Induction of wall ingrowths of transfer cells occurs rapidly and depends upon gene expression in cotyledons of developing Vicia faba seeds

Abstract

Abaxial epidermal cells of developing faba bean (Vicia faba) cotyledons are modified to a transfer cell morphology and function. In contrast, the adaxial epidermal cells do not form transfer cells but can be induced to do so when excised cotyledons are cultured on an agar medium. The first fenestrated layer of wall ingrowths is apparent within 24 h of cotyledon exposure to culture medium. The time course of wall ingrowth formation was examined further. By 2 h following cotyledon excision, a 350 nm thick wall was deposited evenly over the outer periclinal walls of adaxial epidermal cells and densities of cytoplasmic vesicles increased. After 3 h in culture, 10% of epidermal cells contained small projections of wall material on their outer periclinal walls. Thereafter, this percentage rose sharply and reached a maximum of 90% by 15 h. Continuous culture of cotyledons on a medium containing 6-methyl purine (an inhibitor of RNA synthesis) completely blocked wall ingrowth formation. In contrast, if exposure to 6-methyl purine was delayed for 1 h at the start of the culture period, the adaxial epidermal cells were found to contain small wall ingrowths. Treating cotyledons for 1 h with 6-methyl purine at 15 h following cotyledon excision halted further wall ingrowth development. We conclude that transfer cell induction is rapid and that signalling and early events leading to wall ingrowth formation depend upon gene expression. In addition, these gene products have a high turnover rate.

Reactive oxygen species form part of a regulatory pathway initiating trans-differentiation of epidermal transfer cells in Vicia faba cotyledons

“…Thereafter, the net flux of H 2 O 2 production declined up to 3 h, before a second slower rise occurred concomitant with cells being recruited to form WIs between 3 h and 15 h of cotyledon culture (Fig. 2, and see Wardini et al , 2007). Across cotyledon culture, the H 2 O 2 net flux was suppressed comparably in response to DPI and DDC treatments (Fig.…”

“…Following determination of the adaxial epidermal surface area (mm 2 ) of each cotyledon, net fluxes of H 2 O 2 release were computed. The arrow indicates the first appearance of WIs (Wardini et al ., 2007). …”

“…In this context, a series of experimental observations were undertaken to test whether ROS regulated any component of ingrowth wall construction by utilizing adaxial epidermal cells of cultured Vicia faba cotyledons. Upon placing excised cotyledons into culture, their adaxial epidermal cells are rapidly (hours) induced to trans -differentiate to form a polarized ingrowth wall identical to the one formed in planta by their abaxial counterparts (Wardini et al , 2007, and references therein). Such a plant model provides experimental access to a large population of trans -differentiating epidermal cells combined with an ability to visualize developing WI papillae with ease (see Zhou et al , 2010).…”

Intersection of transfer cells with phloem biology—broad evolutionary trends, function, and induction

“…However, when isolated V. faba cotyledons are cultured on nutrient agar, their adaxial epidermal cells are rapidly (h) induced to form wall ingrowths on their outer periclinal walls (Wardini et al, 2007). This system is unique in providing access to a large population of uncommitted epidermal cells (several thousand) that, upon exposure to culture conditions, are synchronously induced to trans -differentiate to a TC morphology across a defined temporal window (Wardini et al, 2007). Developmental synchronicity of the epidermal cell population permits temporal separation of uniform wall deposition from wall ingrowth assembly (see Figure 1D).…”

Green signals for life and death

“…After they have completed their job, the transfer cells die, along with most of the endosperm that frees the stage for the developing embryo. The work by Monjardino et al (2013) in the current issue extends now a series of fine structural studies that have discovered so-called flange ingrowths (Wardini et al 2007;Talbot et al 2002) with respect to the cellular mechanisms using maize as experimental system. They show for the first time that in addition to flange ingrowths, some of the transfer cells at the base of the endosperm show reticulate ingrowths.…”