Leaf shape is one of the key determinants of plant architecture. Leaf shape also affects the amount of sunlight captured and influences photosynthetic efficiency; thus, it is an important agronomic trait in crop plants. Understanding the molecular mechanisms governing leaf shape is a central issue of plant developmental biology and agrobiotechnology. Here, we characterized the narrow-leaf phenotype of FL90, a linkage tester line of rice (Oryza sativa). Light and scanning electron microscopic analyses of FL90 leaves revealed defects in the development of marginal regions and a reduction in the number of longitudinal veins. The narrow-leaf phenotype of FL90 shows a two-factor recessive inheritance and is caused by the loss of function of two WUSCHEL-related homeobox genes, NAL2 and NAL3 (NAL2/3), which are duplicate genes orthologous to maize NS1 and NS2 and to Arabidopsis PRS. The overexpression of NAL2/3 in transgenic rice plants results in wider leaves containing increased numbers of veins, suggesting that NAL2/3 expression regulates leaf width. Thus, NAL2/3 can be used to modulate leaf shape and improve agronomic yield in crop plants.
Auxin flow is important for different root developmental processes such as root formation, emergence, elongation and gravitropism. However, the detailed information about the mechanisms regulating the auxin flow is less well understood in rice. We characterized the auxin transport-related mutants, Ospin-formed2-1 (Ospin2-1) and Ospin2-2, which exhibited curly root phenotypes and altered lateral root formation patterns in rice. The OsPIN2 gene encodes a member of the auxin efflux carrier proteins that possibly regulates the basipetal auxin flow from the root tip toward the root elongation zone. According to DR5-driven GUS expression, there is an asymmetric auxin distribution in the mutants that corresponded with the asymmetric cell elongation pattern in the mutant root tip. Auxin transport inhibitor, N-1-naphthylphthalamic acid and Ospin2-1 Osiaa13 double mutant rescued the curly root phenotype indicating that this phenotype results from a defect in proper auxin distribution. The typical curly root phenotype was not observed when Ospin2-1 was grown in distilled water as an alternative to tap water, although higher auxin levels were found at the root tip region of the mutant than that of the wild-type. Therefore, the lateral root formation zone in the mutant was shifted basipetally compared with the wild-type. These results reflect that an altered auxin flow in the root tip region is responsible for root elongation growth and lateral root formation patterns in rice.
Wild species in the genus Vigna are a great resource of tolerance to various stresses including salinity. We have previously screened the genetic resources of the genus Vigna and identified several accessions that have independently evolved salt tolerance. However, many aspects of such tolerance have remained unknown. Thus, we used autoradiography with radioactive sodium ( 22 Na + ) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS) to visualize and compare Na + allocation in Vigna angularis (Willd.) Ohwi & H.Ohashi (azuki bean), Vigna nakashimae (Ohwi) Ohwi & H.Ohashi, Vigna riukiuensis (Ohwi) Ohwi & H.Ohashi, Vigna luteola (Jacq.) Benth. and Vigna marina (Burm.) Merr.. The results indicated: 1) Tolerant accessions suppress Na + accumulation compared to azuki bean. 2) V. nakashimae and V. marina does so by accumulating higher amount of K + , whereas V. riukiuensis and V. luteola does so by other mechanisms. 3) V. luteola avoids salt-shedding by allocating excess Na + to newly expanded leaves. As the mechanisms of the tolerant species were different, they could be piled up in a single crop via classical breeding or by genetic engineering or genome editing.
Short day length-induced alteration of potassium (K) localization in perennial trees is believed to be a mechanism for surviving and adapting to severe winters. To investigate the relationship between cesium (Cs) and K localizations, a model tree poplar, hybrid aspen T89, was employed. Under short day length conditions, the amount of 137Cs absorbed through the root and translocated to the root was drastically reduced, but 42K was not. Potassium uptake from the rhizosphere is mediated mainly by KUP/HAK/KT and CNGC transporters. In poplar, however, these genes were constantly expressed under short-day conditions except for a slight increase in the expression a KUP/HAK/KT gene six weeks after the onset of the short-day treatment. These results indicated that the suppression of 137Cs uptake was triggered by short day length but not regulated by competitive Cs+ and K+ transport. We hypothesize that there are separately regulated Cs+ and K+ transport systems in poplar.
