Analysis of a dehiscence zone endo-polygalacturonase in oilseed rape (Brassica napus) and Arabidopsis thaliana: evidence for roles in cell separation in dehiscence and abscission zones, and in stylar tissues during pollen tube growth

Characterizing the role of endocarp a and b cells layers during pod (silique) development in Brassicaceae

The process of silique dehiscence is essential for the proper dispersal of seeds at the end of a dehiscent fruit plants lifecycle. Current research focuses on genetic manipulation to mitigate this process and enhance shatter tolerance in crop plants, which has significant economic implications. In this study, we have conducted a time-course analysis of cell patterning and development in valve tissues of Arabidopsis thaliana and closely related Triangle of U species (Brassica juncea, Brassica carinata, Brassica napus, Brassica rapa, and Brassica nigra) from Brassicaceae. The goal was to decipher the detailed temporal developmental patterns of the endocarp a and b cell layers of the valve, specifically their degradation and lignification respectively. Additionally, we propose a new classification system for the lignification of the endocarp a cell layer: L1 indicates the cell closest to the replum, with L2 and L3 representing the second and third cells, respectively, each numerical increment indicating lignified cells farther from the replum. Our findings provide a foundational framework absent in current literature, serving as an effective blueprint for future genomic work aimed at modifying valve structures to enhance agronomic traits, such as reducing fiber (lignin) or increasing shatter tolerance.

Genome-wide identification of GH28 family and insight into its contributions to pod shattering resistance in Brassica napus L

Rapeseed (Brassica napus L.), accounts for nearly 16% of vegetable oil, is the world's second produced oilseed. However, pod shattering has caused significant yield loses in rapeseed production, particularly during mechanical harvesting. The GH28 genes can promote pod shattering by changing the structure of the pod cell wall in Arabidopsis. However, the role of the GH28 gene family in rapeseed was largely unknown. Therefore, a genome-wide comprehensive analysis was conducted to classify the role of GH28 gene family on rapeseed pod shattering. A total of 37 BnaGH28 genes in the rapeseed genome were identified. These BnaGH28s can be divided into five groups (Group A-E), based on phylogenetic and synteny analysis. Protein property, gene structure, conserved motif, cis-acting element, and gene expression profile of BnaGH28 genes in the same group were similar. Specially, the expression level of genes in group A-D was gradually decreased, but increased in group E with the development of silique. Among eleven higher expressed genes in group E, two BnaGH28 genes (BnaA07T0199500ZS and BnaC06T0206500ZS) were significantly regulated by IAA or GA treatment. And the significant effects of BnaA07T0199500ZS variation on pod shattering resistance were also demonstrated in present study. These results could open a new window for insight into the role of BnaGH28 genes on pod shattering resistance in rapeseed.

Evolution and development of fruits of Erycina pusilla and other orchid species

Fruits play a crucial role in seed dispersal. They open along dehiscence zones. Fruit dehiscence zone formation has been intensively studied in Arabidopsis thaliana. However, little is known about the mechanisms and genes involved in the formation of fruit dehiscence zones in species outside the Brassicaceae. The dehiscence zone of A. thaliana contains a lignified layer, while dehiscence zone tissues of the emerging orchid model Erycina pusilla include a lipid layer. Here we present an analysis of evolution and development of fruit dehiscence zones in orchids. We performed ancestral state reconstructions across the five orchid subfamilies to study the evolution of selected fruit traits and explored dehiscence zone developmental genes using RNA-seq and qPCR. We found that erect dehiscent fruits with non-lignified dehiscence zones and a short ripening period are ancestral characters in orchids. Lignified dehiscence zones in orchid fruits evolved multiple times from non-lignified zones. Furthermore, we carried out gene expression analysis of tissues from different developmental stages of E. pusilla fruits. We found that fruit dehiscence genes from the MADS-box gene family and other important regulators in E. pusilla differed in their expression pattern from their homologs in A. thaliana. This suggests that the current A. thaliana fruit dehiscence model requires adjustment for orchids. Additionally, we discovered that homologs of A. thaliana genes involved in the development of carpel, gynoecium and ovules, and genes involved in lipid biosynthesis were expressed in the fruit valves of E. pusilla, implying that these genes may play a novel role in formation of dehiscence zone tissues in orchids. Future functional analysis of developmental regulators, lipid identification and quantification can shed more light on lipid-layer based dehiscence of orchid fruits.

Transcriptomic Time-Course Sequencing: Insights into the Cell Wall Macromolecule-Mediated Fruit Dehiscence during Ripening in Camellia oleifera

Camellia oleifera (C. oleifera), one of the world's four major edible woody oil crops, has been widely planted in southern China's subtropical region for the extremely high nutritional and health benefits of its seed oil. Timing and synchronization of fruit dehiscence are critical factors influencing the oil output and quality, as well as the efficiency and cost of harvesting C. oleifera, yet they extremely lack attention. To gain an understanding of the molecular basis underlying the dehiscence of C. oleifera fruit, we sampled pericarp-replum tissues containing dehiscence zones from fruits at different developmental stages and performed time-series transcriptomic sequencing and analysis for the first time. Statistical and GO enrichment analysis of differentially expressed genes revealed that drastic transcriptional changes occurred over the last short sampling interval (4 days, 18th-22nd October), which directed functional classifications link to cell wall and cell wall macromolecule activity. WGCNA further showed that factors controlling cell wall modification, including endo-1,3;1,4-beta-D-glucanase, WAT1-like protein 37, LRR receptor-like serine/threonine-protein kinase, and cellulose synthase A catalytic subunit, were identified as core members of the co-expression network of the last stage highly related modules. Furthermore, in these modules, we also noted genes that were annotated as coding for polygalacturonase and pectinesterase, two pectinases that were expected to be major players in cell separation during dehiscence. qRT-PCR further confirmed the expression profiles of these cell wall modification relating factors, which possessed a special high transcriptional abundance at the final stage. These results suggested the cell wall associated cell separation, one of the essential processes downstream of fruit dehiscence, happened in dehiscing fruit of C. oleifera during ripening. Hydrolases acting on cell wall components are good candidates for signal mediating dehiscence of C. oleifera fruit. In conclusion, our analysis provided insights into the cell wall macromolecule-mediated fruit dehiscence during ripening in C. oleifera.

Building a Flower: The Influence of Cell Wall Composition on Flower Development and Reproduction

Floral patterning is a complex task. Various organs and tissues must be formed to fulfill reproductive functions. Flower development has been studied, mainly looking for master regulators. However, downstream changes such as the cell wall composition are relevant since they allow cells to divide, differentiate, and grow. In this review, we focus on the main components of the primary cell wall-cellulose, hemicellulose, and pectins-to describe how enzymes involved in the biosynthesis, modifications, and degradation of cell wall components are related to the formation of the floral organs. Additionally, internal and external stimuli participate in the genetic regulation that modulates the activity of cell wall remodeling proteins.

Down-regulation of MANNANASE7 gene in Brassica napus L. enhances silique dehiscence-resistance

MANNANASE7 gene in Brassica napus L. encodes a hemicellulose which located at cell wall or extracellular space and dehiscence-resistance can be manipulated by altering the expression of MANNANASE7. Silique dehiscence is an important physiological process in plant reproductive development, but causes heavy yield loss in crops. The lack of dehiscence-resistant germplasm limits the application of mechanized harvesting and greatly restricts the rapeseed (Brassica napus L.) production. Hemicellulases, together with cellulases and pectinases, play important roles in fruit development and maturation. The hemicellulase gene MANNANASE7 (MAN7) was previously shown to be involved in the development and dehiscence of Arabidopsis (Arabidopsis thaliana) siliques. Here, we cloned BnaA07g12590D (BnMAN7A07), an AtMAN7 homolog from rapeseed, and demonstrate its function in the dehiscence of rapeseed siliques. We found that BnMAN7A07 was expressed in both vegetative and reproductive organs and significantly highly expressed in leaves, flowers and siliques where the abscission or dehiscence process occurs. Subcellular localization experiment showed that BnMAN7A07 was localized in the cell wall. The biological activity of the BnMAN7A07 protein isolated and purified through prokaryotic expression system was verified to catalyse the decomposition of xylan into xylose. Phenotypic studies of RNA interference (RNAi) lines revealed that down-regulation of BnMAN7A07 in rapeseed could significantly enhance silique dehiscence-resistance. In addition, the expression of upstream silique development regulators is altered in BnMAN7A07-RNAi plants, suggesting that a possible feedback regulation mechanism exists in the regulation network of silique dehiscence. Our results demonstrate that dehiscence-resistance can be manipulated by altering the expression of hemicellulase gene BnMAN7A07, which could provide an available genetic resource for breeding practice in rapeseed which is beneficial to mechanized harvest.

