Gerd Bossinger and Antanas V. Spokevicius, School of Ecosystem and Forest Sciences, University of Melbourne
At the end of the first growing season, the stems of perennial plants consist of a parenchymatous pith in the centre of the stem surrounded by primary vascular tissue within a cortex of parenchyma, collenchyma and sclerenchyma cells, surrounded by an epidermis. The vascular tissue, consisting of xylem towards the inside of the stem and phloem to the outside, appears either as discrete vascular bundles (Figure 7.18a), for example as in Arabidopsis thaliana, or as a continuous hollow cylinder as in eucalypts. In either case, both phloem and xylem cells derive from a layer of meristematic cells forming the fascicular cambium which at the end of primary growth join up with meristematic cells between individual bundles, the inter-fascicular cambium, to form a complete meristematic cylinder, the vascular cambium.
At this stage, the primary root consists of a central column of xylem surrounded by a number of phloem strands formed and separated by procambium tissue. This vascular tissue is contained within a central cylinder that is enclosed by a pericycle and an endodermis, surrounded by primary bark and an epidermis (Figure 7.18b).
How did this develop? And how does cambial growth proceed to form the secondary stem tissues, wood to the inside of the stem and bark to the outside?
The procambium forms early during primary growth (in fact as early as the formation of a heart shaped embryo) and, after germination, it can be found in leaf and primary stem tissue. In the embryos of most dicotyledonous plants, the procambium is established from the differentiation of ground meristematic tissue in the hypocotyl and, as the embryo develops cotyledons, the procambium begins its activity by undergoing acropetal (from the base towards the top) divisions toward the shoot apex. After germination has been initiated and leaf formation begins, the procambium starts to develop further and becomes visible as cell bundles from leaf traces formed below the shoot apex. The procambium then begins to advance acropetally toward the newly derived cells below the shoot apical meristem and, basipetally to connect with the vascular tissue in lower, more mature parts of the stem. As the pro-cambial strands mature a lateral initiating or meristematic layer develops, the fascicular cambium (Figure 7.18a). Cells in this region undergo substantial longitudinal elongation prior to completing their development. While interfascicular cambium differentiation is not fully understood some evidence suggests that it forms between distinct vascular bundles from cells predetermined early in development, which under hormonal control re-differentiate to become meristematic.
Auxin is basipetally transported from leaf primordia to the shoot apex and is believed to be the major factor involved in the process of vasculature patterning and differentiation. The initiation of fascicular and interfascicular cambia is believed to be controlled by different and distinct mechanisms. For example, diffusion of auxin in the form of indole-3-acetic acid is involved in the initiation of meristematic activity and vascular tissue differentiation in the interfascicular region of Arabidopsis thaliana. At the molecular level, a mutation in a Class III homeodomain-leucine zipper (HD-ZIP) gene named INTERFASICULAR FIBERLESS (IFL1) leads to the loss of extra xylary fibre cell formation in the interfascicular region but does not have an effect on vascular bundle formation. This gene is expressed preferentially in the interfascicular region and acts as a transcription factor affecting the polar transport of auxin.
In the root, the cambium develops first from procambial tissue in the grooves between the star-shaped primary xylem and individual primary phloem strands (Figure 7.18b). Soon after, pericycle cells opposite the xylem protrusions divide periclinally (the plane of cell division runs parallel to the surface) and the centripetally (towards the inside) derived daughter cells become part of the cambium, now separating primary xylem from primary phloem.
The cambial cells opposite the primary phloem strands begin to produce secondary xylem transforming the star-shaped cambium into a cambial cylinder gradually producing secondary xylem and phloem around its entire circumference.