Nonvascular, Symplasmic Diffusion of Sucrose Cannot Satisfy the Carbon Demands of Growth in the Primary Root Tip of Zea mays L

Abstract

Nonvascular, symplasmic transport of sucrose (Suc) was investigated theoretically in the primary root tip of maize (Zea mays 1.cv WF9 x Mo 17) seedlings. Symplasmic diffusion has been assumed to be the mechanism of transport of Suc to cells in the root apical meristem (R.T. Ciaquinta, W. Lin, N.L. Sadler, V.R. Franceschi [1983] Plant Physiol 72: 362-367), which grow apical to the end of the phloem and must build all biomass with carbon supplied from the shoot or kernel. We derived an expression for the growthsustaining Suc flux, which is the minimum longitudinal flux that would be required to meet the carbon demands of growth in the root apical meristem. We calculated this flux from data on root growth velocity, area, and biomass density, taking into account construction and maintenance respiration and the production of mucilage by the root cap. We then calculated the conductivity of the symplasmic pathway for diffusion, from anatomical data on cellular dimensions and the frequency and dimensions of plasmodesmata, and from two estimates of the diffusive conductance of a plasmodesma, derived from independent data. Then, the concentration gradients required to drive a growth-sustaining Suc flux by diffusion alone were calculated but were found not to be physiologically reasonable. We also calculated the hydraulic conductivity of the plasmodesmatal pathway and found that mass flow of Suc solution through plasmodesmata would also be insufficient, by itself, to satisfy the carbon demands of growth. However, much of the demand for water to cause cell expansion could be met by the water unloaded from the phloem while unloading Suc to satisfy the carbon demands of growth, and the hydraulic conductivity of plasmodesmata is high enough that much of that water could move symplasmically. Either our current understanding of plasmodesmatal ultrastructure and function is flawed, or alternative transport mechanisms must exist for Suc transport to the meristem.Understanding the control of plant growth and development must include an understanding of the transport of photoassimilates from sources into developing sinks. Transport of solutes through the differentiated phloem is explained well by quantitative treatments of Miinch's (1930) pressure flow hypothesis (Christy and Femer, 1973;Tyree et al., 1974a;Ross and Tyree, 1980), and experimental data are in reasonably good agreement with their predictions (cf. Zimmermann and Brown, 1971; Pickard et al.,

The sink-specific and stress-regulated Arabidopsis STP4 gene: enhanced expression of a gene encoding a monosaccharide transporter by wounding, elicitors, and pathogen challenge.

“…In the roots, GUS staining was restricted to the very tips, probably to the root meristem. It has been shown for maize that symplastic diffusion of sugars from the veins toward the primary root tips cannot satisfy the carbon demand of the growing root meristem (Bret-Harte and Silk, 1994). Expression of STP4 mRNA in this tissue could indicate a similar situation for Arabidopsis.…”

Modeling the Hydraulics of Root Growth in Three Dimensions with Phloem Water Sources

“…The phloem develops closer to the root tip than the xylem and channels water from the mature regions into the growth zone (Bret-Harte and Silk, 1994;Frensch and Hsiao, 1995;Pritchard, 1996;Pritchard et al, 2000). We assume the following.…”

“…The nodal roots grow through air and then dry layers of surface soil, making it unlikely that the expanding root cells obtain water from the dry media surrounding the root. Empirical and theoretical studies have concluded that the phloem probably provides water for growth of the primary maize root (Bret-Harte and Silk, 1994;Frensch and Hsiao, 1995;Pritchard, 1996;Pritchard et al, 1996Pritchard et al, , 2000Hukin et al, 2002;Gould et al, 2004).…”

“…8D; Erickson and Sax, 1956;Boyer and Silk, 2004;Silk and Wagner, 1980 (Ginsburg and Ginzburg, 1970;Bret-Harte and Silk, 1994;Frensch and Hsiao, 1995). The empirically based calculation of Frensch and Hsiao (1995), K !…”

RNA Interference of LIN5 in Tomato Confirms Its Role in Controlling Brix Content, Uncovers the Influence of Sugars on the Levels of Fruit Hormones, and Demonstrates the Importance of Sucrose Cleavage for Normal Fruit Development and Fertility

“…While the mechanisms of Suc loading into the phloem have been intensively studied over a similar time period (Riesmeier et al, 1993(Riesmeier et al, , 1994Bü rkle et al, 1998;Meyer et al, 2004;Sauer et al, 2004), those by which it is unloaded into the sink organ have only been clarified relatively recently and only for a subset of plants studied (Bret-Harte and Silk, 1994;Viola et al, 2001;Kü hn et al, 2003;Carpaneto et al, 2005). In the tomato (Solanum lycopersicum) fruit, the path of Suc unloading remains somewhat contentious (Ruan and Patrick, 1995;Hackel et al, 2006).…”