Xylem differentiation, secondary

Hormones and Cuscuta Development Influence of Hormones on Secondary Xylem Differentiation, Phenylalanine Ammonia Lyase (PAL) Activity and Ligniflcation... 

Since new secondary cell wall formation accompanies lignin deposition during secondary xylem differentiation, the possibility exists that GA3 or BA + GA3 enhanced lignification by enhancing amylase activity for starch mobilization. Preliminary experiments (Table 6) showed, contrary to expectation, that all the hormones GA3, BA and NAA, singly or in combination, actually suppressed the enormous (56-fold increase) a-amylase induction that occurs in untreated control segments. The increased amylase activity seen in control segments after 48 h was... 

Shininger TL (1971) The regulation of cambial division and secondary xylem differentiation in Xanthium by auxins and gibberellin. Plant Physiol 47 417-422 Shininger TL (1979) The control of vascular development. Annu Rev Plant Physiol 30 313-337... 

Fig. 7.4 A-C. A model of control of morphogenesis by vectors of auxin wave propagation in the final phase (autolysis) of axial tracheid maturation in secondary xylem of the stem. A Trajectories of vectors of auxin wave propagation (a) in the cambial region and in differentiating secondary tissues as seen in radial (b) and transverse (c) sections. The breakdown of the cytoplasm is initiated when some critical angle (a) between the vector of auxin wave propagation and the cell axis is attained. B and C Vector trajectories associated with a model of regulation of earlywood and latewood differentiation. Ph conducting phloem Ph dividing phloem Cj cambial initial dividing xylem ...

The induction of PAL activity at the onset of vascular differentiation can be shown by the use of plant tissue cultures (37-39). Xylem cells with secondary and lignified walls are differentiated over a time course of 3-14 days by the application of the plant growth factors naphthylene acetic acid (NAA) and kinetin in the ratio 5 1 (1.0 mg/liter NAA, 0.2 mg/liter kinetin) to tissue cultures of bean cells (Phaseolus vulgaris) (37,40). The time for differentiation varies with the type of culture, solid or suspension, and with the frequency and duration of subculture, but for any one culture it is relatively constant (37,41,42). At the time of differentiation when the xylem vessels form, the activity of PAL rises to a maximum. The rising phase of the enzyme activity was inhibited by actinomycin D and by D-2,4-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide (MDMP) applied under carefully controlled conditions (42). This indicated that both transcription and translation were necessary for the response to the hormones. Experiments using an antibody for PAL and a cDNA probe for the PAL-mRNA have also shown that there is an increase in the amount of transcript for PAL during the formation of lignin when Zinnia mesophyll cells are induced to form xylem elements in culture (Lin and Northcote, unpublished work). 

As a plant grows in diameter, secondary xylem is formed from the cambium. Auxin has been implicated in the control of both cambial division and the subsequent differentiation of tracheary element [47]. When vascular bundles are broken, parenchyma cells can redifferentiate into tracheary elements and restore the functional bundles this occurs in response to elevated auxin levels at the wound site [39]. 

When a BR biosynthesis inhibitor, brassinazole, was applied to Arabidopsis thaliana, high levels of ribulose-l,5-bisphosphate carboxylase-oxygenase proteins accumulated in the plastids of the cotyledons. These results suggest that brassinazole treatment in the dark induces the initial steps of plastid differentiation, which occur prior to the development of fhylakoid membranes. This is a novel presumed function of BRs [16]. Brassinazole treament also retards the development of secondary xylem in cress [17]. 

The note by Wareing (1958), reporting no differentiated tracheary cells from added GA3, some from lAA, but more when GA3 was added with lAA, to excised sections of tree stems, was extended in later papers. For five genera, Wareing et al. (1964) found the maximal width of new secondary xylem and phloem tissue when lAA and GA were added together to excised pieces of stem (Fig. 4.12). The cells considered to be on the phloem side of the cambium did not differentiate into sieve tubes or companion cells, however, whether they resulted from GA alone or GA plus lAA. lAA added alone resulted in some tracheary cells but no phloem cells. Kinetin, whether added alone or with IA A, was said to be without effect or perhaps slightly inhibitory. The... 

Thompson NP, Jacobs WP (1966) Polarity of lAA effect on sieve-tube and xylem regeneration in Coleus and tomato stems. Plant Physiol 41 673-682 Torrey JG (1953) The effect of certain metabolic inhibitors on vascular tissue differentiation in isolated pea roots. Am J Bot 40 525-533 Torrey JG, Loomis RS (1967) Auxin-cytokinin control of secondary vascular tissue formation in isolated roots of Raphanus. Am J Bot 54 1098-1106 Torrey JG, Fosket DE, Hepler PK (1971) Xylem formation A paradigm of cyto-differen-tiation in higher plants. Am Sci 59 338-352... 

Samuels, A.L. et al. (2002) Cellular machinery of wood production differentiation of secondary xylem in Pinus contorta var. latifolia. Planta 216, 72-82... 

Treatment with a specific PAO inhibitor attenuates both Spd-induced root cell growth inhibition and Spd-induced cell-cycle arrest. The PAO inhibitor also disrupts differentiation of the secondary wall of meta-xylem elements and xylem parenchymal cells. Overexpression of maize PAO in tobacco plants induces programmed cell death (PCD) in root-cap cells (Tisi et al. 2011). The results suggest that H2O2 produced by Spd oxidation triggers secondary wall deposition and induces PCD. A link between PAs and PCD has been reviewed recently by Moschou and Roubelakis-Angelakis (2014). 

---