1.1 Genetic background
The rice plant belongs to the class Monocotyledonae, the order Glumiflorae, the family Glamineae, and the genus Oryza. The species cultivated in the Asian region is Oryza sativa L., while that cultivated in the savanna of western Africa is O. glaberrima Steud. Besides these two cultivated species, there have been found about 20 wild species belonging to the genus. For the species O. sativa, two subspecies, indica and japonica, and three ecotypes, Indica (continental), Javanica (tropical island) and Japonica (temperate island) are known. The centers of distribution of the three ecotypes are Indian subcontinent and Indochina peninsula, Indonesia, and Japan and Korean peninsula, respectively.
* Abbreviations: A(U)DPG, adenosine (uridine) diphosphate D-glucose; BE, branching enzyme; DAP, day after pollination; DE, debranching enzyme; F6P, D-fructofuranose 6-phosphate; GB, granule bound; G1(6)P, a-D-glucopyranose l(6)-phosphate; GUS, (3-D-glucuronosidase; IT, invertase; PPase, pyrophosphorylase; SDS, sodium dodecyl sulfate; SPPase, sucrose phosphate phosphatase; SPS, sucrose phosphate synthase; SS, starch synthase; SuS, sucrose synthase.
Among five billions of world population, about one-half subsist on rice as the staple food. Together with the other two major cereal crops wheat and maize, rice has a very long history of improvement through selection and breeding by farmers and breeders. Although being the most important carbohydrate supply for the population subsisting on rice-based diet, polished rice grains contain 7 to 10 % of easily digestible proteins closely associated with starch granules. It is thus also an important source of protein when supplies of meat, dairy products, beans, etc., are limited and rice is consumed in quantity. Currently, about 11% of world arable land, or 150 million ha, is used for rice cultivation, and the annual production of hulled grain is over 400 million metric tons. From these figures, we may see that the rice plant has one of the highest productivity among all cereal plants.
Among important cereal plants, rice is endowed with the most versatile properties to be exploited for the development of cultivars for agricultural industry. This is probably due to a wide range of genetic mutability and diversity we find in the rice plant. In spite of genetic diversity, the rice genome is the smallest among main cereal plants. It is natural then, besides being investigated intensively by agronomists as a staple food, the rice has become a model monocotyledonous plant for the basic researches in genetics, cell biology, physiology, biochemistry and molecular biology. Especially the success in establishing the technology for regenerating whole plant from cultured cells or protoplasts has opened up the possibilities of improving the agronomic properties by molecular breeding, and exploiting the rice plant as a reactor for the production of exotic proteins.
From the agricultural point of view, the germination of rice seed is considered as the onset of its life span. Although direct sowing of seeds on the paddy field and allow them to grow until harvest is also practiced, usually seedlings are grown in a seedbed for 20 to 40 days. They are transplanted into a paddy field in a neat pattern for easy management of fertilizer application, weed control, grain harvest, etc., and also for optimizing the growth space to get the highest return from a unit land area. Within a week after transplantation, new roots emerge and the tillering (or branching) starts. The number of tillers reaches the maximum in about a month. The stem of rice plant is consisted of leaf sheath and culm, the latter of which is composed of nodes and internodes. The first stem emerged from seed is the main culm. The branch emerging from the main culm is the primary tiller, the tiller emerging from the primary tiller is the secondary tiller, and sometimes the tertiary tiller may emerge from the secondary tiller. There is a rule in tillering. When the w-th leaf emerges from the leaf sheath beneath, the primary tiller emerges from the axil of (w-5)th leaf. This rule is also observed when the secondary tiller emerges from the primary tiller. The root system comprises the seed root, which develops from the root emerged from seed, and the crown root, which emerges from node afterward. The rule of periodicity as we find for the leaf and tiller developments is also observed in the crown root and node developments.
As the tillering stage comes to conclusion, the rice plant goes into the reproductive stage by differentiating flag leaf to form panicles. The panicle grows and differentiates to panicle axis and primary and secondary branches. From these branches spikelets grow, each of which has a short rachilla and bears a glumous flower. A glumous flower is consisted of two empty glumes, one each of palea and lemma, six stamens and one pistil. It takes about a month from the onset of flag leaf differentiation to the finished panicle to be pushed out of leaf sheath and start flowering. The flowering takes place in the chronological order of the main culm, the primary tiller and the secondary tiller, the upper flower on the primary branch in the same panicle, the uppermost flower, and then from the bottom to the second uppermost flower in the same branch. In one panicle, the earliest to the latest may have a week lag in flowering. The pollination ensues almost instantaneously after flowering, and the fertilization completes within 3 to 4 hours. Therefore the flowering and pollination are recorded as to take place in the same day. After fertilization, the differentiation of embryo primordium completes in 3 days, the differentiation of seed root completes in 5 days, and the differentiation of the primordia of up to the third leaf completes. All of the essential parts of an embryo are formed in about 10 days after pollination (DAP). The spikelet after fertilization is called caryopsis. The cell division in endosperm completes in about 10 DAP. By this time, the aleurone layer is formed on the surface of endosperm, and the former tissue starts accumulating proteins and lipid particles, while the latter starch granules. Although most of the starch in endosperm is derived from the photosynthate produced after panicle formation, a part of photosynthate stored as starch in the stem before panicle formation are also transferred into endosperm. In about one and a half months after panicle emergence, the seed formation comes to a conclusion.
One life span of the rice plant, starting from seed germination to seed maturation, may be from a little less than 100 days to up to 270 days, according to the differences in species variety and growth environment. The major difference in growth rates among rice races with different genetic backgrounds occurs mainly from seed germination to panicle emergence. The rice plant highly tolerates continual cropping on a same piece of land. If one selects appropriate cultivars, it is possible to harvest two to three crops of rice a year from a same piece of land in tropical to subtropical regions. In Taiwan, there are many paddy fields being known to sustain annual double cropping of rice for over a century, and yet the productivity has increased continually because of the improvement in cultivars provided by the agricultural agencies and the advancement in cropping technologies.
The productivity of a cereal plant is measured by the economic yield, or the productivity of plant part useful as food, while the biological productivity concerns with the biological yield, or the total dry matter produced by the plant. The ratio of economic yield over biological yield is called the harvest index. In order to enhance the productivity, it is of prime importance to improve the net assimilation rate of leaves. Then the increase in harvest index may be achieved by optimizing the partitioning of photosynthates between the source, or photosynthetic leaf, and the sink, or the specific storage organ for human consumption. These have been the foci of researches by many agricultural scientists.
The economic yield of a rice plant is determined by the number of panicle, the number of seed a panicle bears, the rate of seed fertilization and seed weight. Excellent rice cultivars with diversified agronomic properties are available. The difference in length of time span to achieve maturity not only determines the adaptability to agroenvironment but also the economic yield if one considers the spatial and temporal efficiencies of rice cultivation. The early or late maturation of rice varieties is determined by the length of vegetative growth period, which is dependent on the photoperiodic sensitivity and thermosensitivity. After the flower bud initiation, there is not much difference in the length of reproductive phase among different varieties.
In case of rice plant, the economic yield is measured by the harvest of seed grains. As described above, from fertilization to maturation of rice seeds takes about one and a half months. From 10 to 14 days after pollination (DAP), if one takes a seed at this stage and tweezes it between fingers, a milky juice, the appearance of which is due to the presence of suspended starch granules, is expressed. This stage of seed growth is called the milky, or milk ripe, stage. The rate of soluble sugar incorporation into insoluble polysaccharides, mainly starch, is the highest at this stage, and thus the milky stage seed is the most suitable sample for the study of starch synthesis in rice.
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