The effect of low temperature on patterns of cell division in developing second leaves of wild-type and slender mutant barley (Hordeum vulgare L.)

This study reports on investigations into the effect of long-term growth at reduced temperatures on cell elongation and cell division in the wild type and a temperature-insensitive ( slender ) mutant of barley. Plants were grown under two temperature regimes (20 and 5 °C) and the mitotic index, cell doubling time and cell lengths over the division and elongation zone were monitored at several stages of development in the second leaf. Leaf length and leaf growth rates were characteristically greater in the slender mutant than in the wild type and this was greatly exaggerated by growth at low temperature. Cell length and the length of the division zone were also greater in the slender mutant than in the wild type, and growing the plants at reduced temperature (5 °C) shortened cell lengths only in the wild type. The slender mutant had a higher mitotic index than the wild type, although in neither genotype was change in the mitotic index observed following growth at reduced temperature. Cell doubling time, on the other hand, was reduced by growth at reduced temperature in the wild type but not in the slender mutant. Thus, the data suggest very different growth responses to low temperature in the two genotypes. The results are discussed in terms of the ability of plants to sense their environment and optimize their metabolism for future growth.     

水稻窄叶突变体zy17的遗传分析和候选基因鉴定

器官大小调控是一个基本的发育生物学过程,受细胞分裂和细胞扩展的影响。然而,植物器官大小调控的遗传和分子机理仍不清楚。为了进一步了解器官大小调控的分子机制,文章分离了一系列水稻叶子宽窄改变的突变体。其中,窄叶突变体zy17叶变窄,同时伴有植株矮化、穗子变小、枝梗数和穗粒数降低的表型。遗传分析表明该窄叶性状受1个隐性基因控制;细胞学分析表明该突变体叶子的细胞数目和维管束数目显著降低,表明ZY17影响了细胞分裂。基因组重测序进一步筛选出ZY17的3个候选基因：Os02g22390基因突变发生在内含子区,编码蛋白为逆转座蛋白;Os02g28280和Os02g29530基因突变都发生在外显子区,其中Os02g28280编码一个功能未知蛋白,该基因突变后,发生碱基置换,产生非同义突变;Os02g29530编码一个含糖基转移酶相关的PFAM结构域的蛋白,该基因突变后,出现两个碱基的缺失,从而导致其蛋白翻译提前终止。对候选基因的深入研究,将揭示水稻叶子大小调控的机制。     

Functional characterization of the Arabidopsis eukaryotic translation initiation factor 5A-2 that plays a crucial role in plant growth and development by regulating cell division, cell growth, and cell death

The eukaryotic translation initiation factor 5A (eIF-5A) is a highly conserved protein found in all eukaryotic organisms. Although originally identified as a translation initiation factor, recent studies in mammalian and yeast (Saccharomyces cerevisiae) cells suggest that eIF-5A is mainly involved in RNA metabolism and trafficking, thereby regulating cell proliferation, cell growth, and programmed cell death. In higher plants, the physiological function of eIF-5A remains largely unknown. Here, we report the identification and characterization of an Arabidopsis (Arabidopsis thaliana) mutant fumonisin B(1)-resistant12 (fbr12). The fbr12 mutant shows an antiapoptotic phenotype and has reduced dark-induced leaf senescence. Moreover, fbr12 displays severe defects in plant growth and development. The fbr12 mutant plant is extreme dwarf with substantially reduced size and number of all adult organs. During reproductive development, fbr12 causes abnormal development of floral organs and defective sporogenesis, leading to the abortion of both female and male germline cells. Microscopic studies revealed that these developmental defects are associated with abnormal cell division and cell growth. Genetic and molecular analyses indicated that FBR12 encodes a putative eIF-5A-2 protein. When expressed in a yeast mutant strain carrying a mutation in the eIF-5A gene, FBR12 cDNA is able to rescue the lethal phenotype of the yeast mutant, indicating that FBR12 is a functional eIF-5A. We propose that FBR12/eIF-5A-2 is fundamental for plant growth and development by regulating cell division, cell growth, and cell death.     

