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Published on June 18, 2008; 10.1104/pp.108.116962

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Received January 27, 2008
Accepted June 6, 2008

Restoration of Mature Etiolated Cucumber Hypocotyl Cell Wall Susceptibility to Expansin by Pretreatment with Fungal Pectinases and EGTA In vitro

Qingxin Zhao , Sheng Yuan *, Xin Wang , Yuling Zhang , Hong Zhu , and Changmei Lu

Nanjing Engineering and Biotechnology Research Center for Microbiology, Jiangsu Key Laboratory for Biodiversity and Biotechnology, College of Life Science, Nanjing Normal University, 1 Wenyuan Road, Nanjing 210046, PR China; Present address: College of Biology and Biotechnology, Yancheng Teachers' University, Yancheng 224002, PR China

* Corresponding author; email: shengyuan{at}email.njnu.edu.cn; yuansheng2006@yahoo.com.cn.

Mature plant cell walls lose their ability to expand and become unresponsive to expansin. This phenomenon is believed to be due to crosslinking of hemicellulose, pectin, or phenolic groups in the wall. By screening various hydrolytic enzymes, we found that pretreatment of non-growing, heat-inactivated, basal cucumber hypocotyls with pectin lyase (Pel1) from Aspergillus japonicus could restore reconstituted exogenous expansin-induced extension in mature cell walls in vitro. Recombinant pectate lyase A (PelA) and polygalacturonase (PG) from Aspergillus spp. exhibited similar capacity to Pel1. Pel1, PelA and PG also enhanced the reconstituted expansin-induced extension of the apical (elongating) segments of cucumber hypocotyls. However, the effective concentrations of PelA and PG for enhancing the reconstituted expansin-induced extension were greater in the apical segments than in the basal segments, whereas Pel1 behaved in the opposite manner. These data are consistent with distribution of more methyl-esterified pectin in cell walls of the apical segments and less esterified pectin in the basal segments. Associated with the degree of esterification of pectin, more calcium was found in cell walls of basal segments compared to apical segments. Pretreatment of the calcium chelator EGTA could also restore mature cell walls' susceptibility to expansin by removing calcium from mature cell walls. Because recombinant pectinases do not hydrolyze other wall polysaccharides, and endoglucanase, xylanase, and protease cannot restore the mature wall's extensibility, we can conclude that the pectin network, especially calcium-pectate bridges, may be the primary factor that determines cucumber hypocotyl mature cell walls' unresponsiveness to expansin.






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