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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Miniature1-Encoded Cell Wall Invertase Is Essential for Assembly and Function of Wall-in-Growth in the Maize Endosperm Transfer Cell^1,[W],[OA]

Department of Microbiology and Cell Science, University of Florida, Gainesville, Florida 32611 (B.-H.K., Y.X., D.S.W.); Interdisciplinary Center for Biotechnology Research, University of Florida, Gainesville, Florida 32610 (B.-H.K.); Department of Plant Pathology, University of Florida, Gainesville, Florida 32611 (D.P.-R., P.S.C.); and United States Department of Agriculture, Agricultural Research Service, Gainesville, Florida 32611 (P.S.C.)

The miniature1 (mn1) seed phenotype in maize (Zea mays) is due to a loss-of-function mutation at the Mn1 locus that encodes a cell wall invertase (INCW2) that localizes exclusively to the basal endosperm transfer cells (BETCs) of developing seeds. A common feature of all transfer cells is the labyrinth-like wall-in-growth (WIG) that increases the plasma membrane area, thereby enhancing transport capacity in these cells. To better understand WIG formation and roles of INCW2 in the BETC development, we examined wild-type and mn1 mutant developing kernels by cryofixation and electron microscopy. In Mn1 seeds, WIGs developed uniformly in the BETC layer during 7 to 17 d after pollination, and the secretory/endocytic organelles proliferated in the BETCs. Mitochondria accumulated in the vicinity of WIGs, suggesting a functional link between them. In the mn1 BETCs, WIGs were stunted and their endoplasmic reticulum was swollen; Golgi density in the mutant BETCs was 51% of the Mn1 Golgi density. However, the polarized distribution of mitochondria was not affected. INCW2-specific immunogold particles were detected in WIGs, the endoplasmic reticulum, Golgi stacks, and the trans-Golgi network in the Mn1 BETCs, while immunogold particles were extremely rare in the mutant BETCs. Levels of WIG development in the empty pericarp4 mutant was heterogeneous among BETCs, and INCW2 immunogold particles were approximately four times more abundant in the larger WIGs than in the stunted WIGs. These results indicate that polarized secretion is activated during WIG formation and that INCW2 is required for normal development of WIGs to which INCW2 is localized.

The authors responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) are: Byung-Ho Kang (bkang{at}ufl.edu) and Prem S. Chourey (psch{at}ufl.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.109.142331

^* Corresponding author; e-mail bkang{at}ufl.edu.

Received June 3, 2009; accepted September 14, 2009; published September 16, 2009.