Small RNAs, such as small interfering RNAs (siRNAs) or microRNAs (miRNAs), regulate gene expression at transcriptional and posttranscriptional levels in eukaryotes. miRNAs are processed from duplexes formed on single-stranded RNA. They regulate expression of their target gene either by cleaving mRNA or supressing translation. In general, the primary miRNA transcripts are synthesized by RNA polymerase II and processed similarly to mRNAs. MIRNA genes are usually located in transcriptionally active euchromatic regions. In contrast, siRNAs are processed from duplexes made of two RNA molecules. One of them is often derived from a transposable element (TE) or from repetitive sequences that reside in heterochromatic regions. The other strand is synthesized by the RNAdependent RNA polymerase on the first strand as a template. siRNAs establish epigenetic marks in parasitic DNA such as TEs, thus they usually act in cis. The rice miRNA miR820, encoded by CACTA TEs (five copies, located on different chromosomes), reduces the expression of the de novo DNA methyltransferase gene OsDRM2. Because miR820 is derived from silent TEs, in which the heterochromatic histone modifications are enriched, the mechanism of MIR820 transcription could be expected to differ from typical miRNAs. Here we show that the primary transcript of MIR820 is mainly derived from the CACTA TE copy on chromosome 7 (MIR820b). Histone modification and DNA methylation status around MIR820b differed from that of the other four loci. These unique epigenetic modifications in MIR820b were only found around the miR820 coding region. We conclude that MIR820b transcription may depend on the unique epigenetic modifications, which in turn may be established by the action of miR820 in cis. This suggests a dual function of miR820 in cis and in trans.
This research provides insight into a unique salt tolerance mechanism of Vigna riukiuensis. V. riukiuensis is one of the great genetic resources of salt tolerance. We have previously reported that V. riukiuensis accumulates a higher amount of sodium in the leaves, whereas V. nakashimae, a close relative of V. riukiuensis, suppresses sodium allocation to the leaves. We first suspected that V. riukiuensis would have developed vacuoles for sodium sequestration, but there were no differences compared to a salt-sensitive accession. However, many starch granules were observed in the chloroplasts of V. riukiuensis. In addition, forced degradation of leaf starch by shading treatment resulted in no radio-Na (22Na) accumulation in the leaves. We performed SEM-EDX to locate Na in leaf sections and detected Na in chloroplasts of V. riukiuensis, especially around the starch granules but not in the middle of those. Our results could provide the second evidence of Na-trapping system by starch granules, following the case of common reed that accumulates starch granule at the shoot base for binding Na.
Natural variation for drought tolerance is a major issue in adaptation and geographic distribution of terrestrial plants. Despite the importance, little is known about the genes and molecular mechanisms that determine its naturally occurring diversity. We analyzed the intraspecific drought tolerance variation between 2 accessions of Arabidopsis thaliana, Columbia (Col)-0 and Wassilewskija (Ws)-2. Measurement of weight loss in detached seedlings demonstrated a clear difference between drought-tolerant Col-0 and drought-sensitive Ws-2. They also differed in their stomatal response under drought condition. Using a quantitative genetic approach, we found a significant quantitative locus on chromosome 1. Surveying in the locus, we extrapolated that the SLAC1 gene, which is associated with stomatal closure, was likely responsible for the difference of drought tolerance. Comparison of their nucleotide and amino acid sequences revealed that there were few differences in regions encoding SLAC1 protein but was a large deletion in SLAC1 promoter of Ws-2. Histochemical GUS staining showed that the SLAC1 expressed dominantly in guard cells of Col-0, but did less in guard cells of Ws-2. Quantitative PCR analysis also showed that transcript level of SLAC1 in guard cells was higher in Col-0 than in Ws-2. The SLAC1 transcription analyses indicate low accumulation of SLAC1 in guard cells of Ws-2. When taken together, our results suggest that the low drought tolerance of Ws-2 was associated with the deletion of the promoter region of Ws-2 SLAC1.
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