Four mutant alleles elucidate the role of the G2 protein in the development of C(4) and C(3) photosynthesizing maize tissues

A common regulatory switch controls a suite of C4 traits in multiple cell types

The C4 photosynthetic pathway provided a major advantage to plants growing in hot, dry environments, including the ancestors of our most productive crops. Two traits were essential for the evolution of this pathway: increased vein density and the functionalization of bundle sheath cells for photosynthesis. Although GRAS transcriptional regulators, including SHORT ROOT (SHR), have been implicated in mediating leaf patterning in both C3 and C4 species, little is known about what controls the specialized features of the cells that mediate C4 metabolism and physiology. We show in the model monocot, Setaria viridis, that SHR regulates components of multiple cell identities, including chloroplast biogenesis and photosynthetic gene expression in bundle sheath cells, a central feature of C4 plants. Furthermore, we found that it also contributes to the two-cell compartmentalization of the characteristic four-carbon shuttle pathway. Disruption of SHR function clearly reduced photosynthetic capacity and seed yield in mutant plants under heat stress. Together, these results show how cell identities are remodeled by SHR to host the suite of traits characteristic of C4 regulation, which are a main engineering target in non-C4 crops to improve climate resilience.

GOLDEN2-like1 is sufficient but not necessary for chloroplast biogenesis in mesophyll cells of C4 grasses

Chloroplasts are the site of photosynthesis. In land plants, chloroplast biogenesis is regulated by a family of transcription factors named GOLDEN2-like (GLK). In C4 grasses, it has been hypothesized that genome duplication events led to the sub-functionalization of GLK paralogs (GLK1 and GLK2) to control chloroplast biogenesis in two distinct cell types: mesophyll and bundle sheath cells. Although previous characterization of golden2 (g2) mutants in maize has demonstrated a role for GLK2 paralogs in regulating chloroplast biogenesis in bundle sheath cells, the function of GLK1 has remained elusive. Here we show that, contrary to expectations, GLK1 is not required for chloroplast biogenesis in mesophyll cells of maize. Comparisons between maize and Setaria viridis, which represent two independent C4 origins within the Poales, further show that the role of GLK paralogs in controlling chloroplast biogenesis in mesophyll and bundle sheath cells differs between species. Despite these differences, complementation analysis revealed that GLK1 and GLK2 genes from maize are both sufficient to restore functional chloroplast development in mesophyll and bundle sheath cells of S. viridis mutants. Collectively our results suggest an evolutionary trajectory in C4 grasses whereby both orthologs retained the ability to induce chloroplast biogenesis but GLK2 adopted a more prominent developmental role, particularly in relation to chloroplast activation in bundle sheath cells.

Progress and prospects of C4 trait engineering in plants

Incorporating C₄ photosynthetic traits into C₃ crops is a rational approach for sustaining future demands for crop productivity. Using classical plant breeding, engineering this complex trait is unlikely to achieve its target. Therefore, it is critical and timely to implement novel biotechnological crop improvement strategies to accomplish this goal. However, a fundamental understanding of C₃ , C₄ , and C₃ -C₄ intermediate metabolism is crucial for the targeted use of biotechnological tools. This review assesses recent progress towards engineering C₄ photosynthetic traits in C₃ crops. We also discuss lessons learned from successes and failures of recent genetic engineering attempts in C₃ crops, highlighting the pros and cons of using rice as a model plant for short-, medium- and long-term goals of genetic engineering. This review provides an integrated approach towards engineering improved photosynthetic efficiency in C₃ crops for sustaining food, fibre and fuel production around the globe.

Mutations in a�Golden2-Like�Gene Cause Reduced Seed Weight in�Barley�albino lemma 1�Mutants

