CONTROL OF XYLEM AND PHOLEM DIFFERENTIATION .pptx

Control of phloem and Xylem

1. CONTROL OF PHLOEM AND XYLEM DIFFERENTIATION

2. The Relation Between Phloem and Xylem Differentiation Plant vascular systems are usually composed of phloem and xylem. In the intact plant, xylem does not differentiate in the absence of phloem, though phloem often develops in the absence of xylem. As already mentioned, the earliest organized vascular system is found in brown algae, and it consists of phloem with no xylem. Along the stem of angiosperms, in addition to the collateral bundles (which consist of both phloem and xylem) there are also bundles of phloem with no adjunct xylem. Here are few example; In Coleus on each collateral bundle , there is usually a bundle of phloem . Phloem anastomoses are lateral sieve tubes with no xylem that occur between the longitudinal bundles ; they are encountered in many plant species, are common in summer conditions, and their differentiation is dependent on light intensity. A mycelium-like network of internal phloem with no xylem was found in the inner mesocarp of the lateral pod walls of the fruit of Vigna unguiculata . In the young organs of intact plants, the phloem always differentiates before the xylem.

3. Coleus Vigna unguiculata

4. Aloni reported that low auxin levels induced sieve elements but not tracheary elements in tissue cultures of Syringa. High auxin levels resulted in the differentiation of both phloem and xylem. In summary, according to report they conclude that low levels of auxin , which is the limiting and controlling factor for both phloem and xylem differentiation in plant as well as in tissue culture, induce phloem but not xylem differentiation. High auxin concentration applied to decapitated Luffa stems induced xylem in the phloem anastomoses. This pattern of vascular development is also proof in tissue culture conditions as well as in vascular regeneration around a wound , where sieve element differentiation is detected a day or more before tracheary differentiation can be observed. Differentiation of secondary phloem may proceed that of secondary xylem by several weeks. Mature needle leaves of Pinus perennially produce secondary phloem but no secondary xylem. In callus grown in culture, sieve elements differentiate with no tracheary elements at low auxin levels.

5. Luffa shows xylem in the presence of high auxin Syringia

6. Role of Gibberellic acid in phloem differentiation There is evidence that gibberellic acid (GA3) promotes phloem differentiation . Digby & Wareing reported that the relative levels of applied auxin and gibberellic acid were important in determining factor for xylem or phloem tissue production in stems of Popolus robusta . High IAA and low GA3 concentrations favoured xylem formation , L ow IAA and high GA3 levels favoured phloem production. On other hand , exceptionally high GA3 applied to the storage root of carrot significantly instead of increase in phloem production it will reduced the amount of secondary phloem production and decreased the phloem/xylem ratio.

7. Role of sugar in xylem and phloem differentiation I n tissue culture of Syringa , Wetmore & Rier have shown that in order to induce phloem and xylem differentiation there is need to apply a sugar together with the auxin. In another example it had been reported that in Coleus stems; A uxin concentration kept constant, low sucrose levels (1.5-2.5%) induced strong xylem differentiation with little or no phloem , whereas differentiation of phloem with little or no xylem was obtained with higher sucrose levels (3-4%); intermediate sucrose concentrations (2.5-3.5%) favoured the formation of phloem and xylem. Subsequent experiments with fern prothalli confirmed that at low sugar concentrations (1.5-3%) xylem was formed, while at higher concentrations (4.5-5%) phloem differentiated. Jeffs & Northcote reported that maltose, lactose ,trehalose induced nodules in Phaseolus that contained both phloem and xylem. They suggested that these three a-glucosyl disaccharides exert a specific effect on vascular differentiation in addition to their value as a carbon source. They concluded that sucrose is important only as a carbon source and that any other sugar that is a sufficiently good carbon source will promote vascular differentiation

8. Induction of Vascular Tissues by Leaves and by Auxin In the spring, developing buds and young growing leaves stimulate cambium reactivation and the formation of phloem and xylem, these cambium reactivation occur due to presence of auxin .These auxin travelled from young developing bud to the root. Young leaves has greatest impact on vascular differentiation in root of the plant. The removal of young leaves from the stem reduces or even prevents vascular differentiation below the excised leaves. Leaves promote a roots-directed vascular regeneration but have no influence on , vascular regeneration in the direction of the shoot tip . These statement is scientifically prove with following experiment;- T he grafting of shoot apices with a few leaf primordia on callus, which results in the formation of vascular tissues below the graft in the callus tissue there is no change in tissue of shoot apical only change occur at below the shoot apical . According to these experiment it concluded that young leaves promote vascular differentiation root

