Dr. Smadar Harpaz-Saad

Plant Cell wall plays a major role in determining plants architecture and plant’s adaptation to different stresses.

Plant cell wall is composed mainly from polysaccharides, proteins and phenolic compounds. The cell wall polysaccharides have immense economic importance as they are central players in determining the mechanical and textural properties of food, wood, paper and textile fibers. Moreover, they serve as major source for the emerging industry of biofuels for renewable energy.

In our lab we use genetic, molecular biology and bioinformatic approaches aimed to identify novel elements involved in cell wall biosynthesis and modification.


Research Interests

The mechanism employed in the regulation of cellulose biosynthesis

Plant cell growth is restricted by its surrounding cell wall. It is therefore, that changes in cell shape and size require the modification of the existing cell walls as well as deposition of newly synthesized cell-wall material to enable growth. Polysaccharides are the most abundant polymers in the cell wall and include cellulose microfibrils, hemicelluloses (e.g. xyloglucan, xylans, and mannans) and pectins. Plants, being a sessile organism, often respond to different environmental signals by modification of their growth pattern.

Cellulose microfibrils, which act as the main load bearing elements in the cell wall, are usually highly oriented, creating mechanical anisotropy in the wall, which in turn determines the growth direction. Despite recent advances, much is still unknown regarding the core mechanism employed in the biosynthesis and assembly of cellulose microfibrils and even less regarding the regulation of this mechanism in response to different developmental and environmental signals.

In our lab we are interested in the identification and characterization of novel elements involved in the regulation of cellulose biosynthesis during plants life cycle and their role in plants accommodation to changing environment. We use genetics, bioinformatics and molecular biology as complementary tools in order to gain better understanding of the mechanism employed in cell wall function, specifically in cellulose biosynthesis.

Bioinformatic approach aimed at the identification of new genes involved in cell wall function

Functionally related genes are often expressed in highly overlapping manner making gene coexpression analysis a powerful tool for rapid gene function prediction. Recently, the accumulating data from transcriptional studies proceeded via microarrays and deep sequencing methods have generated vast amounts of publicly available gene expression data, setting the stage for hypothesis-driven gene function discovery.

In our lab we are developing a combined bioinformatic and genetic approach aimed at the identification of novel elements employed in the regulation of cellulose biosynthesis and cell wall function. Our working hypothesis is that some biologically relevant transcriptional relationships might be revealed only under specific experimental conditions or tissue-specific data sets, and are therefore overlooked by the commonly used global coexpression analyses. For instance, in case of context dependent hetero-dimerization of protein-of-interest with different proteins in different tissues, physiological or developmental contexts, as was previously shown for cellulose synthase or receptor-like kinases.

Preliminary results led to the identification of new genes involved in the regulation of cellulose biosynthesis thus suggesting it is an attractive method to rapidly identify novel genes involved in the regulation of cell wall components biosynthesis.

Seed mucilage as a unique system for the study of cell wall metabolism

Recent studies, established a new role for cellulose in anchoring the pectic component of seed coat mucilage to the seed surface. Seed coat mucilage, composed primarily of pectins, is produced by the maternally derived seed coat epidermal cells (also called mucilage secretory cells; MSCs). A massive induction of pectin biosynthesis and deposition into the apoplast occurs within a narrow developmental window during the course of MSCs differentiation. This phase is followed by extensive cellulose deposition into the radial cell walls and the cytoplasm of MSCs, generating a volcano-shaped secondary cell wall structure called collumela. The sequential induction of cell wall related metabolic pathways over the course of MSCs differentiation makes it an attractive system for the study of cell wall polymers biosynthesis and modification.

In our lab we hypothesize that Arabidopsis seed coat mucilage secretory cells can serve as a unique model system for the study of cellulose microfibrils biosynthesis and assembly. We suggest that specialized, yet equivalent mechanisms are employed in cellulose biosynthesis in different developmental contexts. We therefore engage two complementary approaches in order to use MSC as a tool for the study of cellulose metabolism:

  • i) tissue/condition-dependent coexpression analysis
  • ii) Forward genetics.

