HD-ZIP III genes encode important transcription factors that play a crucial role in plant development and are regulated by microRNAs (miRNA). Although previous studies have examined the functions of rice HD-ZIP III genes using overexpression lines, the developmental consequences of increased expression of the rice OSHB3 gene under native regulatory conditions remain unclear. In this study, we isolated a gain-of-function mutant carrying mutations in the miRNA target site of the OSHB3 gene, a member of the HD-ZIP III family, and examined its morphology at multiple developmental stages. Gene expression was analyzed using quantitative reverse transcription polymerase chain reaction (qRT-PCR) and in situ hybridization. Our results reveal that ectopic expression of the OSHB3 gene is associated with the differentiation pattern of meristems in rice and that a correlation exists between OSHB3 expression levels and phenotypic severity in the mutants. These findings highlight the importance of precise spatial and quantitative regulation of HD-ZIP III genes in rice development.

Cytokinin plays a major role in the regulation of plant development. It is perceived by receptors with histidine kinase activity to regulate the expression of various transcription factors. In a previous study, we reported a semi-dominant mutant, named adaxial-abaxial bipolar leaf1 (abl1)-d, which exhibited a characteristic feature in the fourth leaf of rice, and that the ABL1 gene encodes a cytokinin receptor with histidine kinase activity. Our further analysis suggested that the abl1-d mutation is associated with an active form of histidine kinase and altered cytokinin signaling. However, it remained unclear whether the abl1-d mutation indeed triggers aberrant cytokinin signaling in rice plants, and how the abl1-d mutation affects developmental processes throughout the life cycle of rice. In the present study, we found that homozygous abl1-1d calli have the capacity to regenerate shoots in the absence of cytokinin, suggesting that the abl1-1d homozygous mutation is associated with constitutive cytokinin signaling in rice. We next examined morphological characteristics of both homozygous and heterozygous abl1-1d plants from the post-germination vegetative phase through to reproduction. The results showed that homozygous abl1-1d plants had a reduced number of panicles and were completely sterile, and that leaf size and the midrib structure were altered. Furthermore, the adaxial-abaxial bipolar leaf, a phenotype that is characteristic of the abl1-1d mutant, has previously been observed to resemble two normal leaves fused together at their abaxial sides. Leaves with this particular phenotype exhibited enhanced photosynthetic efficiency under certain environmental conditions. Thus, the abl1-1d mutation, which results in a putative active form of receptor histidine kinase, affects various developmental traits throughout the rice life cycle, probably due to altered cytokinin signaling.

Shoot apical meristems (SAMs) continuously initiate organ formation and maintain pluripotency through dynamic genetic regulations and cell-to-cell communications. The activity of meristems directly affects the plant's structure by determining the number and arrangement of organs and tissues. We have taken a forward genetic approach to dissect the genetic pathway that controls cell differentiation around the SAM. The rice mutants, adaxial-abaxial bipolar leaf 1 and 2 (abl1 and abl2), produce an ectopic leaf that is fused back-to-back with the fourth leaf, the first leaf produced after embryogenesis. The abaxial-abaxial fusion is associated with the formation of an ectopic shoot meristem at the adaxial base of the fourth leaf primordium. We cloned the ABL1 and ABL2 genes of rice by mapping their chromosomal positions. ABL1 encodes OsHK6, a histidine kinase, and ABL2 encodes a transcription factor, OSHB3 (Class III homeodomain leucine zipper). Expression analyses of these mutant genes as well as OSH1, a rice ortholog of the Arabidopsis STM gene, unveiled a regulatory circuit that controls the formation of an ectopic meristem near the SAM at germination.

Cell division is important for organisms to grow and repair damaged tissues. A mutant screen in rice has identified dwarf korpokkur (kor) mutants that code for a novel protein potentially involved in mitosis including cytokinesis in rice. The KOR gene is expressed during the mitotic phase and a defect in the KOR gene induces cells with two nuclei. Analysis of kor mutants suggests that the KOR gene promotes cell division in the rice leaf primordia for a period after initiation, and maintains proper cell morphology especially in non-meristematic tissues. Additionally, kor mutants showed a delayed transition from juvenile phase to adult phase. Future research will shed light on the relationship between the mitotic defect and other features observed in the kor mutants.

