Peter K. Hepler

Peter Klock Hepler HonFRMS 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.

Peter K. Hepler
Born (1936-10-29) October 29, 1936
Dover, New Hampshire
NationalityAmerican
CitizenshipUnited States
Alma materUniversity of New Hampshire, B.S. Chemistry 1958
University of Wisconsin, Ph.D. Plant Cell Biology 1964
Known forCell biology, plant physiology, microscopy
Scientific career
FieldsCell biology, plant physiology, microscopy
InstitutionsStanford University
University of Massachusetts at Amherst
WebsitePeter K. Hepler
Molecular & Cellular Biology

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 by John Gray[10] in 1740[11] and is now a part of the Kestrel Land Trust.[12]

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[13] and then became a postdoctoral fellow with Keith Porter[14] at Harvard University from 1966-1967, 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 1980-1989, 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.[15] Hepler spent many summers teaching and doing research at the Marine Biological Laboratory[16][17] at Woods Hole, Massachusetts. Hepler also participated in a multiyear international collaboration with Brian E. S. Gunning.[18]

Hepler was an Associate Editor of Protoplasma from 1994-2001 and Associate Editor of Plant Physiology from 1998-2000. 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.[19][20] 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,[21][22][23] stomatal function,[24][25][26][27] the role of calcium in plant development[28] and in the development of techniques useful for answering questions using light[29][30][31][32][33] and electron microscopy.[34] Hepler's scientific publications with Barry A. Palevitz are notable for quoting Woody Allen and Yogi Berra.[35]

Hepler described his realization of the influence a review he and Palevitz[4] 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."[4]

Hepler, along with Ledbetter and Porter,[36] is considered to be a co-discoverer of microtubules.[14]

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.[37] Building on the earlier work by Sinnott and Bloch,[38] 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.[39] 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[36] and Green[40] established the importance of cortical microtubules in controlling the alignment of cellulose microfibrils in the cell wall.[41][42] 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.[43] 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.[34][44]

Microtubules and cell motility

Building on the work of Shinya Inoué and Andrew Bajer using polarized light microscopy,[45] 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.[46]

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.[47]

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[48] 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.[49] 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,[50][51] Hepler, along with Dale Callaham, Dahong Zhang, and Patricia Wadsworth, observed calcium ion transients during mitosis[52][53] 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.[54][55][56]

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.[57][58] 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.[59]

Calcium and plant development

Hepler has shown that calcium ions are a central regulator of plant growth and development[60] specifically demonstrating that calcium is important for tip growth[61][62][63] and in phytochrome.[64][65] and cytokinin[66][67][68] 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.[69] The intracellular free calcium ions exist in a gradient dropping from 3000 nM at the tip to 200 nM 20 μm from the tip [70] and the intracellular H+ gradient falls from pH 6.8 at the tip to pH 7.5 10–30 μm from the tip.[71] 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.[72] Energy is required for pollen tube growth[73] 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.[74] 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.[75] The secretion of the vesicles at the growing tip anticipates the increase in growth rate,[76] 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+.[77] 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
  • In 1975, Hepler was the fourth recipient of the Jeanette Siron Pelton Award given by the Botanical Society of America, because his "penetrating analytical and experimental studies of the ultrastructure of differentiating cells have made a significant and lasting contribution to our perception of morphogenesis at the cellular level. In particular his work on the ultrastructure of differentiating xylem elements, on the roles of microtubules and microfibrils, and on the control of the orientation of mitotic spindles in differentiating cells have provided new insights which hold great promise for the future."[78]
  • In 2007, Hepler was named an inaugural Fellow of the American Society of Plant Biologists.[79]
  • In 2010, Hepler was elected as a Fellow of the American Association for the Advancement of Science for his contributions as "one of the most influential plant cell biologists, who has continuously and continues to achieve breakthroughs that have guided research directions of numerous plant scientists."[19][20][80]
  • In 2011, Hepler was honored with the Charles Reid Barnes Life Membership Award from the American Society of Plant Biologists.[81]
  • In 2015, Hepler was named an Honorary Fellow of the Royal Microscopical Society for his contributions to plant science, including publishing the first report suggesting a co-alignment of microtubules with cell wall cellulose microtubules.[82][83]
  • A scholarship was named in honor of Hepler. The Peter K. Hepler Research Scholarship supports undergraduate research on a biological question in a laboratory or field setting outside of the United States.[84]
  • The Plant Biology Graduate Program at the University of Massachusetts Amherst held a symposium on October 14, 2017 entitled: Capturing the dynamic architecture of cells: Honoring the high-resolution career of Peter Hepler. Friends, family, students, and colleagues celebrated his life and contributions to plant cell biology.[85]

