Significance of Root Stem Transition | Plants

In this article we will discuss about the significance of root stem transition.

During vascular transition there is a reorientation of primary xylem and primary phloem. The reorientation of primary vascular tissues commonly takes place in the hypocotyl of the embryo in provascular strands.

Transition regions have their own specific orientation of vascular tissues. This orientation cannot be compared with stem, root and leaf. So it becomes difficult to refer transition region in relation to embryo axis.

Difficulty also arises to trace the relationship between transition region and the axes present above and below it. So several Interpretations have been proposed regarding the structural and evolutionary significance of this region.

One common and earlier concept is that a unit vascular system exists in higher plants at seedling stage. According to this concept the vascular system is morphologically equivalent in all its parts, i.e. root, stem and leaf. It is regarded that the site of formation and the site of maturation of vascular tissues are same. Formation, maturation and expression of vascular tissues at the same site and at different levels imply the unity of vascular system.

The difference in orientation of vascular tissues at various levels of transition ‘may be described, figuratively,’ as branching, twisting, rotation and inversion’. Vascular transition occurs mostly over a short distance except a few instances where it may extend through several internodes, e.g. Pisum sativum. The changes of orientation of vascular tissues may not be conspicuous in the embryo axis.

However it is clearly expressed upon differentiation into primary vascular tissues in a seedling of sporophyte. Eames and MacDaniels illustrated the four types of vascular transition by studying the diagrams of vascular tissue orientation at successive levels through the transition region of a seedling.

Eames and MacDaniels interpreted the four types based on the study of serial sections of partly or fully differentiated tissues. A completely different picture can be obtained if the vascular transition is studied during development. According to this opposite concept the vascular system in a seedling is initially not a continuous unit.

The vascular system consists of radicular-hypocotylary unit and cotyledonary part. These two join in the upper part of hypocotyl to form radicle-hypocotyl-cotyledon unit. The epicotylary trace develops from provascular strand and is superimposed on fully differentiated radicle-hypocotyl- cotyledon unit.

The tissues present between the uniting traces are ‘mutually accommodated’. In favour of opposite view the root-stem transition of Arabidopsis thaliana can be cited. In Arabidopsis the transition region consists of radicle-hypocotyl-cotyledon unit and vascular system of epicotyl.

The root is diarch with exarch protoxylem and with alternate arrangement of xylem and phloem strands. For most of its length hypocotyl has root like structure. At a higher level pith appears interrupting the primary xylem plate. This marks the beginning of vascular transition.

The presence of pith signifies that the region is unoccupied by vascular elements. Slightly above this level the two phloem strands divide into four strands. The split phloem strands move upward through the cotyledonary node.

As they pass upwards two phloem strands become spatially associated with xylem. In the lower portion of cotyledon and in the region of midvein xylem strands complete the transition from exarch to endarch position.

Each pair of phloem strands assumes a position above the endarch xylem. Thus the vascular strands become collateral completing transition. In radicle-hypocotyl- cotyledon unit, by sixth day, the procambial cells form cambium by periclinal division. Cambium is differentiated between primary xylem and primary phloem strands in the region of upper hypocotyl.

This region also corresponds to the portion of cotyledonary node. Secondary vascular tissues are formed as a result of cambial activity and thus the primary vascular structure is disrupted. Then the vascular system of epicotylary trace begins to develop. After differentiation and maturation, the epicotylary trace is superimposed on radicle-hypocotyl- cotyledon unit.

This happens after secondary thickening is initiated in the region of radicle-hypocotyl-cotyledon. Busse et al. examined by light and electron microscopy the transition region of Arabidopsis during vascular development and maturation.

Proponents of opposite view cited the anatomy of transition region of Beta vulgaris and Daucus earlier. They also exhibit radicle-hypocotyl unit and the commencement of epicotylary trace after secondary thickening in the region of hypocotyl-radicle.

The seedling of Beta vulgaris (Fig. 16.2) possesses two’ cotyledons and in between them there is an epicotylary shoot. On the lower side there exist the hypocotyl and the root. Anatomically the root shows diarch xylem and two phloem strands present alternating with xylem. The traces of each cotyledon consist of two vascular bundles that are partially fused along the protoxylem.