Meristem fate is regulated by trehalose 6-phosphate phosphatases (TPPs), but their mechanism of action remains mysterious. Loss of the maize TPPs RAMOSA3 and TPP4 leads to reduced meristem determinacy and more inflorescence branching. However, analysis of an allelic series revealed no correlation between enzymatic activity and branching, and a catalytically inactive version of RA3 complements the ra3 mutant. Together with their nuclear localization, these findings suggest a moonlighting function for TPPs.

important to prevent plants, especially crop plants, from absorbing radiocesium. In Arabidopsis thaliana , cesium ions are transported into root cells by several different potassium transporters such as high-affinity K + transporter 5 (AtHAK5). Therefore, the cesium uptake pathway is thought to be highly redundant, making it difficult to develop plants with low cesium uptake. Here, we isolated rice mutants with low cesium uptake and reveal that the Oryza sativa potassium transporter OsHAK1, which is expressed on the surfaces of roots, is the main route of cesium influx into rice plants, especially in low potassium conditions. During hydroponic cultivation with low to normal potassium concentrations (0–206 m M: the normal potassium level in soil), cesium influx in OsHAK1 -knockout lines was no greater than one-eighth that in the wild type. In field experiments, knockout lines of O. sativa HAK1 ( OsHAK1 ) showed dramatically reduced cesium concentrations in grains and shoots, but their potassium uptake was not greatly affected and their grain yields were similar to that of the wild type. Our results demonstrate that, in rice roots, potassium transport systems other than OsHAK1 make little or no contribution to cesium uptake. These results show that low cesium uptake rice lines can be developed for cultivation in radiocesium-contaminated areas.

Contamination of agricultural fields with heavy metals, such as Cd, is a serious issue for safe food production worldwide. Previous reports indicate that sorghum [Sorghum bicolor (L.) Moench] is useful for phytoextraction of Cd from contaminated soils. To investigate the diversity for Cd accumulation in sorghum germplasm and to select useful cultivars for phytoextraction, we conducted experiments using 106 sorghum landraces and the cultivar BTx623, grown in hydroponic cultures and under field conditions. The Cd concentrations in plant shoots of different landraces showed no correlation between hydroponic and field‐grown plants (r = −0.16). We measured Cd concentrations in various plant parts at different stages of development in field‐grown plants and found that leaf sheaths often had higher Cd concentrations than other plant parts. Our results indicated that screening could be done for plants that might be useful for phytoextraction of Cd by determining the Cd concentrations in leaf sheaths of 1‐mo‐old plants grown in a Cd‐contaminated soil. We also mapped the quantitative trait loci (QTLs) responsible for Cd accumulation in sorghum shoots and found several chromosomal regions that affected Cd concentrations in different parts of the shoot. We were able to identify several promising landraces for developing sorghum cultivars that are capable of accumulating high levels of Cd and phytoextracting Cd from contaminated soils.

Leaf senescence is an important physiological process involving the degradation of a number of metabolites and their remobilization to new reproductive and storage organs. NAC (NAM, ATAF, and CUC) transcription factors are reported as important regulators of the senescence process. Here, we describe the identification and functional characterization of the NAC transcription factor gene, OsY37 (Oryza sativa Yellow37, ONAC011) obtained from Oryza sativa cv. indica, and japonica. We created transgenic plants expressing the OsY37 gene under the control of a strong and constitutive CaMV35S promoter. The resulting transgenic plants overexpressing OsY37 gene showed early heading and precocious senescence phenotype of flag leaves compared with wild-type plants. By contrast, blocking the function of this gene via RNAi (RNA interference) and CRES-T (Chimeric Repressor Silencing Technology) technology, delayed both heading time and leaf senescence. Furthermore, knockdown of OsY37 expression caused dwarfism and high accumulation of chlorophyll during the vegetative phase. Irrespective of early or delayed senescence, transgenic plants showed reduced grain yields. Our results indicate that OsY37 acts as a positive regulator of heading and senescence during the reproductive phase in rice. In addition, OsY37 may be involved in plant development and grain yield.