Patterns of Expansion and Expression Divergence of the Polygalacturonase Gene Family in Brassica oleracea

Plant polygalacturonases (PGs) are closely related to cell-separation events during plant growth and development by degrading pectin. Identifying and investigating their diversification of evolution and expression could shed light on research on their function. We conducted sequence, molecular evolution, and gene expression analyses of PG genes in Brassica oleracea. Ninety-nine B. oleracea PGs (BoPGs) were identified and divided into seven clades through phylogenetic analysis. The exon/intron structures and motifs were conserved within, but divergent between, clades. The second conserved domain (GDDC) may be more closely related to the identification of PGs. There were at least 79 common ancestor PGs between Arabidopsis thaliana and B. oleracea. The event of whole genome triplication and tandem duplication played important roles in the rapid expansion of the BoPG gene family, and gene loss may be an important mechanism in the generation of the diversity of BoPGs. By evaluating the expression in five tissues, we found that most of the expressed BoPGs in clades A, B, and E showed ubiquitous expression characteristics, and the expressed BoPGs in clades C, D, and F were mainly responsible for reproduction development. Most of the paralogous gene pairs (76.2%) exhibited divergent expression patterns, indicating that they may have experienced neofunctionalization or subfunctionalization. The cis-elements analysis showed that up to 96 BoPGs contained the hormone response elements in their promoters. In conclusion, our comparative analysis may provide a valuable data foundation for the further functional analysis of BoPGs during the development of B. oleracea.

Comprehensive analysis of polygalacturonase gene family highlights candidate genes related to pollen development and male fertility in wheat (Triticum aestivum L.)

Four polygalacturonase gene family members were highlighted that contribute to elucidate the roles of polygalacturonase during the fertility conversion process in male-sterile wheat. Polygalacturonase (PG) belongs to a large family of hydrolases with important functions in cell separation during plant growth and development via the degradation of pectin. Specific expressed PGs in anthers may be significant for male sterility research and hybrid wheat breeding, but they have not been characterized in wheat (Triticum aestivum L.). In this study, we systematically studied the PG gene family using the latest published wheat reference genomic information. In total, 113 wheat PG genes were identified, which could be classified into six categories A-F according to their structure characteristics and phylogenetic comparisons with Arabidopsis and rice. Polyploidy and segmental duplications in wheat were proved to be mainly responsible for the expansion of the wheat PG gene family. RNA-seq showed that TaPGs have specific temporal and spatial expression characteristics, in which 12 TaPGs with spike-specific expression patterns were detected by qRT-PCR in different fertility anthers of KTM3315A, a thermo-sensitive cytoplasmic male-sterile wheat. Four of them specific upregulated (TaPG09, TaPG95, and TaPG93) or downregulated (TaPG87) at trinucleate stage of fertile anthers, and further aligning with the homologous in Arabidopsis revealed that they may undertake functions such as anther dehiscence, separation of pollen, pollen development, and pollen tube elongation, thereby inducing male fertility conversion in KTM3315A. These findings facilitate function investigations of the wheat PG gene family and provide new insights into the fertility conversion mechanism in male-sterile wheat.

Mechanism and Regulation of Silique Dehiscence, Which Affects Oil Seed Production

Silique dehiscence is an important physiological process during natural growth that enables mature seeds to be released from plants, which then undergo reproduction and ensure the survival of future generations. In agricultural production, the time and degree of silique dehiscence affect the harvest time and processing of crops. Premature silique dehiscence leads to seeds being shed before harvest, resulting in substantial reductions to yields. Conversely, late silique dehiscence is not conducive to harvesting, and grain weight and oil content will be reduced due to the respiratory needs of seeds. In this paper, the mechanisms and regulation of silique dehiscence, and its application in agricultural production is reviewed.

CRISPR/Cas9-mediated genome editing reveals differences in the contribution of INDEHISCENT homologues to pod shatter resistance in Brassica napus L

The INDEHISCENT (IND) and ALCATRAZ (ALC) gene homologues have been reported to be essential for dehiscence of fruits in Brassica species. But their functions for pod shatter resistance in Brassica napus, an important oil crops, are not well understood. Here, we assessed the functions of these two genes in rapeseed using CRISPR/Cas9 technology. The induced mutations were stably transmitted to successive generations, and a variety of homozygous mutants with loss-of-function alleles of the target genes were obtained for phenotyping. The results showed that the function of BnIND gene is essential for pod shatter and highly conserved in Brassica species, whereas the BnALC gene appears to have limited potential for rapeseed shatter resistance. The homoeologous copies of the BnIND gene have partially redundant roles in rapeseed pod shatter, with BnA03.IND exhibiting higher contributions than BnC03.IND. Analysis of data obtained from the gene expression and sequence variations of gene copies revealed that cis-regulatory divergences alter gene expression and underlie the functional differentiation of BnIND homologues. Collectively, our results generate valuable resources for rapeseed breeding programs, and more importantly provide a strategy to improve polyploid crops.

Genome-Wide Identification and Expression Profiling of the Polygalacturonase (PG) and Pectin Methylesterase (PME) Genes in Grapevine (Vitis vinifera L.)

In pectin regulation, polygalacturonases (PGs) and pectin methylesterases (PMEs) are critical components in the transformation, disassembly network, and remodeling of plant primary cell walls. In the current study, we identified 36 PG and 47 PME genes using the available genomic resources of grapevine. Herein, we provide a comprehensive overview of PGs and PMEs, including phylogenetic and collinearity relationships, motif and gene structure compositions, gene duplications, principal component analysis, and expression profiling during developmental stages. Phylogenetic analysis of PGs and PMEs revealed similar domain composition patterns with Arabidopsis. The collinearity analysis showed high conservation and gene duplications with purifying selection. The type of duplications also varied in terms of gene numbers in PGs (10 dispersed, 1 proximal, 12 tandem, and 13 segmental, respectively) and PMEs (23 dispersed, 1 proximal, 16 tandem, and 7 segmental, respectively). The tissue-specific response of PG and PME genes based on the reported transcriptomic data exhibited diverged expression patterns in various organs during different developmental stages. Among PGs, VvPG8, VvPG10, VvPG13, VvPG17, VvPG18, VvPG19, VvPG20, VvPG22, and VvPG23 showed tissue- or organ-specific expression in majority of the tissues during development. Similarly, in PMEs, VvPME3, VvPME4, VvPME5, VvPME6, VvPME19, VvPME21, VvPME23, VvPME29, VvPME31, and VvPME32 suggested high tissue-specific response. The gene ontology (GO), Kyoto Encyclopedia of Genes and Genomics (KEGG) enrichment, and cis-elements prediction analysis also suggested the putative functions of PGs and PMEs in plant development, such as pectin and carbohydrate metabolism, and stress activities. Moreover, qRT-PCR validation of 32 PG and PME genes revealed their role in various organs of grapevines (i.e., root, stem, tendril, inflorescence, flesh, skins, and leaves). Therefore, these findings will lead to novel insights and encourage cutting-edge research on functional characterization of PGs and PMEs in fruit crop species.

