Saeko Konishi

A silicon transporter in rice

Silicon is beneficial to plant growth and helps plants to overcome abiotic and biotic stresses by preventing lodging (falling over) and increasing resistance to pests and diseases, as well as other stresses. Silicon is essential for high and sustainable production of rice, but the molecular mechanism responsible for the uptake of silicon is unknown. Here we describe the Low silicon rice 1 (Lsi1) gene, which controls silicon accumulation in rice, a typical silicon-accumulating plant. This gene belongs to the aquaporin family and is constitutively expressed in the roots. Lsi1 is localized on the plasma membrane of the distal side of both exodermis and endodermis cells, where casparian strips are located. Suppression of Lsi1 expression resulted in reduced silicon uptake. Furthermore, expression of Lsi1 in Xenopus oocytes showed transport activity for silicon only. The identification of a silicon transporter provides both an insight into the silicon uptake system in plants, and a new strategy for producing crops with high resistance to multiple stresses by genetic modification of the roots silicon uptake capacity.

Deletion in a gene associated with grain size increased yields during rice domestication

The domestication of crops involves a complex process of selection in plant evolution and is associated with changes in the DNA regulating agronomically important traits. Here we report the cloning of a newly identified QTL, qSW5 (QTL for seed width on chromosome 5), involved in the determination of grain width in rice. Through fine mapping, complementation testing and association analysis, we found that a deletion in qSW5 resulted in a significant increase in sink size owing to an increase in cell number in the outer glume of the rice flower; this trait might have been selected by ancient humans to increase yield of rice grains. In addition, we mapped two other defective functional nucleotide polymorphisms of rice domestication-related genes with genome-wide RFLP polymorphisms of various rice landraces. These analyses show that the qSW5 deletion had an important historical role in artificial selection, propagation of cultivation and natural crossings in rice domestication, and shed light on how the rice genome was domesticated.

An SNP Caused Loss of Seed Shattering During Rice Domestication

Loss of seed shattering was a key event in the domestication of major cereals. We revealed that the qSH1 gene, a major quantitative trait locus of seed shattering in rice, encodes a BEL1-type homeobox gene and demonstrated that a single-nucleotide polymorphism (SNP) in the 5′ regulatory region of the qSH1 gene caused loss of seed shattering owing to the absence of abscission layer formation. Haplotype analysis and association analysis in various rice collections revealed that the SNP was highly associated with shattering among japonica subspecies of rice, implying that it was a target of artificial selection during rice domestication.

An efflux transporter of silicon in rice

Silicon is an important nutrient for the optimal growth and sustainable production of rice. Rice accumulates up to 10% silicon in the shoot, and this high accumulation is required to protect the plant from multiple abiotic and biotic stresses. A gene, Lsi1, that encodes a silicon influx transporter has been identified in rice. Here we describe a previously uncharacterized gene, low silicon rice 2 (Lsi2), which has no similarity to Lsi1. This gene is constitutively expressed in the roots. The protein encoded by this gene is localized, like Lsi1, on the plasma membrane of cells in both the exodermis and the endodermis, but in contrast to Lsi1, which is localized on the distal side, Lsi2 is localized on the proximal side of the same cells. Expression of Lsi2 in Xenopus oocytes did not result in influx transport activity for silicon, but preloading of the oocytes with silicon resulted in a release of silicon, indicating that Lsi2 is a silicon efflux transporter. The identification of this silicon transporter revealed a unique mechanism of nutrient transport in plants: having an influx transporter on one side and an efflux transporter on the other side of the cell to permit the effective transcellular transport of the nutrients.

Characterization of the Silicon Uptake System and Molecular Mapping of the Silicon Transporter Gene in Rice

Rice (Oryza sativa L. cv Oochikara) is a typical silicon-accumulating plant, but the mechanism responsible for the high silicon uptake by the roots is poorly understood. We characterized the silicon uptake system in rice roots by using a low-silicon rice mutant (lsi1) and wild-type rice. A kinetic study showed that the concentration of silicon in the root symplastic solution increased with increasing silicon concentrations in the external solution but saturated at a higher concentration in both lines. There were no differences in the silicon concentration of the symplastic solution between the wild-type rice and the mutant. The form of soluble silicon in the root, xylem, and leaf identified by 29Si-NMR was also the same in the two lines. However, the concentration of silicon in the xylem sap was much higher in the wild type than in the mutant. These results indicate that at least two transporters are involved in silicon transport from the external solution to the xylem and that the low-silicon rice mutant is defective in loading silicon into xylem rather than silicon uptake from external solution to cortical cells. To map the responsible gene, we performed a bulked segregant analysis by using both microsatellite and expressed sequence tag-based PCR markers. As a result, the gene was mapped to chromosome 2, flanked by microsatellite marker RM5303 and expressed sequence tag-based PCR marker E60168.

