IntroductionMaize (Zea mays L.) is one of the major cereal crops with high yield and economic value. Maize contributes to the steady production of cereals in the world and has an important role in economy and international trade as it is used for food, animal feed, and materials for many industries. The demands for food, animal feed, and materials as well as fuel in the world are growing rapidly, and it is estimated that about 200 million tons of maize would need to be produced annually to meet demands in 2017 (Edgertom, 2009). Similar to many other crops, maize is less productive due to weevils. Though maize weevils (Sitophilus zeamais Motsch.) eat most types of cereals, legumes, oil seeds, and other plant products, they prefer corn. The adult weevils drill a hole into the grain, lay eggs, and secrete a sticky mucus to block the hole. The larvae hatch in the seed and consume the embryo and other parts until only the testa is left. Once large enough, the maggot will drill holes to get out, grow wings, and infect other plants (Gutierrez-Campos et al., 1999).Recently there has been increasing interest in searching for plants' natural peptides that are able to inhibit harmful agents. One group of natural peptides are plant defensins. They are small cysteine-rich proteins containing about 45-54 amino acids. Plant defensins attract great interest as they are reported to be involved in different defense pathways in plants (Selitrennikoff, 2001). Plant defensins are multifunctional, and, according to many studies, they show antifungal and antibacterial activity (Wang et al., 2011), inhibition of trypsin and α-amylase activities (Melo et al., 2002), inhibition of protein synthesis (Colilla et al., 1990), increased tolerance to heavy metals (Mirouze et al., 2006), and regulation of plant growth and development (Stotz et al., 2009). Based on their sequential characteristics and functions (van der Weerden and Anderson, 2013), plant defensins are divided into 18 groups. The first group consists of defensins functioning as inhibitors of α-amylase or trypsin. Liu et al. (2003) isolated defensin from chickpea (VrD1) and demonstrated that VrD1 could

The EXP1 gene encodes expansin, which has the ability to loosen the plant cell wall. The soybean expansin gene GmEXP1 is activated specifically during the root elongation process, and thus it plays important roles in root development. During the drought period, changes in pressure within the cell and the fast development of the root allow plants to collect water from deep soil, which in turn helps plants grow and develop. In this study, we have successfully cloned and generated a GmEXP1 construct expressing recombinant expansin protein in tobacco plants. GmEXP1 is expressed in transgenic tobacco plants and passed on to the next generation. The transgenic tobacco plants have improved drought tolerance, which is demonstrated in both the length and volume of roots. From these promising results, we applied the same approach to generate drought-tolerant plants.