Comparative analysis of the male inflorescence transcriptome profiles of an ms22 mutant of maize

In modern agricultural production, maize is the most successful crop utilizing heterosis. 712C-ms22 is an important male sterile material in maize. In this study, we performed transcriptome sequencing analysis of the V10 stage of male inflorescence. Through this analysis, 27.63 million raw reads were obtained, and trimming of the raw data revealed 26.63 million clean reads, with an average match rate of 94.64%. Using Tophat software, we matched these clean reads to the maize reference genome. The abundance of 39,622 genes was measured, and 35,399 genes remained after filtering out the non-expressed genes across all the samples. These genes were classified into 19 categories by clusters of orthologous groups of protein annotation. Transcriptome sequencing analysis of the male sterile and fertile 712C-ms22 maize revealed some key DEGs that may be related to metabolic pathways. qRT-PCR analysis validated the gene expression patterns identified by RNA-seq. This analysis revealed some of the essential genes responsible for pollen development and for pollen tube elongation. Our findings provide useful markers of male sterility and new insights into the global mechanisms mediating male sterility in maize.

The effect of INDEHISCENT point mutations on silique shatter resistance in oilseed rape (Brassica napus)

This study elucidates the influence of indehiscent mutations on rapeseed silique shatter resistance. A phenotype with enlarged replum-valve joint area and altered cell dimensions in the dehiscence zone is described. Silique shattering is a major factor reducing the yield stability of oilseed rape (Brassica napus). Attempts to improve shatter resistance often include the use of mutations in target genes identified from Arabidopsis (Arabidopsis thaliana). A variety of phenotyping methods assessing the level of shatter resistance were previously described. However, a comparative and comprehensive evaluation of the methods has not yet been undertaken. We verified the increase of shatter resistance in indehiscent double knock-down mutants obtained by TILLING with a systematic approach comparing three independent phenotyping methods. A positive correlation of silique length and shatter resistance was observed and accounted for in the analyses. Microscopic studies ruled out the influence of different lignification patterns. Instead, we propose a model to explain increased shattering resistance of indehiscent rapeseed mutants by altered cell shapes and sizes within the contact surfaces of replum and valves.

Genome-Wide Assessment of Polygalacturonases-Like (PGL) Genes of Medicago truncatula, Sorghum bicolor, Vitis vinifera and Oryza sativa Using Comparative Genomics Approach

The polygalacturonases (PG) is one of the important members of pectin-degrading glycoside hydrolases of the family GH28. In plants, PG represents multigene families associated with diverse processes. In the present study, an attempt has been made to investigate the diversity of PG genes among monocots and dicots with respect to phylogeny, gene duplication and subcellular localization to get an insight into the evolutionary and functional attributes. The genome-wide assessment of Medicago truncatula, Vitis vinifera Sorghum bicolor, and Oryza sativa L. ssp. japonica genomes revealed 53, 49, 38 and 35 PG-like (PGL) genes, respectively. The predominance of glyco_hydro_28 domain, hydrophilic nature and genes with multiple introns were uniformly observed. The subcellular localization showed the presence of signal sequences targeting the secretory pathways. The phylogenetic tree constructed marked uniformity with three distinct clusters for each plant irrespective of the variability in the genome sizes. The site-specific selection pressure analysis based on K _a/K _s values showed predominance of purifying selection pressures among different groups identified in these plants. The functional divergence analysis revealed significant site-specific selective constraints. Results of site-specific selective pressure analysis throw light on the functional diversity of PGs in various plant processes and hence its constitutive nature. These findings are further strengthened by functional divergence analysis which reveals functionally diverse groups in all the four species representing monocots and dicots. The outcome of the present work could be utilized for deciphering the novel functions of PGs in plants.

Histological Characteristics, Cell Wall Hydrolytic Enzymes Activity and Candidate Genes Expression Associated with Seed Shattering of Elymus sibiricus Accessions

Elymus sibiricus (siberian wildrye) is a perennial, cool-season, self-pollinating, and allotetraploid grass. As an economically important species, it has been widely grown and used for pasture and hay in northern China. Because of serious seed shattering (SS), however, E. sibiricus is difficult to grow for commercial seed production. To better understand the underlying mechanism of SS, we investigated the differences in SS of cultivars and wild accessions in relation to morphological and genetic diversity, histological characteristics, lignin staining, cell wall hydrolytic enzymes activity and candidate genes expressions. We found high level of morphological and genetic diversity among E. sibiricus accessions. In general, cultivars had higher average pedicel breaking tensile strength (BTS) value than wild accessions, of which PI655199 had the highest average BTS value (144.51 gf) and LQ04 had the lowest average BTS value (47.17 gf) during seed development. SS showed a significant correlation with seed length, awn length and 1000-seed weight. SS was caused by degradation of abscission layers that formed at early heading stage, and degradation of abscission layers occurred at 14 days after heading. Histological analysis of abscission zone (AZ) showed a smooth fracture surface on the rachilla in high SS genotype, suggesting higher degradation degree of abscission layers. This may resulted from the increased cellulase and polygalacturonase activity found in AZ at seed physiological maturity. Staining of pedicels of two contrasting genotypes suggested more lignin deposition in low SS genotype may play a role in resistance of SS. Furthermore, candidate genes that involved in cell wall-degrading enzyme and lignin biosynthesis were differentially expressed in AZ, indicating the involvement and role in SS. This study provided novel insights into the mechanism of SS in E. sibiricus.

Cell Wall Remodeling in Abscission Zone Cells during Ethylene-Promoted Fruit Abscission in Citrus

Abscission is a cell separation process by which plants can shed organs such as fruits, leaves, or flowers. The process takes place in specific locations termed abscission zones. In fruit crops like citrus, fruit abscission represents a high percentage of annual yield losses. Thus, understanding the molecular regulation of abscission is of capital relevance to control production. To identify genes preferentially expressed within the citrus fruit abscission zone (AZ-C), we performed a comparative transcriptomics assay at the cell type resolution level between the AZ-C and adjacent fruit rind cells (non-abscising tissue) during ethylene-promoted abscission. Our strategy combined laser microdissection with microarray analysis. Cell wall modification-related gene families displayed prominent representation in the AZ-C. Phylogenetic analyses of such gene families revealed a link between phylogenetic proximity and expression pattern during abscission suggesting highly conserved roles for specific members of these families in abscission. Our transcriptomic data was validated with (and strongly supported by) a parallel approach consisting on anatomical, histochemical and biochemical analyses on the AZ-C during fruit abscission. Our work identifies genes potentially involved in organ abscission and provides relevant data for future biotechnology approaches aimed at controlling such crucial process for citrus yield.

Transcriptome profiling of differentially expressed genes in floral buds and flowers of male sterile and fertile lines in watermelon

Background

Male sterility is an important mechanism for the production of hybrid seeds in watermelon. Although fruit development has been studied extensively in watermelon, there are no reports on gene expression in floral organs. In this study, RNA-sequencing (RNA-seq) was performed in two near-isogenic watermelon lines (genic male sterile [GMS] line, DAH3615-MS and male fertile line, DAH3615) to identify the differentially expressed genes (DEGs) related to male sterility.

Results

DEG analysis showed that 1259 genes were significantly associated with male sterility at a FDR P-value of < 0.01. Most of these genes were only expressed in the male fertile line. In addition, 11 functional clusters were identified using DAVID functional classification analysis. Of detected genes in RNA-seq analysis, 19 were successfully validated by qRT-PCR.

Conclusions

In this study, we carried out a comprehensive floral transcriptome sequence comparison of a male fertile line and its near-isogenic male sterile line in watermelon. This analysis revealed essential genes responsible for stamen development, including pollen development and pollen tube elongation, and allowed their functional classification. These results provided new information on global mechanisms related to male sterility in watermelon.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2186-9) contains supplementary material, which is available to authorized users.

How to let go: pectin and plant cell adhesion

Plant cells do not, in general, migrate. They maintain a fixed position relative to their neighbors, intimately linked through growth and differentiation. The mediator of this connection, the pectin-rich middle lamella, is deposited during cell division and maintained throughout the cell's life to protect tissue integrity. The maintenance of adhesion requires cell wall modification and is dependent on the actin cytoskeleton. There are developmental processes that require cell separation, such as organ abscission, dehiscence, and ripening. In these instances, the pectin-rich middle lamella must be actively altered to allow cell separation, a process which also requires cell wall modification. In this review, we will focus on the role of pectin and its modification in cell adhesion and separation. Recent insights gained in pectin gel mechanics will be discussed in relation to existing knowledge of pectin chemistry as it relates to cell adhesion. As a whole, we hope to begin defining the physical mechanisms behind a cells' ability to hang on, and how it lets go.

