Maize domestication phenotypes reveal strigolactone networks coordinating grain size evolution with kernel-bearing cupule architecture.

The maize (Zea mays) ear represents one of the most striking domestication phenotypes in any crop species, with the cob conferring an exceptional yield advantage over the ancestral form of teosinte. Remodeling of the grain-bearing surface required profound developmental changes. However, the underlying mechanisms remain unclear and can only be partly attributed to the known domestication gene Teosinte glume architecture 1 (Tga1). Here we show that a more complete conversion involves strigolactones (SLs), and that these are prominent players not only in the Tga1 phenotype but also other domestication features of the ear and kernel. Genetic combinations of a teosinte tga1 allele with three SL-related mutants progressively enhanced ancestral morphologies. The SL mutants, in addition to modulating the tga1 phenotype, also reshaped kernel-bearing pedicels and cupules in a teosinte-like manner. Genetic and molecular evidence are consistent with SL regulation of TGA1, including direct interaction of TGA1 with components of the SL-signaling system shown here to mediate TGA1 availability by sequestration. Roles of the SL network extend to enhancing maize seed size and, importantly, coordinating increased kernel growth with remodeling of protective maternal tissues. Collectively, our data show that SLs have central roles in releasing kernels from restrictive maternal encasement and coordinating other factors that increase kernel size, physical support, and their exposure on the grain-bearing surface.

Independent evolution of transposase and TIRs facilitated by recombination between Mutator transposons from divergent clades in maize.

Nearly all eukaryotes carry DNA transposons of the Robertson's Mutator (Mu) superfamily, a widespread source of genome instability and genetic variation. Despite their pervasive impact on host genomes, much remains unknown about the evolution of these transposons. Transposase recognition of terminal inverted repeats (TIRs) is thought to drive and constrain coevolution of MuDR transposase genes and TIRs. To address the extent of this relationship and its impact, we compared separate phylogenies of TIRs and MuDR gene sequences from Mu elements in the maize genome. Five major clades were identified. As expected, most Mu elements were bound by highly similar TIRs from the same clade (homomorphic type). However, a subset of elements contained dissimilar TIRs derived from divergent clades. These "heteromorphs" typically occurred in multiple copies indicating active transposition in the genome. In addition, analysis of internal sequences showed that exchanges between elements having divergent TIRs produced new mudra and mudrb gene combinations. In several instances, TIR homomorphs had been regenerated within a heteromorph clade with retention of distinctive internal MuDR sequence combinations. Results reveal that recombination between divergent clades facilitates independent evolution of transposase (mudra), transposase-binding targets (TIRs), and capacity for insertion (mudrb) of active Mu elements. This mechanism would be enhanced by the preference of Mu insertions for recombination-rich regions near the 5' ends of genes. We suggest that cycles of recombination give rise to alternating homo- and heteromorph forms that enhance the diversity on which selection for Mu fitness can operate.

The sugar transporter ZmSUGCAR1 of the nitrate transporter 1/peptide transporter family is critical for maize grain filling.

Maternal-to-filial nutrition transfer is central to grain development and yield. nitrate transporter 1/peptide transporter (NRT1-PTR)-type transporters typically transport nitrate, peptides, and ions. Here, we report the identification of a maize (Zea mays) NRT1-PTR-type transporter that transports sucrose and glucose. The activity of this sugar transporter, named Sucrose and Glucose Carrier 1 (SUGCAR1), was systematically verified by tracer-labeled sugar uptake and serial electrophysiological studies including two-electrode voltage-clamp, non-invasive microelectrode ion flux estimation assays in Xenopus laevis oocytes and patch clamping in HEK293T cells. ZmSUGCAR1 is specifically expressed in the basal endosperm transfer layer and loss-of-function mutation of ZmSUGCAR1 caused significantly decreased sucrose and glucose contents and subsequent shrinkage of maize kernels. Notably, the ZmSUGCAR1 orthologs SbSUGCAR1 (from Sorghum bicolor) and TaSUGCAR1 (from Triticum aestivum) displayed similar sugar transport activities in oocytes, supporting the functional conservation of SUGCAR1 in closely related cereal species. Thus, the discovery of ZmSUGCAR1 uncovers a type of sugar transporter essential for grain development and opens potential avenues for genetic improvement of seed-filling and yield in maize and other grain crops.

