Organelle genome and its relevance in Plant Breeding

 

 

 Besides the nuclear genome, plants possess two small extrachromosomal genomes in mitochondria and chloroplasts. DNA present in the cytoplasm, known as Plasmon, is located as plastome in plastids (pt-DNA) and as chondriome in mitochondria (mt-DNA). The size of the organelle DNA is small compared to the genomic DNA. Small amounts of organelle DNA must be multiplied by the number of organelles per cell, up to a hundred in the case of plastids and several hundred in the case of mitochondria.

The endosymbiotic theory states that some of the organelles in today's eukaryotic cells were once prokaryotic microbes. As per the theory, chloroplasts and mitochondria originated from cyanobacterium and α-proteobacterium, respectively. cpDNA regions include Large Single-Copy (LSC) and Small Single-Copy (SSC) regions along with inverted Repeats (IRA & IRB), whereas mtDNA is multipartite. Inheritance of traits controlled by genes located in the cytoplasm is known as cytoplasmic inheritance. Characters of only one of the two parents (usually the female parent) are transmitted to the progeny. As a result, progenies obtained from reciprocal crosses exhibit consistent differences in trait expression, and there is a lack of segregation in F2 and subsequent generations.

Some examples of the economic traits influenced by cytoplasmic factors are cytoplasmic male sterility, biotic resistance and abiotic tolerance, yield and quality parameters, tissue culture and regeneration ability, heterosis, and plant adaptation.

The ability of transformation is reported to be influenced by cytoplasmic factors. In a study, Japonica rice line 19 was transformed using a vector harboring the smGFP gene transferred into the rice plastid genome by bombardment. The resistant callus was obtained after long-lasting multiple selections and proved to be in homoplastomic status by molecular testing.

Genes residing in mitochondria and chloroplasts are known to confer abiotic stress tolerance. A subgroup of class I Caseinolytic proteases (Clps) function as molecular chaperones and confer thermotolerance to plants. In bread wheat, class I Clp family consisting of five ClpB, two ClpC, and two ClpD genes were identified. Phylogenetic analysis showed that these genes were highly conserved across grass genomes. Subcellular localization prediction revealed that TaClpC and TaClpD subgroup proteins and TaClpB1 proteins are potentially targeted to chloroplasts, while TaClpB5 to mitochondria, and TaClpB2, TaClpB3, and TaClpB4 to cytoplasm.

Most crop improvement programs are focused on genetic information related to traits controlled by genomic DNA. However, there are well-documented economically important traits controlled by the organelle genome. Work on the genetic basis of the Plasmon is still difficult, but it could have enormous benefits for the genetic improvement of feed, food, and non-food crops. mt-DNA through CMS-based hybrid breeding highlights the potential of non-nuclear genes. The expression of transgenes in chloroplasts has opened the door to a new era in biotechnology, has proven its potential in agricultural crops, and offers several advantages over their expression in the nucleus.

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References:

1. FREI, U., PEIRETTI, E.G., & WENZEL, G., 2010, Significance of cytoplasmic DNA in plant breeding. Plant Breed. Rev., 23: 175-210.
2. WANG, Y., WEI, Z., & XING, S., 2018, Stable plastid transformation of rice, a monocot cereal crop. Biochem. Biophys. Res. Commun., 503(4): 2376-2379.
3. MUTHUSAMY, S. K., DALAL, M., CHINNUSAMY, V., & BANSAL, K. C., 2016, Differential regulation of genes coding for organelle and cytosolic ClpATPases under biotic and abiotic stresses in wheat. Front. Plant Sci., 7(929): 189-195.