Expansins in Plant Growth and Development: an Update on an Emerging Topic

Abstract: Expansins are a class of proteins identified by their ability to induce the extension of isolated plant cell walls. Expansins are encoded by an extensive multigene family in higher plants, several members of which have been shown to be expressed in a tissue-specific manner. Besides playing an apparently key role in wall expansion, and hence in cell growth, expansins have been implicated in an increasing number of processes during plant growth and development. These include: leaf organogenesis, fruit softening, and wall disassembly. A second class of closely related proteins (referred to as β-expansins) has been identified. Other recent advances in expansin research include the recovery of transgenic plants with altered level of expansins, and the production of recombinant expansins in het-erologous expression systems.     

Wall-yielding properties of cell walls from elongating cucumber hypocotyls in relation to the action of expansin

The wall-yielding properties of cell walls were examined using frozen-thawed and pressed segments (FTPs) obtained from the elongation zones of cucumber hypocotyls with a newly developed programmable creep meter. The rate of wall extension characteristically changed depending on both tension and pH. By treatment of the FTPs with acid, the yield tension (y) was shifted downward and the extensibility (phi) was increased. However, the downward shift of y was greatly suppressed and the increase in phi was partly inhibited in boiled FTPs. The boiled FTPs reconstituted with expansin fully recovered the acid-induced downward y shift as well as the increase in phi. Even under the tension below y, wall extension took place pH dependently. Such extension was markedly slower (low-rate extension) than that under the tension above y (high-rate extension). At a higher concentration (8 M), urea markedly inhibited the creep ascribable to the inhibition of the acid-induced downward y shift and increase in phi. Moderate concentrations (2 M) of urea promoted wall creep pH dependently. The promotion was equivalent to a 0.5 decrease in pH. The promotion of creep by 2 M urea was observed in boiled FTPs reconstituted with expansin but not in boiled FTPs. These findings indicated that the acid-facilitated creep was controlled by y as well as in cucumber cell walls. However, y and phi might be inseparable and mutually related parameters because the curve of the stress extension rate (SER) showed a gradual change from the low-rate extension to the high-rate extension. Expansin played a role in pH-dependent regulation of both y and phi. The physiological meaning of the pH-dependent regulation of wall creep under different creep tensions is also discussed with reference to a performance chart obtained from the SER curves.     

Ectopic expression of Expansin3 or Expansinβ1 causes enhanced hormone and salt stress sensitivity in Arabidopsis

Expansins are cell wall loosening proteins that appear to permit the microfibril matrix network to slide in growing plant cell walls, thereby enabling the wall to expand. To scrutinize possible impacts on plant growth and development when expansins are over-expressed, we characterized phenotypic alterations of the transgenic plants that constitutively expressed AtEXP3 or AtEXP-beta1 under control of 35S-CaMV promoter. Our results suggest that both AtEXP3-OX and AtXPbeta1-OX are very sensitive to salt stress. However, the mechanisms underlying their enhanced salt sensitivity appear to be different.     

植物细胞壁伸展测定仪在蚕豆扩张蛋白特性研究中的应用

扩张蛋白(expansin)在细胞扩张和果实成熟中起着极为重要的作用.植物细胞壁伸展测定仪是研究扩张蛋白必不可少的仪器.为此以电涡流传感器为核心部件装配了一种具有结构简单、操作方便和测量准确等优点的新型测定仪,并利用该仪器研究了蚕豆(Vicia faba)扩张蛋白的特性.结果表明蚕豆根、茎、上胚轴和成熟叶片中均存在扩张蛋白,而且叶片和幼根的扩张蛋白活性最强;免疫印迹证实在蚕豆根、茎、上胚轴和成熟叶片中确实存在扩张蛋白.以上结果说明本仪器灵敏且可靠,用此仪器首次发现在成熟叶片中存在扩张蛋白.     

Differential location of α-expansin proteins during the accommodation of root cells to an arbuscular mycorrhizal fungus

-Expansins are extracellular proteins that increase plant cell-wall extensibility. We analysed their pattern of expression in cucumber roots in the presence and in the absence of the mycorrhizal fungus, Glomus versiforme. The distribution of -expansins was investigated by use of two polyclonal antibodies (anti-EXPA1 and anti-EXPA2, prepared against two different cucumber -expansins) in immunoblotting, immunofluorescence, and immunogold experiments. Immunoblot results indicate the presence of a 30-kDa band specific for mycorrhizal roots. The two antibodies identify antigens with a different distribution in mycorrhizal roots: anti-EXPA1 labels the interface zone, but the plant cell walls only weakly. By contrast, the anti-EXPA2 labels only the plant cell walls. In order to understand the potential role of -expansins during the accommodation of the fungus inside root cells, we prepared semi-thin sections to measure the size of cortical cells and the thickness of cortical cell walls in mycorrhizal and non-mycorrhizal root. Mycorrhizal cortical cells were significantly larger than non-mycorrhizal cells and had thicker cell walls. In double-labelling experiments with cellobiohydrolase–gold complex, we observed that cellulose was co-localized with -expansins. Taken together, the results demonstrate that -expansins are more abundant in the cucumber cell walls upon mycorrhizal infection; we propose that these wall-loosening proteins are directly involved in the accommodation of the fungus by infected cortical cells.     

