Minisymposium 16: Cell walls

Abs # 31002: Biochemical property and in vivo function of rice β-expansins

Presenter:	Choi, Dongsu , choid@msu.edu
Authors	Choi, Dongsu  (A)   Lee, Yi  (A) (B)  Kende, Hans  (A)
Affiliations:	(A): Michigan State University (B): Chungbuk National University

Expansins are proteins involved in long-term extension of isolated plant cell walls. β-Expansins are more abundant in monocots than in dicots and show higher activity on monocot cell walls than on dicot cell walls. However, little is known about the role of β-expansins in vegetative growth and their biochemical properties. Here, we present recent progress in elucidating the properties and action of OsEXPB3, a vegetative β-expansin of rice. α-Expansins are bound to the cell wall of plants and can be solubilized with an extraction buffer containing 1 M NaCl. Using antibodies raised against OsEXPB3, we found that OsEXPB3 is tightly bound to the cell wall and, unlike α-expansins, cannot be solubilized with extraction buffer containing 1 M NaCl. OsEXPB3 protein could only be extracted with buffer containing SDS. Localization of α-expansins in the cell wall was confirmed by immunogold labeling and electron microscopy. The subcellular localization of vegetative β-expansins, however, has not yet been studied. Using immunogold labeling and electron microscopy, we found that OsEXPB3 is exclusively localized in the cell walls. Gold particles were mainly distributed over the primary cell walls. Immunohistochemistry showed that OsEXPB3 was present in all regions of the coleoptiles and root tissues tested. In previous work, we observed that expression of OsEXP4, an α-expansin, is closely correlated with growth and development in transgenic rice. The biological role of β-expansins, however, has not been determined. To investigate the in vivo function of ет-expansins, we generated transgenic rice plants carrying sense OsEXPB3 cDNA constructs. In seedling tests using control transgenic T₁ plants and sense transgenic T₁ plants, we observed enhanced coleoptile and root growth by up to 35.6% and 29%, respectively. Interestingly, OsEXPB3 transgenic seedlings did not show any increase in mesocotyl growth, while over-expression of OsEXP4 greatly promoted mesocotyl growth in transgenic seedlings. This result suggests that β-expansins also act as primary wall-loosening factors in rice cell walls as do α-expansins but that they may have different organ specificities.