Search

The BASIC PENTACYSTEINE (BPC) GAGA (C-box) binding proteins belong to a small plant transcription factor family. We previously reported that BPCs of class I bind directly to C-boxes in the SEEDSTICK (STK) promoter and the mutagenesis of these cis-elements affects STK expression in the flower. The MADS-domain factor SHORT VEGETATIVE PHASE (SVP) is another key regulator of STK. Direct binding of SVP to CArG-boxes in the STK promoter are required to repress its expression during the first stages of flower development, Postprint (updated version)

[EN] Plant domestication is the process of adapting plants to human use by selecting specific traits. The selection process often involves the modification of some components of the plant reproductive mechanisms. Allelic variants of genes associated with flowering time, vernalization, and the circadian clock are responsible for the adaptation of crops, such as rice, maize, barley, wheat, and tomato, to non-native latitudes. Modifications in the plant architecture and branching have been selected for higher yields and easier harvests. These phenotypes are often produced by alterations in the regulation of the transition of shoot apical meristems to inflorescences, and then to floral meristems. Floral homeotic mutants are responsible for popular double-flower phenotypes in Japanese cherries, roses, camellias, and lilies. The rise of peloric flowers in ornamentals such as snapdragon and florists' gloxinia is associated with non-functional alleles that control the relative expansion of lateral and ventral petals. Mechanisms to force outcrossing such as self-incompatibility have been removed in some tree crops cultivars such as almonds and peaches. In this review, we revisit some of these important concepts from the plant domestication perspective, focusing on four topics related to the pre-fertilization mechanisms: flowering time, inflorescence architecture, flower development, and pre-fertilization self-incompatibility mechanisms.

The plastid-localized phosphoglucose isomerase isoform PGI1 is an important determinant of growth in Arabidopsis thaliana, likely due to its involvement in the biosynthesis of plastidial isoprenoid-derived hormones. Here, we investigated whether PGI1 also influences seed yields. PGI1 is strongly expressed in maturing seed embryos and vascular tissues. PGI1-null pgi1-2 plants had ∼60% lower seed yields than wild-type plants, with reduced numbers of inflorescences and thus fewer siliques and seeds per plant. These traits were associated with low bioactive gibberellin (GA) contents. Accordingly, wild-type phenotypes were restored by exogenous GA application. pgi1-2 seeds were lighter and accumulated ∼50% less fatty acids (FAs) and ∼35% less protein than wild-type seeds. Seeds of cytokinin-deficient plants overexpressing CYTOKININ OXIDASE/DEHYDROGENASE1 (35S:AtCKX1) and GA-deficient ga20ox1 ga20ox2 mutants did not accumulate low levels of FAs, and exogenous application of the cytokinin 6-benzylaminopurine and GAs did not rescue the reduced weight and FA content of pgi1-2 seeds. Seeds from reciprocal crosses between pgi1-2 and wild-type plants accumulated wild-type levels of FAs and proteins. Therefore, PGI1 is an important determinant of Arabidopsis seed yield due to its involvement in two processes: GA-mediated reproductive development and the metabolic conversion of plastidial glucose-6-phosphate to storage reserves in the embryo.

