Rice (Oryza sativa L.) is a staple food for more than 50% of the world's population, including regions of high population density and rapid growth. Rice is produced under both upland and lowland ecosystems with about 76% of the global rice produced from irrigated lowland rice systems. The objective of this article is to discuss growth and formation of yield components in rice during crop growth cycles. The yield components of rice are the number of panicles per unit area, number of spikelets per panicle, weight of spikelet and spikelet sterility or filled spikelet. In addition, shoot dry weight, grain harvest index, and nitrogen (N) harvest index are also positively associated with grain yield. These yield components and yield associated parameters are formed during crop growth cycle. Growth cycle of the rice plant is divided into three stages. These stages are designated as vegetative, reproductive and spikelet filling or ripening. Yield potential of rice is formed or defined during these growth stages. Plant height, tillering (associated with panicle number), root growth, leaf area, and morphology are the main features of vegetative growth stage. In the reproductive growth stage panicle development takes place. Booting and flowering are part of the reproductive growth stage. Panicle size or spikelets per panicle are determined in the reproductive growth stage. Spikelet size or weight is determined during the spikelet filling growth stage. The reproductive growth stage is the most sensitive to biotic and abiotic stresses, followed by spikelet filling and vegetative growth stage. Recent advances in molecular linkage maps of rice and other developments of molecular biology offer new opportunities for improving rice yield components in favor of higher yield.

Yield Physiology of Rice

In mammals, cadmium is widely considered as a non-genotoxic carcinogen acting through a methylation-dependent epigenetic mechanism. Here, the effects of Cd treatment on the DNA methylation patten are examined together with its effect on chromatin reconfiguration in Posidonia oceanica. DNA methylation level and pattern were analysed in actively growing organs, under short- (6 h) and long- (2 d or 4 d) term and low (10 mM) and high (50 mM) doses of Cd, through a Methylation-Sensitive Amplification Polymorphism technique and an immunocytological approach, respectively. The expression of one member of the CHROMOMETHYLASE (CMT) family, a DNA methyltransferase, was also assessed by qRT-PCR. Nuclear chromatin ultrastructure was investigated by transmission electron microscopy. Cd treatment induced a DNA hypermethylation, as well as an up-regulation of CMT, indicating that de novo methylation did indeed occur. Moreover, a high dose of Cd led to a progressive heterochromatinization of interphase nuclei and apoptotic figures were also observed after long-term treatment. The data demonstrate that Cd perturbs the DNA methylation status through the involvement of a specific methyltransferase. Such changes are linked to nuclear chromatin reconfiguration likely to establish a new balance of expressed/repressed chromatin. Overall, the data show an epigenetic basis to the mechanism underlying Cd toxicity in plants.

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In Posidonia oceanica cadmium induces changes in DNA methylation and chromatin patterning

The publication of draft sequences for the two subspecies of Oryza sativa (rice), japonica (cv. Nipponbare) and indica (cv. 93-11)1,2, provides a unique opportunity to study the dynamics of transposable elements in this important crop plant. Here we report the use of these sequences in a computational approach to identify the first active DNA transposons from rice and the first active miniature inverted-repeat transposable element (MITE) from any organism. A sequence classified as a Tourist-like MITE of 430 base pairs, called miniature Ping (mPing), was present in about 70 copies in Nipponbare and in about 14 copies in 93-11. These mPing elements, which are all nearly identical, transpose actively in an indica cell-culture line. Database searches identified a family of related transposase-encoding elements (called Pong), which also transpose actively in the same cells. Virtually all new insertions of mPing and Pong elements were into low-copy regions of the rice genome. Since the domestication of rice mPing MITEs have been amplified preferentially in cultivars adapted to environmental extremes—a situation that is reminiscent of the genomic shock theory for transposon activation3.