α-Expansins in the semiaquatic ferns Marsilea quadrifolia and Regnellidium diphyllum: evolutionary aspects and physiological role in rachis elongation

To investigate the evolutionary history of expansins and their role in cell elongation in early land plants, we isolated two α-expansin genes, Mq-EXP1 and Rd-EXP1, respectively, from the semiaquatic ferns Marsilea quadrifolia L. and Regnellidium diphyllum Lindm. The deduced amino acid sequences of the fern expansins exhibit a high degree of identity to those of seed plants, showing that expansin genes were conserved during the evolution of vascular plants. Gel-blot analysis of M. quadrifolia and R. diphyllum genomic DNA indicated that, in both ferns, α-expansins are encoded by multigene families. Expression of α-expansin genes probed with Mq-EXP1 was confined to the elongating region of the Marsilea rachis. Cell-wall proteins of M.␣quadrifolia induced in-vitro extension of acidified cucumber cell walls. In R. diphyllum, expression of Rd-EXP1 increased when elongation of the rachis was enhanced by submergence or ethylene. These results indicate that α-expansins act as wall-loosening proteins in ferns, as has been proposed for angiosperms. In addition, Rd-EXP1 may play a role in mediating elongation of the rachis in submerged plants. Received: 7 March 2000 / Accepted: 29 April 2000     

Portrait of the expansin superfamily in Physcomitrella patens: comparisons with angiosperm expansins

BACKGROUND AND AIMS: Expansins are plant cell wall loosening proteins important in a variety of physiological processes. They comprise a large superfamily of genes consisting of four families (EXPA, EXPB, EXLA and EXLB) whose evolutionary relationships have been well characterized in angiosperms, but not in basal land plants. This work attempts to connect the expansin superfamily in bryophytes with the evolutionary history of this superfamily in angiosperms. METHODS: The expansin superfamily in Physcomitrella patens has been assembled from the Physcomitrella sequencing project data generated by the Joint Genome Institute and compared with angiosperm expansin superfamilies. Phylogenetic, motif, intron and distance analyses have been used for this purpose. KEY RESULTS: A gene superfamily is revealed that contains similar numbers of genes as found in arabidopsis, but lacking EXLA or EXLB genes. This similarity in gene numbers exists even though expansin evolution in Physcomitrella diverged from the angiosperm line approx. 400 million years ago. Phylogenetic analyses suggest that there were a minimum of two EXPA genes and one EXPB gene in the last common ancestor of angiosperms and Physcomitrella. Motif analysis seems to suggest that EXPA protein function is similar in bryophytes and angiosperms, but that EXPB function may be altered. CONCLUSIONS: The EXPA genes of Physcomitrella are likely to have maintained the same biochemical function as angiosperm expansins despite their independent evolutionary history. Changes seen at normally conserved residues in the Physcomitrella EXPB family suggest a possible change in function as one mode of evolution in this family.     

Promoters of two anther-specific genes confer organ-specific gene expression in a stage-specific manner in transgenic systems

Differential screening of a stage-specific cDNA library of Indica rice has been used to identify two genes expressed in pre-pollination stage panicles, namely OSIPA and OSIPK coding for proteins similar to expansins/pollen allergens and calcium-dependent protein kinases (CDPK), respectively. Northern analysis and in situ hybridizations indicate that OSIPA expresses exclusively in pollen while OSIPK expresses in pollen as well as anther wall. Promoters of these two anther-specific genes show the presence of various cis-acting elements (GTGA and AGAAA) known to confer anther/pollen-specific gene expression. Organ/tissue-specific activity and strength of their regulatory regions have been determined in transgenic systems, i.e., tobacco and Arabidopsis. A unique temporal activity of these two promoters was observed during various developmental stages of anther/pollen. Promoter of OSIPA is active during the late stages of pollen development and remains active till the anthesis, whereas, OSIPK promoter is active to a low level in developing anther till the pollen matures. OSIPK promoter activity diminishes before anthesis. Both promoters show a potential to target expression of the gene of interest in developmental stage-specific manner and can help engineer pollen-specific traits like male-sterility in plants. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Accessions: OSIPA cDNA, AF220610; OSIPK cDNA, AF312920; OSIPA partial gene and upstream promoter region, AY166659; OSIPK gene-specific and upstream sequence, AY168440.