The albino lemma 1 (alm1) mutants of barley (Hordeum vulgare L.) exhibit obvious chlorophyll-deficient hulls. Hulls are seed-enclosing tissues on the spike, consisting of the lemma and palea. The alm1 phenotype is also expressed in the pericarp, culm nodes and basal leaf sheaths, but leaf blades and awns are normal green. A single recessive nuclear gene controls tissue-specific alm1 phenotypic expression. Positional cloning revealed that the ALM1 gene encodes a Golden 2-like (GLK) transcription factor, HvGLK2, belonging to the GARP subfamily of Myb transcription factors. This finding was validated by genetic evidence indicating that all 10 alm1 mutants studied had a lesion in functionally important regions of HvGLK2, including the three alpha-helix domains, an AREAEAA motif and the GCT box. Transmission electron microscopy revealed that, in lemmas of the alm1.g mutant, the chloroplasts lacked thylakoid membranes, instead of stacked thylakoid grana in wild-type chloroplasts. Compared with wild type, alm1.g plants showed similar levels of leaf photosynthesis but reduced spike photosynthesis by 34%. The alm1.g mutant and the alm1.a mutant showed a reduction in 100-grain weight by 15.8% and 23.1%, respectively. As in other plants, barley has HvGLK2 and a paralog, HvGLK1. In flag leaves and awns, HvGLK2 and HvGLK1 are expressed at moderate levels, but in hulls, HvGLK1 expression was barely detectable compared with HvGLK2. Barley alm1/Hvglk2 mutants exhibit more severe phenotypes than glk2 mutants of other plant species reported to date. The severe alm1 phenotypic expression in multiple tissues indicates that HvGLK2 plays some roles that are nonredundant with HvGLK1.

Understanding the Genetic Basis of C4 Kranz Anatomy with a View to Engineering C3 Crops

One of the most remarkable examples of convergent evolution is the transition from C₃ to C₄ photosynthesis, an event that occurred on over 60 independent occasions. The evolution of C₄ is particularly noteworthy because of the complexity of the developmental and metabolic changes that took place. In most cases, compartmentalized metabolic reactions were facilitated by the development of a distinct leaf anatomy known as Kranz. C₄ Kranz anatomy differs from ancestral C₃ anatomy with respect to vein spacing patterns across the leaf, cell-type specification around veins, and cell-specific organelle function. Here we review our current understanding of how Kranz anatomy evolved and how it develops, with a focus on studies that are dissecting the underlying genetic mechanisms. This research field has gained prominence in recent years because understanding the genetic regulation of Kranz may enable the C₃-to-C₄ transition to be engineered, an endeavor that would significantly enhance crop productivity.

Insights into the regulation of C4 leaf development from comparative transcriptomic analysis

C4 photosynthesis is more efficient than C3 photosynthesis for two reasons. First, C4 plants have evolved a repertoire of C4 enzymes to enhance CO2 fixation. Second, C4 leaves have Kranz anatomy with a high vein density in which the veins are surrounded by one layer of bundle sheath (BS) cells and one layer of mesophyll (M) cells. The BS and M cells are not only functionally well differentiated, but also well-coordinated for rapid transport of photo-assimilates between the two types of photosynthetic cells. Recent comparative transcriptomic and anatomical analyses of C3 and C4 leaves have revealed early onset of C4-related processes in leaf development, suggesting that delayed mesophyll differentiation contributes to higher C4 vein density, and have identified some candidate regulators for the higher vein density in C4 leaves. Moreover, comparative transcriptomics of maize husk (C3) and foliar leaves (C4) has identified a cohort of candidate regulators of Kranz anatomy development. In addition, there has been major progress in the identification of transcription factor binding sites, greatly increasing our knowledge of gene regulation in plants.

Characterizing regulatory and functional differentiation between maize mesophyll and bundle sheath cells by transcriptomic analysis

To study the regulatory and functional differentiation between the mesophyll (M) and bundle sheath (BS) cells of maize (Zea mays), we isolated large quantities of highly homogeneous M and BS cells from newly matured second leaves for transcriptome profiling by RNA sequencing. A total of 52,421 annotated genes with at least one read were found in the two transcriptomes. Defining a gene with more than one read per kilobase per million mapped reads as expressed, we identified 18,482 expressed genes; 14,972 were expressed in M cells, including 53 M-enriched transcription factor (TF) genes, whereas 17,269 were expressed in BS cells, including 214 BS-enriched TF genes. Interestingly, many TF gene families show a conspicuous BS preference in expression. Pathway analyses reveal differentiation between the two cell types in various functional categories, with the M cells playing more important roles in light reaction, protein synthesis and folding, tetrapyrrole synthesis, and RNA binding, while the BS cells specialize in transport, signaling, protein degradation and posttranslational modification, major carbon, hydrogen, and oxygen metabolism, cell division and organization, and development. Genes coding for several transporters involved in the shuttle of C(4) metabolites and BS cell wall development have been identified, to our knowledge, for the first time. This comprehensive data set will be useful for studying M/BS differentiation in regulation and function.

