Cell Biology & Molecular Genetics
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Heven Sze

Plant Biologist

Ph.D., Purdue University, 1975
Telephone: (301) 405-1645
Fax: (301) 314-9081


Research Interests:  Cell and molecular biology. Understand plant growth, development, adaptation and reproduction through studies of membrane transport.

Membrane transport is essential for almost all cellular processes, and disruption often results in disease or death. New information, emerging from the study of completed genomes, has revealed a surprising degree of conservation of structure of many important membrane transporter families. Yet we do not know the substrates and mode of action of most membrane transporters, their regulation, and their impact on cellular and organ function. Model systems have several advantages. The genomes have been sequenced, and they can be genetically manipulated, thus simplifying the study of those transporters that are conserved among organisms. We are using plants and yeast to identify key transporters and to understand how organisms sense and respond to changes in nutrients and toxic compounds in the cell or in the environment.

The essential functions of Ca2+ depend on the spatial and temporal distribution of this ion within each cell. The dynamic changes of this ion within the cytosol and internal stores suggest that Ca channels and pumps are diverse and tightly regulated. Of multiple Ca-ATPases in Arabidopsis, only a few have been characterized. Major goals are to (1) identify and characterize Ca pumps after functional expression of plant genes in yeast mutants; (2) understand how expression and activity of transporters are regulated, and (3) understand the in vivo functions in plants. T-DNA disrupted mutants of pumps are being identified, thus allowing us to test for the first time whether growth, signaling responses, and male fertility are impaired.

Proton pumps occupy a prominent position among all transporters in plants and yeast. Without the primary motive force to energize coupled carriers or ion channels, all other transport, and thus life, would cease. In spite of this, it is not understood how proton pumps are integrated into the signal transduction networks that govern growth and adaptation. A major proton pump (vacuolar H+-ATPase) acidifies the vacuole and endomembrane compartments, and provides the driving force for transport of many ions and metabolites across the vacuolar membrane. Many novel H+-coupled cation transporters were uncovered in plant genomes, though their functions are largely unknown. A working model is that H+ pumps and cation/H exchangers affect development, reproduction and tolerance to stress through their effect on endomembrane trafficking. One goal is to understand these functions at the biochemical and cellular level using a combination of biochemical, cellular, molecular, genetic and genomic tools.

(The research programs have been supported by the National Science Foundation, USDA and the Department of Energy.)


Dr. Sze has taught courses in Plant Physiology, Advanced Cell Biology, and a seminar in plant cell biology. Current courses include
  • Plant Physiology- A 4-credit course consisting of 3 h of lecture and 3 h of laboratory taught every fall. Plant Physiology is the study of plant growth, survival and development and the physical, biochemical and molecular mechanisms underlying these processes. The course covers how plants take up inorganic nutrients and water and how nutrients and water are distributed throughout the plant body, how plants convert solar energy to food energy, and how plants respond to environmental and hormonal stimuli in order to develop and to survive.
  • Advanced Plant Development and Physiology is a 2 credit course consisting of lectures and journal discussions aimed at graduate students majoring in plant biology. The course takes an integrated experimental approach, including cellular, biochemical, genetic, molecular and functional genomic methods to understand the concepts and principles of major processes, such as plant defense, and plant responses to hormonal and environmental stimuli.


Usually consists of 3-4 members, including postdoctoral fellows and graduate students. Current members are Dr. Senthilkumar Padmanaban, Yongxian Lu, Salil Chanroj, and Kevin Bock.

Selected Recent Publications:

Liang F, KW Cunningham, JF Harper & H Sze. (1997) ECA1 complements yeast mutants defective in Ca pumps and encodes an ER-type Ca-ATPase in Arabidopsis thaliana. Proc. Natl. Acad. Sci. 94, 8579-8584

Li, X, H-t Hsu, RT Su & H Sze. (1998) The molecular chaperone calnexin associates with the vacuolar H+-ATPase from oat seedling. The Plant Cell 10, 119-130

Hwang, I., F. Liang, JF Harper, and H. Sze (2000). Calmodulin interacts with the autoinhibitory domain at the N terminus of ACA2, a novel Ca pump from Arabidopsis. Plant Physiol. 122, 157-167

Sze, H., Xuhang Li, and M. Palmgren. (1999) Energization of the plant cell membranes by H+-pumping ATPases: biosynthesis and regulation.The Plant Cell 11, 677-689 (review)

Hwang I, H. Sze, JF Harper (2000) A Ca-dependent protein kinase can inhibit a calmodulin stimulated Ca-pump (ACA2) located in the ER of Arabidopsis . Proc Natl Acad Sci 97, 6224-6229

Sze H, Liang F, Hwang I, Curran A, Harper JF (2000) Diversity and Regulation of Plant Ca2+ Pumps: Insights from expression in yeast. Annu Rev Plant Physiol Plant Mol Biol 51: 433-462 (Review)

Sze H, K Schumacher, ML Muller, S. Padmanaban and Lincoln Taiz. (2002) A simple nomenclature for a complex proton pump: VHA genes encode the Vacuolar H+-ATPase. Trends Plant Sci 7, 157-161

Wu Z, F Liang, B Hong, JC Young, MR Sussman, JF Harper and H Sze (2002). An ER-bound Ca2+/Mn2+ pump, AtECA1, supports plant growth and confers tolerance to Mn2+ stress. Plant Physiol. 130:128-37

Padmanaban S, X Lin, I Perera, Y Kawamura and H Sze. (2004)  Differential expression of vacuolar H+-ATPase subunit c genes in tissues active in membrane trafficking and their roles in plant growth as revealed by RNAi.  Plant Physiol. 134:1514-26

Sze, H, Padmanaban S, Cellier F, Honys D, Cheng NH, KW. Bock, G. Conejero, X Li, Twell D, Ward J, K Hirschi (2004).  Expression pattern of a novel gene family, AtCHX, highlights their potential roles in osmotic adjustment and K+ homeostasis in pollen biology.  Plant Physiol. 136:2532-47

Bock KW, D Honys, JM. Ward, S Padmanaban, EP Nawrocki, KD Hirschi, D Twell, and H Sze (2006)  Integrating Membrane Transport with Male Gametophyte Development and Function through Transcriptomics.  Plant Physiol. 140, 1151-1168

Sze H, Frietsch S, Li X, Bock KW, Harper JF (2006)  Genomic and Molecular Analyses of Transporters in the Male Gametophyte.  In ‘The Pollen Tube’ Ed. R. Malho.  Plant Cell Monograph 3, 71 Springer-Verlag. (Review)

Padmanaban S, Chanroj S, Kwak J, Li X, Ward JM, Sze H.  (2007) Participation of an endomembrane cation/H+ exchanger AtCHX20 in osmoregulation of guard cells.  Plant Physiol. 144(1): 82-93


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