References
  1. Hepler, P. K.; R. O. Wayne (1993). "This Week's Citation Classic" (PDF). Current Contents (July 26, 1993) (30): 8. Retrieved October 6, 2016.
  2. Hepler, P. K., S. M. Wick and S. M. Wolniak (1981). The structure and role of membranes in the mitotic apparatus. in: International Cell Biology 1980–1981, H.G. Schweiger, ed. Berlin: Springer-Verlag. pp. 673–686.CS1 maint: multiple names: authors list (link)
  3. Hepler, P. K.; B. A. Palevitz (1974). "Microtubules and microfilaments". Annual Review of Plant Physiology. 25: 309–362. doi:10.1146/annurev.pp.25.060174.001521.
  4. Hepler, P. K.; B. A. Palevitz (1986). "Microtubules and microfilaments" (PDF). Current Contents (August 11, 1986) (32): 20. Retrieved October 7, 2016.
  5. Hepler, J. R. (1922). Methods in Forcing Rhubarb: M.S. Thesis. University of Wisconsin. ISBN 978-1273396984.
  6. Hepler, Billy (2012). "America's Youngest Seed Grower" (PDF). Heritage Farm Companion (Summer): 6–9.
  7. "A Bean Collector's Window". Retrieved October 18, 2016.
  8. Hepler, Anna. "Anna Hepler Intricate Universe". Retrieved October 7, 2016.
  9. "Membership Corner" (PDF) (31(5), 22). APBS News September/October 2004. Archived from the original (PDF) on 2016-04-04. Retrieved 2016-10-07.
  10. "John Gray". Find a Grave. Retrieved August 24, 2019.
  11. "Hepler Family (Pelham, MA)". UmassAmherst: MassWoods. Retrieved October 6, 2016.
  12. "Kestrel Land Trust: Conserve the Valley You Love". Kestrel Land Trust. Retrieved October 6, 2016.
  13. VandenBosch, K. A., W. Becker and B. A Palevitz (1996). "The natural history of a scholar and gentleman: A biography of Eldon H. Newcomb". Protoplasma. 195 (1–4): 4–11. doi:10.1007/bf01279181. S2CID 32568416.CS1 maint: multiple names: authors list (link)
  14. Hepler, P. K., J. D. Pickett-Heaps and B. E. S. Gunning (2013). "Some retrospectives on early studies of plant microtubules". The Plant Journal. 75 (2): 189–201. doi:10.1111/tpj.12176. PMID 23496242.CS1 maint: multiple names: authors list (link)
  15. Hepler, Peter K. (2016). "Founders' Review: The Cytoskeleton and Its Regulation by Calcium and Protons". Plant Physiology. 170 (1): 3–22. doi:10.1104/pp.15.01506. PMC 4704593. PMID 26722019.
  16. "MBL Society Members". Marine Biological Laboratory. Retrieved October 6, 2016.
  17. "Physiology 1981". History of the Marine Biological Laboratory. Retrieved October 6, 2016.
  18. Hepler, P. K.; B. E. S. Gunning (1998). "Confocal fluorescence microscopy of plant cells". Protoplasma. 201 (3): 121–157. doi:10.1007/bf01287411. S2CID 1258312.
  19. "AAAS Members Elected as Fellows". AAAS. Retrieved October 6, 2016.
  20. "Members in the News". ASPB Newsletter 33(3), 26. April 2010. Archived from the original on October 9, 2016. Retrieved October 6, 2016.
  21. Hepler, P. K.; E. H. Newcomb (1967). "Fine structure of cell plate formation in the apical meristem of Phaseolus roots". Journal of Ultrastructure Research. 19 (5–6): 498–513. doi:10.1016/s0022-5320(67)80076-5. PMID 6055780.
  22. Palevitz, B. A.; P. K. Hepler (185). "Changes in dye coupling of stomatal cells of Allium and Commelina demonstrated by microinjection of Lucifer yellow". Planta. 164 (4): 473–479. doi:10.1007/bf00395962. PMID 24248219. S2CID 30377452.
  23. Turgeon, R.; P. K. Hepler (1989). "Symplastic continuity between mesophyll and companion cells in minor veins of mature Cucurbita pepo L. leaves". Planta. 179 (1): 24–31. doi:10.1007/bf00395767. PMID 24201418. S2CID 21975131.
  24. Zeiger, E.; P. K. Hepler (1976). "Production of Guard Cell Protoplasts from Onion and Tobacco". Plant Physiology. 58 (4): 492–498. doi:10.1104/pp.58.4.492. PMC 543252. PMID 16659703. Retrieved October 6, 2016.