C4 cycles: past, present, and future research on C4 photosynthesis

In the late 1960s, a vibrant new research field was ignited by the discovery that instead of fixing CO(2) into a C(3) compound, some plants initially fix CO(2) into a four-carbon (C(4)) compound. The term C(4) photosynthesis was born. In the 20 years that followed, physiologists, biochemists, and molecular and developmental biologists grappled to understand how the C(4) photosynthetic pathway was partitioned between two morphologically distinct cell types in the leaf. By the early 1990s, much was known about C(4) biochemistry, the types of leaf anatomy that facilitated the pathway, and the patterns of gene expression that underpinned the biochemistry. However, virtually nothing was known about how the pathway was regulated. It should have been an exciting time, but many of the original researchers were approaching retirement, C(4) plants were proving recalcitrant to genetic manipulation, and whole-genome sequences were not even a dream. In combination, these factors led to reduced funding and the failure to attract young people into the field; the endgame seemed to be underway. But over the last 5 years, there has been a resurgence of interest and funding, not least because of ambitious multinational projects that aim to increase crop yields by introducing C(4) traits into C(3) plants. Combined with new technologies, this renewed interest has resulted in the development of more sophisticated approaches toward understanding how the C(4) pathway evolved, how it is regulated, and how it might be manipulated. The extent of this resurgence is manifest by the publication in 2011 of more than 650 pages of reviews on different aspects of C(4). Here, I provide an overview of our current understanding, the questions that are being addressed, and the issues that lie ahead.

The grass leaf developmental gradient as a platform for a systems understanding of the anatomical specialization of C(4) leaves

C(4) photosynthesis relies on spatial and quantitative specializations of common features of leaf anatomy, including venation pattern, bundle sheath cell and chloroplast differentiation, plasmodesmatal abundance, and secondary cell wall enhancement. It has thus far been challenging to dissect the molecular basis for these C(4)-specific alterations in spatial and quantitative patterns of regulation. The target downstream networks of genes and protein interactions that produce these fundamental anatomical features in both C(4) and C(3) species are poorly understood. The developing leaves of monocot grasses provide a base-to-tip gradient of developmental stages that can provide the platform for comprehensive molecular and anatomical data that can yield a better understanding both of the regulators and the targets that produce C(4) patterns, through a variety of gene discovery and systems analysis strategies.

Structural and metabolic transitions of C4 leaf development and differentiation defined by microscopy and quantitative proteomics in maize

C(4) grasses, such as maize (Zea mays), have high photosynthetic efficiency through combined biochemical and structural adaptations. C(4) photosynthesis is established along the developmental axis of the leaf blade, leading from an undifferentiated leaf base just above the ligule into highly specialized mesophyll cells (MCs) and bundle sheath cells (BSCs) at the tip. To resolve the kinetics of maize leaf development and C(4) differentiation and to obtain a systems-level understanding of maize leaf formation, the accumulation profiles of proteomes of the leaf and the isolated BSCs with their vascular bundle along the developmental gradient were determined using large-scale mass spectrometry. This was complemented by extensive qualitative and quantitative microscopy analysis of structural features (e.g., Kranz anatomy, plasmodesmata, cell wall, and organelles). More than 4300 proteins were identified and functionally annotated. Developmental protein accumulation profiles and hierarchical cluster analysis then determined the kinetics of organelle biogenesis, formation of cellular structures, metabolism, and coexpression patterns. Two main expression clusters were observed, each divided in subclusters, suggesting that a limited number of developmental regulatory networks organize concerted protein accumulation along the leaf gradient. The coexpression with BSC and MC markers provided strong candidates for further analysis of C(4) specialization, in particular transporters and biogenesis factors. Based on the integrated information, we describe five developmental transitions that provide a conceptual and practical template for further analysis. An online protein expression viewer is provided through the Plant Proteome Database.

The GLK1 'regulon' encodes disease defense related proteins and confers resistance to Fusarium graminearum in Arabidopsis

Overexpression (OE) was used to study the role of the Arabidopsis Golden2-like (GLK1) transcriptional activator in regulating gene expression. Affymetrix Gene Chip and RT-PCR analyses indicated that GLK1 OE in Arabidopsis reprogrammed gene expression networks to enhance a high constitutive expression of genes encoding disease defense related proteins. These include PR10, isochorismate synthase, antimicrobial peptides, glycosyl hydrolases, MATE efflux and other genes associated with pathogen response and detoxification. However, PR1, an indicator of systemic acquired resistance (SAR), was downregulated in GLK1 OE. GLK1 OE in Arabidopsis confers resistance to Fusarium graminearum, a broad host pathogen responsible for major losses in cereal crops. This is the first identification of the GLK1 'regulon' and a novel role for GLK1 in plant defense, suggesting its potential use for providing disease resistance in crop plants.

