To Ti Potency & Plasticity

TOTIPOTENCY • The ability of a single plant cell to give rise to a whole plant, including metabolism, in a suitable given stimulus. This property is unique to plants.

PRACTICAL APPLICATIONS OF TOTIPOTENCY

• Stem, leaf or any other explant can be used for propagation of horticultural crops. • Germplasm preservation of endangered plant species. • Haploid plant production. • Propagation of hybrid crops where seed setting is not possible. • Introduction of cellular organelles from different sources.

Plasticity • Processes involved in plant growth and development adapt to environmental conditions. This plasticity allows plants to alter their metabolism, growth and development to best suit their environment. Particularly important aspects of this adaptation, as far as plant tissue culture and regeneration are concerned, are the abilities to initiate cell division from almost any tissue of the plant and to regenerate lost organs or undergo different developmental pathways in response to particular stimuli.

• When plant cells and tissues are cultured in vitro they generally exhibit a very high degree of plasticity, which allows one type of tissue or organ to be initiated from another type. In this way, whole plants can be subsequently regenerated.

SOMATIC EMBRYOGENESIS • Process of embryo formation from somatic plant tissue is called somatic embryogenesis. • Direct embryogenesis (without callus) • Indirect embryogenesis (callus)

FACTORS AFFECTIN SOMATIC EMBRYOGENESIS

A. Chemical: Nutrients (macro & micro) Plant growth regulators. B. Physical: Oxygen, temperature, light etc.

C. Genotype D. Age of explant E. Explant Source

APPLICATIONS OF SOMATIC EMBRYOGENESIS • Propagation: the rate of propagation is much greater than in macropropagation (1 ml of settled cell volume can yield 100- 3000000 embryos) • Artificial seeds • Genetic transformation: Homogeneous embryogenic cell suspension cultures are used for transformation because most of the cells are totipotent and regeneration proceeds through somatic embryogenesis. As such low risk of chimerism was expected during the selection of transgenic plants. The main bottleneck was the difficult and time consuming initiation process and the maintenance of embryogenic cell suspensions.

STAGES OF EMBRYOGENESIS

ORGANOGENESIS • Formation of differentiated tissues from undifferentiated one is called organogenesis. (plasticity)

STEPS IN ORGANOGENESIS • Studies have shown that induction of shoots or roots from explants could be divided into three stages. In the first stage, the cells acquire competence for subsequent cell proliferation and differentiation; during the second stage, the developmental fate of competent cells is determined; and the third stage is devoted for differentiation and development of determined organs. However, the molecular mechanism(s) that govern the developmental fate of cells during organogenesis, remains poorly understood.

FACTORS AFFECTING ORGANOGENESIS • Explant: 1. Size (smaller size will have homogeneous tissues while larger size will have heterogeneous tissues) • 2. Age. • Phytohormones: Organogenesis occurs in various plant tissue cultures in response to exogenously added phytohormones, mainly auxin and cytokinin. High auxin/cytokinin ratios in the medium usually induce root formation whereas low auxin/ cytokinin ratios promote shoot formation. On the other hand, media containing intermediate auxin/cytokinin ratios promote disorganized cellular proliferation and callus formation. –

• Ph: 5.6 – 5.8 • Temperature: Tropical species need higher temperature while temperate crops need lower temperatures for organogenesis. • Light: It varies from crop to crop and organ to organ but in general blue spectrum promotes shoot formation and red light induces rooting.

IMPOTANCE OF ORGANOGENESIS • Regeneration of fertile plants from callus. • Mutational breeding of sexually and asexually reproduced plants was achieved through cell culture. • Fertile plants can be regenerated from hybrid callus culture. • Plants having variation in chromosome number (triploids, tetraploids) are regenerated.

CYTODIFFERENTIATION • Differentiation of cells into specific organs, generally connecting tissues is called cytodifferentiation. • IMPORTANCE: Grafting process of immature plants. • Hybridization manipulation can create more desirable vegetatively propagated plants. •