Miniature1-Encoded Cell Wall Invertase Is Essential for Assembly and Function of Wall-in-Growth in the Maize Endosperm Transfer Cell

Byung-Ho Kang,Donna S Williams,Yuqing Xiong,Prem S Chourey,Diego Pozueta-Romero

doi:10.1104/pp.109.142331

Abstract

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.

Highlights

The miniature1 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
WIGs are observed in 12- and 17-d after pollination (DAP) Mn1 BETCs after staining by toluidine blue
Accumulation of cell wall material on the basal cell wall was clearly resolved in the 12-DAP BETCs (Fig. 1B, asterisks), but Mn1 BETCs at 7 DAP were devoid of such basal cell wall thickening (Fig. 1A)

Summary

Introduction

The miniature (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. Though, only a few have been associated with well-defined functions unique to the BETL These include a family of BETL genes that encode proteins with antimicrobial function (Hueros et al, 1995; Serna et al, 2001; Cai et al, 2002), BETL-specific transcriptional activation (Gomez et al, 2002), and the miniature (Mn1)-encoded cell wall invertase (Cheng et al, 1996). The lack of the Mn1-encoded cell wall invertase (INCW2) in the BETL causes the mn seed phenotype (Cheng et al, 1996), which is associated with reduced cell size and cell number in developing endosperm (Vilhar et al, 2002) and a loss of nearly 70% of the seed weight relative to the wild type. The genetic analyses suggest that the INCW2-mediated Suc hydrolysis is a major physiological function of the enzyme in the BETL, exogenously supplied hexose sugars to in vitro kernel cultures of the mn seeds failed to restore the Mn1 seed phenotype (Cheng et al, 1999)

Methods

Results

Conclusion