Fruit tree physiology

Personally, as I grew up in a fruit trees growers family, I see as one of our main goals to incorporate novel genome-based technics into the study of fruit trees physiology. We aim to develop a research expertise in studying fruit trees physiology as a tool for knowledge-based development of agro-technical treatments.


Selected Publications

  • Harpaz-Saad, S., Western, T.L. and Kieber, J.J. (2012). 
    The FEI2-SOS5 pathway and CELLULOSE SYNTHASE 5 are required for cellulose biosynthesis in the Arabidopsis seed coat and affect pectin mucilage structure. Plant Signaling and Behavior, 7:285-288.
  • Harpaz-Saad, S., Yoon, G. M., Mattoo, A. K., and Kieber , J. J. (2012). 
    The Formation of ACC and Competition Between Polyamines and Ethylene for SAM. Annual Plant Reviews 44: 53–81.
  • Harpaz-Saad, S., McFarlane, H.E., Xu, S., Divi, U.K., Forward, B., Western, T.L. and Kieber, J.J. (2011). 
    Cellulose synthesis via the FEI2 RLK/SOS5 pathway and CELLULOSE SYNTHASE 5 is required for the structure of seed coat mucilage in Arabidopsis. Plant Journal, 68:941-953. (Featured article; cover photo).
  • Azoulay-Shemer, T., Harpaz-Saad, S., Cohen-Peer, R., Mett, A., Spicer, V., Lovat, N., Krokhin, O., Brand, A., Gidoni, D., Standing, K.G., Goldschmidt, E.E. and Eyal, Y. (2011). 
    Dual N- and C-terminal processing of citrus chlorophyllase precursor within the plastid membranes leads to the mature enzyme. Plant and Cell Physiology, 52:70-83.
  • Berger, Y., Harpaz-Saad, S., Brand, A., Melnik, H., Sirding, N., Alvarez, J. P., Zinder, M., Samach, A., Eshed, Y. and Ori, N. (2009). 
    The NAC-domain transcription factor GOBLET specifies leaflet boundaries in compound tomato leaves. Development. 136:823-832. (Featured article; cover photo).
  • Harpaz-Saad, S., Azoulay, T., Arazi, T., Ben-Yaakov, E., Mett, A., Shibolet, Y. M., Hortensteiner, S., Gidoni, D., Gal-On, A., Goldschmidt, E. E., and Eyal, Y. (2007). 
    Chlorophyllase is a rate limiting enzyme in chlorophyll catabolism and is posttranslationally regulated. Plant Cell, 19:1007-1022.
  • Azoulay Shemer, T., Harpaz-Saad, S., Belausov, E., Lovat, N., Krokhin, O., Spicer, V., Standing, K. G., Goldschmidt, E. E. and Eyal, Y. (2008). 
    Citrus chlorophyllase dynamics at ethylene-induced fruit color-break; a study of chlorophyllase expression, posttranslational processing kinetics and in-situ intracellular localization. Plant Physiology, 148:108-118.
  • Ben-Yaakov, E., Harpaz-Saad, S., Galili, D., Eyal, Y. and Goldschmidt, E. E. (2006). 
    Chlorophyllase activity and chlorophyll degradation during the course of leaf senescence. Israeli J. Plant Sciences, 54:129-135.
  • Harpaz, S., Goldschmidt, E. E., Gizis, A., Vaya, Y. and Ben-Arie, R. (2001). 
    Post-harvest treatments of lychee fruit as revealed in fruit surface morphology and enzyme activity. Acta Horticulture, 313-315.

Lab Members

Eran Barack Undergraduate student

Lea Katz M.Sc. student

Yael Ozik Undergraduate student

Daniela Ben-Tov M.Sc. student

Important Links

The Arabidopsis Information Resource (TAIR)

The Salk Institute Genomic Analysis Laboratory (SIGNAL) for the identification of insertion mutants and cloned genes of interest 
http://signal.salk.edu/ (T-DNA Express) 

The BAR Arabidopsis eFP browser (University of Toronto) for exploring gene expression pattern 

Gene expression network in seed development (Goldberg and Harada) 

Open Positions

As our lab was only recently established, on July 2012, 
we are looking for M.Sc. and Ph.D. students.

Please contact us for more details.