Polyploidy in Stevia

Genes and Chromosomes

We all know that all hereditary characteristics of all living beings are encoded into their genes. All the genes are, in turn, coded in long DNA molecules in the cells. DNA molecules contain the instructions an organism needs to develop, live and reproduce. These instructions are found inside every cell, and are passed down from parents to their offspring.   

 

All the plant cells contain a very important organelle in them – which is called nucleus. The cell nucleus​ is a membrane bound structure that contains the cell's genes and controls the cell's growth and reproduction. It is usually the most prominent organelle in a cell. The nucleus is small and round, and it works as the cell's control center.

 

In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure.

 

Chromosomes are not visible in the cell’s nucleus—not even under a microscope—when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Polyploidy

A plant cell typically has two copies of a set of chromosomes. Mathematically speaking, a plant cell should have 2n number of chromosomes when n is the number of chromosome in one set. In an offspring born through sexual reproduction, n number of chromosomes comes from each parent and the resultant number of chromosomes becomes 2n. Plants having 2n number of chromosomes are called “Diploid”. During production of male and female germ cells, a special type of cell division takes place, which halves the number of chromosomes in those cells. Thus, a plant having 2n number of chromosomes in its cells produces pollen cell and female germ cells with n number of chromosomes in each of them. The germ cells having n number of chromosomes are termed “Haploid”.

Structure of a plant cell
The chromosome

It is possible to produce progeny plants with 4n or more number of chromosomes through a chemical treatment of seeds or growing tissues (meristem) of plants. A chemical – colchicine, is typically used to induct polyploidy in plants. The plants having 4n number of chromosomes are called “Tetraploid”. Plants having more sets of chromosomes than that of normal plants are called “Polyploid”.

 

Colchicine inhibits chromosome segregation during meiosis; half the resulting gametes, therefore, contain no chromosomes, while the other half contains double the usual number of chromosomes (i.e., diploid instead of haploid, as gametes usually are), and lead to embryos with double the usual number of chromosomes (i.e., tetraploid instead of diploid). While this would be fatal in most higher animal cells, in plant cells it is not only usually well tolerated, but also frequently results in larger, hardier, faster-growing, and in general more desirable plants than the normally diploid parents; for this reason, this type of genetic manipulation is frequently used in breeding plants commercially.

In stevia, induction of polyploidy is often an important tool in its genetic improvement. In some cases, the polyploid plants have higher growth vigour, larger and thicker leaves and greater biomass production potential. Sometimes, polyploidy in plants have negative outcomes also. Polyploid plants with odd number of sets of chromosomes (n, 3n, 5n etc.) are generally sterile and can only be multiplied vegetatively (through cuttings and tissue culture).

In case of Stevia n = 11. Thus number of chromosomes in normal diploid variety is 2x11=22

Work done on polyploidy induction in Stevia

Triployed plants (3n) were produced by mating tetraployed plants (4n) with normal diploid plants (2n). The triployed plants showed higher Reb A content. The tetraployed plants had larger leaves (Sanyo K, 1990; Shuichi et al, 2001)

 

Tetraploids in stevia had significantly increased leaf size, thickness and chlorophyll content and reduced internode length (Yadav et al, 2013)

 

Polyploid plants recorded higher numbers of secondary branches, more leaf thickness and area, delayed flowering and higher steviol glycoside content in leaf (Hegde et al, 2015)

In 2018, Hong Zhang et al induced polyploidy in stevia by colchicine treatment of germinating seeds. They have adopted the following method - 

Hong Zhang et al (2018) : A - Diploid control B- Tetraploid

They successfully developed some tetraploid plants - which has 4n or double the number of chromosomes of a normal plant. The tetraploid plants had significantly larger stomata (the openings on leaf surface for breathing and releasing of water vapor) and higher chlorophyll content. Higher chlorophyll content indicates higher photosynthetic activity. Larger stomata are indicative of more efficient transport of water and nutrients through plant vascular tissues. The tetraploid plants were vigorous with large, thick leaves, stouter stems, shorter internodes and had higher glandular trichome (stem and leaf hair) density.

 

The authors reported up to 267% increase in stevioside content and 183% increase in Reabudioside A in tetraploid plants.  The remarkable differences in leaf morphology and microscopic structure in the tetraploid plants may explain their elevated levels of steviol glycosides, because S. rebaudiana leaves are the main tissue for both synthesis and primary accumulation of steviol glycosides

References : 

 

Sanyo Kokusaku. (1990) New triploid of Stevia Rebaudiana Bertoni contains sweet diterpenoid. Patent Number(s): JP2242622-A; JP2748141-B2.

 

Shuichi,H.,Tsuneo,Y. and Satoshi,F. (2001) Breeding of triploid plants of stevia (Stevia rebaudiana Bertoni) with high rebaudioside A content. Jpn. J. Trop. Agric. 45: 281289.

 

Yadav, AK; Singh, S; Yadav, SC; Dhyani, D; Bhardwaj, G; Sharma, A, Singh, B. (2013) Induction and morpho-chemical characterization of Stevia rebaudiana colchiploids: Indian Journal of Agricultural Sciences 83 (2): 159–65,

 

Hegde SN, Rameshsing CN, Vasundhara M. (2015) Characterization of Stevia rebaudiana Bertoni polyploids for growth and quality; International Journal of Phytomedicines and Related Industries, Volume 7, Issue  3 : 188-195

Hong Zhang, Shaoya An, Juan Hu, Zhe Lin, Xiang Liu, Han Bao, Ren Chen (2018) Induction, identification and characterization of polyploidy in Stevia rebaudiana Bertoni Plant Biotechnology 35, 81–86