Poster: Cell walls

Abs # 608: Post-pollination changes in maize silk cell wall biophysical properties

Presenter:	Sella Kapu, Nuwan U., nus102@psu.edu
Authors	Sella Kapu, Nuwan U. (A) (B)  Cosgrove, Daniel J. (B)
Affiliations:	(A): Integrative Biosciences Program, The Pennsylvania State University (B): Department of Biology, The Pennsylvania State University

Maize silks cease elongating upon pollination whereas un-pollinated silks continue to grow. Under greenhouse conditions, elongation of un-pollinated silks continued beyond 6 days after emergence (DAE) and senescence started around 13 DAE. Field observations indicate that elongation slows approximately 30 h after pollination (HAP). Under field conditions, the silk-ovary junction starts to collapse about 24 HAP. By 48 HAP the collapse is complete and silks detach from the ovaries. To investigate the biophysical basis for growth cessation after pollination, we performed several assays with cell walls, including a) acid-induced extension, b) expansin-induced extension, c) stress/strain (Instron) analysis, and d) stress relaxation analysis. Un-pollinated cell walls showed high rates of acid-induced extension, even up to 10 DAE. In comparison, pollinated silks were much less extensible. Similarly, pollinated silk walls collected 32 HAP showed significantly less extension in response to maize pollen extract (crude β-expansin). Plastic compliances of pollinated silks decreased by 65% (34 HAP) compared with controls. Moreover, pollinated silk walls showed reduced stress relaxation, particularly at times <1 s. Our results support the hypothesis that cessation of maize silk growth upon pollination involves the stiffening of cell walls, probably through polymer cross-linking and perhaps a reduction in β-expansin activity.