The SUMO ligase MMS21 profoundly influences maize development through its impact on genome activity and stability.

The post-translational addition of SUMO plays essential roles in numerous eukaryotic processes including cell division, transcription, chromatin organization, DNA repair, and stress defense through its selective conjugation to numerous targets. One prominent plant SUMO ligase is METHYL METHANESULFONATE-SENSITIVE (MMS)-21/HIGH-PLOIDY (HPY)-2/NON-SMC-ELEMENT (NSE)-2, which has been connected genetically to development and endoreduplication. Here, we describe the potential functions of MMS21 through a collection of UniformMu and CRISPR/Cas9 mutants in maize (Zea mays) that display either seed lethality or substantially compromised pollen germination and seed/vegetative development. RNA-seq analyses of leaves, embryos, and endosperm from mms21 plants revealed a substantial dysregulation of the maize transcriptome, including the ectopic expression of seed storage protein mRNAs in leaves and altered accumulation of mRNAs associated with DNA repair and chromatin dynamics. Interaction studies demonstrated that MMS21 associates in the nucleus with the NSE4 and STRUCTURAL MAINTENANCE OF CHROMOSOMES (SMC)-5 components of the chromatin organizer SMC5/6 complex, with in vitro assays confirming that MMS21 will SUMOylate SMC5. Comet assays measuring genome integrity, sensitivity to DNA-damaging agents, and protein versus mRNA abundance comparisons implicated MMS21 in chromatin stability and transcriptional controls on proteome balance. Taken together, we propose that MMS21-directed SUMOylation of the SMC5/6 complex and other targets enables proper gene expression by influencing chromatin structure.

The number of catalytic cycles in an enzyme's lifetime and why it matters to metabolic engineering.

Metabolic engineering uses enzymes as parts to build biosystems for specified tasks. Although a part's working life and failure modes are key engineering performance indicators, this is not yet so in metabolic engineering because it is not known how long enzymes remain functional in vivo or whether cumulative deterioration (wear-out), sudden random failure, or other causes drive replacement. Consequently, enzymes cannot be engineered to extend life and cut the high energy costs of replacement. Guided by catalyst engineering, we adopted catalytic cycles until replacement (CCR) as a metric for enzyme functional life span in vivo. CCR is the number of catalytic cycles that an enzyme mediates in vivo before failure or replacement, i.e., metabolic flux rate/protein turnover rate. We used estimated fluxes and measured protein turnover rates to calculate CCRs for ∼100-200 enzymes each from Lactococcus lactis, yeast, and Arabidopsis CCRs in these organisms had similar ranges (<103 to >107) but different median values (3-4 × 104 in L. lactis and yeast versus 4 × 105 in Arabidopsis). In all organisms, enzymes whose substrates, products, or mechanisms can attack reactive amino acid residues had significantly lower median CCR values than other enzymes. Taken with literature on mechanism-based inactivation, the latter finding supports the proposal that 1) random active-site damage by reaction chemistry is an important cause of enzyme failure, and 2) reactive noncatalytic residues in the active-site region are likely contributors to damage susceptibility. Enzyme engineering to raise CCRs and lower replacement costs may thus be both beneficial and feasible.

Structural variation affecting DNA backbone interactions underlies adaptation of B3 DNA binding domains to constraints imposed by protein architecture.

