Unique and overlapping expression patterns among members of photosynthesis-associated nuclear gene families in Arabidopsis

The functional network of the Arabidopsis plastoglobule proteome based on quantitative proteomics and genome-wide coexpression analysis

Plastoglobules (PGs) in chloroplasts are thylakoid-associated monolayer lipoprotein particles containing prenyl and neutral lipids and several dozen proteins mostly with unknown functions. An integrated view of the role of the PG is lacking. Here, we better define the PG proteome and provide a conceptual framework for further studies. The PG proteome from Arabidopsis (Arabidopsis thaliana) leaf chloroplasts was determined by mass spectrometry of isolated PGs and quantitative comparison with the proteomes of unfractionated leaves, thylakoids, and stroma. Scanning electron microscopy showed the purity and size distribution of the isolated PGs. Compared with previous PG proteome analyses, we excluded several proteins and identified six new PG proteins, including an M48 metallopeptidase and two Absence of bc1 complex (ABC1) atypical kinases, confirmed by immunoblotting. This refined PG proteome consisted of 30 proteins, including six ABC1 kinases and seven fibrillins together comprising more than 70% of the PG protein mass. Other fibrillins were located predominantly in the stroma or thylakoid and not in PGs; we discovered that this partitioning can be predicted by their isoelectric point and hydrophobicity. A genome-wide coexpression network for the PG genes was then constructed from mRNA expression data. This revealed a modular network with four distinct modules that each contained at least one ABC1K and/or fibrillin gene. Each module showed clear enrichment in specific functions, including chlorophyll degradation/senescence, isoprenoid biosynthesis, plastid proteolysis, and redox regulators and phosphoregulators of electron flow. We propose a new testable model for the PGs, in which sets of genes are associated with specific PG functions.

Impact of defoliation severity on photosynthesis, carbon metabolism and transport gene expression in perennial ryegrass

Defoliation severity affects grass regrowth. The changes to biological processes affecting regrowth induced by severe defoliation are not fully understood, nor have they been investigated at a molecular level in field-grown plants. Field-grown perennial ryegrass (Lolium perenne L.) plants were defoliated to 20, 40 or 60mm during winter. Throughout regrowth, transcript profiles of 17 genes involved in photosynthesis and carbon metabolism or transport were characterised in stubble and lamina tissue. Although defoliation to 20mm reduced residual lamina area and stubble water-soluble carbohydrate reserves compared with plants defoliated to 40 or 60mm, net herbage regrowth was not reduced. Transcript profiles indicated a potential compensatory mechanism that may have facilitated regrowth. At the one-leaf regrowth stage, plants defoliated to 20mm had greater abundance of photosynthesis-related gene transcripts (rca, rbcS1, rbcS2, fba, fbp and fnr) and 20% greater stubble total nitrogen than plants defoliated to 60mm. A greater capacity for photosynthesis in outer leaf sheaths may be one potential mechanism used by severely defoliated plants to compensate for the reduced residual lamina area; however, this premise requires further investigation.

Transcriptome analysis of potato leaves expressing the trehalose-6-phosphate synthase 1 gene of yeast

Transgenic lines of the potato cultivar White Lady expressing the trehalose-6-phosphate synthase (TPS1) gene of yeast exhibit improved drought tolerance, but grow slower and have a lower carbon fixation rate and stomatal density than the wild-type. To understand the molecular basis of this phenomenon, we have compared the transcriptomes of wild-type and TPS1-transgenic plants using the POCI microarray containing 42,034 potato unigene probes. We show that 74 and 25 genes were up-, and down-regulated, respectively, in the mature source leaves of TPS1-transgenic plants when compared with the wild-type. The differentially regulated genes were assigned into 16 functional groups. All of the seven genes, which were assigned into carbon fixation and metabolism group, were up-regulated, while about 42% of the assigned genes are involved in transcriptional and post-transcriptional regulation. Expression of genes encoding a 14-3-3 regulatory protein, and four transcription factors were down-regulated in the TPS1-transgenic leaves. To verify the microarray results, we used RNA gel blot analysis to examine the expression of eight genes and found that the RNA gel blot and microarray data correlated in each case. Using the putative Arabidopsis orthologs of the assigned potato sequences we have identified putative transcription binding sites in the promoter region of the differentially regulated genes, and putative protein-protein interactions involving some of the up- and down-regulated genes. We have also demonstrated that starch content is lower, while malate, inositol and maltose contents are higher in the TPS1-transgenic than in the wild-type leaves. Our results suggest that a complex regulatory network, involving transcription factors and other regulatory proteins, underpins the phenotypic alterations we have observed previously in potato when expressing the TPS1 gene of yeast.

