Po-Pu Liu

A Novel Endo-β-Mannanase Gene in Tomato LeMAN5 Is Associated with Anther and Pollen Development

Endo-β-mannanase (EC 3.2.1.78) is involved in cell wall disassembly and the weakening of plant tissues by degrading mannan polymers in the cell walls. Endo-β-mannanase genes are expressed in tomato (Lycopersicon esculentum) seeds (LeMAN1 and LeMAN2) and fruits (LeMAN3 and LeMAN4). A novel endo-β-mannanase gene (termed LeMAN5) was found in the tomato genome by genome-walking PCR and bacterial artificial chromosome library screening. The 5′-upstream region of this endo-β-mannanase gene contained four copies of the pollen-specific cis-acting elements POLLEN1LELAT52 (AGAAA). A GUS-reporter gene driven with the putative LeMAN5 promoter (-543 to +38) was activated in anthers and pollen of transgenic Arabidopsis, with the highest β-glucuronidase activity detected in pollen. β-Glucuronidase expression was detected in mature pollen retained in sporangia, discharged pollen, and elongating pollen tubes in transgenic Arabidopsis. Consistently, expression of LeMAN5 mRNA and endo-β-mannnanase activity was detected in tomato anthers and pollen. In anthers, the highest mRNA expression and endo-β-mannanase activity were detected during late stages of anther development, when pollen maturation occurred. Endo-β-mannanase activity was present in discharged pollen, which was easily eluted in a buffer, indicating that the enzyme proteins are probably secreted from, and deposited on, the surface of pollen. These data suggest that the LeMAN5 endo-β-mannanase is associated with anther and pollen development.

microRNA, seeds, and Darwin?: diverse function of miRNA in seed biology and plant responses to stress

microRNAs (miRNAs) are small, single-stranded RNAs that down-regulate target genes at the post-transcriptional level. miRNAs regulate target genes by guiding mRNA cleavage or by repressing translation. miRNAs play crucial roles in a broad range of developmental processes in plants. Multiple miRNAs are present in germinating seeds and seedlings of Arabidopsis, some of which are involved in the regulation of germination and seedling growth by plant hormones such as abscisic acid (ABA) and auxin. The involvement of miRNAs in ABA responses is not limited to the early stages of plant development but seems to be important for general stress responses throughout the plant life cycle. This Darwin review summarizes recent progress in miRNA research focusing on seed and stress biology, two topics which were of interest to Charles Darwin.

The microRNA156 and microRNA172 gene regulation cascades at post-germinative stages in Arabidopsis

MicroRNAs (miRNAs) are involved in developmental programmes of plants, including seed germination and post-germination. Here, we provide evidence that two different miRNA pathways, miR156 and miR172, interact during the post-germination stages in Arabidopsis. Mutant seedlings expressing miR156-resistant SQUAMOSA PROMOTER-BINDING PROTEIN-LIKE13 (mSPL13), which has silent mutations in the miR156 complementary sequence, over-accumulated SPL13 mRNA and exhibited a delay in seedling development. Microarray analysis indicated that SCHNARCHZAPFEN (SNZ), an AP2-like gene targeted by miR172, was downregulated in these mutants. Quantitative reverse transcription-polymerase chain reaction (RT-PCR) and miRNA gel blot analyses showed that the MIR172 genes were up-regulated in mSPL13 mutants. These results suggest that the miRNA regulation cascades (miR156 s SPL13 ! miR172 s SNZ) play a critical role during the post-germination developmental stages in Arabidopsis.

