**1. Introduction**

Wheat (*Triticum* spp.) is one of the most important and widely cultivated crops with the annual yield of 694 million metric tons. More than 40 countries and over 35% of the world population use wheat as their staple food [1, 2]. Wheat is cultivated on larger area than other

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cereals and modified to different climatic conditions [3, 4]. Bread wheat (2n = 6x = 42) and durum wheat (4x = 28) are the two common cultivated species around the world. Bread wheat supplies about 95% wheat globally, while durum and other wheats (emmer (4x = 28), einkorn (2x = 14) and spelt (6x = 42)) provide only 5% of the world wheat [2, 5]. Human population is increasing rapidly and is estimated to reach 9.4 billion by 2050. Therefore, food production will require a greater yield from the present cropland without horizontal expansion [6]. Population growth, environmental pollution and utilization of croplands for other purposes may reduce the croplands by 10–20% [7]. To meet the growing demand of global food shortage of 2050, total food production must increase by 50% at least to meet out demands of 2050. Among the crop plants, wheat is an economic and rich source of energy and proteins and supplies one fifth of all human calories for the world population [8]. Plant breeders are always trying to find wheat germplasm having desirable traits such as tolerance to diseases and other abiotic stresses [7, 9]. There is no doubt that cereals such as wheat, rice and maize are the world's leading food crops for all humans and are the principal resources that have led to the emergence of human civilization.
