Roles of extensins in cotyledon primordium formation and shoot apical meristem activity in Nicotiana tabacum

Extensins are cell wall basic glycoproteins with a polypeptide backbone that is extremely rich in hydroxyproline. In this paper, the function of extensins in embryo development was studied in Nicotiana tabacum. By using Western blot and immunohistochemistry, the extensin JIM20 epitopes were found to express in different developmental stages of embryos, and specifically in the top of the embryo proper (EP) and the suspensor of the late globular embryos. In order to clarify the functions of extensins, a potent hydroxyproline synthesis inhibitor, 3,4-dehydro-L-proline (3,4-DHP), was used in ovule and embryo culture. The results showed that the addition of 3,4-DHP caused abnormal embryos with single, asymmetry and supernumerary cotyledon primordia, and continuous culture led to cotyledon defects in the germinated seedlings. Histological sections showed that the shoot apical meristem (SAM) of the abnormal seedlings was dissimilar from the controls, especially in the seedlings with cup-shaped cotyledons. Furthermore, the vasculature of the abnormal cotyledons was in an out-of-order format and contained at least two main veins. Finally, both the hydroxyproline assay and fluorescent immunolocalization confirmed that 3,4-DHP treatment reduced the level of extensins in the cultured ovules and embryos. These results indicate that extensins may play important roles in the cotyledon primordium formation, SAM activity, and vasculature differentiation during embryo development.     

Whole-genome comparison of leucine-rich repeat extensins in Arabidopsis and rice. A conserved family of cell wall proteins form a vegetative and a reproductive clade

We have searched the Arabidopsis and rice (Oryza sativa) genomes for homologs of LRX1, an Arabidopsis gene encoding a novel type of cell wall protein containing a leucine-rich repeat (LRR) and an extensin domain. Eleven and eight LRX (LRR/EXTENSIN) genes have been identified in these two plant species, respectively. The LRX gene family encodes proteins characterized by a short N-terminal domain, a domain with 10 LRRs, a cysteine-rich motif, and a variable C-terminal extensin-like domain. Phylogenetic analysis performed on the conserved domains indicates the existence of two major clades of LRX proteins that arose before the eudicot/monocot divergence and then diversified independently in each lineage. In Arabidopsis, gene expression studies by northern hybridization and promoter::uidA fusions showed that the two phylogenetic clades represent a specialization into "reproductive" and "vegetative" LRXs. The four Arabidopsis genes of the "reproductive" clade are specifically expressed in pollen, whereas the seven "vegetative" genes are predominantly expressed in various sporophytic tissues. This separation into two expression classes is also supported by previous studies on maize (Zea mays) and tomato (Lycopersicon esculentum) LRX homologs and by information on available rice ESTs. The strong conservation of the amino acids responsible for the putative recognition specificity of the LRR domain throughout the family suggests that the LRX proteins interact with similar ligands.     

Differential retention and expansion of the ancestral genes associated with the paleopolyploidies in modern rosid plants,as revealed by analysis of the extensins super-gene family

Background

All modern rosids originated from a common hexapolyploid ancestor, and the genomes of some rosids have undergone one or more cycles of paleopolyploidy. After the duplication of the ancient genome, wholesale gene loss and gene subfunctionalization has occurred. Using the extensin super-gene family as an example, we tracked the differential retention and expansion of ancestral extensin genes in four modern rosids, Arabidopsis, Populus, Vitis and Carica, using several analytical methods.

Results

The majority of extensin genes in each of the modern rosids were found to originate from different ancestral genes. In Arabidopsis and Populus, almost half of the extensins were paralogous duplicates within the genome of each species. By contrast, no paralogous extensins were detected in Vitis and Carica, which have only undergone the common γ-triplication event. It was noteworthy that a group of extensins containing the IPR006706 domain had actively duplicated in Arabidopsis, giving rise to a neo-extensin around every 3 million years. However, such extensins were absent from, or rare in, the other three rosids. A detailed examination revealed that this group of extensins had proliferated significantly in the genomes of a number of species in the Brassicaceae. We propose that this group of extensins might play important roles in the biology and in the evolution of the Brassicaceae. Our analyses also revealed that nearly all of the paralogous and orthologous extensin-pairs have been under strong purifying selection, leading to the strong conservation of the function of extensins duplicated from the same ancestral gene.

Conclusions

Our analyses show that extensins originating from a common ancestor have been differentially retained and expanded among four modern rosids. Our findings suggest that, if Arabidopsis is used as the model plant, we can only learn a limited amount about the functions of a particular gene family. These results also provide an example of how it is essential to learn the origination of a gene when analyzing its function across different plant species.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-612) contains supplementary material, which is available to authorized users.     

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北京市第22中学的生物学教师陈珊问：动物细胞培养时为什么会出现贴壁现象？ 答：第1,绝大部分细胞在机体内的环境下是与其他细胞或者细胞外基质结合的,不是孤立存在的。第2。细胞与细胞或者细胞外基质结合的物质基础以及在体外培养中贴壁的物质基础是什么。[第一段]     

细胞体外培养时为什么会贴到培养皿壁上

北京市第22中学的生物学教师陈珊问:动物细胞培养时为什么会出现贴壁现象? 答:第1,绝大部分细胞在机体内的环境下是与其他细胞或者细胞外基质结合的,不是孤立存在的.第2,细胞与细胞或者细胞外基质结合的物质基础以及在体外培养中贴壁的物质基础是什么.