BcMF26a and BcMF26b Are Duplicated Polygalacturonase Genes with Divergent Expression Patterns and Functions in Pollen Development and Pollen Tube Formation in Brassica campestris

Polygalacturonase (PG) is one of the cell wall hydrolytic enzymes involving in pectin degradation. A comparison of two highly conserved duplicated PG genes, namely, Brassica campestris Male Fertility 26a (BcMF26a) and BcMF26b, revealed the different features of their expression patterns and functions. We found that these two genes were orthologous genes of At4g33440, and they originated from a chromosomal segmental duplication. Although structurally similar, their regulatory and intron sequences largely diverged. QRT-PCR analysis showed that the expression level of BcMF26b was higher than that of BcMF26a in almost all the tested organs and tissues in Brassica campestris. Promoter activity analysis showed that, at reproductive development stages, BcMF26b promoter was active in tapetum, pollen grains, and pistils, whereas BcMF26a promoter was only active in pistils. In the subcellular localization experiment, BcMF26a and BcMF26b proteins could be localized to the cell wall. When the two genes were co-inhibited, pollen intine was formed abnormally and pollen tubes could not grow or stretch. Moreover, the knockout mutants of At4g33440 delayed the growth of pollen tubes. Therefore, BcMF26a/b can participate in the construction of pollen wall by modulating intine information and BcMF26b may play a major role in co-inhibiting transformed plants.

Identification and expression analysis of BoMF25, a novel polygalacturonase gene involved in pollen development of Brassica oleracea

BoMF25 acts on pollen wall. Polygalacturonase (PG) is a pectin-digesting enzyme involved in numerous plant developmental processes and is described to be of critical importance for pollen wall development. In the present study, a PG gene, BoMF25, was isolated from Brassica oleracea. BoMF25 is the homologous gene of At4g35670, a PG gene in Arabidopsis thaliana with a high expression level at the tricellular pollen stage. Collinear analysis revealed that the orthologous gene of BoMF25 in Brassica campestris (syn. B. rapa) genome was probably lost because of genome deletion and reshuffling. Sequence analysis indicated that BoMF25 contained four classical conserved domains (I, II, III, and IV) of PG protein. Homology and phylogenetic analyses showed that BoMF25 was clustered in Clade F. The putative promoter sequence, containing classical cis-acting elements and pollen-specific motifs, could drive green fluorescence protein expression in onion epidermal cells. Quantitative RT-PCR analysis suggested that BoMF25 was mainly expressed in the anther at the late stage of pollen development. In situ hybridization analysis also indicated that the strong and specific expression signal of BoMF25 existed in pollen grains at the mature pollen stage. Subcellular localization showed that the fluorescence signal was observed in the cell wall of onion epidermal cells, which suggested that BoMF25 may be a secreted protein localized in the pollen wall.

Ultrastructural localization of polygalacturonase in ethylene-stimulated abscission of tomato pedicel explants

Polygalacturonase (PG) is crucial in plant organ abscission process. This paper investigated the cellular and subcellular localization of PG in ethylene-stimulated abscission of tomato pedicel explants. Confocal laser scanning microscopy of abscission zone sections with the fluorescent probe Cy3 revealed that PG was initially accumulated in parenchyma cells in cortical and vascular tissues after 8 h of ethylene treatment and then extended throughout the abscission zone when the abscission zone separated at 24 h after ethylene treatment. At the subcellular level, transmission electron microscopy with immunogold staining showed that PG showed abundant accumulation in the cortical and vascular tissues at 8 h after ethylene treatment, and the distribution area extended to the central parenchyma cells at 16 h after ethylene treatment. In addition, PGs were observed in the distal and proximal parts of the tomato pedicel explants throughout the abscission process. The results provided a visualized distribution of PG in the pedicel abscission zone and proved that PG was closely related to abscission.

A comprehensive transcriptome analysis of silique development and dehiscence in Arabidopsis and Brassica integrating genotypic, interspecies and developmental comparisons

Asynchronous flowering of Brassica napus (canola) leads to seeds and siliques at varying stages of maturity as harvest approaches. This range of maturation can result in premature silique dehiscence (pod shattering), resulting in yield losses, which may be worsened by environmental stresses. Therefore, a goal for canola crop improvement is to reduce shattering in order to maximize yield. We performed a comprehensive transcriptome analysis on the dehiscence zone (DZ) and valve of Arabidopsis and Brassica siliques in shatter resistant and sensitive genotypes at several developmental stages. Among known Arabidopsis dehiscence genes, we confirmed that homologs of SHP1/2, FUL, ADPG1, NST1/3 and IND were associated with shattering in B. juncea and B. napus. We noted a correlation between reduced pectin degradation genes and shatter-resistance. Tension between lignified and non-lignified cells in the silique DZ plays a major role in dehiscence. Light microscopy revealed a smaller non-lignified separation layer in relatively shatter-resistant B. juncea relative to B. napus and this corresponded to increased expression of peroxidases involved in monolignol polymerization. Sustained repression of auxin biosynthesis, transport and signaling in B. juncea relative to B. napus may cause differences in dehiscence zone structure and cell wall constituents. Tension on the dehiscence zone is a consequence of shrinkage and loss of flexibility in the valves, which is caused by senescence and desiccation. Reduced shattering was generally associated with upregulation of ABA signaling and down-regulation of ethylene and jasmonate signaling, corresponding to more pronounced stress responses and reduced senescence and photosynthesis. Overall, we identified 124 cell wall related genes and 103 transcription factors potentially involved in silique dehiscence.

Letting go is never easy: abscission and receptor-like protein kinases

Abscission is the process by which plants discard organs in response to environmental cues/stressors, or as part of their normal development. Abscission has been studied throughout the history of the plant sciences and in numerous species. Although long studied at the anatomical and physiological levels, abscission has only been elucidated at the molecular and genetic levels within the last two decades, primarily with the use of the model plant Arabidopsis thaliana. This has led to the discovery of numerous genes involved at all steps of abscission, including key pathways involving receptor-like protein kinases (RLKs). This review covers the current knowledge of abscission research, highlighting the role of RLKs. [Figure: see text] John C. Walker (Corresponding author).

IDA: a peptide ligand regulating cell separation processes in Arabidopsis

In contrast to animals, plants continuously produce new organs, such as leaves, flowers, and lateral roots (LRs), and may shed organs that have served their purpose. In the model plant Arabidopsis thaliana the peptide INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) signals through the leucine-rich repeat-receptor-like kinases (LRR-RLKs) HAESA (HAE), and HAESA-LIKE2 (HSL2) to control the abscission of floral organs after pollination. Recent work from other plant species indicates that this signalling system is conserved and could regulate leaf abscission in soybean and tomato. Abscission is a cell separation process involving the breakdown of cell walls between adjacent files of abscission zone (AZ) cells at the base of organs to be shed. The emergence of new lateral root primordia (LRP), initiated deep inside the root under the influence of the phytohormone auxin, is similarly dependent on cell wall dissolution to separate cells in the overlying tissues. It has been shown that this process also requires IDA, HAE, and HSL2. The receptors are redundant in function during floral organ abscission, but during lateral root emergence (LRE) they are differentially involved in regulating cell wall remodelling (CWR) genes. An overview is given here of the similarities and differences of IDA signalling during floral organ abscission and LRE.

Temporal and spatial expression of polygalacturonase gene family members reveals divergent regulation during fleshy fruit ripening and abscission in the monocot species oil palm

BACKGROUND

Cell separation that occurs during fleshy fruit abscission and dry fruit dehiscence facilitates seed dispersal, the final stage of plant reproductive development. While our understanding of the evolutionary context of cell separation is limited mainly to the eudicot model systems tomato and Arabidopsis, less is known about the mechanisms underlying fruit abscission in crop species, monocots in particular. The polygalacturonase (PG) multigene family encodes enzymes involved in the depolymerisation of pectin homogalacturonan within the primary cell wall and middle lamella. PG activity is commonly found in the separation layers during organ abscission and dehiscence, however, little is known about how this gene family has diverged since the separation of monocot and eudicots and the consequence of this divergence on the abscission process.

