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CROP SCIENCE

Cover illustration includes the transmission electron micrographs of Dr. Utku Avci

written by: Dr. Candace Haigler
Professor of Crop Science and Plant Biology,
North Carolina State University

This cover illustration includes the transmission electron micrographs of Dr. Utku Avci, formerly a Ph.D. student and now a Postdoctoral Research Associate in the Department of Crop Science working in the laboratory of Dr. Candace Haigler. It highlights an article that provides novel information about the genetic regulation of xylem differentiation. The research study was collaborative with Dr. Eric Beers and co-workers at Virginia Polytechnic Institute and State University.

Water-conducting xylem cells are essential for plant growth and function in order to move water from the soil to the most aerial parts of the plant. The number, size, shape, and placement of the water conducting cells also has strong effects on plant adaptation in natural and crop production systems. This paper supports the role of a NAC domain transcription factor, XND1, in the negative regulation of xylem cell differentiation. This was considered a particularly novel finding because several other NAC transcription factors are positive effectors of the same differentiation program. The results were obtained in the model plant, Arabidopsis, taking advantage of its rich genetic resources as well as its small size and predictable development that facilitate microscopic observations.

The differentiation of xylem tracheary elements requires the coordination of cell elongation (to define the final length of the conducting cell), secondary cell wall synthesis (to strengthen the conducting cell), and programmed cell death (to leave a hollow cell wall that can support water transport). When XND1 was over-expressed in Arabidopsis, conducting xylem elements did not form and the plants were extremely dwarfed because they could not transport water. In the cover image, the larger two-week-old control plants contrast dramatically with the severely dwarfed XND1 over-expressors of the same age. The corresponding transmission electron micrographs illustrate that cells that should have become conducting xylem elements did not deposit secondary walls or die; instead they remained alive with thin cell walls. In contrast, the sugar conducting cells (phloem) continued to form and even had thicker cell walls than in controls.

These results, plus other genetic manipulations described in the paper, support the contention that XND1 affects xylem differentiation through regulation of secondary wall synthesis and programmed cell death. We now hypothesize that in stressful natural or crop environments in other species, the activity of XND1-type transcription factors may help to regulate the timing and balance of growth vs. survival metabolic activities by regulating xylem differentiation.

For more information see http://www.blackwell-synergy.com/loi/TPJ and: Zhao C, Avci U, Grant EH, Haigler CH, Beers EP (2008) XND1, a member of the NAC domain family in Arabidopsis thaliana, negatively regulates lignocellulose synthesis and programmed cell death in xylem. Plant Journal, 53: 425-436.

Crop Science contacts are: Candace_haigler@ncsu.edu and uavci@unity.ncsu.edu.