Abstract

Phytoglobin, Mineral Nutrient Deficiency and Abiotic Stress in Plants: What we are Learning from Functional Genomic Approach?

Girdhar Pandey, Professor, University of Delhi

Recent studies on soil fertility index are indicating a severe deficiency of mineral nutrients in it due to exhaustive usage causing a decline in the total growth output. Apart from mineral nutrient deficiency, abiotic and biotic stresses are also contributing in reducing the crop yield. Plants on the other hand have evolved a variety of adaptive mechanisms to combat such harsh and hostile environmental conditions. It has been established that the mineral nutrient ion balance in plants is crucial for plants survival and provide resistance towards stress conditions. In the post-genomic era, by the usage of functional genomics techniques  resulted in the identification of several key genes, which may play significant roles in sensing and adaptation to nutrient deprivation conditions and are useful to further explore this field. The global transcriptome expression profiling, proteome and metabolome analysis are very important and  crucial for elucidating the gene regulatory networks of essential nutrients and probably will help in developing different strategies, which will pave the way to improve the agronomic traits in crops. Potassium (K+) and calcium (Ca2+) are essential plant nutrients, required in abundance for optimal growth and development of plants. Earlier reports have shown that deficiency of any of the two cations is damaging for the plant growth and hence crop yield and production. Our whole transcriptome analysis of K+ and Ca2+ deficiency in rice have identified large number of differentially expressing genes (DEGs). Our findings highlight altered expression patterns of genes majorly involved in metabolic processes, stress responses, signaling pathways, transcriptional regulation, and transport of multiple molecules in both K+ and Ca2+ deficiency conditions in rice. Interestingly, one of the classes of DEGs include non-symbiotic Hbs, also known as Phytoglobins (Pgbs) in plants under both K+ and Ca2+ deficiency conditions in rice. In plants, Pgbs have been categorized into 6 different classes, namely, Phytogb0 (Pgb0), Phytogb1 (Pgb1), Phytogb2 (Pgb2), SymPhytogb (sPgb), Leghaemoglobin (Lb), and Phytogb3 (Pgb3). Among the 6 Phytogbs, sPgb and Lb have been functionally characterized, whereas understanding of the roles of other Pgbs is still evolving. From this juncture, we have undertaken the functional characterization of 2 rice Pgbs (OsPgb1.1 and OsPgb1.2). Based on the expression analysis studies, OsPgbs exhibited increased transcript levels under salt, drought, cold stresses and ABA treatment. In addition, promoter analysis of OsPgb1.1 and OsPgb1.2 revealed that these genes are ubiquitously expressed in plant tissues and induced by nutrient deficiency as well as abiotic stresses. Overexpression of OsPgb1.1 and OsPgb1.2 in Arabidopsis resulted in tolerant phenotype to low sucrose and low K+ condition, respectively and exhibited better root growth in both the conditions. Similarly, overexpression of OsPgb1.1 and OsPgb1.2 in rice also manifested in tolerant phenotype under abiotic stresses, K+ and Ca2+ deficiency. Overall, our results indicated the potential involvement of rice phytoglobins in signaling networks triggered by these stresses. We speculate that rice phytoglobins play significant role in plant growth and development by regulating reactive oxygen species (ROS), nitric oxide (NO)  and energy homeostasis in plant cell. In future, these rice phytoglobins can be used as potential candiate to develop crop, which can tolerate K+ and Ca2+ deficiency as well as abiotic stresses without lossing yield and productivity.