RESULTS

The objective of the current study was to identify PGs responsible for the high activity previously observed in the abscission zone (AZ) during fruit shedding of the tropical monocot oil palm, and to analyze PG gene expression during oil palm fruit ripening and abscission. We identified 14 transcripts that encode PGs, all of which are expressed in the base of the oil palm fruit. The accumulation of five PG transcripts increase, four decrease and five do not change during ethylene treatments that induce cell separation. One PG transcript (EgPG4) is the most highly induced in the fruit base, with a 700-5000 fold increase during the ethylene treatment. In situ hybridization experiments indicate that the EgPG4 transcript increases preferentially in the AZ cell layers in the base of the fruit in response to ethylene prior to cell separation.

CONCLUSIONS

The expression pattern of EgPG4 is consistent with the temporal and spatial requirements for cell separation to occur during oil palm fruit shedding. The sequence diversity of PGs and the complexity of their expression in the oil palm fruit tissues contrast with data from tomato, suggesting functional divergence underlying the ripening and abscission processes has occurred between these two fruit species. Furthermore, phylogenetic analysis of EgPG4 with PGs from other species suggests some conservation, but also diversification has occurred between monocots and eudicots, in particular between dry and fleshy fruit species.

Preventing unwanted breakups: using polygalacturonases to regulate cell separation

Cell separation is an important biological process in plants that is precisely regulated both spatially and temporally. Key separation events include abscission of organs such as leaves and fruit and dehiscence events such as pod shatter in canola and other Brassicas. Polygalacturonases (PGs) are enzymes essential for the degradation of pectin, an important component of the adhesive material between cells. Although there are around 70 PG genes with overlapping expression domains, recent analysis has revealed the function of several PGs in specific aspects of Arabidopsis reproductive development. Upstream regulators that control the expression domain of some of these PGs during reproductive development have also been identified. This information provides new strategies to control unwanted cell separation events in various crops.

Major changes in the cell wall during silique development in Arabidopsis thaliana

Fruit development is a highly complex process, which involves major changes in plant metabolism leading to cell growth and differentiation. Changes in cell wall composition and structure play a major role in modulating cell growth. We investigated the changes in cell wall composition and the activities of associated enzymes during the dry fruit development of the model plant Arabidopsis thaliana. Silique development is characterized by several specific phases leading to fruit dehiscence and seed dispersal. We showed that early phases of silique growth were characterized by specific changes in non-cellulosic sugar content (rhamnose, arabinose, xylose, galactose and galacturonic acid). Xyloglucan oligosaccharide mass profiling further showed a strong increase in O-acetylated xyloglucans over the course of silique development, which could suggest a decreased capacity of xyloglucans to be associated with each other or to cellulose. The degree of methylesterification, mediated by the activity of pectin methylesterases (PMEs), decreased over the course of silique growth and dehiscence. The major changes in cell wall composition revealed by our analysis suggest that it could be major determinants in modulating cell wall rheology leading to growth or growth arrest.

Transcriptome analysis of various flower and silique development stages indicates a set of class III peroxidase genes potentially involved in pod shattering in Arabidopsis thaliana

BACKGROUND

Plant class III peroxidases exist as a large multigenic family involved in numerous functions suggesting a functional specialization of each gene. However, few genes have been linked with a specific function. Consequently total peroxidase activity is still used in numerous studies although its relevance is questionable. Transcriptome analysis seems to be a promising tool to overcome the difficulties associated with the study of this family. Nevertheless available microarrays are not completely reliable for this purpose. We therefore used a macroarray dedicated to the 73 class III peroxidase genes of A. thaliana to identify genes potentially involved in flower and fruit development.

RESULTS

The observed increase of total peroxidase activity during development was actually correlated with the induction of only a few class III peroxidase genes which supports the existence of a functional specialization of these proteins. We identified peroxidase genes that are predominantly expressed in one development stage and are probable components of the complex gene networks involved in the reproductive phase. An attempt has been made to gain insight into plausible functions of these genes by collecting and analyzing the expression data of different studies in plants. Peroxidase activity was additionally observed in situ in the silique dehiscence zone known to be involved in pod shattering. Because treatment with a peroxidase inhibitor delayed pod shattering, we subsequently studied mutants of transcription factors (TF) controlling this mechanism. Three peroxidases genes -AtPrx13, AtPrx30 and AtPrx55- were altered by the TFs involved in pod shatter.

CONCLUSIONS

Our data illustrated the problems caused by linking only an increase in total peroxidase activity to any specific development stage or function. The activity or involvement of specific class III peroxidase genes needs to be assessed. Several genes identified in our study had not been linked to any particular development stage or function until now. Notably AtPrx13, which is one of the peroxidase genes not present on commercially available microarrays. A systematic survey of class III peroxidase genes expression is necessary to reveal specific class III peroxidase gene functions and the regulation and evolution of this key multifunctional enzyme family. The approach used in this study highlights key individual genes that merit further investigation.

ps-2, the gene responsible for functional sterility in tomato, due to non-dehiscent anthers, is the result of a mutation in a novel polygalacturonase gene

The recessive mutation ps-2, which appeared spontaneously in tomato, confers functional male sterility due to non-dehiscent anthers. In this study, we isolated and characterized the PS-2 gene. A single nucleotide mutation in a novel tomato polygalacturonase gene is responsible for the ps-2 phenotype. The mutation in ps-2 is responsible for an alternative splicing during maturation of the pre-mRNA, which leads to an aberrant mRNA. Differentiation between ps-2 and wild type (PS-2) anthers only appears in the final developmental stage in which the stomium remains closed in the mutant. To our knowledge, this is the first functional sterility gene isolated in the Solanaceae family. The specific expression of the Arabidopsis homolog of PS-2 in the anther dehiscence zone suggests a conserved mode of action over the plant kingdom, which means that the repression of PS-2 homologs may be a potential way to introduce functional sterility in other species.

Specific functions of individual class III peroxidase genes

In higher plants, class III peroxidases exist as large multigene families (e.g. 73 genes in Arabidopsis thaliana). The diversity of processes catalysed by peroxidases as well as the large number of their genes suggests the possibility of a functional specialization of each isoform. In addition, the fact that peroxidase promoter sequences are very divergent and that protein sequences contain both highly conserved domains and variable regions supports this hypothesis. However, two difficulties are associated with the study of the function of specific peroxidase genes: (i) the modification of the expression of a single peroxidase gene often results in no visible mutant phenotype, because it is compensated by redundant genes; and (ii) peroxidases show low substrate specificity in vitro resulting in an unreliable indication of peroxidase specific activity unless complementary data are available. The generalization of molecular biology approaches such as whole transcriptome analysis and recombinant DNA combined with biochemical approaches provide unprecedented tools for overcoming these difficulties. This review highlights progress made with these new techniques for identifying the specific function of individual class III peroxidase genes taking as an example the model plant A. thaliana, as well as discussing some other plants.

ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1), ADPG2, and QUARTET2 are Polygalacturonases required for cell separation during reproductive development in Arabidopsis

Cell separation is thought to involve degradation of pectin by several hydrolytic enzymes, particularly polygalacturonase (PG). Here, we characterize an activation tagging line with reduced growth and male sterility caused by increased expression of a PG encoded by QUARTET2 (QRT2). QRT2 is essential for pollen grain separation and is part of a small family of three closely related endo-PGs in the Arabidopsis thaliana proteome, including ARABIDOPSIS DEHISCENCE ZONE POLYGALACTURONASE1 (ADPG1) and ADPG2. Functional assays and complementation experiments confirm that ADPG1, ADPG2, and QRT2 are PGs. Genetic analysis demonstrates that ADPG1 and ADPG2 are essential for silique dehiscence. In addition, ADPG2 and QRT2 contribute to floral organ abscission, while all three genes contribute to anther dehiscence. Expression analysis is consistent with the observed mutant phenotypes. INDEHISCENT (IND) encodes a putative basic helix-loop-helix required for silique dehiscence, and we demonstrate that the closely related HECATE3 (HEC3) gene is required for normal seed abscission and show that IND and HEC3 are required for normal expression of ADPG1 in the silique dehiscence zone and seed abscission zone, respectively. We also show that jasmonic acid and ethylene act together with abscisic acid to regulate floral organ abscission, in part by promoting QRT2 expression. These results demonstrate that multiple cell separation events, including both abscission and dehiscence, require closely related PG genes.