The role of secondary metabolites in the determination of cell identity has been an area of particular interest over recent years, and studies strongly indicate a connection between cell fate and the regulation of enzymes involved in secondary metabolism. In Arabidopsis thaliana, the maternally derived seed coat plays pivotal roles in both the protection of the developing embryo and the first steps of germination. In this regard, a characteristic feature of seed coat development is the accumulation of proanthocyanidins (PAs - a class of phenylpropanoid metabolites) in the innermost layer of the seed coat. Our genome-wide transcriptomic analysis suggests that the ovule identity factor SEEDSTICK (STK) is involved in the regulation of several metabolic processes, providing a strong basis for a connection between cell fate determination, development and metabolism. Using phenotypic, genetic, biochemical and transcriptomic approaches, we have focused specifically on the role of STK in PA biosynthesis. Our results indicate that STK exerts its effect by direct regulation of the gene encoding BANYULS/ANTHOCYANIDIN REDUCTASE (BAN/ANR), which converts anthocyanidins into their corresponding 2,3-cis-flavan-3-ols. Our study also demonstrates that the levels of H3K9ac chromatin modification directly correlate with the active state of BAN in an STK-dependent way. This is consistent with the idea that MADS-domain proteins control the expression of their target genes through the modification of chromatin states. STK might thus recruit or regulate histone modifying factors to control their activity. In addition, we show that STK is able to regulate other BAN regulators. Our study demonstrates for the first time how a floral homeotic gene controls tissue identity through the regulation of a wide range of processes including the accumulation of secondary metabolites.

Synthesized by glycogen synthase and starch synthases (SS) using ADP-glucose as the sugar donor molecule, glycogen and starch accumulate as predominant storage carbohydrates in most bacteria and plants, respectively. We have recently shown that the so-called >starch-less> Arabidopsis thaliana adg1-1 and aps1 mutants impaired in ADP-glucose pyrophosphorylase do indeed accumulate low starch content in normal growth conditions, and relatively high starch content when plants were cultured in the presence of microbial volatiles. Our results were strongly supported by data obtained using a highly sensitive method for confocal fluorescence microscopic visualization of iodine stained starch granules. Using Arabidopsis leaves from WT plants, aps1 plants, ss3/ss4 plants lacking both class III and class IV SS, gbss plants lacking the granule-bound SS, and sus1/sus2/sus3/sus4 plants lacking four genes that code for proteins with sucrose synthase activity, in this work we precisely describe the method for preparation of plant samples for starch microscopic examination. Furthermore, we show that this method can be used to visualize glycogen in bacteria, and pure starch granules, amylose and amylopectin. © 2012 Landes Bioscience.

ADP-glucose pyrophosphorylase (AGP) is a heterotetrameric enzyme comprising two small and two large subunits that catalyze the production of ADP-glucose linked to starch biosynthesis. The current paradigm on leaf starch metabolism assumes that post-translational redox modification of AGP in response to light is a major determinant of fine regulation of transitory starch accumulation. According to this view, under oxidizing conditions occurring during the night the two AGP small subunits (APS1) are covalently linked via an intermolecular disulfide bridge that inactivates the protein, whereas under reducing conditions occurring during the day NADP-thioredoxin reductase C (NTRC)-dependent reductive monomerization of APS1 activates the enzyme. In this work we have analyzed changes in the redox status of APS1 during dark-light transition in leaves of plants cultured under different light intensities. Furthermore, we have carried out time-course analyses of starch content in ntrc mutants, and in aps1 mutants expressing the Escherichia coli redox-insensitive AGP (GlgC) in the chloroplast. We also characterized aps1 plants expressing a redox-insensitive, mutated APS1 (APS1mut) form in which the highly conserved Cys81 residue involved in the formation of the intermolecular disulfide bridge has been replaced by serine. We found that a very moderate, NTRC-dependent APS1 monomerization process in response to light occurred only when plants were cultured under photo-oxidative conditions. We also found that starch accumulation rates during the light in leaves of both ntrc mutants and GlgC-expressing aps1 mutants were similar to those of wild-type leaves. Furthermore, the pattern of starch accumulation during illumination in leaves of APS1mut-expressing aps1 mutants was similar to that of APS1-expressing aps1 mutants at any light intensity. The overall data demonstrate that post-translational redox modification of AGP in response to light is not a major determinant of fine regulat

