With global warming, plant high temperature injury is becoming an increasingly serious problem. In wheat, barley, and various other commercially important crops, the early phase of anther development is especially susceptible to high temperatures. Activation of auxin biosynthesis with increased temperatures has been reported in certain plant tissues. In contrast, we here found that under high temperature conditions, endogenous auxin levels specifically decreased in the developing anthers of barley and Arabidopsis . In addition, expression of the YUCCA auxin biosynthesis genes was repressed by increasing temperatures. Application of auxin completely reversed male sterility in both plant species. These findings suggest that tissue-specific auxin reduction is the primary cause of high temperature injury, which leads to the abortion of pollen development. Thus, the application of auxin may help sustain steady yields of crops despite future climate change.

Nedd4 antagonizes Notch signaling by promoting degradation of Notch and Deltex. This Nedd4 function may be important for protecting unstimulated cells from sporadic activation of Notch signaling.

The prc gene, which is involved in cleavage of the C-terminal peptide from the precursor form of penicillin-binding protein 3 (PBP 3) of Escherichia coli, was cloned and mapped at 40.4 min on the chromosome. The gene product was identified as a protein of about 80 kDa in maxicell and in vitro systems. Fractionation of the maxicells producing the product suggested that the product was associated with the periplasmic side of the cytoplasmic membrane. This was consistent with the notion that the C-terminal processing of PBP 3 probably occurs outside the cytoplasmic membrane: the processing was found to be dependent on the secY and secA functions, indicating that the prc product or PBP 3 or both share the translocation machinery with other extracytoplasmic proteins. DNA sequencing analysis of the prc gene region identified an open reading frame, with two possible translational starts 6 bp apart from each other, that could code for a product with a calculated molecular weight of 76,667 or 76,432. The prc mutant was sensitive to thermal and osmotic stresses. Southern analysis of the chromosomal DNA of the mutant unexpectedly revealed that the mutation was a deletion of the entire prc gene and thus that the prc gene is conditionally dispensable. The mutation resulted in greatly reduced heat shock response at low osmolarity and in leakage of periplasmic proteins.

Plant male reproductive development is highly organized and sensitive to various environmental stressors, including high temperature. We have established an experimental procedure to evaluate high temperature injury in japonica rice plants. High temperature treatment (39 degrees C/30 degrees C) starting at the microspore stage repeatedly reduced spikelet fertility in our system. Morphological observations revealed that pollen viability in plants exposed to high temperatures was lower than that in control plants. Most pollen grains in high temperature-treated plants displayed a normal round shape and stained reddish purple with Alexander's reagent; however, the pollen grains were very poorly attached and displayed limited germination on the stigma. To investigate gene regulatory mechanisms in the anther in high temperature environments, DNA microarray analysis was performed by comparing non-treated samples with samples treated with 2-4 d of high heat. Genes responsive to high temperatures were identified from clustering of microarray data. Among these, at least 13 were designated as high temperature-repressed genes in the anther. Expression analyses revealed that these genes were expressed specifically in the immature anther mainly in the tapetum at the microspore stage and down-regulated after 1 d of high temperature. The expression levels of Osc6, OsRAFTIN and TDR, which are tapetum-specific genes, were unaffected by high temperatures. These results suggest that not all tapetal genes are inhibited by increased temperatures and the tapetum itself is not degraded in such an environment. However, high temperatures may disrupt some of the tapetum functions required for pollen adhesion and germination on the stigma.

Roots display hydrotropism in response to moisture gradients, which is thought to be important for controlling their growth orientation, obtaining water, and establishing their stand in the terrestrial environment. However, the molecular mechanism underlying hydrotropism remains unknown. Here, we report that roots of the Arabidopsis mutant mizu-kussei1 (miz1), which are impaired in hydrotropism, show normal gravitropism and elongation growth. The roots of miz1 plants showed reduced phototropism and a modified wavy growth response. There were no distinct differences in morphological features and root structure between miz1 and wild-type plants. These results suggest that the pathway inducing hydrotropism is independent of the pathways used in other tropic responses. The phenotype results from a single recessive mutation in MIZ1, which encodes a protein containing a domain (the MIZ domain) that is highly conserved among terrestrial plants such as rice and moss. The MIZ domain was not found in known genomes of organisms such as green algae, red algae, cyanobacteria, or animals. We hypothesize that MIZ1 has evolved to play an important role in adaptation to terrestrial life because hydrotropism could contribute to drought avoidance in higher plants. In addition, a pMIZ1::GUS fusion gene was expressed strongly in columella cells of the root cap but not in the elongation zone, suggesting that MIZ1 functions in the early phase of the hydrotropic response.Arabidopsis ͉ columella cells ͉ drought avoidance ͉ MIZU-KUSSEI1 (MIZ1) ͉ root tropism