Polygalacturonase-inhibiting protein is a structural component of plant cell wall

It is generally believed that plants "evolved a strategy of defending themselves from a phytopathogen attack" during evolution. This metaphor is used frequently, but it does not facilitate understanding of the mechanisms providing plant resistance to the invasion of foreign organisms and to other unfavorable external factors, as well as the role of these mechanisms in plant growth and development. Information on processes involving one of the plant resistance factors--polygalacturonase-inhibiting protein (PGIP)--is considered in this review. The data presented here indicate that PGIP, being an extracellular leucine-rich repeat-containing protein, performs important functions in the structure of plant cell wall. Amino acid residues participating in PGIP binding to homogalacturonan in the cell wall have been determined. The degree of methylation and the mode of distribution of homogalacturonan methyl groups are responsible for the formation of a complex structure, which perhaps determines the specificity of PGIP binding to pectin. PGIP is apparently one of the components of plant cell wall determining some of its mechanical properties; it is involved in biochemical processes related to growth, expansion, and maceration, and it influences plant morphology. Polygalacturonase (PG) is present within practically all plant tissues, but the manifestation of its activity varies significantly depending on physiological conditions in the tissue. Apparently, the regulation of PG functioning in apoplast significantly affects the development of processes associated with the modification of the structure of plant cell wall. PGIP can regulate PG activity through binding to homogalacturonan. The genetically determined structure of PGIP in plants determines the mode of its interaction with an invader and perhaps is one of the factors responsible for the set of pathogens causing diseases in a given plant species.

Stamen abscission zone transcriptome profiling reveals new candidates for abscission control: enhanced retention of floral organs in transgenic plants overexpressing Arabidopsis ZINC FINGER PROTEIN2

Organ detachment requires cell separation within abscission zones (AZs). Physiological studies have established that ethylene and auxin contribute to cell separation control. Genetic analyses of abscission mutants have defined ethylene-independent detachment regulators. Functional genomic strategies leading to global understandings of abscission have awaited methods for isolating AZ cells of low abundance and very small size. Here, we couple laser capture microdissection of Arabidopsis thaliana stamen AZs and GeneChip profiling to reveal the AZ transcriptome responding to a developmental shedding cue. Analyses focus on 551 AZ genes (AZ(551)) regulated at the highest statistical significance (P < or = 0.0001) over five floral stages linking prepollination to stamen shed. AZ(551) includes mediators of ethylene and auxin signaling as well as receptor-like kinases and extracellular ligands thought to act independent of ethylene. We hypothesized that novel abscission regulators might reside in disproportionately represented Gene Ontology Consortium functional categories for cell wall modifying proteins, extracellular regulators, and nuclear-residing transcription factors. Promoter-beta-glucuronidase expression of one transcription factor candidate, ZINC FINGER PROTEIN2 (AtZFP2), was elevated in stamen, petal, and sepal AZs. Flower parts of transgenic lines overexpressing AtZFP2 exhibited asynchronous and delayed abscission. Abscission defects were accompanied by altered floral morphology limiting pollination and fertility. Hand-pollination restored transgenic fruit development but not the rapid abscission seen in wild-type plants, demonstrating that pollination does not assure normal rates of detachment. In wild-type stamen AZs, AtZFP2 is significantly up-regulated postanthesis. Phenotype data from transgene overexpression studies suggest that AtZFP2 participates in processes that directly or indirectly influence organ shed.

Physiological roles of plant glycoside hydrolases

The functions of plant glycoside hydrolases and transglycosidases have been studied using different biochemical and molecular genetic approaches. These enzymes are involved in the metabolism of various carbohydrates containing compounds present in the plant tissues. The structural and functional diversity of the carbohydrates implies a vast spectrum of enzymes involved in their metabolism. Complete genome sequence of Arabidopsis and rice has allowed the classification of glycoside hydrolases in different families based on amino acid sequence data. The genomes of these plants contain 29 families of glycoside hydrolases. This review summarizes the current research on plant glycoside hydrolases concerning their principal functional roles, which were attributed to different families. The majority of these plant glycoside hydrolases are involved in cell wall polysaccharide metabolism. Other functions include their participation in the biosynthesis and remodulation of glycans, mobilization of energy, defence, symbiosis, signalling, secondary plant metabolism and metabolism of glycolipids.

Expression of polygalacturonases and evidence to support their role during cell separation processes in Arabidopsis thaliana

Polygalacturonases (PGs) have been proposed to play an important role in the process of cell separation. The Arabidopsis thaliana genome contains 69 annotated genes that by amino acid homology and transcript organization could be classified as putative PGs and these can be grouped into multiple clades. An analysis of five members located in two separate clades, using reporter fusion constructs and reverse transcription-PCR, revealed that whilst these PGs exhibit high sequence similarity they have distinct patterns of spatial and temporal expression. Sites of expression include the aleurone and endosperm cells surrounding the emerging radicle in a germinating seed, the cortical cells adjacent to the developing lateral root, the abscission zones of floral organs, the dehiscence zone of anthers and siliques, and pollen grains. Silencing of an abscission-related PG (At2g41850), using a T-DNA insertion strategy, delayed the time-course of floral organ loss but did not prevent shedding from taking place. These observations are discussed with regard to the contribution that PGs may play during the life cycle of a plant.

Detection and identification of rhamnogalacturonan lyase activity in intercellular spaces of expanding cotton cotyledons

Rhamnogalacturonan lyase (RG lyase) activity has been detected and its relative activity measured in vivo during the expansion of cotton (Gossypium hirsutum L.) cotyledons. Rhamnogalacturonan (RG) oligomers labeled with a fluorescent tag were injected into the intercellular spaces of cotton cotyledons and, after incubation, the digested substrate was rinsed out. Enzyme digestion products were detected and identified by capillary zone electrophoresis. Rhamnogalacturonan lyase products were identified as such by co-migration with the digestion products of linear RG oligomers when the oligomers were treated with fungal RG lyase but not when treated with fungal RG hydrolase. In addition, reaction of plant RG lyase digestion products of RG oligomers with I(2)/KI, which selectively removes unsaturated galactopyranosyluronic acid (GaLap) residues formed at the non-reducing end of the oligomer, converted the plant digestion products into RG oligomers that co-migrated with fungal RG hydrolase products. The activity of the enzyme in the intercellular spaces of cotton cotyledons is very low and could be detected most easily when not >0.03 nmol of substrate was injected in a approximately 0.7-cm(2) area and incubated in vivo for 2-6 h. Rhamnogalacturonan lyase activity was the highest in rapidly expanding 3- to 4-day-old cotyledons and gradually decreased during the slow-down in expansion over the next 2-3 days. The RG lyase activity was also detected when the APTS (8-aminopyrene-1,3,6-trisulfonic acid, trisodium salt)-labeled substrates were introduced into intercellular spaces by infiltration instead of injection, indicating that the activity was not induced by wounding or released into the apoplast by cell damage. An exo-RG galacturonohydrolase activity was also found, but RG hydrolase and exo-RG rhamnohydrolase were not detected.

Patterns of expansion and expression divergence in the plant polygalacturonase gene family

Analysis of Arabidopsis and rice polygalacturonases suggests that polygalacturonases duplicates underwent rapid expression divergence and that the mechanisms of duplication affect the divergence rate.

Background

Polygalacturonases (PGs) belong to a large gene family in plants and are believed to be responsible for various cell separation processes. PG activities have been shown to be associated with a wide range of plant developmental programs such as seed germination, organ abscission, pod and anther dehiscence, pollen grain maturation, fruit softening and decay, xylem cell formation, and pollen tube growth, thus illustrating divergent roles for members of this gene family. A close look at phylogenetic relationships among Arabidopsis and rice PGs accompanied by analysis of expression data provides an opportunity to address key questions on the evolution and functions of duplicate genes.