Incidents at the Fukushima and Chernobyl nuclear power stations have resulted in widespread environmental contamination by radioactive nuclides. Among them, 137cesium has a 30 year half-life, and its persistence in soil raises serious food security issues. It is therefore important to prevent plants, especially crop plants, from absorbing radiocesium. In Arabidopsis thaliana, cesium ions are transported into root cells by several different potassium transporters such as high-affinity K+ transporter 5 (AtHAK5). Therefore, the cesium uptake pathway is thought to be highly redundant, making it difficult to develop plants with low cesium uptake. Here, we isolated rice mutants with low cesium uptake and reveal that the Oryza sativa potassium transporter OsHAK1, which is expressed on the surfaces of roots, is the main route of cesium influx into rice plants, especially in low potassium conditions. During hydroponic cultivation with low to normal potassium concentrations (0-206 µM: the normal potassium level in soil), cesium influx in OsHAK1-knockout lines was no greater than one-eighth that in the wild type. In field experiments, knockout lines of O. sativa HAK1 (OsHAK1) showed dramatically reduced cesium concentrations in grains and shoots, but their potassium uptake was not greatly affected and their grain yields were similar to that of the wild type. Our results demonstrate that, in rice roots, potassium transport systems other than OsHAK1 make little or no contribution to cesium uptake. These results show that low cesium uptake rice lines can be developed for cultivation in radiocesium-contaminated areas.

Germinability under low temperature is one of the most important traits in seedling establishment in direct-sowing culture of rice. The objective of this study was the identification of genes responsible for higher and faster germination under low temperature, with the aim of breeding new rice varieties for direct-sowing culture. We identified four quantitative trait loci (QTLs) responsible for germination properties under low temperature in an East European rice variety, Maratteli. Maratteli-derived alleles at qLTG3-1 and qLTG3-2 on chromosome 3 and qLTG11-1 on chromosome 11 increased germination rate under low temperature. In contrast, the Maratteli allele at qLTG1-1 on chromosome 1 reduced the rate. Near-isogenic lines (NILs) with the Akitakomachi genetic background harboring qLTG3-1 and qLTG11-1 alleles from Maratteli accelerated germination and a NIL with qLTG1-1 delayed germination under low temperature, resulting in enhanced or suppressed germination rates, respectively. These QTLs altered the germination profile of Akitakomachi. Hence, Maratteli is a better source of genes for the improvement of germination under low temperature. We conclude that the low temperature germinability of various Japanese varieties inferior in this character can be improved by marker-assisted selection without modification of other superior traits. These NILs may also contribute to uncovering the molecular mechanisms of cold tolerance at the germination stage.

Rice OsHMA3 is a vacuolar cadmium (Cd) transporter belonging to the P 1B ‐ATPase family and has a long (273aa) C‐terminal region. We analyzed the function of the region related to Cd using the transgenic Arabidopsis Col‐0 ecotype, which is sensitive to Cd. The OsHMA3 variant containing a truncated (58aa) C‐terminal region did not confer Cd tolerance, whereas an OsHMA3 variant containing a longer truncated (105aa) C‐terminal region conferred Cd tolerance to transgenic Arabidopsis . We conclude that the C‐terminal region, particularly the region containing the first 105aa, has an important role in OsHMA3 activity.

Domesticated maize make 8–20 rows of kernels, whereas its ancestor teosinte makes 2 rows. David Jackson and colleagues report that variation at the FEA2 locus in maize influences kernel row number and kernels per ear, which are important crop yield traits.

Trehalose is a non-reducing disaccharide that is used as an osmolyte, transport sugar, carbon reserve and stress protectant in a wide range of organisms. In plants, trehalose 6-phosphate (Tre6P), the intermediate of trehalose biosynthesis, is thought to be a signal of sucrose status. Trehalose itself may play a role in pathogenic and symbiotic plant-microbe interactions, in responses to abiotic stress and in developmental signalling, but its precise functions are unknown. A major obstacle to investigating its function is the technical difficulty of measuring the very low levels of trehalose usually found in plant tissues, as most of the established trehalose assays lack sufficient specificity and/or sensitivity.