Functional and architectural diversification of transcription factor families has played a central role in the independent evolution of complex development in plants and animals. Here, we investigate the role of architectural constraints on evolution of B3 DNA binding domains that regulate plant embryogenesis. B3 domains of ABI3, FUS3, LEC2 and VAL1 proteins recognize the same cis-element. Complex architectures of ABI3 and VAL1 integrate cis-element recognition with other signals, whereas LEC2 and FUS3 have reduced architectures conducive to roles as pioneer activators. In yeast and plant in vivo assays, B3 domain functions correlate with architectural complexity of the parent transcription factor rather than phylogenetic relatedness. In a complex architecture, attenuated ABI3-B3 and VAL1-B3 activities enable integration of cis-element recognition with hormone signaling, whereas hyper-active LEC2-B3 and FUS3-B3 over-ride hormonal control. Three clade-specific amino acid substitutions (ß4-triad) implicated in interactions with the DNA backbone account for divergence of LEC2-B3 and ABI3-B3. We find a striking correlation between differences in in vitro DNA binding affinity and in vivo activities of B3 domains in plants and yeast. Our results highlight the role of DNA backbone interactions that preserve DNA sequence specificity in adaptation of B3 domains to functional constraints associated with domain architecture.

Emb15 encodes a plastid ribosomal assembly factor essential for embryogenesis in maize.

Ribosome assembly factors guide the complex process by which ribosomal proteins and the ribosomal RNAs form a functional ribosome. However, the assembly of plant plastid ribosomes is poorly understood. In the present study, we discovered a maize (Zea mays) plastid ribosome assembly factor based on our characterization of the embryo defective 15 (emb15) mutant. Loss of function of Emb15 retards embryo development at an early stage, but does not substantially affect the endosperm, and causes an albino phenotype in other genetic backgrounds. EMB15 localizes to plastids and possesses a ribosome maturation factor M (RimM) domain in the N-terminus and a predicted UDP-GlcNAc pyrophosphorylase domain in the C-terminus. The EMB15 RimM domain originated in bacteria and the UDP-GlcNAc pyrophosphorylase domain originated in fungi; these two domains came together in the ancestor of land plants during evolution. The N-terminus of EMB15 complemented the growth defect of an Escherichia coli strain with a RimM deletion and rescued the albino phenotype of emb15 homozygous mutants. The RimM domain mediates the interaction between EMB15 and the plastid ribosomal protein PRPS19. Plastid 16S rRNA maturation is also significantly impaired in emb15. These observations suggest that EMB15 functions in maize seed development as a plastid ribosome assembly factor, and the C-terminal domain is not important under normal conditions.

Transposable elements employ distinct integration strategies with respect to transcriptional landscapes in eukaryotic genomes.

Transposable elements (TEs) are ubiquitous DNA segments capable of moving from one site to another within host genomes. The extant distributions of TEs in eukaryotic genomes have been shaped by both bona fide TE integration preferences in eukaryotic genomes and by selection following integration. Here, we compare TE target site distribution in host genomes using multiple de novo transposon insertion datasets in both plants and animals and compare them in the context of genome-wide transcriptional landscapes. We showcase two distinct types of transcription-associated TE targeting strategies that suggest a process of convergent evolution among eukaryotic TE families. The integration of two precision-targeting elements are specifically associated with initiation of RNA Polymerase II transcription of highly expressed genes, suggesting the existence of novel mechanisms of precision TE targeting in addition to passive targeting of open chromatin. We also highlight two features that can facilitate TE survival and rapid proliferation: tissue-specific transposition and minimization of negative impacts on nearby gene function due to precision targeting.

The Moderately (D)efficient Enzyme: Catalysis-Related Damage In Vivo and Its Repair.

Enzymes have in vivo life spans. Analysis of life spans, i.e., lifetime totals of catalytic turnovers, suggests that nonsurvivable collateral chemical damage from the very reactions that enzymes catalyze is a common but underdiagnosed cause of enzyme death. Analysis also implies that many enzymes are moderately deficient in that their active-site regions are not naturally as hardened against such collateral damage as they could be, leaving room for improvement by rational design or directed evolution. Enzyme life span might also be improved by engineering systems that repair otherwise fatal active-site damage, of which a handful are known and more are inferred to exist. Unfortunately, the data needed to design and execute such improvements are lacking: there are too few measurements of in vivo life span, and existing information about the extent, nature, and mechanisms of active-site damage and repair during normal enzyme operation is too scarce, anecdotal, and speculative to act on. Fortunately, advances in proteomics, metabolomics, cheminformatics, comparative genomics, and structural biochemistry now empower a systematic, data-driven approach for identifying, predicting, and validating instances of active-site damage and its repair. These capabilities would be practically useful in enzyme redesign and improvement of in-use stability and could change our thinking about which enzymes die young in vivo, and why.

