Growth and differentiation of roots

Four different zones can be distinguished longitudinally according to the developmental stages and histological characters of cells on the tip of primary root: root cap, zone of cell division (proximal meristem), zone of elongation and zone of maturation (Fig. 8.2.). The root apical meristem produces the cells of the root cap and also cells which differentiate to fulfil specific function. The file of cells with similar origin building up the tissues of roots can be observed microscopically especially on a young root (Fig. 8.2.).

The tip of root is covered and protected by the root cap. Between the root cap and proximal

meristem can be found the quiescent centre, that has cells with low dividing capacity [12]. For

the maintenance of the root apical meristem (RAM) the WUS-homologue WOX5 transcription

factor is responsible. The

WOX5 gene expresses in the quiescent centre, but affect in the

neighbouring cells inhibiting their differentiation [13]. Similar molecular mechanisms support

the maintenance of the RAM than in SAM. The key component in the development of primary

and lateral roots is the auxin. The movement of auxin in the tip of the primary root has a

specific pattern The auxin is transported from the shoot into the root and can be found in high concentration in the proximal meristem (Fig. 8.3). Among the identified very important proteins determining the identity and differentiation of Arabidopsis root cells can be found e.g. PLETHORA (PLT1 and PLT2), ARABIDOPSIS CRINKLY4 (ACR4), CLE44, SCARECROW (SCR) and SHORTROOT (SHR) [3, 5].

Fig. 8.2. The main structural element, zones and tissues of onion root tips. The root apical meristem (RAM) and the cell lines originated from it are highlighted. From [3] with permission.

Fig. 8.3. The polar auxin transport in the tip of Arabidopsis primary root. Auxin is transported from the shoot apex to the root cap through the vascular tissues and

moves up in the endodermis. From [3]

with permission.

87 Summary

1. The initiation of the plant organs begins during the embryogenesis, but they are able to produce new organs thorough their whole life. The formation of plant body jointly depends on the growth, differentiation and development.

2. The growth is defined as irreversible increase in volume and mass. It is a consequence of cell division and enlargement. Formation of cells, tissues and organs with specialized structures and function is called differentiation. Morphogenesis is the forming of the whole plant and involves growth and differentiation. The morphogenesis is driven by the genetic program and modified by environmental conditions.

3. Cells in plant meristems possess high developmental capacity. Shoot apical meristem (SAM) and root apical meristem (RAM) are found at the tips of stems and roots, respectively.

4. The plant body has a modular structure. The module produced by shoot apical meristem is called phytomer and comprises a node with an associated leaf, axillary bud and the internode segment.

5. In meristems the number of cells is rather constant, the cell division and differentiation are balanced. Stem-cells in vegetative meristems divide and provide new cells that will specialize, but they also regenerate themselves.

6. A negative feedback loop ensures the maintenance of the function and size of SAM.

The CLAVATA1 (CLV1) receptor kinase is activated by the small CVL3 peptide and inhibits the expression of the

WUSCHEL (WUS) gene in the central zone of SAM by

inducing a signalling cascade. If the level of WUS transcript enhances, it increases the expression of CLV3, thus decreasing later its own transcription.

7. SAM produces leaves at regular intervals and in predictable arrangements, the phenomenon is the phyllotaxy. The leaf primordia initiate at the local auxin maxima that are forming due to auxin-regulated asymmetric localization and stability of the PIN1 auxin transporters. Development of the adaxial/abaxial polarity of leaves is regulated by antagonistic interactions between different transcription factors.

8. Four different zones can be distinguished longitudinally on the tip of primary root: root

cap, zones of cell division (proximal meristem), elongation and maturation. The

WUS-homologue

WOX5 transcription factor gene expresses in the quiescent centre, but

inhibit the differentiation of the neighbouring cells. A similar molecular mechanism

supports the maintenance of the root apical meristem (RAM) than that in SAM. The

key component in the development of primary and lateral roots is the auxin. The auxin

moves from the shoot into the tip of the primary root, but direction will turn and

moves up in the endodermis.