Functional differentiation of bundle sheath and mesophyll maize chloroplasts determined by comparative proteomics

Chloroplasts of maize (Zea mays) leaves differentiate into specific bundle sheath (BS) and mesophyll (M) types to accommodate C4 photosynthesis. Consequences for other plastid functions are not well understood but are addressed here through a quantitative comparative proteome analysis of purified M and BS chloroplast stroma. Three independent techniques were used, including cleavable stable isotope coded affinity tags. Enzymes involved in lipid biosynthesis, nitrogen import, and tetrapyrrole and isoprenoid biosynthesis are preferentially located in the M chloroplasts. By contrast, enzymes involved in starch synthesis and sulfur import preferentially accumulate in BS chloroplasts. The different soluble antioxidative systems, in particular peroxiredoxins, accumulate at higher levels in M chloroplasts. We also observed differential accumulation of proteins involved in expression of plastid-encoded proteins (e.g., EF-Tu, EF-G, and mRNA binding proteins) and thylakoid formation (VIPP1), whereas others were equally distributed. Enzymes related to the C4 shuttle, the carboxylation and regeneration phase of the Calvin cycle, and several regulators (e.g., CP12) distributed as expected. However, enzymes involved in triose phosphate reduction and triose phosphate isomerase are primarily located in the M chloroplasts, indicating that the M-localized triose phosphate shuttle should be viewed as part of the BS-localized Calvin cycle, rather than a parallel pathway.

The effect of overexpression of two Brassica CBF/DREB1-like transcription factors on photosynthetic capacity and freezing tolerance in Brassica napus

The effects of overexpression of two Brassica CBF/DREB1-like transcription factors (BNCBF5 and 17) in Brassica napus cv. Westar were studied. In addition to developing constitutive freezing tolerance and constitutively accumulating COR gene mRNAs, BNCBF5- and 17-overexpressing plants also accumulate moderate transcript levels of genes involved in photosynthesis and chloroplast development as identified by microarray and Northern analyses. These include GLK1- and GLK2-like transcription factors involved in chloroplast photosynthetic development, chloroplast stroma cyclophilin ROC4 (AtCYP20-3), beta-amylase and triose-P/Pi translocator. In parallel with these changes, increases in photosynthetic efficiency and capacity, pigment pool sizes, increased capacities of the Calvin cycle enzymes, and enzymes of starch and sucrose biosynthesis, as well as glycolysis and oxaloacetate/malate exchange are seen, suggesting that BNCBF overexpression has partially mimicked cold-induced photosynthetic acclimation constitutively. Taken together, these results suggest that BNCBF/DREB1 overexpression in Brassica not only resulted in increased constitutive freezing tolerance but also partially regulated chloroplast development to increase photochemical efficiency and photosynthetic capacity.

Quantification of photosynthetic gene expression in maize C(3) and C(4) tissues by real-time PCR

Carbon assimilation in maize follows the C(4) mechanism. This requires the tissue-specific and light-induced expression of a set of different genes involved in CO(2) fixation as well as adaptations in the leaf anatomy including a reduced distance between vascular bundles compared to C(3) plants. However, several maize tissues exist with larger bundle distances and there is significant evidence that CO(2) fixation follows the C(3) mechanism in these tissues. We isolated maize C(3) and C(4) tissues and quantified the accumulation of mRNAs encoding PEPC, ME, the small subunit of Rubisco, and PPDK. For this, primer systems for the specific and sensitive detection by real-time PCR were established. The observed patterns show the expected distribution for foliar leaf tissues. Also in total husk leaves, all transcripts under investigation were detected, albeit at a lower level. When mesophyll cells which are located distant from bundles were isolated from husk leaves, only accumulation of RbcS was observed. Comparing the expression of two genes encoding for isoenzymes of the small subunit of RbcS in the different tissues differential patterns of relative transcript abundance were observed. Transcripts for the DOF1 transcription factor involved in the activation of photosynthetic genes in maize were found in leaf tissues performing both C(4) and C(3) photosynthesis with highest accumulation levels in C(4) mesophyll cells, whereas the homologous DOF2 gene was not expressed in any of the investigated samples. The results provide novel insights into the regulation of C(3) and C(4) carbon fixation pathways in maize.