Construction and applications of a B vitamin genetic resource for investigation of vitamin-dependent metabolism in maize.

The B vitamins provide essential co-factors for central metabolism in all organisms. In plants, B vitamins have surprising emerging roles in development, stress tolerance and pathogen resistance. Hence, there is a paramount interest in understanding the regulation of vitamin biosynthesis as well as the consequences of vitamin deficiency in crop species. To facilitate genetic analysis of B vitamin biosynthesis and functions in maize, we have mined the UniformMu transposon resource to identify insertional mutations in vitamin pathway genes. A screen of 190 insertion lines for seed and seedling phenotypes identified mutations in biotin, pyridoxine and niacin biosynthetic pathways. Importantly, isolation of independent insertion alleles enabled genetic confirmation of genotype-to-phenotype associations. Because B vitamins are essential for survival, null mutations often have embryo lethal phenotypes that prevent elucidation of subtle, but physiologically important, metabolic consequences of sub-optimal (functional) vitamin status. To circumvent this barrier, we demonstrate a strategy for refined genetic manipulation of vitamin status based on construction of heterozygotes that combine strong and hypomorphic mutant alleles. Dosage analysis of pdx2 alleles in endosperm revealed that endosperm supplies pyridoxine to the developing embryo. Similarly, a hypomorphic bio1 allele enabled analysis of transcriptome and metabolome responses to incipient biotin deficiency in seedling leaves. We show that systemic pipecolic acid accumulation is an early metabolic response to sub-optimal biotin status highlighting an intriguing connection between biotin, lysine metabolism and systemic disease resistance signaling. Seed-stocks carrying insertions for vitamin pathway genes are available for free, public distribution via the Maize Genetics Cooperation Stock Center.

The thick aleurone1 Gene Encodes a NOT1 Subunit of the CCR4-NOT Complex and Regulates Cell Patterning in Endosperm.

Maize (Zea mays) thick aleurone1 (thk1-R) mutants form multiple aleurone layers in the endosperm and have arrested embryogenesis. Prior studies suggest that thk1 functions downstream of defective kernel1 (dek1) in a regulatory pathway that controls aleurone cell fate and other endosperm traits. The original thk1-R mutant contained an ∼2-Mb multigene deletion, which precluded identification of the causal gene. Here, ethyl methanesulfonate mutagenesis produced additional alleles, and RNA sequencing from developing endosperm was used to identify a candidate gene based on differential expression compared with the wild-type progenitor. Gene editing confirmed the gene identity by producing mutant alleles that failed to complement existing thk1 mutants and that produced multiple-aleurone homozygous phenotypes. Thk1 encodes a homolog of NEGATIVE ON TATA-LESS1, a protein that acts as a scaffold for the CARBON CATABOLITE REPRESSION4-NEGATIVE ON TATA-LESS complex. This complex is highly conserved and essential in all eukaryotes for regulating a wide array of gene expression and cellular activities. Maize also harbors a duplicate locus, thick aleurone-like1, which likely accounts for the ability of thk1 mutants to form viable cells. Transcriptomic analysis indicated that THK1 regulates activities involving cell division, signaling, differentiation, and metabolism. Identification of thk1 provides an important new component of the DEK1 regulatory system that patterns cell fate in endosperm.

BonnMu: A Sequence-Indexed Resource of Transposon-Induced Maize Mutations for Functional Genomics Studies.