  25. Zeiger, E., W. Moody, P. Hepler and F. Varela (1977). "Light-sensitive membrane potentials in onion guard cells". Nature. 270 (5634): 270–271. Bibcode:1977Natur.270..270Z. doi:10.1038/270270a0. S2CID 4162345.CS1 maint: multiple names: authors list (link)
  26. Zeiger, E.; P. K. Hepler (1977). "Light and stomatal function: blue light stimulates swelling of guard cell protoplasts". Science. 196 (4292): 887–889. Bibcode:1977Sci...196..887Z. doi:10.1126/science.196.4292.887. PMID 17821809. S2CID 13433483.
  27. Zeiger, E.; P. K. Hepler (1979). "Blue light-induced, intrinsic vacuolar fluorescence in onion guard cells". Journal of Cell Science. 37: 1–10. PMID 479318. Retrieved October 6, 2016.
  28. Hepler, Peter (2005). "Calcium: An essential regulator of plant growth and development". The Plant Cell. 17 (8): 2142–2155. doi:10.1105/tpc.105.032508. PMC 1182479. PMID 16061961.
  29. Zhang, D., P. Wadsworth, and P. K. Hepler (1990). "Microtubule dynamics in living dividing cells: Confocal imaging of microinjected fluorescent brain tubulin". Proc. Natl. Acad. Sci. USA. 87 (22): 8820–8824. Bibcode:1990PNAS...87.8820Z. doi:10.1073/pnas.87.22.8820. PMC 55051. PMID 11607116.CS1 maint: multiple names: authors list (link)
  30. Zhang, D., P. Wadsworth and P. K. Hepler (1993). "Dynamics of microfilaments are similar, but distinct from microtubules during cytokinesis in living, dividing plant cells". Cell Motility and the Cytoskeleton. 24 (3): 151–155. doi:10.1002/cm.970240302.CS1 maint: multiple names: authors list (link)
  31. Valster, A. H., E. S. Pierson, Valenta, P. K. Hepler and A. M. C. Emons (1997). "Probing the Plant Actin Cytoskeleton during Cytokinesis and Interphase by Profilin Microinjection". The Plant Cell. 9 (10): 1815–1824. doi:10.1105/tpc.9.10.1815. PMC 157024. PMID 12237348.CS1 maint: multiple names: authors list (link)
  32. Vos, J. W., A. H. Valster and P. K. Hepler (1988). Methods for Studying Cell Division in Higher Plants. Methods in Cell Biology. 61. pp. 413–437. doi:10.1016/S0091-679X(08)61992-5. ISBN 9780125441636. PMID 9891326.CS1 maint: multiple names: authors list (link)
  33. Hepler, P. K.; J. Hush (1996). "Behavior of Microtubules in Living Plant Cells". Plant Physiology. 112 (2): 455–461. doi:10.1104/pp.112.2.455. PMC 157968. PMID 12226402.
  34. Lancelle, S. A., D. A. Callaham and P. K. Hepler (1986). "A method for rapid freeze fixation of plant cells". Protoplasma. 131 (2): 153–165. doi:10.1007/bf01285037. S2CID 19236616.CS1 maint: multiple names: authors list (link)
  35. "Poems and Quotations About the MicroWorld". Microscopy Society of America. Retrieved October 6, 2016.
  36. Ledbetter, M. C.; K. R. Porter (1963). "A 'microtubule' in plant cell fine structure". Journal of Cell Biology. 19 (1): 239–250. doi:10.1083/jcb.19.1.239. PMC 2106853. PMID 19866635.
  37. Newcomb, E. H. (1996). "A career in science: Fulfillment of a dream". Protoplasma. 195 (1–4): 1–3. doi:10.1007/bf01279180. S2CID 12850336.
  38. Sinnott, E. W.; R. Bloch (1945). "The cytoplasmic basis of intercellular patterns in vascular differentiation". American Journal of Botany. 32 (3): 151–156. doi:10.2307/2437535. JSTOR 2437535.
  39. Hepler, P. K.; E. H. Newcomb (1964). "The Fine Structure of Young Tracheary Xylem Elements Arising by Redifferentiation of Parenchyma in Wounded Coleus Stem". Journal of Experimental Botany. 14 (3): 496–503. doi:10.1093/jxb/14.3.496. Retrieved October 6, 2016.