88 Questions

1. What is the difference between growth, differentiation and development?

2. What does it mean that the shoot has a modular structure?

3. What are common in meristematic tissues and in their maintenance?

4. Which plant hormone is the most important in plant morphogenesis?

5. How develop a leaf?

6. Which longitudinal regions or zones can be identified in roots?

Questions to discuss

1. What kind of environmental factors influence the morphogenesis of plants?

2. Why are important the growth and development of plants for the mankind?

Suggested reading

1. The seed plant body plan II. Form and function of organ systems. In: Jones R, Ougham H, Thomas H, Waaland S (Eds) The molecular life of plants. Wiley-Blackwell, American Society of Plant Biologists, 2013. pp. 27-32.

2. Growth and development. In: L. Taiz and E. Zeiger (Eds), Plant physiology. Sinauer Associates Inc., Publishers Sunderland, Massachusetts, U.S.A., 5th Edition, 2014., pp.

339-374.

References

[1] The seed plant body plan II. Form and function of organ systems. In: R. Jones, H.

Ougham, H. Thomas and S. Waaland (Eds), The molecular life of plants. Wiley-Blackwell, American Society of Plant Biologists, pp. 27-32, 2013.

[2] J. E. Bidlack, S. H. Jansky, Stern's Introductory Plant Biology, 12th Edition.McGraw-Hill Companies Inc., New York, U.S.A., 2011. pp. 189-212.

[3] Growth and development. In: R. Jones, H. Ougham, H. Thomas and S. Waaland (Eds), The molecular life of plants. Wiley-Blackwell, American Society of Plant Biologists, pp. 405-454, 2013.

[4]

A. Fehér, Callus, dedifferentiation, totipotency, somatic embryogenesis: What these

terms mean in the era of molecular plant biology? Front. Plant Sci., vol. 26, Nr. 10, p. 536. doi:

10.3389/fpls.2019.00536. eCollection 2019. Apr.

[5] Growth and development. In: L. Taiz and E. Zeiger (Eds), Plant physiology. Sinauer

Associates Inc., Publishers Sunderland, Massachusetts, U.S.A., 5th Edition, 2014., pp. 339-374.

89

[6] K.F.X.

Mayer, H. Schoof, A. Haecker, M. Lenhard, G. Jürgens and T. Laux, Role of

WUSCHEL in regulating stem cell fate in the Arabidopsis shoot meristem. Cell, vol. 95, pp. 805–

815, 1998.

[7]

M. Lenhard, G. Jürgens and T. Laux, The WUSCHEL and SHOOTMERISTEMLESS genes

fulfil complementary roles in Arabidopsis shoot meristem regulation. Development, vol. 129, pp. 3195–3206. 2002.

[8] C.W. Vroemen, A.P. Mordhorst, C. Albrecht, M.A.C.J. Kwaaitaal and S.C. de Vries, The CUP-SHAPED COTYLEDON3 Gene Is Required for Boundary and Shoot Meristem Formation in Arabidopsis. Plant Cell, vol. 15(7), pp. 1563–1577, 2003.

[9] S.E. Clark, R.W. Williams and E.M. Meyerowitz, The CLAVATA1 Gene encodes a putative receptor kinase that controls shoot and floral meristem size in Arabidopsis. Cell, vol. 89, pp.

575–585, 1997.

[10] S.E. Clark, M.P. Running and E.M. Meyerowitz, CLAVATA3 is a specific regulator of shoot and floral meristem development affecting the same processes as CLAVATA1.

Development, vol. 121, pp. 2057–2067, 1995.

[11] D. Reinhardt, E.R. Pesce, P. Stieger, T. Mandel, K. Baltensperger, M. Bennett, J. Traas, J. Friml and C. Kuhlemeier, Regulation of phyllotaxis by polar auxin transport. Nature, vol. 426, pp. 255–260, 2003.

[12] L. Dolan, K. Janmaat, V. Willemsen, P. Linstead, S. Poethig, K. Roberts et al., Cellular organisation of the Arabidopsis thaliana root. Development, vol. 119, pp. 71–84, 1993.

[13] C. van den Berg, V. Willemsen, W. Hage, P. Weisbeek, B. Scheres, Cell fate in the

Arabidopsis root meristem determined by directional signalling. Nature, vol. 378, pp. 62–65,

1995.

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