Sequence-indexed insertional libraries in maize (Zea mays) are fundamental resources for functional genetics studies. Here, we constructed a Mutator (Mu) insertional library in the B73 inbred background designated BonnMu A total of 1,152 Mu-tagged F2-families were sequenced using the Mu-seq approach. We detected 225,936 genomic Mu insertion sites and 41,086 high quality germinal Mu insertions covering 16,392 of the annotated maize genes (37% of the B73v4 genome). On average, each F2-family of the BonnMu libraries captured 37 germinal Mu insertions in genes of the Filtered Gene Set (FGS). All BonnMu insertions and phenotypic seedling photographs of Mu-tagged F2-families can be accessed via MaizeGDB.org Downstream examination of 137,410 somatic and germinal insertion sites revealed that 50% of the tagged genes have a single hotspot, targeted by Mu By comparing our BonnMu (B73) data to the UniformMu (W22) library, we identified conserved insertion hotspots between different genetic backgrounds. Finally, the vast majority of BonnMu and UniformMu transposons was inserted near the transcription start site of genes. Remarkably, 75% of all BonnMu insertions were in closer proximity to the transcription start site (distance: 542 bp) than to the start codon (distance: 704 bp), which corresponds to open chromatin, especially in the 5' region of genes. Our European sequence-indexed library of Mu insertions provides an important resource for functional genetics studies of maize.

Effects of long-term exposure to elevated temperature on Zea mays endosperm development during grain fill.

Cereal yields decrease when grain fill proceeds under conditions of prolonged, moderately elevated temperatures. Endosperm-endogenous processes alter both rate and duration of dry weight gain, but underlying mechanisms remain unclear. Heat effects could be mediated by either abnormal, premature cessation of storage compound deposition or accelerated implementation of normal development. This study used controlled environments to isolate temperature as the sole environmental variable during Zea mays kernel-fill, from 12 days after pollination to maturity. Plants subjected to elevated day, elevated night temperatures (38°C day, 28°C night (38/28°C])) or elevated day, normal night (38/17°C), were compared with those from controls grown under normal day and night conditions (28/17°C). Progression of change over time in endosperm tissue was followed to dissect contributions at multiple levels, including transcriptome, metabolome, enzyme activities, product accumulation, and tissue ultrastructure. Integrated analyses indicated that the normal developmental program of endosperm is fully executed under prolonged high-temperature conditions, but at a faster rate. Accelerated development was observed when both day and night temperatures were elevated, but not when daytime temperature alone was increased. Although transcripts for most components of glycolysis and respiration were either upregulated or minimally affected, elevated temperatures decreased abundance of mRNAs related to biosynthesis of starch and storage proteins. Further analysis of 20 central-metabolic enzymes revealed six activities that were reduced under high-temperature conditions, indicating candidate roles in the observed reduction of grain dry weight. Nonetheless, a striking overall resilience of grain filling in the face of elevated temperatures can be attributed to acceleration of normal endosperm development.

Identification of a metabolic disposal route for the oncometabolite S-(2-succino)cysteine in Bacillus subtilis.

Cellular thiols such as cysteine spontaneously and readily react with the respiratory intermediate fumarate, resulting in the formation of stable S-(2-succino)-adducts. Fumarate-mediated succination of thiols increases in certain tumors and in response to glucotoxicity associated with diabetes. Therefore, S-(2-succino)-adducts such as S-(2-succino)cysteine (2SC) are considered oncometabolites and biomarkers for human disease. No disposal routes for S-(2-succino)-compounds have been reported prior to this study. Here, we show that Bacillus subtilis metabolizes 2SC to cysteine using a pathway encoded by the yxe operon. The first step is N-acetylation of 2SC followed by an oxygenation that we propose results in the release of oxaloacetate and N-acetylcysteine, which is deacetylated to give cysteine. Knockouts of the genes predicted to mediate each step in the pathway lose the ability to grow on 2SC as the sulfur source and accumulate the expected upstream metabolite(s). We further show that N-acetylation of 2SC relieves toxicity. This is the first demonstration of a metabolic disposal route for any S-(2-succino)-compound, paving the way toward the identification of corresponding pathways in other species.

Autonomous and non-autonomous functions of the maize Shohai1 gene, encoding a RWP-RK putative transcription factor, in regulation of embryo and endosperm development.

