Journal of Plant Molecular Breeding (2015), 3(2): 44-55 --------------------------------------------------------------------------------- *Corresponding Author (): yamchi@gau.ac.ir Received: 13 August 2015/ Revised: 19 December 2015 Accepted: 24 December 2015 Proline accumulation and osmotic stress: an overview of P5CS gene in plants Sahand Amini 1 , Cyrus Ghobadi 2 and Ahad Yamchi 3* 1 Department of Agricultural Biotechnology, College of Agriculture Isfahan University of Technology, Isfahan, Iran 2 Department of Horticultural Sciences, College of Agriculture Isfahan University of Technology, Isfahan, Iran 3 Department of Plant Breeding and Biotechnology, College of Plant Production Gorgan University of Agriculture Science and Natural Recourses, Gorgan, Iran ABSTRACT: Under osmotic stresses, proline accumulation is an important response of plants to these conditions. Proline is a compatible osmolyte which affects many cellular and molecular aspects of a plant in both normal and stressful situations. Proline is shown to be involved in plant development in normal conditions and in conferring resistance to a plant under biotic and abiotic stresses. Therefore, many surveys have already been conducted to unveil its mechanisms and signaling pathways, so that it might be considered as an insight into resolving growing challenges of agriculture, drought and soil salinity. Δ 1 -pyrroline-5-carboxylate synthetase (P5CS), one of the two main enzymes in the proline biosynthesis pathway of the glutamate precursor, has been demonstrated to play a significant role in proline accumulation in plants under water stresses. Regarding the role of P5CS under the osmotic stress, there are controversial observations in various plants which casts doubts regarding whether P5CS is a rate-limiting enzyme in the pathway or not. Obviously, transgene P5CS is proved to give higher resistance to transgenic plants under drought and salinity, by elevating proline content. In this review of literature, proline and its identified various functions in plants, characteristics of P5CS enzyme, signals, inducers and inhibitors of P5CS gene and the expression pattern of P5CS under differential conditions in studied plant species are discussed. Finally, some of the important features of the transgenic plants overexpressing P5CS have been summarized. KEYWORDS: Abiotic Stress, Overexpressing, Proline, P5CS, Transgenic Plants INTRODUCTION It is not impenetrable today that the most influential ob- stacles to achieve high-yielding crops are osmotic stresses, particularly drought and soil salinity whereby dehydration causes loss of millions of tons of crops every year and half of the arable lands has already become arid due to soil salinization (66) inasmuch as they are growing at the worrisome rate of 3 hectare per minute (39). These stresses affect the plant with lowering the amount of water available for it, as well as osmotic potential of cells. These conditions cause deficiency in normal development and growth of the plant, reduction in its fertility and even death of the plant in severe and prolonged stresses (8). Therefore, in- vestigation on improving plants resistance to osmotic stresses has drawn much attention among researchers. Proline, the most accumulated osmolyte, which accumulates to high levels in many plants in various stressful conditions such as drought, salinity, high and low temperature. Photo-damage, heavy metals and even pathogens, have been proven to play a significant role in adapting the plants to water stresses (30, 66, 69). Proline which is conspicuously more than just an osmo- protectant, has many functions under normal and stressful conditions in plants. Proline, as a cyclic amino