Starch accumulation mechanisms in Cassava roots revealed!

Husna, Amity University Kolkata

Cassava (Manihot esculenta Crantz) is a tropical perennial root crop species that can efficiently accumulate starch in its storage roots. It ranks the fifth most important food crop worldwide. Approximately one billion people rely on the starch-rich roots of Cassava. It is harvested yearly with an approximately 12 months growth cycle. High-efficiency photosynthesis, tolerance to drought, and extraordinarily high starch accumulation in storage roots are some necessary biological characteristics of cassava. 

For an economically important crop, photosynthesis and the transport and accumulation of carbohydrates are three essential physiological processes of yield formation. Cassava has three types of roots: primary and secondary fibrous roots and storage roots. Storage roots develop from primary fibrous roots (PFRs) and starch can constitute up to 85% of the dry weight of the storage roots. However, how such a large amount of carbohydrates can be loaded into the phloem in the leaves and unloaded into parenchymal cells in the storage roots is not known and needs to be determined.

Various sucrose unloading models in plants:

In a recent study, the general transport models of carbohydrates in cassava were determined.

To date, there are three sucrose unloading models that have been identified in plants. They are:

• Symplastic model
• Apoplastic model, and
• Mixed model.

Symplastic phloem unloading happens when sucrose passes through the plasmodesmata between phloem companion cells (CCs) and parenchyma cells (PCs) and into sink tissues. This is the principal pathway for most plant species.

Apoplastic unloading depends on sucrose transporters (SUTs), the sucrose transporters can take up sucrose from the phloem to parenchyma cells in storage roots. This process is accompanied by post-unloading invertases. Invertases play a very important role in mediating the sink strength because they catalyze the irreversible cleavage of sucrose to glucose and fructose. Alternatively, the sucrose in the apoplastic space between cells can also be transported via sucrose transporters (SUTs) that are located on the plasma membrane. The unloading processes usually vary based on the sink type, developmental stage, and the function of the genes involved. Soluble sugar content and the invertase activity determine the transition between two unloading pathways.

Method of determining the transport model in Cassava:

The sucrose unloading pattern and its impact on cassava storage root development were investigated using microstructural and physiological analyses, namely, carboxyfluorescein (CF) and C14 isotope tracing. The expression profiling of genes involved in the symplastic and the apoplastic transport was performed. This included enzyme activity, protein gel blot analysis, and transcriptome sequencing analyses.

The findings showed that carbohydrates are transported mainly in the form of sucrose, and over 54.6% was present within the stem phloem of Cassava. These findings were based on structural, physiological, and biochemical experiments on developing storage roots. It included measurements of the activity of enzymes and the expression of mRNAs of genes involved in transport and post-transport events.

Results of the study:

1.       Sucrose was found to be the major transported component because there were increasing numbers of plasmodesmata in the developing storage roots of cultivated cassava. This allowed sucrose to be efficiently unloaded into the parenchyma cells in the storage roots.

2.       There was a shift in transport from the apoplastic to the symplastic unloading at the beginning of the fibrous root swelling stage, which is crucial for a highly efficient starch accumulation and high-yield cassava variety. Hence, sucrose was predominantly unloaded from the phloem into their storage roots via symplastic unloading patterns.

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References:  

1. Pan, Kun, et al. “Predominantly Symplastic Phloem Unloading of Photosynthates Maintains Efficient Starch Accumulation in the Cassava Storage Roots (Manihot Esculenta Crantz).” BMC Plant Biology, vol. 21, no. 1, July 2021, p. 318. BioMed Central, doi:10.1186/s12870-021-03088-1. https://bmcplantbiol.biomedcentral.com/articles/10.1186/s12870-021-03088-1#citeas

About author:

Husna is an undergraduate student of BTech Biotechnology at Amity University Kolkata. She is a research enthusiast in Immunology and Immunotherapy but she has a keen interest in various other bioscience subjects as well. She is constantly focused on improving her knowledge and laboratory skills through various internships. She is a Scientific content writer who has knowledge in diverse backgrounds of Biotechnology.

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5. https://bioxone.in/news/an-early-warning-sign-for-the-silent-killer-heart-disease-discovered/

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