In plants, establishment of the basic body plan during embryogenesis involves complex processes of axis formation, cell fate specification and organ differentiation. While molecular mechanisms of embryogenesis have been well studied in the eudicot Arabidopsis, only a small number of genes regulating embryogenesis has been identified in grass species. Here, we show that a RKD-type RWP-RK transcription factor encoded by Shohai1 (Shai1) is indispensable for embryo and endosperm development in maize. Loss of Shai1 function causes variable morphological defects in the embryo including small scutellum, shoot axis bifurcation and arrest during early organogenesis. Analysis of molecular markers in mutant embryos reveals disturbed patterning of gene expression and altered polar auxin transport. In contrast with typical embryo-defective (emb) mutants that expose a vacant embryo pocket in the endosperm, the endosperm of shai1 kernels conforms to the varied size and shape of the embryo. Furthermore, genetic analysis confirms that Shai1 is required for autonomous formation of the embryo pocket in endosperm of emb mutants. Analyses of genetic mosaic kernels generated by B-A translocation revealed that expression of Shai1 in the endosperm could partially rescue a shai1 mutant embryo and suggested that Shai1 is involved in non-cell autonomous signaling from endosperm that supports normal embryo growth. Taken together, we propose that the Shai1 gene functions in regulating embryonic patterning during grass embryogenesis partly by endosperm-to-embryo interaction.

Arabidopsis TH2 Encodes the Orphan Enzyme Thiamin Monophosphate Phosphatase.

To synthesize the cofactor thiamin diphosphate (ThDP), plants must first hydrolyze thiamin monophosphate (ThMP) to thiamin, but dedicated enzymes for this hydrolysis step were unknown and widely doubted to exist. The classical thiamin-requiring th2-1 mutation in Arabidopsis thaliana was shown to reduce ThDP levels by half and to increase ThMP levels 5-fold, implying that the THIAMIN REQUIRING2 (TH2) gene product could be a dedicated ThMP phosphatase. Genomic and transcriptomic data indicated that TH2 corresponds to At5g32470, encoding a HAD (haloacid dehalogenase) family phosphatase fused to a TenA (thiamin salvage) family protein. Like the th2-1 mutant, an insertional mutant of At5g32470 accumulated ThMP, and the thiamin requirement of the th2-1 mutant was complemented by wild-type At5g32470 Complementation tests in Escherichia coli and enzyme assays with recombinant proteins confirmed that At5g32470 and its maize (Zea mays) orthologs GRMZM2G148896 and GRMZM2G078283 are ThMP-selective phosphatases whose activity resides in the HAD domain and that the At5g32470 TenA domain has the expected thiamin salvage activity. In vitro and in vivo experiments showed that alternative translation start sites direct the At5g32470 protein to the cytosol and potentially also to mitochondria. Our findings establish that plants have a dedicated ThMP phosphatase and indicate that modest (50%) ThDP depletion can produce severe deficiency symptoms.

A plastidial pantoate transporter with a potential role in pantothenate synthesis.

The pantothenate (vitamin B5) synthesis pathway in plants is not fully defined because the subcellular site of its ketopantoate → pantoate reduction step is unclear. However, the pathway is known to be split between cytosol, mitochondria, and potentially plastids, and inferred to involve mitochondrial or plastidial transport of ketopantoate or pantoate. No proteins that mediate these transport steps have been identified. Comparative genomic and transcriptomic analyses identified Arabidopsis thaliana BASS1 (At1g78560) and its maize (Zea mays) ortholog as candidates for such a transport role. BASS1 proteins belong to the bile acid : sodium symporter family and share similarity with the Salmonella enterica PanS pantoate/ketopantoate transporter and with predicted bacterial transporters whose genes cluster on the chromosome with pantothenate synthesis genes. Furthermore, Arabidopsis BASS1 is co-expressed with genes related to metabolism of coenzyme A, the cofactor derived from pantothenate. Expression of Arabidopsis or maize BASS1 promoted the growth of a S. enterica panB panS mutant strain when pantoate, but not ketopantoate, was supplied, and increased the rate of [3H]pantoate uptake. Subcellular localization of green fluorescent protein fusions in Nicotiana tabacum BY-2 cells demonstrated that Arabidopsis BASS1 is targeted solely to the plastid inner envelope. Two independent Arabidopsis BASS1 knockout mutants accumulated pantoate ∼10-fold in leaves and had smaller seeds. Taken together, these data indicate that BASS1 is a physiologically significant plastidial pantoate transporter and that the pantoate reduction step in pantothenate biosynthesis could be at least partly localized in plastids.

