Plant vascular systems are usually composed of phloem and xylem. Phloem often develops without xylem, whereas xylem does not form without phloem. Low levels of auxin induce phloem differentiation but not xylem, while high auxin induces both. Other factors like gibberellic acid, sugars, leaves, roots, cytokinins, pressure and ethylene also influence phloem and xylem differentiation. The dynamic interplay between these various signals precisely controls vascular development.

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;
1. 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.
2. 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.
3. In the young organs of intact plants, the phloem always differentiates before the xylem.

3. Coleus
Vigna unguiculata

4. 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.
5. High auxin concentration applied to decapitated Luffa stems induced xylem in the phloem anastomoses.
6. 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.
7. 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.
1. 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,
Low IAA and high GA3 levels favoured phloem production.
2. 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
1. In 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.
2. In another example it had been reported that in Coleus stems;
Auxin 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.
3. 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.
4. 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;-
► The 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 . This 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.
► There 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.