Categorías Emergentes

Genetic modification (GM) is a very efficient method for introducing foreign genes into rice genome. Protocols for efficient rice transformation using bombardment or Agrobacterium have been well developed (Christou et al., 1991; Hiei et al., 1994).

Two GM rice varieties with herbicide resistance, LLRice60 and LLRice62, were approved in the United States in 2000. Subsequent approval of these and other types of herbicide-resistant GM rice occurred across Canada, Australia, Mexico, and Colombia. However, none of these approvals resulted in commercialization. Varieties containing the Bt genes (cry1Ab, 1Ac 1Aa, 2A, 1B, or a combination of these genes) for resistance against lepidopteran pests were developed in different laboratories (Breitler et al., 2004; High et al.,

13

2004; Ho et al., 2006). The first field testing of the Bt rice was conducted in China in 1998 (Shu et al., 2000; Ye et al., 2001). Systematic field trials on these GM rice have shown that using the GM rice can reduce the use of pesticide significantly compared with using the conventional varieties. In 2009, China granted biosafety approval to GM rice varieties Bt Huahui No. 1 and Bt Shanyou 63 with pest resistance (Li et al., 2015). It is expected that China will be the first country to commercially release insect resistant transgenic rice.

Rice has also been engineered to withstand different abiotic stress conditions, such as drought, heat, cold, salinity, and mineral deficiency. Abscisic acid (ABA) is a phytohormone which plays important roles in the regulation of seed dormancy and adaptation to abiotic stresses. OsPYL/RCARs were identified as functional ABA receptors regulating ABA- dependent gene expression in rice. The overexpression of OsPYL/RCAR5 in rice driven by the maize ubiquitin promoter enhances improved drought and salt stress tolerance in rice (Kim et al., 2014). However, the plant height was slightly reduced under paddy field conditions and the grain yield severely decreased. It is necessary to fine regulating the expression level of OsPYL/RCAR5 to avoid deleterious effects on agricultural traits.

Nutritional improvement of rice grains has been a hot area in rice genetic engineering. The most well-known example is the development of Golden Rice. Vitamin A deficiency is one of the most prevalent deficiency diseases in developing countries, affecting more than 4 million children each year, up to 500,000 of whom become partially or totally blind (Ye et al., 2000). The entire β-carotene biosynthesis pathway has been engineered into rice endosperm, which is known as Golden Rice (Ye et al., 2000). Since then, efforts have been made to introduce genes for provitamin A biosynthesis into commercial rice varieties, including the genes for phytoene synthase (psy) and lycopene β-cyclase (β-lcy) originated from the daffodil and the gene for phytoene desaturase (crt1) of bacterial origin. ‘Golden Rice 2’ in which maize psy replaced daffodil psy contained carotenoids up to 23-fold of Golden Rice (Paine et al., 2005). Similarly, rice has been engineered to increase iron content, which is a very useful supplement for children and women in developing countries. At IRRI, two genes, ferritin and transporter gene, were added to IR64. The ferritin coding for iron storage is from soybean. Rice has its own ferritin gene, but adding additional ferritin gene increases the plant’s iron storage capacity (Oliva et al., 2014). The transporter gene from another rice variety allows iron in the leaf to be transported to the grain. IRRI is also developing iron-rich rice and drought tolerant rice by genetic modification. Rice has also been transgenically modified to increase quantities of various amino acids and improve starch biosynthesis and oil quality (Newell-mcgloughlin, 2008).

14

Sun et al. (2015) engineered rice that stores more sugar in its grains and stems by adding a gene (SUSIBA2) from barley that affects starch storage. The new plant emits as little as 1% of the methane. What’s more, the new rice may also boost food security as it produces significantly higher yield per plant. In a three-year-long trial, the rice grew well and led to drops in paddy field methane emissions.

Much effort has been made in the past decade to engineer C4 rice for higher photosynthesis efficiency and nitrogen and water use efficiency (Leegood, 2013). The International C4 Rice Consortium (http://c4rice.irri.org/) was formed to transfer high-yield C4 metabolism to rice and altering rice leaf anatomy and morphology in order to make it comparable with the Kranz-type biochemistry. The main approaches used in this ambitious project include: (i) integrating of the genes typical for the C4 metabolism into rice to increase photosynthesis efficiency, e.g. the genes encoding phosphoenolpyruvate carboxylase in mesophyll cells (MCs) and enzymes from Calvin-Benson cycle in bundle sheath cells (BSCs), (ii) down-regulating endogenous genes in rice, such as encoding MC enzymes of the Calvin- Benson cycle and photorespiration, (iii) introducing C4 cell-type specific gene expression and protein accumulation in rice, including the identification of suitable regulatory elements to ensure the protein's compartmentalization between MCs and BSCs, and (iv) identifying C4 transporters which transfer metabolites between subcellular compartments and introducing corresponding genes into rice (Karki et al., 2013; Leegood, 2013). Significant progress has been made through the identification of gene promoters for compartmental gene expression (Wang et al., 2013), gene cloning for the main enzymes of the C4 metabolic pathway from maize and their transformation into rice (Kajala et al., 2011), and the determination of candidate transporters of intermediate metabolites between MCs and BSCs (Karki et al., 2013).