Oryza sativa heavy metal P-type ATPase 3 (OsHMA3) plays a major role in the hyperaccumulation of cadmium (Cd) by the rice cultivars `Cho-Ko-Koku (CKK)'and `Anjana Dhan.' On the other hand, mutants that accumulate Cd in the shoots at a very low level have suggested the function of several genes such as OsHMA2 (which is neccesary to load Cd into xylem) or O. sativa natural resistance-associated macrophage protein 5 (OsNRAMP5, which regulates the uptake of Cd from soil into the roots). Although many studies have examined metal transporters, few have investigated the function of combination of multiple gene in plant. Herein, first, we used an RNA interference (RNAi) method to down regulate OsHMA3 and recovered plants in which the expression of OsHMA3 was strongly suppressed. Second, crossing of OsHMA3 RNAi plants with oshma2 mutants revealed a functional relationship between OsHMA3 and OsHMA2 in plant. We concluded that a complete loss of function of OsHMA2 is crucial to minimize Cd translocation from the roots to the shoots. The current findings also implied that the activity of OsHMA2 can be accelerated in plant in the roots without the C-terminal region of the protein.

• The cadmium (Cd) over-accumulating rice (Oryza sativa) cv Cho-Ko-Koku was previously shown to have an enhanced rate of root-to-shoot Cd translocation. This trait is controlled by a single recessive allele located at qCdT7. • In this study, using positional cloning and transgenic strategies, heavy metal ATPase 3 (OsHMA3) was identified as the gene that controls root-to-shoot Cd translocation rates. The subcellular localization and Cd-transporting activity of the gene products were also investigated. • The allele of OsHMA3 that confers high root-to-shoot Cd translocation rates (OsHMA3mc) encodes a defective P(1B) -ATPase transporter. OsHMA3 fused to green fluorescent protein was localized to vacuolar membranes in plants and yeast. An OsHMA3 transgene complemented Cd sensitivity in a yeast mutant that lacks the ability to transport Cd into vacuoles. By contrast, OsHMA3mc did not complement the Cd sensitivity of this yeast mutant, indicating that the OsHMA3mc transport function was lost. • We propose that the root cell cytoplasm of Cd-overaccumulating rice plants has more Cd available for loading into the xylem as a result of the lack of OsHMA3-mediated transportation of Cd to the vacuoles. This defect results in Cd translocation to the shoots in higher concentrations. These data demonstrate the importance of vacuolar sequestration for Cd accumulation in rice.

Widespread soil contamination with heavy metals has fostered the need for plant breeders to develop new crops that do not accumulate heavy metals. Metal-transporting transmembrane proteins that transport heavy metals across the plant plasma membrane are key targets for developing these new crops. Oryza sativa heavy metal ATPase 3 (OsHMA3) is known to be a useful gene for limiting cadmium (Cd) accumulation in rice. OsHMA2 is a close homolog of OsHMA3, but the function of OsHMA2 is unknown. To gain insight into the function of OsHMA2, we analyzed three Tos17 insertion mutants. The translocation ratios of zinc (Zn) and Cd were clearly lower in all mutants than in the wild type, suggesting that OsHMA2 is a major transporter of Zn and Cd from roots to shoots. By comparing each allele in the OsHMA2 protein structure and measuring the Cd translocation ratio, we identified the C-terminal region as essential for Cd translocation into shoots. Two alleles were identified as good material for breeding rice that does not contain Cd in the grain but does contain some Zn, and that grows normally.

We are interested in elucidating the molecular mechanisms underlying plant reactions to the toxic heavy metal cadmium (Cd). To this end, we devised a new screening strategy using agar plates with a gradient of Cd concentrations, termed Cd-gradient agar plates (CGAPs), to isolate Arabidopsis mutants that displayed altered reactions to the metal. Arabidopsis M2 seeds, derived from ethyl methanesulfonate (EMS) treated seeds, were germinated on the CGAPs such that the primary root of each seedling elongated against increasing concentrations of Cd on the surface of the plate. Under these conditions, the lengths of the primary roots reliably demonstrated the degree of Cd tolerance of individual seedlings. The use of CGAPs also allowed close observation of the root reaction of each seedling to Cd without causing lethal damage. The screen identified three mutant lines, MRC-32, MRC-22 and MRC-26, which showed distinctly different characteristics. MRC-32 plants exhibited enhanced tolerance to Cd and contained Cd at higher concentrations than wild-type (WT) plants treated with the heavy metal. The whole root system of MRC-22 plants showed a Cd-phobic response. MRC-26 plants accumulated less Cd in their aboveground tissues than WT plants, suggesting that they were defective in transporting the heavy metal from roots to aboveground tissues. We also determined the likely chromosomal location of each mutation.