Sucrose xylem cells

In some cases GA3 or cytokinins increased the number of secondary xylem cells, particularly when combined with added auxin (Hejnowicz and Tomas-ZEWSKi 1969, Hess and Sachs 1972) in other cases they were without effect or even decreased the number (Robards et al. 1969). Noting that earlier workers had shown that GA increased lAA transport (e.g., Jacobs and Case 1965), as did cytokinin (e.g.. Black and Osborne 1965), Hejnowicz and Tomaszewski demonstrated that such increased transport of auxin was correlated with the increased formation of xylem from such hormonal combinations added to their woody stems. Sucrose also increased the number of xylem cells resulting from auxin (Robards et al. 1969, Zajaczkowski 1973), an effect which similarly might be due to the increased lAA transport that is elicited by sucrose (Smith and Jacobs 1968). The decrease in xylem area caused by the addition of abscisic acid (Hess and Sachs 1972) is likewise potentially explainable as due to abscisic acid s effect in decreasing lAA transport (Chang and Jacobs 1973). 

The cambium layer of plant stems (Fig. 1-16) differentiates continuously to form phloem on the outside of the cambium and xylem on the inside. At the same time, cambium cells are retained. Thus, at each cell division one daughter cell becomes a differentiated cell, while another remains the less differentiated cambium. This pattern of continuous differentiation from a line of stem cells with constant properties is found in animals as well as in plants. In the differentiation of cambium it appears that chemical signals obtained from the surrounding cells on either the inside or the outside of the cambium layer determine whether the differentiated cell becomes phloem or xylem. Sucrose, auxin, and cytokinins are all involved. 

Figure 1. Source leaf minor vein phloem. (A) Autoradiograph of leaf tissues following l C-sucrose accumulation showing radioactivity (white) in veins. (B) Tracing of an electron micrograph of a cross section of minor vein, x, xylem, vp, vascular parenchyma cc, companion cell se, sieve element pp, phloem parenchyma, me, mesophyll cell. Reproduced with permission from Ref. 6. Copyright 1983. Annual Reviews.

The xylem ray cells of the sugar maple (Acer saccharum) contain -10% starch (172). In birch (Betula spp.) stored starch is converted into sucrose when the ambient temperature drops to - 5 °C (50), perhaps to provide cryoprotection. 

Wiese A, Elzinga N, Wobbes B et al (2004) A conserved upstream open reading frame mediates sucrose-induced repression of translation. Plant Cell 16 1717—1729 YosMmoto K, Noutoshi Y, Hayashi Ket al (2012) A chemical biology approach reveals an opposite action between thermospermine and auxin in xylem development in Arabidopsis thaliana. Plant CeU Physiol 53 635-645... 

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