Developmental regulation of CYCA2s contributes to tissue-specific proliferation in Arabidopsis

In multicellular organisms, morphogenesis relies on a strict coordination in time and space of cell proliferation and differentiation. In contrast to animals, plant development displays continuous organ formation and adaptive growth responses during their lifespan relying on a tight coordination of cell proliferation. How developmental signals interact with the plant cell-cycle machinery is largely unknown. Here, we characterize plant A2-type cyclins, a small gene family of mitotic cyclins, and show how they contribute to the fine-tuning of local proliferation during plant development. Moreover, the timely repression of CYCA2;3 expression in newly formed guard cells is shown to require the stomatal transcription factors FOUR LIPS/MYB124 and MYB88, providing a direct link between developmental programming and cell-cycle exit in plants. Thus, transcriptional downregulation of CYCA2s represents a critical mechanism to coordinate proliferation during plant development.

Proteomics - The key to understanding systems biology of Arabidopsis trichomes

Every multicellular organism consists of numerous organs, tissues and specific cell types. To gain detailed knowledge about the morphogenesis of these complex structures, it is inevitable to advance biochemical analyses to ultimate spatial and temporal resolution since individual cell types contribute differently to the overall performance of living objects. Single cell sampling combined with systems biological approaches was recently applied to investigations of Arabidopsis thaliana trichomes (leaf hairs). These are single celled structures that provide ideal model systems to address various aspects of plant cell development and differentiation at the level of individual cells. A previously suggested function of trichomes in plant stress responses could thus be confirmed. Furthermore, trichome-specific "omics" data collected in several laboratories are mutually conclusive which demonstrates the applicability of systems biological approaches at the single cell level.

GIGANTEA directly activates Flowering Locus T in Arabidopsis thaliana

Plants perceive environmental signals such as day length and temperature to determine optimal timing for the transition from vegetative to floral stages. Arabidopsis flowers under long-day conditions through the CONSTANS (CO)-FLOWERING LOCUS T (FT) regulatory module. It is thought that the environmental cues for photoperiodic control of flowering are initially perceived in the leaves. We have previously shown that GIGANTEA (GI) regulates the timing of CO expression, together with FLAVIN-BINDING, KELCH REPEAT, F BOX protein 1. Normally, CO and FT are expressed exclusively in vascular bundles, whereas GI is expressed in various tissues. To better elucidate the role of tissue-specific expression of GI in the flowering pathway, we established transgenic lines in which GI is expressed exclusively in mesophyll, vascular bundles, epidermis, shoot apical meristem, or root. We found that GI expressed in either mesophyll or vascular bundles rescues the late-flowering phenotype of the gi-2 loss-of-function mutant under both short-day and long-day conditions. Interestingly, GI expressed in mesophyll or vascular tissues increases FT expression without up-regulating CO expression under short-day conditions. Furthermore, we examined the interaction between GI and FT repressors in mesophyll. We found that GI can bind to three FT repressors: SHORT VEGETATIVE PHASE (SVP), TEMPRANILLO (TEM)1, and TEM2. Finally, our chromatin immunoprecipitation experiments showed that GI binds to FT promoter regions that are near the SVP binding sites. Taken together, our data further elucidate the multiple roles of GI in the regulation of flowering time.

Comparative evolution of photosynthetic genes in response to polyploid and nonpolyploid duplication

The likelihood of duplicate gene retention following polyploidy varies by functional properties (e.g. gene ontologies or protein family domains), but little is known about the effects of whole-genome duplication on gene networks related by a common physiological process. Here, we examined the effects of both polyploid and nonpolyploid duplications on genes encoding the major functional groups of photosynthesis (photosystem I, photosystem II, the light-harvesting complex, and the Calvin cycle) in the cultivated soybean (Glycine max), which has experienced two rounds of whole-genome duplication. Photosystem gene families exhibit retention patterns consistent with dosage sensitivity (preferential retention of polyploid duplicates and elimination of nonpolyploid duplicates), whereas Calvin cycle and light-harvesting complex gene families do not. We observed similar patterns in barrel medic (Medicago truncatula), which shared the older genome duplication with soybean but has evolved independently for approximately 50 million years, and in Arabidopsis (Arabidopsis thaliana), which experienced two nested polyploidy events independent from the legume duplications. In both soybean and Arabidopsis, Calvin cycle gene duplicates exhibit a greater capacity for functional differentiation than do duplicates within the photosystems, which likely explains the greater retention of ancient, nonpolyploid duplicates and larger average gene family size for the Calvin cycle relative to the photosystems.

A gene duplication/loss event in the ribulose-1,5-bisphosphate-carboxylase/oxygenase (rubisco) small subunit gene family among accessions of Arabidopsis thaliana

Rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase; EC 4.1.1.39), the most abundant protein in nature, catalyzes the assimilation of CO(2) (worldwide about 10(11) t each year) by carboxylation of ribulose-1,5-bisphosphate. It is a hexadecamer consisting of eight large and eight small subunits. Although the Rubisco large subunit (rbcL) is encoded by a single gene on the multicopy chloroplast genome, the Rubisco small subunits (rbcS) are encoded by a family of nuclear genes. In Arabidopsis thaliana, the rbcS gene family comprises four members, that is, rbcS-1a, rbcS-1b, rbcS-2b, and rbcS-3b. We sequenced all Rubisco genes in 26 worldwide distributed A. thaliana accessions. In three of these accessions, we detected a gene duplication/loss event, where rbcS-1b was lost and substituted by a duplicate of rbcS-2b (called rbcS-2b*). By screening 74 additional accessions using a specific polymerase chain reaction assay, we detected five additional accessions with this duplication/loss event. In summary, we found the gene duplication/loss in 8 of 100 A. thaliana accessions, namely, Bch, Bu, Bur, Cvi, Fei, Lm, Sha, and Sorbo. We sequenced an about 1-kb promoter region for all Rubisco genes as well. This analysis revealed that the gene duplication/loss event was associated with promoter alterations (two insertions of 450 and 850 bp, one deletion of 730 bp) in rbcS-2b and a promoter deletion (2.3 kb) in rbcS-2b* in all eight affected accessions. The substitution of rbcS-1b by a duplicate of rbcS-2b (i.e., rbcS-2b*) might be caused by gene conversion. All four Rubisco genes evolve under purifying selection, as expected for central genes of the highly conserved photosystem of green plants. We inferred a single positive selected site, a tyrosine to aspartic acid substitution at position 72 in rbcS-1b. Exactly the same substitution compromises carboxylase activity in the cyanobacterium Anacystis nidulans. In A. thaliana, this substitution is associated with an inferred recombination. Functional implications of the substitution remain to be evaluated.

Control of leaf and vein development by auxin

Leaves are the main photosynthetic organs of vascular plants and show considerable diversity in their geometries, ranging from simple spoon-like forms to complex shapes with individual leaflets, as in compound leaves. Leaf vascular tissues, which act as conduits of both nutrients and signaling information, are organized in networks of different architectures that usually mirror the surrounding leaf shape. Understanding the processes that endow leaves and vein networks with ordered and closely aligned shapes has captured the attention of biologists and mathematicians since antiquity. Recent work has suggested that the growth regulator auxin has a key role in both initiation and elaboration of final morphology of both leaves and vascular networks. A key feature of auxin action is the existence of feedback loops through which auxin regulates its own transport. These feedbacks may facilitate the iterative generation of basic modules that underlies morphogenesis of both leaves and vasculature.

The molecular basis for distinct pathways for protein import into Arabidopsis chloroplasts

The translocons at the outer envelope membrane of chloroplasts (TOCs) initiate the import of thousands of nucleus-encoded proteins into the organelle. The identification of structurally and functionally distinct TOC complexes has led to the hypothesis that the translocons constitute different import pathways that are required to coordinate the import of sets of proteins whose expression varies in response to organelle biogenesis and physiological adaptation. To test this hypothesis, we examined the molecular basis for distinct TOC pathways by analyzing the functional diversification among the Toc159 family of TOC receptors. We demonstrate that the N-terminal A-domains of the Toc159 receptors regulate their selectivity for preprotein binding. Furthermore, the in vivo function of the two major Toc159 family members (atToc159 and atToc132) can be largely switched by swapping their A-domains in transgenic Arabidopsis thaliana. On the basis of these results, we propose that the A-domains of the Toc159 receptors are major determinants of distinct pathways for protein import into chloroplasts.

Regulation of preprocambial cell state acquisition by auxin signaling in Arabidopsis leaves

The principles underlying the formation of veins in the leaf have long intrigued developmental biologists. In Arabidopsis leaves, files of anatomically inconspicuous subepidermal cells that will elongate into vein-forming procambial cells selectively activate ATHB8 gene expression. The biological role of ATHB8 in vein formation and the molecular events that culminate in acquisition of the ATHB8preprocambial cell state are unknown, but intertwined pathways of auxin transport and signal transduction have been implicated in defining paths of vascular strand differentiation. Here we show that ATHB8 is required to stabilize preprocambial cell specification against auxin transport perturbations, to restrict preprocambial cell state acquisition to narrow fields and to coordinate procambium formation within and between veins. We further show that ATHB8 expression at preprocambial stages is directly and positively controlled by the auxin-response transcription factor MONOPTEROS (MP) through an auxin-response element in the ATHB8promoter. We finally show that the consequences of loss of ATHB8function for vein formation are masked by MP activity. Our observations define, at the molecular level, patterning inputs of auxin signaling in vein formation.

Web-queryable large-scale data sets for hypothesis generation in plant biology

The approaching end of the 21st century's first decade marks an exciting time for plant biology. Several National Science Foundation Arabidopsis 2010 Projects will conclude, and whether or not the stated goal of the National Science Foundation 2010 Program-to determine the function of 25,000 Arabidopsis genes by 2010-is reached, these projects and others in a similar vein, such as those performed by the AtGenExpress Consortium and various plant genome sequencing initiatives, have generated important and unprecedented large-scale data sets. While providing significant biological insights for the individual laboratories that generated them, these data sets, in conjunction with the appropriate tools, are also permitting plant biologists worldwide to gain new insights into their own biological systems of interest, often at a mouse click through a Web browser. This review provides an overview of several such genomic, epigenomic, transcriptomic, proteomic, and metabolomic data sets and describes Web-based tools for querying them in the context of hypothesis generation for plant biology. We provide five biological examples of how such tools and data sets have been used to provide biological insight.