9. The pioneering study of Jacobs clearly showed that auxin, indole-3-acetic acid (lAA), produced by the young growing leaves was the main factor in limiting and controlling xylem and phloem regeneration around the wound in Coleus stems. Auxin alone could, both qualitatively and quantitatively, supplant the effect of the leaves on vascular regeneration in Coleus . The polar movement of auxin from the young leaves towards the roots through procambium, cambium, or parenchyma tissues triggers a complex sequence of changes that ultimately results in the formation of a vascular strand along the flow of auxin. Once developed, this vascular strand remains the preferable pathway of auxin transport, in as much as cells possessing the ability to transport auxin are associated with vascular tissues. Consequently, new streams of auxin emanating from young developing leaves are directed towards the vascular strands. Sachs has shown that a pre-existing vascular strand that is not supplied with auxin (e.g. one descending from an old leaf) acts as a sink for any new stream of auxin. In case of low auxin level it take auxin from neighbouring strand and shows vascular differentiation in pre-existing strand and regeneration of new tissue formation occur inside the pre-existing strand. On the other hand, a strand that is well supplied with auxin (e.g. one descending from a young leaf) prevents the expression of another source of auxin in its neighbourhood and will not form any kind of transport and vascular regeneration in neighbouring strand as long as it is well supplied with auxin .

10. An additional factor in vascular control is the auxin-transport capacity of mature vascular tissues. Auxin from mature leaves moves more rapidly in a nonpolar fashion in the sieve tubes. Due to rapid transport of auxin through phloem they will help in vascular differentiation in plant . When the phloem below mature leaves are damaged, there is a quantitative increase in vascular differentiation in damaged part, that leads to replacement of long non-functional, damaged tissues and formation of new tissue . T his fast wound healing effect is due to transport of additional auxin in the wound region that arrives from the mature phloem.

11. The Role of Roots in Vascular Differentiation The root need not be present to obtain vascular differentiation in stem. This is true also for vascular differentiation in tissue culture, which likewise occurs in the absence of roots. Roots do, however, have two major known functions in vascular differentiation, namely: (a) The root orients the pattern of vascular differentiation from the leaves towards the root tip by acting as a sink for the continuous flow of auxin deriving from the leaves; (b) the root apices are sources of inductive stimuli that promote vascular development. The role of cytokinin in vascular differentiation Cytokinins (CK) are a class of plant hormones that promote cell division, or cytokinesis, in plant roots and shoots. They are involved primarily in cell growth and differentiation, but also affect apical dominance, axillary bud growth, and leaf senescence. T here are two types of cytokinins: adenine-type cytokinins represented by kinetin, zeatin, and 6-benzylaminopurine, and phenyl urea-type cytokinins like phenyl urea and thidiazuron (TDZ). Phenyl urea cytokinins have been not found in plants Most adenine-type cytokinins are synthesized in roots . Typically, cytokinins are transported in the xylem. Cambium and other actively dividing tissues also synthesize cytokinins. It has special role in cell division in plant.

12. Effects of Pressure and Ethylene on Vascular Tissues Brown & Sax were the first to demonstrate the need for mechanical pressure for normal development of secondary vascular tissues. Plant tissues synthesize ethylene in response to external pressures. These external pressure created in stem due to specific kind of stress .eg, wind stress. The bending of shoots is known to induce the formation of reaction wood in the stressed shoots. This phenomenon of formation of reaction wood in bending shoot occur naturally in environment . Nelson & Hillis have shown that when seedlings of Eucalyptus gomphocephala are placed in the horizontal position they produce higher amounts of ethylene in their upper halves, which leads to formation reaction wood. In additional experiment with combination of auxin and ethylene showed that auxin and ethylene are involved in the elicitation of reaction wood. REACTION WOOD

13. Ethylene is known to affect xylem differentiation, and it was proved by Robert and Miller. Miller & Roberts have shown that the ethylene-releasing agent 2-chloroethy1phosphonic acid (CEPA) or the ethylene precursor L-methionine promoted xylem differentiation in Lactuca sativa in culture. Addition of ethylene inhibitor such as silver to the culture medium inhibited lignification and xylem differentiation. The inhibition of silver was completely reversed by the addition of L-methionine to the medium. Miller, et al suggested that ethylene may play a role in controlling lignification during xylogenesis by inducing wall-bound peroxidase activity.