An active DNA transposon family in rice

Allelopathy studies the interactions among plants, fungi, algae and bacteria with the organisms living in a certain ecosystem, interactions that are mediated by the secondary metabolites produced and exuded into the environment. Consequently, allelopathy is a multidisciplinary science where ecologists, chemists, soil scientists, agronomists, biologists, plant physiologists and molecular biologists offer their skills to give an overall view of the complex interactions occurring in a certain ecosystem. As a result of these studies, applications in weed and pest management are expected in such different fields as development of new agrochemicals, cultural methods, developing of allelopathic crops with increased weed resistance, etc. The present paper will focus on the chemical aspects of allelopathy, pointing out the most recent advances in the chemicals disclosed, their mode of action and their fate in the ecosystem. Also, attention will be paid to achievements in genomics and proteomics, two emerging fields in allelopathy. Rather than being exhaustive, this paper is intended to reflect a critical vision of the current state of allelopathy and to point to future lines of research where in the authors' opinion the main advances and applications could and should be expected.

Allelopathy--a natural alternative for weed control.

Allelopathy can be defined as an important mechanism of plant interference mediated by the addition of plant-produced secondary products to the soil rhizosphere. Allelochemicals are present in all types of plants and tissues and are released into the soil rhizosphere by a variety of mechanisms, including decomposition of residues, volatilization, and root exudation. Allelochemical structures and modes of action are diverse and may offer potential for the development of future herbicides. We have focused our review on a variety of weed and crop species that establish some form of potent allelopathic interference, either with other crops or weeds, in agricultural settings, in the managed landscape, or in naturalized settings. Recent research suggests that allelopathic properties can render one species more invasive to native species and thus potentially detrimental to both agricultural and naturalized settings. In contrast, allelopathic crops offer strong potential for the development of cultivars that are ...

Weed and Crop Allelopathy

Momilactones, Growth inhibitors from rice, Oryza sativa L

Growth and germination inhibitors in rice husks

Biology of Rice.

Differentiation of Ecotypes in Oryza Sativa L.

In order to isolate growth and germination inhibitors from the hulls of dormant rice seeds, the seeds of a Japonica cultivar 'Koshihikari' and an Indica one 'Surjamukhi', which have strong dormancy, were used. The rice hull (ca 200kg) was soaked in 80% methanol for about 2 months. The methanol extracts were concentrated to an aqueous phase by the flash evaporator at below 40°C. The aqueous phase was extracted with chloroform and the active fraction was moved in chloroform. The active chloroform fraction was successively fractionated by organic solvents : n-hexane, benzene, ether, ethylene chloride, and ethyl acetate. The biological activity of each extract was checked by lettuce germination test. The active fraction checked was further separated according to solubility in dilute acid or base. The neutral fraction of benzene and ether extracts showed a strong inhibitory activity. The active fraction was used for further chromatographic study. By the silica gel thin layer chromatography with a developing solvent, benzene : ethyl acetate (10 : 1), two active compounds were crystallized and were identified as momilactone-A (C20H2603) and -B (C20H26O4) The momilactones inhibited the seed germination and the seedling growth of rice and lettuce at a concentration of 10-3M. Especially, these substances remarkably inhibited the root growth of rice seedling at 10-5-10-3M. Momilactone-B had stronger inhibition than momilactone-A. The inhibitory activity of momilactone-B was similar to that of a typical growth regulator, abscisic acid. 'Koshihikari' contained more momilactone-A than momilactone-B, while 'Surjamukhi, which has stronger dormancy than 'Koshihikari', held more mornilactone-B than momi-lactone-A. It was supposed that the content of momilactone-A and -B in rice seeds might vary with the rice cultivar. The inhibitory activity of momilactones was partially reduced by GA3 or IAA. In considering the breaking of dormacy in rice seeds, it may be neces-sary to check the interaction between momilactones and GA or IAA which has an important role during rice seed germination.

Norindo Takahashi

Papers

Momilactones, Growth inhibitors from rice, Oryza sativa L

Growth and germination inhibitors in rice husks

Biology of Rice.

Differentiation of Ecotypes in Oryza Sativa L.

Mechanisms of Dormancy in Rice Seeds : II. New Growth Inhibitors, Momilactone-A and-B Isolated from the Hulls of Rice Seeds