Characterization and Fine Mapping of a Novel Rice Narrow Leaf Mutant nal9

A narrow leaf mutant was isolated from transgenic rice （Oryza sativa L.） lines carrying a T-DNA insertion. The mutant is characterized by narrow leaves during its whole growth period, and was named nal9 （narrow leaf 9）. The mutant also has other phenotypes, such as light green leaves at the seedling stage, reduced plant height, a small panicle and increased tillering. Genetic analysis revealed that the mutation is controlled by a single recessive gene. A hygromycin resistance assay showed that the mutation was not caused by T-DNA insertion, so a map-based cloning strategy was employed to isolate the nal9 gene. The mutant individuals from the F2 generations of a cross between the nal9mutant and Longtepu were used for mapping. With 24 F2 mutants, the nal9 gene was preliminarily mapped near the marker RM156 on the chromosome 3. New INDEL markers were then designed based on the sequence differences between japonica and indica at the region near RM156. The nal9 gene was finally located in a 69.3 kb region between the markers V239B and V239G within BAC OJ1212_C05 by chromosome walking. Sequence and expression analysis showed that an ATP-dependent CIp protease proteolytic subunit gene （CIpP） was most likely to be the nal9 gene. Furthermore, the nal9 mutation was rescued by transformation of the CIpP cDNA driven by the 35S promoter. Accordingly, the CIpP gene was identified as the NAL9 gene. Our results provide a basis for functional studies of NAL9 in future work.     

The global translation profile in a ribosomal protein mutant resembles that of an eIF3 mutant

Background

Genome-wide assays performed in Arabidopsis and other organisms have revealed that the translation status of mRNAs responds dramatically to different environmental stresses and genetic lesions in the translation apparatus. To identify additional features of the global landscape of translational control, we used microarray analysis of polysomal as well as non-polysomal mRNAs to examine the defects in translation in a poly(A) binding protein mutant, pab2 pab8, as well as in a mutant of a large ribosomal subunit protein, rpl24b/shortvalve1.

Results

The mutation of RPL24B stimulated the ribosome occupancy of mRNAs for nuclear encoded ribosomal proteins. Detailed analysis yielded new insights into the translational regulon containing the ribosomal protein mRNAs. First, the ribosome occupancy defects in the rpl24b mutant partially overlapped with those in a previously analyzed initiation factor mutant, eif3h. Second, a group of mRNAs with incomplete coding sequences appeared to be uncoupled from the regulon, since their dependence on RPL24B differed from regular mRNAs. Third, different sister paralogs of the ribosomal proteins differed in their translation state in the wild-type. Some sister paralogs also differed in their response to the rpl24b mutation. In contrast to rpl24b, the pab2 pab8 mutant revealed few gene specific translational defects, but a group of seed storage protein mRNAs were stimulated in their ribosome occupancy. In the course of this work, while optimizing the statistical analysis of ribosome occupancy data, we collected 12 biological replicates of translation states from wild-type seedlings. We defined 20% of mRNAs as having a high variance in their translation state. Many of these mRNAs were functionally associated with responses to the environment, suggesting that subtle variation in the environmental conditions is sensed by plants and transduced to affect the translational efficiency of hundreds of mRNAs.

Conclusions

These data represent the first genome-wide analysis of translation in a eukaryote defective in the large ribosomal subunit. RPL24 and eIF3h play similar but non-identical roles in eukaryotic translation. The data also shed light on the fine structure of the regulon of ribosomal protein mRNAs.

     

Reduced enzyme activity and starch level in an induced mutant of chloroplast phosphoglucose isomerase

Ethyl methane sulfonate treatment was used to induce a mutation in the nuclear gene encoding the chloroplast isozyme of phosphoglucose isomerase in Clarkia xantiana. The mutation, which proved allelic to wild type activity, was backcrossed to wild type for five generations so that the two could be compared in a near isogenic background. An immunological analysis showed that the mutant, when homozygous, reduced the activity of the isozyme by about 50%. In contrast to wild type, the mutant showed little change in leaf starch level over a diurnal period or following a 72-hour continuous light treatment. By the end of the diurnal light period, the mutant accumulated only about 60% as much starch as wild type. However, mutant leaves had an increased sucrose level presumably because photosynthate was directly exported from the chloroplasts. The mutant also exhibited reduced leaf weight. These changes in metabolism and growth suggest that the wild type level of plastid phosphoglucose isomerase activity is necessary to achieve wild type carbohydrate status.     

LONGIFOLIA1 and LONGIFOLIA2, two homologous genes, regulate longitudinal cell elongation in Arabidopsis.

Plants have diversified their leaf morphologies to adapt to diverse ecological niches. The molecular components responsible for regulating leaf morphology, however, have not been fully elucidated. By screening Arabidopsis activation-tagging lines, we identified a dominant mutant, which we designated longifolia1-1D (lng1-1D). lng1-1D plants were characterized by long petioles, narrow but extremely long leaf blades with serrated margins, elongated floral organs, and elongated siliques. The elongated leaves of the mutant were due to increased polar cell elongation rather than increased cell proliferation. Molecular characterization revealed that this phenotype was caused by overexpression of the novel gene LNG1, which was found to have a homolog, LNG2,in Arabidopsis. To further examine the role of the LNG genes, we characterized lng1 and lng2 loss-of-function mutant lines. In contrast to the elongated leaves of lng1-1D plants, the lng1 and lng2 mutants showed slightly decreased leaf length. Furthermore, the lng1-3 lng2-1 double mutant showed further decreased leaf length associated with less longitudinal polar cell elongation. The leaf widths in lng1-3 lng2-1 mutant plants were similar to those in wild type, implying that the role of LNG1 and LNG2 on polar cell elongation is similar to that of ROTUNDIFOLIA3 (ROT3). However, analysis of a lng1-3 lng2-1 rot3-1 triple mutant and of a lng1-1D rot3-1 double mutant indicated that LNG1 and LNG2 promote longitudinal cell elongation independently of ROT3. Taken together, these findings indicate that LNG1 and LNG2 are new components that regulate leaf morphology by positively promoting longitudinal polar cell elongation independently of ROT3 in Arabidopsis.     