  40. Green, P. B. (1962). "Mechanism for plant cellular morphogenesis". Science. 138 (3548): 1404–1405. Bibcode:1962Sci...138.1404G. doi:10.1126/science.138.3548.1404. PMID 17753861. S2CID 39081841.
  41. Torrey, J. G., D. E. Fosket and P. K. Hepler (1971). "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. 59 (3): 338–352. JSTOR 27829621.CS1 maint: multiple names: authors list (link)
  42. Wasteneys, G. O.; F. Brandizzi (2013). "A Glorious Half-Century of Microtubules". The Plant Journal. 75 (2): 185–188. doi:10.1111/tpj.12260. PMID 23834223.
  43. Palevitz, B. A.; P. K. Hepler (1976). "Cellulose microfibril orientation and cell shaping in developing guard cells of Allium: The role of microtubules and ion accumulation". Planta. 132 (1): 71–93. doi:10.1007/BF00390333. PMID 24424910. S2CID 2744599.
  44. Lancelle, S. A., M. Cresti and P. K. Hepler (1987). "Ultrastructure of the cytoskeleton in freeze-substituted pollen tubes of Nicotiana alata". Protoplasma. 140 (2–3): 141–150. doi:10.1007/bf01273723. S2CID 6452268.CS1 maint: multiple names: authors list (link)
  45. Inoué, S.; A. Bajer (1961). "Birefringence in endosperm mitosis". Chromosoma. 12: 48–63. doi:10.1007/bf00328913. PMID 13717778. S2CID 5069716.
  46. Hepler, P. K.; W. T. Jackson (1968). "Microtubules and early stages of cell plate formation in the endosperm of Haemanthus katherinae Baker". Journal of Cell Biology. 38 (2): 437–446. doi:10.1083/jcb.38.2.437. PMC 2107485. PMID 5664211.
  47. Hepler, P. K. (1976). "The blepharoplast of Marsilea: Its de novo formation and spindle association". Journal of Cell Science. 21 (2): 361–390. PMID 972175. Retrieved October 6, 2016.
  48. Weisenberg, R. C. (1972). "Microtubule formation in vitro in solutions containing low calcium concentration". Science. 177 (4054): 1104–1105. Bibcode:1972Sci...177.1104W. doi:10.1126/science.177.4054.1104. PMID 4626639. S2CID 34875893.
  49. Hepler, P. K. (1980). "Membranes in the mitotic apparatus of barley cells". Journal of Cell Biology. 86 (2): 490–499. doi:10.1083/jcb.86.2.490. PMC 2111505. PMID 7400216.
  50. Wick, S. M.; P. K. Hepler (1980). "Localization of Ca++-containing antimonate precipitates during mitosis". Journal of Cell Biology. 86 (2): 500–513. doi:10.1083/jcb.86.2.500. PMC 2111497. PMID 7400217.
  51. Wolniak, S. M., P. K. Hepler, and W. T. Jackson (1980). "Detection of the membrane-calcium distribution during mitosis in Haemanthus endosperm with chlorotetracycline". Journal of Cell Biology. 87 (1): 23–32. doi:10.1083/jcb.87.1.23. PMC 2110715. PMID 7419592.CS1 maint: multiple names: authors list (link)
  52. Hepler, P. K.; D. A. Callaham (1987). "Free calcium increases during anaphase in stamen hair cells of Tradescantia". Journal of Cell Biology. 105 (5): 2137–2143. doi:10.1083/jcb.105.5.2137. PMC 2114859. PMID 3680374.
  53. Hepler, P. K. (1989). "Calcium transients during mitosis: Observations in flux". Journal of Cell Biology. 109 (6): 2567–2573. doi:10.1083/jcb.109.6.2567. PMC 2115931. PMID 2687283. Retrieved October 7, 2016.
  54. Zhang, D. H. (1990). "Regulation of anaphase chromosome motion in Tradescantia stamen hair cells by calcium and related signaling agents". Journal of Cell Biology. 111 (1): 171–182. doi:10.1083/jcb.111.1.171. PMC 2116166. PMID 2114409.
  55. Zhang, D. H., P. Wadsworth, and P. K. Hepler (1990). "Microtubule dynamics in living dividing plant cells: Confocal imaging of microinjected fluorescent brain tubulin". Proc. Natl. Acad. Sci. USA. 87 (22): 8820–8824. Bibcode:1990PNAS...87.8820Z. doi:10.1073/pnas.87.22.8820. PMC 55051. PMID 11607116.CS1 maint: multiple names: authors list (link)