The regulation of post-germinative transition from the cotyledon- to vegetative-leaf stages by microRNA-targeted SQUAMOSA PROMOTER-BINDING PROTEIN LIKE13 in Arabidopsis

Germination and early seedling development are critical for successful stand establishment of plants. Following germination, the cotyledons, which are derived from embryonic tissue, emerge from the seed. Arabidopsis seedlings at post-germinative stages are supported mainly by the supply of nutrition from the cotyledons until vegetative leaves emerge and initiate photosynthesis. The switch to autotrophic growth is a significant transition at the post-germinative stage. Here, we provide evidence that down-regulation of SQUAMOSA PROMOTER-BINDING PROTEIN LIKE13 (SPL13) by microRNA156 (miR156) plays an important role in the regulation of the post-germinative switch from the cotyledon stage to the vegetative-leaf stage. Silent mutations created in the SPL13 sequence in the region that is complementary to the miR156 sequence caused the deregulation of the mutant form of SPL13 (mSPL13) mRNA from miR156. Mutant seedlings over-accumulated miRNA-resistant messages and exhibited a delay in the emergence of vegetative leaves compared to wild-type seedlings. The delay was not observed in control transgenic plants expressing nonmutated SPL13, indicating that the phenotype was caused specifically by the silent mutations and deregulation of SPL13 from miR156. Characterization of the SPL13 promoter indicated that this gene is expressed mainly in the hypocotyl and affects leaf primordium development. These results suggest that the repression of SPL13 by miR156 is essential for normal post-germinative growth in Arabidopsis.

Simple purification of small RNAs from seeds and efficient detection of multiple microRNAs expressed in Arabidopsis thaliana and tomato (Lycopersicon esculentum) seeds

MicroRNAs (miRNAs) play critical roles in the development of animals and plants. Characterizing the stage- and tissue-specific expression of miRNAs that potentially regulate target transcription factor expression is becoming more important for understanding the regulatory mechanisms of critical events during plant development. A simple method for purifying small RNAs from seeds is described, as well as an efficient non-radioactive labelling system for making miRNA probes. In Arabidopsis thaliana seed extracts, low molecular-weight (LMW) RNAs (e.g. 5S rRNA, tRNA and miRNA) were separated from high molecular-weight (HMW) nucleic acids (e.g. 28S and 18S rRNA, mRNA and genomic DNA) by fractionation using isopropanol. HMW RNAs precipitated in 20% isopropanol, while most LMW RNAs remained in the supernatant. The purified LMW RNAs were used successfully for RNA gel blotting to detect miRNAs expressed in Arabidopsis and tomato ( Lycopersicon esculentum ) seeds. To increase the detection sensitivity of the microRNA probes, additional digoxigenin-labelled uridine triphosphates (UTPs) were incorporated into the miRNA probes by designing template oligo DNAs with three extra adenines (A) at each end of their sequence. These DNA oligomers were used to make double-stranded DNA templates for miRNA probe synthesis. This probe (termed AAA PLUS ) exhibited stronger signals than normal probes. A technique was also developed to quickly screen expressed miRNAs in seeds using a miniblot system, which enabled simultaneous examination with multiple miRNA probes. This method provides a simple alternative to microRNA microarrays to identify the major miRNAs expressed in seeds.

An Arabidopsis thaliana embryo arrest mutant exhibiting germination potential

The ability to initiate radicle elongation, or germination potential, occurs in developing embryos before the completionofseedmaturation.Greenembryosafterthe walking-stick stage in developing Arabidopsis thaliana seeds germinate when excised from seeds and incubated in Murashige–Skoog (MS) medium containing 1% sucrose. Germination potential is not observed during early embryogenesis at the globular, heart and torpedo stages. Here, we describe an Arabidopsis mutant with embryos arrested at early stages of development, but still exhibiting germination potential. The mutant, termed embryo ball (eb), produced shrunken seeds containing round or irregularly shaped embryos that did not germinate. The round embryos excised from developing eb seeds were capable of growinga primary root withroothairswhen incubatedin media. In contrast,cotyledons were absent at the apical region of the eb embryos, although the apical region produced leaf-like structures with trichomes, indicating vegetative leaf identity. These observations suggested that morphological maturation was not essential for the induction of germination potential. The eb embryos exhibited partial desiccation tolerance that is characteristic of mature embryos at later stages of development, suggesting that cell maturation was also independent of morphological maturation. The eb