Starch is a branched homopolisaccharide of ¿-1,4-linked glucose subunits with ¿-1,6- linked glucose subunits with ¿-1,6-linked glucose at the branching points. It represents the major storage carbohydrate in most higher plants. In addition, this polysaccharide represents the most common carbohydrate in the human diet. Starch is abundant in storage organs such as cereal seeds and tubers, and has multiple applications in the paper, textiles, cosmetics, pharmaceutical and construction industries. Plants are able to perceive biotic stimuli by recognizing a multitude of different signaling compounds originating from the interacting organisms. Contact with these substances not only adjusts plant defense reactions, but very often leads to decrease in photosynthesis, and to a transition from source to sink status in photosynthetic tissues. Some of these compounds are volatiles with molecular masses less than 300 Da and a high vapor pressure. Given the lack of knowledge regarding how microbial volatiles may affect plant primary carbohydrate metabolism, in this study I explored the effect of volatiles released from different microbial species on starch metabolism., In the First chapter of the work I explored the effect on leaf starch metabolism of volatiles released from different microbial species ranging from Gram-negative and Gram-positive bacteria, and fungi. All microbial species tested (including plant pathogens and species normally not interacting with plants) emitted volatiles that strongly promoted starch accumulation in leaves of both mono- and di-cotyledonous plants. Starch content in leaves of plants treated during 2 days with microbial volatiles was comparable or even higher than that of reserve organs such as potato tubers. Microbial volatiles promoting starch accumulation are sensed in leaves. Starch structure and amylose content were severely affected during MIcrobial VOlatiles Induced Starch Accumulation Process (MIVOISAP). Treatment of leaves with fungal volatiles (FVs) emitted by Alternaria alternata resulted in enhanced levels of sucrose, ADP-glucose (ADPG), UDP-glucose and 3-phosphoglycerate (3PGA). MIVOISAP occurs independently of the presence of sucrose in the culture medium and is strongly repressed by cysteine supplementation. Transcriptome and enzyme activity analyses of potato leaves exposed to FVs emitted by A. alternata showed that starch over-accumulation was accompanied by up-regulation of sucrose synthase (SuSy), invertase inhibitors, starch synthases (SS) class III and IV, starch branching enzyme, and glucose-6-phosphate transporter. This phenomenon, was also accompanied by down-regulation of acid invertase, plastidial thioredoxins, starch breakdown enzymes, proteins involved in internal amino acid provision, and less well defi ned mechanisms involving bacteria-type stringent response. In chapter 2, time-course analyses of starch accumulation in Arabidopsis leaves exposed to FVs emitted by A. alternata revealed that MIVOISAP is due to stimulation of starch biosynthesis during illumination. The increase of starch content in illuminated leaves of FVs-treated hy1/cry1, hy1/cry2 and hy1/cry1/cry2 A, In attempting to solve a 20 years-old controversy on the possible occurrence of starch biosynthetic pathway(s) additional/alternative to the "classic" AGP pathway, in chapter 3 I carried out a judicious characterization of the starch-less adg1-1 and aps1 mutants impaired in AGP. Quantitative measurement analyses revealed that both mutants accumulated WT ADPG and ca. 2% of WT starch, as further confi rmed by confocal fl uorescence microscopic observation of iodine stained leaves and of leaves expressing granule bound starch synthase fused with green fl uorescent protein. Introduction of the sex1 mutation affecting starch breakdown into adg1-1 and aps1 increased the starch content to 8-10% of the WT starch. Importantly, aps1 leaves exposed to FVs emitted by A. alternata for 10 h accumulated ca. 60% of the WT starch. aps1 plants expressing the bacterial ADP-glucose hydrolase (EcASPP) in the plastid accumulated normal ADPG and reduced starch when compared with aps1 plants, whereas aps1 plants expressing EcASPP in the cytosol showed reduced ADPG and starch. Moreover, aps1 plants expressing a bacterial AGP in the plastid accumulated WT starch and ADPG. The overall data showed that (a) there occur important source(s), other than AGP, of ADPG linked to starch biosynthesis, and (b) AGP is a major determinant of starch accumulation but not of intracellular ADPG content in Arabidopsis.