DNA changes tell us about rice domestication

Crop domestication can be considered a model system of plant evolution. Genome analyses of rice have revealed the fine population structure of this major crop associated with local origins of landraces. Recent cloning of rice domestication-related genes and identification of the responsible functional nucleotide polymorphisms in landraces, while taking into account their population structures, have revealed the existence of historical signatures of the DNA involved in the domestication process. These signatures imply the importance of multiple selection steps wherein natural variants were combined to improve crop performance during domestication. These analyses will provide new insights into the relationship between Darwinian selection for agronomical phenotypes and DNA changes in terms of plant evolution.

Inference of the japonica Rice Domestication Process from the Distribution of Six Functional Nucleotide Polymorphisms of Domestication-Related Genes in Various Landraces and Modern Cultivars

Crop domestication can serve as a model of plant evolutionary processes. It involves a series of selection events from standing natural variation and newly occurring mutations and combinations of mutations as a result of natural crossings in populations during local adaptation and propagation of plant lines to other cultivation areas. Our earlier identification of three functional nucleotide polymorphisms (FNPs) of distinct genes involved in the rice domestication process led us to propose a model of the japonica rice domestication process. Here, we examined three more FNPs in two domestication-related genes involved in pigment synthesis during the development of seed pericarp color (Rc and Rd) in 91 landraces (and some modern cultivars) of japonica rice collected from throughout the area of distribution of rice. These polymorphisms were assigned by using genome-wide patterns of restriction fragment length polymorphisms (RFLPs) and the local origins of the landraces. The results led us to infer the process of japonica rice domestication in more detail and propose a more refined model of the japonica domestication process. In this model, the critical role of the Rc FNP at an early step of the domestication process was highlighted. Independent artificial selections of two defective Rd alleles were found, suggesting a role for Rd other than in pigment synthesis during rice domestication.

Reply to “ Japonica rice carried to, not from, Southeast Asia”

makes the assumption that the genes for primitive traits are preserved in the putative homeland, as in the case of the dominant allele qSW5, whereas humans have selected for the recessive qsw5 encoding a broader seed size1. Whether this was the only major gene involved in grain-size increase during domestication is unknown, and other grain-size QTLs have been identified9. However, the high percentage of dominant carriers (qSW5) in the Philippines and Indonesia may be explained by local selection conditions and/or introgression from wild populations. The archeological record from the Lower Yangzte region in China charts an evolutionary trend toward increasing grain size between 6000 and 4000 bc2. Although nonshattering was a key domestication trait in rice, qsh1 documented by Shomura et al.1 is only one of the alleles that may have caused this; sh4 is another important target of early selection and is more widespread in rices, including indica, and might therefore have been selected earlier9. The evolution of nonshattering genes in rice is therefore complex. The third mutation considered by Shomura et al.1 is the wx mutation, which results in sticky, low-amylose rice grains. This is not a trait related to initial domestication but rather a later diversification allele that has been the target of selection under cultural food preference. The dominant allele Wx is seen not only in wild rice but also in widespread cultivated rices in South, East and Southeast Asia. High frequencies of wx are correlated with cultural preferences for sticky cereals, which also extends to waxy genotypes of Setaria italica, Panicum miliaceum and several other species within Eastern Asia10. Thus, all the genes discussed here, qsw5, qsh1 and wx, have been targets of selection within some cultivated rices, but none of them are clearly linked to the beginnings of cultivation or domestication. Because all of the genes considered by Shomura et al.1 were targets of cultural selection, they have been subjected to differing selection histories within different regional cultural histories. As such, they are perhaps less useful for phylogeographic reconstruction than neutral loci. Dorian Q Fuller1 & Yo-Ichiro Sato2 1Institute of Archaeology, University College London, 31-34 Gordon Square, London WC1H 0PY, UK. 2Research Institute for Humanity & Nature, 457-4 Motoyama, Kamigamo, Kita-ku, Kyoto 603-8047, Japan. Correspondence should be addressed to D.Q.F. (d.fuller@ucl.ac.uk).