High-temperature stress causes abortive male reproductive development in many plant species. Here, we report a putative mechanism of high-temperature injury during anther early development in barley plants (Hordeum vulgare L). Under high-temperature conditions (30 degrees C day/25 degrees C night), cell-proliferation arrest, increased vacuolization, over-development of chloroplasts, and certain abnormalities of the mitochondria, nuclear membrane, and rough endoplasmic reticulum (RER) were observed in developing anther cells, but not in developing ovule cells. Moreover, premature degradation of tapetum cells and premature progression to meiotic prophase in pollen mother cells (PMCs) were also observed. To monitor transcriptional alterations during high-temperature injury, we performed DNA microarray analysis using the 22K Barley1 GeneChip. Expression profiles were captured at four time points during the early development of panicles, and during vegetative growth of seedlings as a control, with or without high-temperature treatment. Abiotic or biotic stress related genes were equally or more dominantly up-regulated in the seedlings exposed to high temperatures compared with the panicles. In contrast, certain genes associated with histones, DNA replication initiation, mitochondria, and ribosomes were specifically repressed in the exposed panicles. In situ hybridization studies indicated that repression locally occurred on the developing anther cells exposed to high temperatures. Microarray analysis also indicated that a series of genes, including a meiosis-specific gene Asy1 and anther-specific lipid transfer protein genes, was prematurely up-regulated at an earlier stage under high-temperature conditions. Real-time quantitative RT-PCR analyses well confirmed the expression differences of certain key genes predicted by the DNA microarrays. These results suggest that high-temperature causes premature progression of anther early development program and fate, such as progression to meiosis of PMCs, cell-proliferation arrest and degradation in anther wall cells, accompanied by comprehensive alterations in transcription.

The development of the inflorescence, microspores and anthesis were well synchronized among individuals or in the panicles of barley under controlled environmental conditions. To study the effects of high-temperature stress on the development of pollen mother cells (PMCs) and microspores, the plants were subjected to high temperature treatment at different stages of reproductive growth. When plants were exposed to high temperature for five days at the early differentiation stage of the panicle, pollen grains had apparently normal exine but no or little cytoplasm. At the pre-meiotic stage of PMCs, high temperature caused subsequent development of short anthers possessing no pollen grains. When plants were exposed to high temperature during meiosis of PMCs, all pollen grains possessed exine and were swollen but showed little starch accumulation. In these plants treated at high temperature, the panicles at the heading stage had a normal appearance, but their seeds were virtually sterile. These results indicated that there are at least three stages of reproductive growth hypersensitive to high temperature, which resulted in abnormal terminal phenotypes different from one another.

In response to a moisture gradient, roots exhibit hydrotropism to control the orientation of their growth. To exhibit hydrotropism, however, they must overcome the gravitropism that is dominant on Earth. We found that moisture gradient or water stress caused immediate degradation of the starch anchors, amyloplasts, in root columella cells of Arabidopsis and radish (Raphanus sativus). Namely, development of hydrotropic response was accompanied by a simultaneous reduction in starch content in columella cells. Rapid degradation of amyloplasts in columella cells also occurred in the water-stressed roots with sorbitol or mannitol. Both hydrotropically stimulated and water-stressed roots showed a reduced responsiveness to gravity. Roots of a starchless mutant, pgm1-1, showed an enhanced hydrotropism compared with that of the wild type. These results suggest that the reduced responsiveness to gravity is, at least in part, attributable to the degradation of amyloplasts in columella cells. Thus, the reduction in gravitropism allows the roots to exhibit hydrotropism.