In the root the protoxylem is exarch and this peripheral position is maintained up to hypocotyl. But at higher-level two elements of metaxylem are differentiated toward periphery on the lateral side of protoxylem. This orientation gradually leads to the formation of endarch xylem. The diarch xylem of root is thus gradually became four stranded that after fusion along the protoxylem form the cotyledonary traces.

There are two phloem strands in the root. As they pass towards hypocotyl they branch and thus four phloem strands are differentiated. Each phloem strand associates with one metaxylem plate outside. Thus collateral vascular bundles are produced which persists in the cotyledons.

The epicotylary trace develops after the root-hypocotyl-cotyledon vascular strand is partly differentiated. The epicotylary traces are collateral with endarch xylem and they are connected with similarly oriented tissues present in the root.

The epicotylary traces seem to be superimposed over the root-hypocotyl- cotyledon vascular traces. So in Beta vulgaris there exists no vascular transition between root and stem, but rather a mutual accommodation of vascular tissues.

In Daucus carota, like Beta vulgaris, the vascular continuity exists between radicle and cotyledon. The epicotvlary traces join with root-cotyledon traces.

Daucus carota is dicotyledonous plant and each cotyledon of the seedling possesses three vascular bundles. The median bundle consists of xylem with exarch protoxylem that is continuous with the protoxylem of root. Two phloem strands are present lateral to the median xylem strand in root.

The other two lateral vascular bundles are collateral with endarch xylem. These two bundles owe their origin from the diarch xylem of root. So the primary vascular tissues of cotyledon and root are continuous without any inversion. The epicotylary traces develop later. They are collateral and ultimately join with radicle-hypocotyl-cotyledon traces.

The ontogenetic studies of vascular transition of Beta, Daucus and Arabidopsis reveal that there is no root-stem transition and the phenomenon like twisting, inversion etc. are not operative as was illustrated by Eames and MacDaniels in the earlier or common concept. The modern concept, which is based on developmental studies, believes in the double origin of vascular system.

It reveals that during ontogeny vascular connections are established between radicle and cotyledon via hypocotyl. The epicotylary traces are developed later from procambium strand. The traces ultimately join with the fully differentiated radicle-hypocotyl-cotyledon unit and the tissues between the traces are mutually accommodated.

There is a physiologic interpretation of transition region. A seedling axis has two ends-root and shoot. Root end has its own root apical meristem. The derivative cells of this meristem form the structure of root including vascular tissues. The shoot end consists of cotyledons, epicotyl and foliar primordia.

Epicotyl (if it is precocious), cotyledons and first foliar primordia influence in the formation of upper parts of the seedling axis. The derivative cells of root apical meristem form an impression at the upper part of seedling axis. The cells present at the base of upper part of seedling axis and the derivate cells of root apical meristem merge with each other thus forming a continuous vascular strand.

From evolutionary point of view it is postulated that the different arrangement of vascular tissues as exhibited by different plant parts are not equivalent. The alternate arrangement of xylem and phloem strands in root represents its primitive nature. In shoot the superimposed or collateral arrangement of vascular tissues is thought to be advanced.

The different internal structures at the successive levels of transition region exhibit the evolutionary types. In this respect advanced evolutionary stages are exhibited by collateral arrangement of vascular tissues in shoot. In this region the primitive stage, i.e. alternate arrangement of vascular tissue in root is completely omitted.

An understanding of the structure of transition region is important due to followings:

i. To interpret the homologies between root and shoot.

ii. To trace evolutionary stages of vascular tissues.

iii. To study the structure of radicle-hypocotyl-cotyledon unit.

iv. To study the relationship between epicotylary trace and radicle-hypocotyl-cotyledon unit.

v. To make a comparative study of epidermis and cortex of root and shoot.

vi. To study the significance of pericycle.

vii. To study the differences between protoxylem and metaxylem, and protophloem and metaphloem.

viii. To study the developmental relationship between primary and secondary tissues.