Small kernel2 Encodes a Glutaminase in Vitamin B6 Biosynthesis Essential for Maize Seed Development.

Vitamin B6, an essential cofactor for a range of biochemical reactions and a potent antioxidant, plays important roles in plant growth, development, and stress tolerance. Vitamin B6 deficiency causes embryo lethality in Arabidopsis (Arabidopsis thaliana), but the specific role of vitamin B6 biosynthesis in endosperm development has not been fully addressed, especially in monocot crops, where endosperm constitutes the major portion of the grain. Through molecular characterization of a small kernel2 (smk2) mutant in maize, we reveal that vitamin B6 has differential effects on embryogenesis and endosperm development in maize. The B6 vitamer pyridoxal 5'-phosphate (PLP) is drastically reduced in both the smk2 embryo and the endosperm. However, whereas embryogenesis of the smk2 mutant is arrested at the transition stage, endosperm formation is nearly normal. Cloning reveals that Smk2 encodes the glutaminase subunit of the PLP synthase complex involved in vitamin B6 biosynthesis de novo. Smk2 partially complements the Arabidopsis vitamin B6-deficient mutant pdx2.1 and Saccharomyces cerevisiae pyridoxine auxotrophic mutant MML21. Smk2 is constitutively expressed in the maize plant, including developing embryos. Analysis of B6 vitamers indicates that the endosperm accumulates a large amount of pyridoxamine 5'-phosphate (PMP). These results indicate that vitamin B6 is essential to embryogenesis but has a reduced role in endosperm development in maize. The vitamin B6 required for seed development is synthesized in the seed, and the endosperm accumulates PMP probably as a storage form of vitamin B6.

Conserved Functions of the MATE Transporter BIG EMBRYO1 in Regulation of Lateral Organ Size and Initiation Rate.

Genetic networks that determine rates of organ initiation and organ size are key regulators of plant architecture. Whereas several genes that influence the timing of lateral organ initiation have been identified, the regulatory pathways in which these genes operate are poorly understood. Here, we identify a class of genes implicated in regulation of the lateral organ initiation rate. Loss-of-function mutations in the MATE transporter encoded by maize (Zea mays) Big embryo 1 (Bige1) cause accelerated leaf and root initiation as well as enlargement of the embryo scutellum. BIGE1 is localized to trans-Golgi, indicating a possible role in secretion of a signaling molecule. Interestingly, phenotypes of bige1 bear striking similarity to cyp78a mutants identified in diverse plant species. We show that a CYP78A gene is upregulated in bige1 mutant embryos, suggesting a role for BIGE1 in feedback regulation of a CYP78A pathway. We demonstrate that accelerated leaf formation and early flowering phenotypes conditioned by mutants of Arabidopsis thaliana BIGE1 orthologs are complemented by maize Bige1, showing that the BIGE1 transporter has a conserved function in regulation of lateral organ initiation in plants. We propose that BIGE1 is required for transport of an intermediate or product associated with the CYP78A pathway.

Does Abiotic Stress Cause Functional B Vitamin Deficiency in Plants?

B vitamins are the precursors of essential metabolic cofactors but are prone to destruction under stress conditions. It is therefore a priori reasonable that stressed plants suffer B vitamin deficiencies and that certain stress symptoms are metabolic knock-on effects of these deficiencies. Given the logic of these arguments, and the existence of data to support them, it is a shock to realize that the roles of B vitamins in plant abiotic stress have had minimal attention in the literature (100-fold less than hormones) and continue to be overlooked. In this article, we therefore aim to explain the connections among B vitamins, enzyme cofactors, and stress conditions in plants. We first outline the chemistry and biochemistry of B vitamins and explore the concept of vitamin deficiency with the help of information from mammals. We then summarize classical and recent evidence for stress-induced vitamin deficiencies and for plant responses that counter these deficiencies. Lastly, we consider potential implications for agriculture.