  56. Zhang, D. H., P. Wadsworth and P. K. Hepler (1992). "Modulation of anaphase spindle microtubule structure in stamen hair cells of Tradescantia by calcium and related agents". Journal of Cell Science. 102 (1): 79–89. Retrieved October 6, 2016.CS1 maint: multiple names: authors list (link)
  57. Palevitz, B. A., J. F. Ash, and P. K. Hepler (1974). "Actin in the green alga, Nitella". Proc. Natl. Acad. Sci. USA. 71 (2): 363–366. Bibcode:1974PNAS...71..363P. doi:10.1073/pnas.71.2.363. PMC 388005. PMID 4592689.CS1 maint: multiple names: authors list (link)
  58. Palevitz, B. A.; P. K. Hepler (1975). "Identification of actin in situ at the ectoplasm-endoplasm interface of Nitella. Microfilament-chloroplast association". Journal of Cell Biology. 65 (1): 29–38. doi:10.1083/jcb.65.1.29. PMC 2111164. PMID 1127014.
  59. Kersey, Y. M., P. K. Hepler, B. A. Palevitz, and N. K. Wessells (1976). "Polarity of actin filaments in Characean algae". Proc. Natl. Acad. Sci. USA. 73 (1): 165–167. Bibcode:1976PNAS...73..165K. doi:10.1073/pnas.73.1.165. PMC 335861. PMID 1061112.CS1 maint: multiple names: authors list (link)
  60. Hepler, P. K. (2005). "Historical Perspective Essay: Calcium: a central regulator of plant growth and development". Plant Cell. 17 (8): 2142–55. doi:10.1105/tpc.105.032508. PMC 1182479. PMID 16061961.
  61. Miller, D. D., D. A. Callaham, D. J. Gross and P. K. Hepler (1992). "Free Ca2+ gradient in growing pollen tubes of Lilium". Journal of Cell Science. 101: 7–12. Retrieved October 7, 2016.CS1 maint: multiple names: authors list (link)
  62. Wilsen, K. L.; P. K. Hepler (2007). "Sperm Delivery in Flowering Plants: The Control of Pollen Tube Growth". BioScience. 57 (10): 835–844. doi:10.1641/b571006.
  63. P. K. Hepler; J. G. Kunkel; C. M. Rounds; L. J. Winship (2012). "Calcium entry into pollen tubes". Trends in Plant Science. 17 (1): 32–38. doi:10.1016/j.tplants.2011.10.007. PMID 22104406.
  64. Wayne, R.; P. K. Hepler (1984). "The Role of Calcium Ions in Phytochrome-mediated germination of spores of Onoclea sensibilis L.". Planta. 160 (1): 12–20. doi:10.1007/bf00392460. PMID 24258366. S2CID 14789256.
  65. Wayne, R.; P. K. Hepler (1985). "Red Light Stimulates and Increase in Intracellular Calcium in the Spores of Onoclea sensibilis". Plant Physiology. 77 (1): 8–11. doi:10.1104/pp.77.1.8. PMC 1064446. PMID 16664033.
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  72. Feijó, J. A., Sainhas, J., Holdaway-Clarke, T., Cordiero, M. S., Kunkel, J. G., and Hepler, P. K. (2001). "Cellular oscillations and the regulation of growth: The pollen tube paradigm". BioEssays. 23 (1): 86–94. doi:10.1002/1521-1878(200101)23:1<86::AID-BIES1011>3.0.CO;2-D. PMID 11135313.CS1 maint: multiple names: authors list (link)
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  74. Holdaway-Clark, T.L., Feijo, J.A., Hackett, G.R., Kunkel, J.G., Hepler, P. K. (1997). "Pollen tube growth and the intracellular cytosolic calcium gradient oscillate in phase while extracellular calcium influx is delayed" (PDF). Plant Cell. 9 (11): 1999–2010. doi:10.2307/3870560. JSTOR 3870560. PMC 157053. PMID 12237353. Retrieved August 8, 2019.CS1 maint: multiple names: authors list (link)
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  76. McKenna, S.T., Kunkel, J.G., Bosch, M., Rounds, C.M., Vidali, L., Winship, L.J., and Hepler, P.K. (2009). "Exocytosis precedes and predicts the increase in growth in oscillating pollen tubes". Plant Cell. 21 (10): 3026–40. doi:10.1105/tpc.109.069260. PMC 2782290. PMID 19861555.CS1 maint: multiple names: authors list (link)
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