Results

We found that both tandem and whole-genome duplications contribute significantly to the expansion of this gene family but are associated with substantial gene losses. In addition, there are at least 21 PGs in the common ancestor of Arabidopsis and rice. We have also determined the relationships between Arabidopsis and rice PGs and their expression patterns in Arabidopsis to provide insights into the functional divergence between members of this gene family. By evaluating expression in five Arabidopsis tissues and during five stages of abscission, we found overlapping but distinct expression patterns for most of the different PGs.

Conclusion

Expression data suggest specialized roles or subfunctionalization for each PG gene member. PGs derived from whole genome duplication tend to have more similar expression patterns than those derived from tandem duplications. Our findings suggest that PG duplicates underwent rapid expression divergence and that the mechanisms of duplication affect the divergence rate.

Expression divergence and functional redundancy of polygalacturonases in floral organ abscission

The importance of cell separation in plant development cannot be overemphasized. The polygalacturonases (PGs) are the one of cell wall hydrolytic enzyme families that has been associated with various cell separation processes in plant development including seed germination, dehiscence, organ abscission, and fruit ripening. Both Arabidopsis and rice PG gene family have expanded in a lineage-specific fashion after the split more than 150 million years ago. Tandem duplications and large-scale duplications are the major contributors to the current PG family size in Arabidopsis. The spatial expression analysis of the 66 Arabidopsis PG family members have led us to conclude that different duplication mechanisms affect the expression divergence differently. This becomes more apparent when temporal examination of expression is conducted in five developmental stages of floral organ abscission in Arabidopsis. Nine distinct patterns of PGs are identified during floral organ abscission in Arabidopsis. Four PGs are specifically upregulated during abscission associated with cell separation process. Careful understanding of relationships among Arabidopsis PGs in a context of evolution together with expression analysis of these PGs will facilitate the functional study of PGs specifically in floral organ abscission in Arabidopsis.

Ethylene-dependent and -independent pathways controlling floral abscission are revealed to converge using promoter::reporter gene constructs in the ida abscission mutant

The process of floral organ abscission in Arabidopsis thaliana can be modulated by ethylene and involves numerous genes contributing to cell separation. One gene that is absolutely required for abscission is INFLORESCENCE DEFICIENT IN ABSCISSION, IDA, as the ida mutant is completely blocked in abscission. To elucidate the genetic pathways regulating floral abscission, molecular markers expressed in the floral abscission zone have been studied in an ida mutant background. Using plants with promoter-reporter gene constructs including promoters of a novel FLORAL ABSCISSION ASSOCIATED gene (FAA) encoding a putative single-stranded binding protein (BASIL), chitinase (CHIT::GUS) and cellulase (BAC::GUS), it is shown that IDA acts in the last steps of the abscission process. These markers, as well as HAESA, encoding a receptor-like kinase, were unaffected in their temporal expression patterns in ida compared with wild-type plants; thus showing that different regulatory pathways are active in the abscission process. In contrast to BASIL, CHIT::GUS and BAC::GUS showed, however, much weaker induction of expression in an ida background, consistent with a reduction in pathogen-associated responses and a lack of total dissolution of cell walls in the mutant. IDA, encoding a putative secreted peptide ligand, and HAESA appeared to have identical patterns of expression in floral abscission zones. Lastly, to address the role of ethylene, IDA::GUS expression in the wild type and the ethylene-insensitive mutant etr1-1 was compared. Similar temporal patterns, yet restricted spatial expression patterns were observed in etr1-1, suggesting that the pathways regulated by IDA and by ethylene act in parallel, but are, to some degree, interdependent.

Plant glycoside hydrolases involved in cell wall polysaccharide degradation

The cell wall plays a key role in controlling the size and shape of the plant cell during plant development and in the interactions of the plant with its environment. The cell wall structure is complex and contains various components such as polysaccharides, lignin and proteins whose composition and concentration change during plant development and growth. Many studies have revealed changes in cell walls which occur during cell division, expansion, and differentiation and in response to environmental stresses; i.e. pathogens or mechanical stress. Although many proteins and enzymes are necessary for the control of cell wall organization, little information is available concerning them. An important advance was made recently concerning cell wall organization as plant enzymes that belong to the superfamily of glycoside hydrolases and transglycosidases were identified and characterized; these enzymes are involved in the degradation of cell wall polysaccharides. Glycoside hydrolases have been characterized using molecular, genetic and biochemical approaches. Many genes encoding these enzymes have been identified and functional analysis of some of them has been performed. This review summarizes our current knowledge about plant glycoside hydrolases that participate in the degradation and reorganisation of cell wall polysaccharides in plants focussing particularly on those from Arabidopsis thaliana.

Comprehensive approach to genes involved in cell wall modifications in Arabidopsis thaliana

The plant cell wall is of supermolecular architecture, and is composed of various types of heterogeneous polymers. A few thousand enzymes and structural proteins are directly involved in the construction processes, and in the functional aspects of the dynamic architecture in Arabidopsis thaliana. Most of these proteins are encoded by multigene families, and most members within each family share significant similarities in structural features, but often exhibit differing expression profiles and physiological functions. Thus, for the molecular dissection of cell wall dynamics, it is necessary to distinguish individual members within a family of proteins. As a first step towards characterizing the processes involved in cell wall dynamics, we have manufactured a gene-specific 70-mer oligo microarray that consists of 765 genes classified into 30 putative families of proteins that are implicated in the cell wall dynamics of Arabidopsis. By using this array system, we identified several sets of genes that exhibit organ preferential expression profiles. We also identified gene sets that are expressed differentially at certain specific growth stages of the Arabidopsis inflorescence stem. Our results indicate that there is a division of roles among family members within each of the putative cell wall-related gene families.

The final step of the ethylene biosynthesis pathway in turnip tops (Brassica rapa): molecular characterization of the 1-aminocyclopropane-1-carboxylate oxidase BrACO1 throughout zygotic embryogenesis and germination of heterogeneous seeds

In a previous report from the present authors, it was shown that the 1-aminocyclopropane-1-carboxylate (ACC) oxidation may play a crucial role during zygotic embryogenesis of turnip tops seeds. The present study was performed to elucidate the contribution of the silique-wall and seeds in ethylene production during this developmental process. ACC content in the silique wall is only higher than in seeds during the middle phases of zygotic embryogenesis. The ACC-oxidase (ACO) activity peaks in the silique-wall and seeds during the onset of embryogenesis, declining gradually afterwards, being undetectable during desiccation period. Using reverse transcriptase-polymerase chain reaction, one cDNA clone coding for an ACO and called BrACO1, was isolated. The deduced protein for BrACO1 has a molecular weight of 36.8 kDa and a high homology with other crucifer ACOs. The heterologous expression of this cDNA confirmed that BrACO1 is an ACO. The expression of this gene was high during the first phases of silique-wall development, low during the middle phases and undetectable during desiccation. By contrast, BrACO1 transcript was accumulated only in the earliest phases of seed embryogenesis and may participate in the highest ACO activity and ethylene production by seeds at the beginning of embryogenesis. Finally, in this work a correlation between the heterogeneity of Brassica rapa L. cv. Rapa seeds and the ability to oxidize the ACC to ethylene has been demonstrated.

