. USA . 1976 . 73 . 1 . 165–167 . 335861 . 10.1073/pnas.73.1.165 . 1061112 . 1976PNAS...73..165K. free.
  59. Hepler, P. K. . Historical Perspective Essay: Calcium: a central regulator of plant growth and development . Plant Cell . 2005 . 17 . 8 . 2142–55 . 10.1105/tpc.105.032508 . 16061961 . 1182479.
  60. Miller, D. D., D. A. Callaham, D. J. Gross and P. K. Hepler . Free Ca2+ gradient in growing pollen tubes of Lilium . Journal of Cell Science . 1992 . 101 . 7–12 . 10.1242/jcs.101.1.7 . October 7, 2016.
  61. Wilsen, K. L. . P. K. Hepler . Sperm Delivery in Flowering Plants: The Control of Pollen Tube Growth . BioScience . 2007 . 57 . 10 . 835–844 . 10.1641/b571006. free.
  62. P. K. Hepler . J. G. Kunkel . C. M. Rounds . L. J. Winship . Calcium entry into pollen tubes . Trends in Plant Science . 2012 . 17 . 1 . 32–38 . 10.1016/j.tplants.2011.10.007 . 22104406.
  63. Wayne, R. . P. K. Hepler . The Role of Calcium Ions in Phytochrome-mediated germination of spores of Onoclea sensibilis L. . Planta . 1984 . 160 . 1 . 12–20 . 10.1007/bf00392460 . 24258366 . 14789256.
  64. Wayne, R. . P. K. Hepler . Red Light Stimulates and Increase in Intracellular Calcium in the Spores of Onoclea sensibilis . Plant Physiology . 1985 . 77 . 1 . 8–11 . 10.1104/pp.77.1.8 . 16664033 . 1064446.
  65. Saunders, M. J. . P. K. Hepler . Calcium ionophore A23187 stimulates cytokinin-like mitosis in Funaria . Science . 1982 . 217 . 4563 . 943–945 . 10.1126/science.217.4563.943 . 17747957 . 1982Sci...217..943S . 24442631.
  66. Saunders, M. J. . P. K. Hepler . Localization of membrane-associated calcium following cytokinin treatment in Funaria using chlortetracycline . Planta . 1981 . 152 . 3 . 272–281 . 10.1007/bf00385156 . 24302427 . 8122384.
  67. Conrad, P. A. . P. K. Hepler . The effect of 1,4-dihydropyridines on the initiation and development of gametophore buds in the moss Funaria . Plant Physiology . 1988 . 86 . 3 . 684–687 . 10.1104/pp.86.3.684 . 16665970 . 1054552.
  68. Book: Hepler, P. K., Lovy-Wheeler, A., McKenna, S. T., and Kunkel, J. G. . Ions and pollen tube growth. . The Pollen Tube . 2006 . 3 . 47–69 . https://www.bio.umass.edu/biology/kunkel/pub/reprints/HLMK_IonsPTG2006.pdf . August 8, 2019 . 10.1007/7089_043 . Plant Cell Monographs . 3-540-31121-1.
  69. Holdaway-Clarke, T. L., and Hepler, P. K. . . Control of pollen tube growth: Role of ion gradients and fluxes. . New Phytol. . 2003 . 159 . 3 . 539–563 . 10.1046/j.1469-8137.2003.00847.x . 33873604. 86549036 .
  70. Lovy-Wheeler, A., Kunkel, J. G., Allwood, E. G., Hussey, P. J., and Hepler, P. K. . Oscillatory increases in alkalinity anticipate growth and may regulate actin dynamics in pollen tubes of lily. . Plant Cell . 2006 . 18 . 9 . 2182–93 . 10.1105/tpc.106.044867 . 16920777 . 1560910.
  71. Feijó, J. A., Sainhas, J., Holdaway-Clarke, T., Cordiero, M. S., Kunkel, J. G., and Hepler, P. K. . Cellular oscillations and the regulation of growth: The pollen tube paradigm. . BioEssays . 2001 . 23 . 1 . 86–94 . 10.1002/1521-1878(200101)23:1<86::AID-BIES1011>3.0.CO;2-D . 11135313. 17904147 .
  72. Winship, L.J., Rounds, C., and Hepler, P. K. . Perturbation analysis of calcium, alkalinity and secretion during growth of lily pollen tubes . Plants . 2017 . 6 . 4 . 3 . 10.3390/plants6010003 . 28042810 . 5371762 . free.
  73. Holdaway-Clark, T.L., Feijo, J.A., Hackett, G.R., Kunkel, J.G., Hepler, P. K. . Pollen tube growth and the intracellular cytosolic calcium gradient oscillate in phase while extracellular calcium influx is delayed . Plant Cell . 1997 . 9 . 11 . 1999–2010 . August 8, 2019 . 10.2307/3870560 . 12237353 . 157053 . 3870560.
  74. Rounds, C.M., Hepler, P.K., and Winship, L.J. . The apical actin fringe contributes to localized cell wall deposition and polarized growth in the lily pollen tube . Plant Physiology . 2014 . 166 . 1 . 139–51 . 10.1104/pp.114.242974 . 25037212 . 4149702.
  75. McKenna, S.T., Kunkel, J.G., Bosch, M., Rounds, C.M., Vidali, L., Winship, L.J., and Hepler, P.K. . Exocytosis precedes and predicts the increase in growth in oscillating pollen tubes . Plant Cell . 21 . 10 . 3026–40 . 10.1105/tpc.109.069260 . 19861555 . 2782290 . 2009.
  76. Hepler, P.K., Rounds, C.M., and Winship, L.J. . Control of cell wall extensibility during pollen tube growth . Molecular Plant . 2013 . 6 . 4 . 998–1017 . 10.1093/mp/sst103 . 23770837 . 4043104.
  77. Web site: Jeanette Siron Pelton Award . Botanical Society of America . October 8, 2016.
  78. Web site: Hepler named fellow of American Society of Plant Biologists . UmassAmherst News & Media Relations . October 6, 2016.
  79. Web site: Peter K. Hepler . AAAS . October 8, 2016.
  80. Web site: Hepler wins national award for plant discoveries . UmassAmherst News & Media Relations . October 6, 2016.
  81. Web site: RMS Honorary Fellows . Royal Microscopical Society . October 6, 2016.
  82. Web site: Hepler Named Honorary Fellow of Royal Microscopical Society . UmassAmherst News & Media Relations . October 6, 2016.
  83. Web site: Peter K. Hepler Research Scholarship . UmassAmherst . 4 February 2016 . October 6, 2016.
  84. Web site: Plant Biology Annual Symposium History .

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