Response of Rice genotypes to Phosphorus-starvation stress

Avani Dave, Jai Hind College

Out of the multiple essential macronutrients that help the living organisms at every step of their survival, Phosphorus (P) is ranked of paramount importance. Mainly due to its role in several biomolecules like the nucleic acids, phospholipid membrane, an important factor for the working of the enzymes, and various energy-rich molecules and carriers. Plants utilize phosphorus for their seamless functioning. They acquire this macronutrient by absorption via the roots from the soil, in the form of H2PO4− or HPO4−2. This is where the problem of P-deficiency starts from, although the soil contains an adequate amount of phosphorus, the plants can’t absorb it in its elemental form. 

Pi-deficient soil type is currently mapped as a global concern, leading to decreased availability of productive land. India amongst the other countries is facing this now more than ever, with an estimation of further reduction of available Pi-content in the soil. The countries are currently overcoming this issue with the utilization of fertilizers acting as a source of phosphorus. P-fertilizers would not be environmentally sustainable as it bothers the ecology, due to the increased usage of rocks and demands for large economic investments. So, we would have to amplify various mechanisms in plants to maximize the P-uptake followed by P-use efficiency.

Rice – Oryza sativa L. acts as one of the staple foods for the majority of people around the globe. This makes it very crucial to tackle issues of reduced crop production due to an inefficient level of essential nutrients in the soil. The plants over time, have acquired designated responses in order to tolerate the effects of deficiency of phosphorus. Identification of these responses may come in handy for the upcoming breeding approach so as to allow better growth of rice cultivars in P-deficient soils. 

Similar transcriptome analysis under conditions of  P-starvation stress has been carried out in Arabidopsis, Wheat, and Maize as well. That being said, the current study varies from the previous studies in the aspect of an increased study time. Starting from the seedling stage and ending at the vegetative stage. 

Method applied

Two contrasting rice genotypes – P-deficiency sensitive (Pusa-44) and P-deficiency tolerant (NIL-23) were subjected to three different growth conditions: P-sufficient, P-deficient, or P-starvation. These were allowed to grow hydroponically for 45 days, the sample was later used for further analysis of the effect of P-starvation stress on gene expression in contrasting rice genotypes. The following were estimated,

• Activity of acid phosphatase, phosphorus content in shoot and root tissues.
• Chlorophyll content in the leaf. 

Seven differentially expressed genes (DEG) were selected, four up-regulated and three down-regulated) for quantitative RT-qPCR analysis which would eventually be used to confirm the results derived from the RNA sequence. 

Results 

The following changes were observed amongst the contrasting rice genotypes used in the study:

• The changes in the extent of growth in roots and shoots were noticed during the seedling stage however, the changes were more significant at the vegetative stage.
• Shoot biomass seemed to be more affected under stress as compared to root biomass which contributed to an increased root-shoot biomass ratio.
• The morphology and molecular content of various parts of the plants underwent changes.
• As the phosphorus content kept decreasing, the roots increased the APase activity.
• In the case of deficiency tolerant plants, the mobilization of the phosphorus from the roots up to the shoots with the help of transporters was very well observed.
• The genes involved in epigenetic regulation were differentially expressed by either up or down-regulation, to a varying degree across the different genotypes.
• Similar trend was observed in the case of expression of genes that were involved 

• Phytohormone and signal transduction
• Carbohydrate and lipid metabolism
• Photosynthesis
• Cell wall and associated activities
• Phosphorus response

• Around 50 – 70% of the differentially expressed genes were up-regulated under stress caused by phosphorus starvation. 
• The data obtained from the RT-qPCR validated the results of the RNA sequence.

Highlighting key features that help in improved phosphorus acquisition

Ever since germination up until maturity, a plant would show constant signs of stress if made to grow in P-deficient soil. The morphological analysis of rice plants grown in P-deficient soil highlights multiple key features that help in the improved acquisition of phosphorus:

• The number of roots reduced accompanied by an increase in their length, which provides a larger area for P absorption.
• A reduction in the biomass synthesis further alters root and shoot growth, which is regulated by several mechanisms 
• When the plant reaches the tillering stage under starvation, a prominent reduction in height was observed. 
• Starvation also affects Leaves, A reduction in the size of the leaf lamina was observed which eventually contributed to reduced chlorophyll content.
• Phosphorus content in the soil is inversely proportional to APase (Acid phosphatase) activity 
• Several molecular mechanisms (for e.g. modulation of gene expression) are incorporated to deal with the stress caused by deficiency of phosphorus 

Conclusion

We are down on our luck to not have enough understanding of some of the crucial P-uptake, transport, and use-efficiency factors. That being said, we do have a fair insight into the distribution of P by transporters having known functional characteristics as well as the differential expression of the genes involved in P-deficiency tolerance. 

The current study divulges molecular information and mechanisms by studying notable features that aid in P-starvation stress tolerance in rice. For instance, the transporters, signalling molecules, auxin-responsive protein, LTPL-protease inhibitor family proteins, glycerophosphoryl diester phosphodiesterase family proteins, phosphatases, and transcription factors. 

Cohesive data derived from this will eventually help in the mapping of the regulatory network supported by an amalgamation of biochemical, physiological, genetic, and molecular mechanisms to assist the plants with the restricted presence of P in the soil. However, there is a scope of genetic and epigenetic techniques, further providing increased productivity of the cultivars even in P-deficient soils.

Also read: Marine methane-eating microbes regulate the global temperature

Source:

Kumar, S., Pallavi, Chugh, C. et al. Characterization of contrasting rice (Oryza sativa L.) genotypes reveals the inorganic phosphate -efficient schema for phosphate starvation tolerance. BMC Plant Biol 21, 282 (2021). https://doi.org/10.1186/s12870-021-03015-4

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Author Info: Avani Dave is currently in the final year of her bachelor’s degree, majoring in Life Sciences. Holding a good academic and extra-curricular record, she is on a constant journey of acquiring exposure in her field of interest while simultaneously not limiting herself to just that. Avani likes studying Diseases and Syndromes and everything under this umbrella! That being said, she is adept at working across departments and promises to deliver.