Primary cell wall metabolism: tracking the careers of wall polymers in living plant cells

Numerous examples have been presented of enzyme activities, assayed in vitro, that appear relevant to the synthesis of structural polysaccharides, and to their assembly and subsequent degradation in the primary cell walls (PCWs) of higher plants. The accumulation of the corresponding mRNAs, and of the (immunologically recognized) proteins, has often also (or instead) been reported. However, the presence of these mRNAs, antigens and enzymic activities has rarely been shown to correspond to enzyme action in the living plant cell. In some cases, apparent enzymic action is observed in vivo for which no enzyme activity can be detected in in-vitro assays; the converse also occurs. Methods are reviewed by which reactions involving structural wall polysaccharides can be tracked in vivo. Special attention is given to xyloglucan endotransglucosylase (XET), one of the two enzymic activities exhibited in vitro by xyloglucan endotransglucosylase/hydrolase (XTH) proteins, because of its probable importance in the construction and restructuring of the PCW's major hemicellulose. Attention is also given to the possibility that some reactions observed in the PCW in vivo are not directly enzymic, possibly involving the action of hydroxyl radicals. It is concluded that some proposed wall enzymes, for example XTHs, do act in vivo, but that for other enzymes this is not proven. Contents I. Primary cell walls: composition, deposition and roles 642 II. Reactions that have been proposed to occur in primary cell walls 645 III. Tracking the careers of wall components in vivo: evidence for action of enzymes in the walls of living plant cells 656 IV. Evidence for the occurrence of nonenzymic polymer scission in vivo? 666 VI. Conclusion 667 References 667.

Cloning and analysis of a cDNA encoding an endo-polygalacturonase expressed during the desiccation period of the silique-valves of turnip-tops (Brassica rapa L. cv. Rapa)

During zygotic embryogenesis of turnip-tops (Brassica rapa L. cv. Rapa), the polygalacturonase activity (PG; EC 3.2.1.15), measured as a decrease in viscosity of polygalacturonic acid, reached a high when the desiccation process in the seeded silique was triggered and the valves had lost more than 70-75% of their moisture (45-50 DPA). The PG activity was not detected in any phases of developing seeds. This work also characterizes a cDNA with an open reading frame of 1303 bp and that codes for a putative PG called BrPG1. This falls into the category of clade-B, which includes PG related to shattering and abscission processes. The deduced BrPG1 sequence predicted a 434-residue-long precursor protein (46.7kDa) with a transit peptide sequence 23 amino acids long. A molecular mass of 44.3 kDa was calculated for the mature form of BrPG1, which showed high sequence similarity to PGA1 (97%) of B. napus (X98373) and ADPG1 (87%) of Arabidopsis thaliana (AJ002532). All conserved amino acids at the catalytic site of PGs belonging to clade-B were preserved on BrPG1. This BrPG1 gene was specifically expressed in the silique valves of turnip-tops and was temporally expressed at the beginning of its desiccation.

Ethylene-dependent and -independent processes associated with floral organ abscission in Arabidopsis

Abscission is an important developmental process in the life cycle of the plant, regulating the detachment of organs from the main body of the plant. This mechanism can be initiated in response to environmental cues such as disease or pathogen, or it can be a programmed shedding of organs that no longer provide essential functions to the plant. We have identified five novel dab (delayed floral organ abscission) mutants (dab1-1, dab2-1, dab3-1, dab3-2, and dab3-3) in Arabidopsis. These mutants each display unique anatomical and physiological characteristics and are governed by three independent loci. Scanning electron microscopy shows delayed development of the flattened fracture plane in some mutants and irregular elongation in the cells of the fracture plane in other mutants. The anatomical observations are also supported by breakstrength measurements that show high breakstrength associated with broken cells, moderate levels for the flattened fracture plane, and low levels associated with the initial rounding of cells. In addition, observations on the expression patterns in the abscission zone of cell wall hydrolytic enzymes, chitinase and cellulose, show altered patterns in the mutants. Last, we have compared these mutants with the ethylene-insensitive mutants etr1-1 and ein2-1 to determine if ethylene is an essential component of the abscission process and find that although ethylene can accelerate abscission under many conditions, the perception of ethylene is not essential. The role of the dab genes and the ethylene response genes during the abscission process is discussed.

Two Brassica napus polygalacturonase inhibitory protein genes are expressed at different levels in response to biotic and abiotic stresses

Plants encode a distinct set of polygalacturonase inhibitory proteins (PGIPs) that function to inhibit polygalacturonase enzymes produced by soft-rot fungal pathogens. We characterized two PGIP-encoding genes ( Bnpgip1 and Bnpgip2) from Brassica napus DH12075 (a double-haploid line derived from a cross between 'Crésor' and 'Westar'). The two proteins exhibit 67.4% identity at the amino acid level and contain 10 imperfect leucine-rich repeats. The pgip genes are present as a small multigene family in B. napus with at least four members. Bnpgip1 and Bnpgip2 are constitutively expressed in roots, stems, flower buds and open flowers. In mature leaf tissue, different levels of induction were observed in response to biotic and abiotic stresses. Bnpgip1 expression was highly responsive to flea beetle feeding and mechanical wounding, weakly responsive to Sclerotinia sclerotiorum infection and exposure to cold but not to dehydration. Conversely, Bnpgip2 expression was strongly induced by S. sclerotiorum infection and to a lesser degree by wounding but not by flea beetle feeding. Application of jasmonic acid to leaves induced both Bnpgip1 and Bnpgip2 gene expression; however, salicylic acid did not activate either gene. Taken together, these results suggest that separate pathways regulate Bnpgip1 and Bnpgip2, and that their roles in plant development or resistance to biotic and abiotic stress differ.

Dehydration and dehiscence in siliques of Brassica napus and Brassica rapa

Silique dehiscence (shattering) in Brassica species has a pronounced effect on agricultural yields. Shattering is highly variable and difficult to quantify, and consequently there have been few studies that explore interspecies variation in shattering in relation to silique development. In this paper, a rapid and simple method has been developed for quantifying silique dehiscence. The variable-speed pod splitter is a mechanical device that provides a measure of the impact force required to trigger shattering of individual siliques. We have used the variable-speed pod splitter to show that siliques of Brassica rapa cv. Parkland were significantly more resistant to shattering than those of Brassica napus cv. Quantum. Siliques of both species became prone to shattering following a short period of rapid dehydration during which their water content fell from approx. 70% to approx. 10% (based on weight). Magnetic resonance imaging of individual siliques of varying ages revealed that water loss occurred from the inside the pericarp in B. napus cv. Quantum and from the outside of the pericarp in B. rapa cv. Parkland. We suggest a mechanism for how this difference in the pattern of water loss contributed to the difference in susceptibility to shatter between the two cultivars.Key words: shattering, magnetic resonance imaging, canola, valve.

Temporal and spatial expression of a polygalacturonase during leaf and flower abscission in oilseed rape and Arabidopsis

During leaf abscission in oilseed rape (Brassica napus), cell wall degradation is brought about by the action of several hydrolytic enzymes. One of these is thought to be polygalacturonase (PG). Degenerate primers were used to isolate a PG cDNA fragment by reverse transcriptase-polymerase chain reaction from RNA extracted from ethylene-promoted leaf abscission zones (AZs), and in turn a full-length clone (CAW471) from an oilseed rape AZ cDNA library. The highest homology of this cDNA (82%) was to an Arabidopsis sequence that was predicted to encode a PG protein. Analysis of expression revealed that CAW471 mRNA accumulated in the AZ of leaves and reached a peak 24 h after ethylene treatment. Ethylene-promoted leaf abscission in oilseed rape was not apparent until 42 h after exposure to the gas, reaching 50% at 48 h and 100% by 56 h. In floral organ abscission, expression of CAW471 correlated with cell separation. Genomic libraries from oilseed rape and Arabidopsis were screened with CAW471 and the respective genomic clones PGAZBRAN and PGAZAT isolated. Characterization of these PG genes revealed that they had substantial homology within both the coding regions and in the 5'-upstream sequences. Fusion of a 1,476-bp 5'-upstream sequence of PGAZAT to beta-glucuronidase or green fluorescent protein and transformation of Arabidopsis revealed that this fragment was sufficient to drive expression of these reporter genes in the AZs at the base of the anther filaments, petals, and sepals.

Abscission, dehiscence, and other cell separation processes

Cell separation is a critical process that takes place throughout the life cycle of a plant. It enables roots to emerge from germinating seeds, cotyledons, and leaves to expand, anthers to dehisce, fruit to ripen, and organs to be shed. The focus of this review is to examine how processes such as abscission and dehiscence are regulated and the ways new research strategies are helping us to understand the mechanisms involved in bringing about a reduction in cell-to-cell adhesion. The opportunities for using this information to manipulate cell separation for the benefit of agriculture and horticulture are evaluated.