Wall ingrowth architecture in epidermal transfer cells of Vicia faba cotyledons.

We describe the use of scanning electron microscopy to provide novel views of the three-dimensional morphology of the ingrowth wall in epidermal transfer cells of cotyledons of developing Vicia faba seed. Wall ingrowth deposition in these cells amplifies the surface area of plasma membrane available for transport of solutes during cotyledon development. Despite the physiological importance of such amplification, little is known about wall ingrowth morphology and deposition in transfer cells. A detailed morphological analysis of wall deposition in this study clearly established for the first time that wall ingrowths are deposited at scattered, discrete loci as papillate ingrowth projections. The new views of the ingrowth wall revealed that these projections branch and fuse laterally, and fusion occurs by fine connections to form a fenestrated sheet or layer. This sheet of wall material then provides a base for further deposition of ingrowth projections to progressively build many interconnected, fenestrated layers. Consolidations, or filling-in, of the fenestrae in these layers appears to occur from small fingerlike protrusions of wall material which extend laterally from the most recently deposited surface of the fenestrae. We propose that deposition of fenestrated layers may provide a mechanism for maintaining continuous amplification of plasma membrane surface area in the face of turnover of the plasma membrane and transporter proteins associated with it. The techniques reported in this paper will provide new opportunities to investigate wall ingrowth deposition and its regulation in transfer cells.