Phenotypical and structural characterization of the Arabidopsis mutant involved in shoot apical meristem

An Arabidopsis mutant induced by T-DNA insertion was studied with respect to its phenotype, micro-structure of shoot apical meristem (SAM) and histo-chemical localization of the GUS gene in comparison with the wild type. Phenotypical observation found that the mutant exhibited a dwarf phenotype with smaller organs (such as smaller leaves, shorter petioles), and slower development and flowering time compared to the wild type. Optical microscopic analysis of the mutant showed that it had a smaller and more flattened SAM, with reduced cell layers and a shortened distance between two leaf primordia compared with the wild type. In addi-tion, analysis of the histo-chemical localization of the GUS gene revealed that it was specifically expressed in the SAM and the vascular tissue of the mutant, which suggests that the gene trapped by T-DNA may function in the SAM, and T-DNA insertion could influence the functional activity of the related gene in the mutant, lead-ing to alterations in the SAM and a series of phenotypes in the mutant.     

An ABA-regulated and Golgi-localized protein phosphatase controls water loss during leaf senescence in Arabidopsis

It is known that a senescing leaf loses water faster than a non-senescing leaf and that ABA has an important role in promoting leaf senescence. However, questions such as why water loss is faster, how water loss is regulated, and how ABA functions in leaf senescence are not well understood. Here we report on the identification and functional analysis of a leaf senescence associated gene called SAG113. The RNA blot and GUS reporter analyses all show that SAG113 is expressed in senescing leaves and is induced by ABA in Arabidopsis. The SAG113 expression levels are significantly reduced in aba2 and abi4 mutants. A GFP fusion protein analysis revealed that SAG113 protein is localized in the Golgi apparatus. SAG113 encodes a protein phosphatase that belongs to the PP2C family and is able to functionally complement a yeast PP2C-deficient mutant TM126 (ptc1Δ). Leaf senescence is delayed in the SAG113 knockout mutant compared with that in the wild type, stomatal movement in the senescing leaves of SAG113 knockouts is more sensitive to ABA than that of the wild type, and the rate of water loss in senescing leaves of SAG113 knockouts is significantly reduced. In contrast, inducible over-expression of SAG113 results in a lower sensitivity of stomatal movement to ABA treatment, more rapid water loss, and precocious leaf senescence. No other aspects of growth and development, including seed germination, were observed. These findings suggest that SAG113, a negative regulator of ABA signal transduction, is specifically involved in the control of water loss during leaf senescence.     

Effect of the Rht3 dwarfing gene on dynamics of cell extension in wheat leaves, and its modification by gibberellic acid and paclobutrazol

The second leaf of wheat was used as a model system to examinethe effects of the Rht3 dwarfing gene on leaf growth. Comparedto the rht3 wild type, the Rht3allele decreased final leaf length,surface area and dry mass by reducing the maximum growth rates,but without affecting growth duration. Gibberellic acid (GA₃)increased final leaf length and maximum growth rate in the rht3wild type, but was without effect on the Rht3 mutant, whichis generally regarded as being non-responsive to gibberellin(GA). Paclobutrazol, an inhibitor of GA biosynthesis, decreasedfinal leaf length and maximum growth rate in the rht3 wild typeto values similar to those in the untreated Rht3 mutant. NeitherGA₃ nor paclobutrazol affected the duration of leaf growth.The decrease in leaf length was produced by reduction of celllength rather than cell number. The maximum relative elementalgrowth rate (REGR) for cell extension was essentially the samein all treatments, as was the time between the cells leavingthe meristem and achieving maximum extension rate. The differencesbetween the genotypes and treatments were all almost entirelydue to differences in the time taken from the attainment ofmaximum REGR of cell extension to the cessation of extension.This was reflected in the length of the extension zone, whichwas approximately 6–8 per cent of final leaf length. Theeffects of the Rht3 allele, GA₃ and paclobutrazol all appearto be on the processes which promote the cessation of cell elongation. Key words: Cell extension, gibberellin, leaf growth, Rht3 gene, Triticum, wheat