Cell differentiation in the longitudinal veins and formation of commissural veins in rice (Oryza sativa) and maize (Zea mays)

The WIP6 transcription factor TOO MANY LATERALS specifies vein type in C4 and C3 grass leaves

Grass leaves are invariantly strap shaped with an elongated distal blade and a proximal sheath that wraps around the stem. Underpinning this shape is a scaffold of leaf veins, most of which extend in parallel along the proximo-distal leaf axis. Differences between species are apparent both in the vein types that develop and in the distance between veins across the medio-lateral leaf axis. A prominent engineering goal is to increase vein density in leaves of C3 photosynthesizing species to facilitate the introduction of the more efficient C4 pathway. Here, we discover that the WIP6 transcription factor TOO MANY LATERALS (TML) specifies vein rank in both maize (C4) and rice (C3). Loss-of-function tml mutations cause large lateral veins to develop in positions normally occupied by smaller intermediate veins, and TML transcript localization in wild-type leaves is consistent with a role in suppressing lateral vein development in procambial cells that form intermediate veins. Attempts to manipulate TML function in rice were unsuccessful because transgene expression was silenced, suggesting that precise TML expression is essential for shoot viability. This finding may reflect the need to prevent the inappropriate activation of downstream targets or, given that transcriptome analysis revealed altered cytokinin and auxin signaling profiles in maize tml mutants, the need to prevent local or general hormonal imbalances. Importantly, rice tml mutants display an increased occupancy of veins in the leaf, providing a step toward an anatomical chassis for C4 engineering. Collectively, a conserved mechanism of vein rank specification in grass leaves has been revealed.

Photosynthesis of rice leaves with a parallel venation is highly tolerant to vein severing

Vein severing in plants caused by leaf damage is common in fields where crops are cultivated. It is hypothesized that leaves with complex reticulate venation can withstand hydraulic disturbances caused by vein severing, thereby preserving leaf carbon assimilation. However, limited research focuses on vein damage of leaves with parallel venation. We studied how vein-severing affected the photosynthetic traits of rice (Oryza sativa) leaves in seconds, minutes and days, under varying water-demand conditions and differing extents of water supply disruption. Rice leaves completely lost their photosynthetic capacity within 2.5 minutes after excision. Severing the midrib resulted in reduced light-saturated photosynthetic rate (A), stomatal conductance (gsw ) and transpiration rate (E) by 2.6, 6.8 and 5.9%, respectively, already after thirty minutes. We further investigated the photosynthetic trait responses to various extents of leaf width severing, while keeping the midrib functional. Surprisingly, A, gsw and E in the downstream area of the severed leaves largely remained stable, showing minimal variation across different leaf width severing ratios. These traits declined only slightly even under increased ambient light intensity and leaf-to-air vapor pressure deficit. This sustained photosynthesis post-severing is attributed to the efficient lateral water transport. Long-term leaf damage slightly but not significantly, impacted the downstream photosynthetic traits within five days post-severing. However, a more pronounced reduction in gas exchange during leaf senescence was observed nine days after severing. These findings suggested that rice leaves can tolerate hydraulic disturbances from vein severing and maintain functionality under various conditions, which is crucial for crop yield stability. However, long-term consequences require further investigation.

Elevated CO2 concentration increases maize growth under water deficit or soil salinity but with a higher risk of hydraulic failure

Climate change presents a challenge for plants to acclimate their water relations under changing environmental conditions, and may increase the risks of hydraulic failure under stress. In this study, maize plants were acclimated to two different CO2 concentrations ([CO2]; 400 ppm and 700 ppm) while under either water stress (WS) or soil salinity (SS) treatments, and their growth and hydraulic traits were examined in detail. Both WS and SS inhibited growth and had significant impacts on hydraulic traits. In particular, the water potential at 50% loss of stem hydraulic conductance (P50) decreased by 1 MPa in both treatments at 400 ppm. When subjected to elevated [CO2], the plants under both WS and SS showed improved growth by 7-23%. Elevated [CO2] also significantly increased xylem vulnerability (measured as loss of conductivity with decreasing xylem pressure), resulting in smaller hydraulic safety margins. According to the plant desiccation model, the critical desiccation degree (time×vapor pressure deficit) that the plants could tolerate under drought was reduced by 43-64% under elevated [CO2]. In addition, sensitivity analysis showed that P50 was the most important trait in determining the critical desiccation degree. Thus, our results demonstrated that whilst elevated [CO2] benefited plant growth under WS or SS, it also interfered with hydraulic acclimation, thereby potentially placing the plants at a higher risk of hydraulic failure and increased mortality.

A rapid method to quantify vein density in C4 plants using starch staining

C4 photosynthesis has evolved multiple times in the angiosperms and typically involves alterations to the biochemistry, cell biology and development of leaves. One common modification found in C4 plants compared with the ancestral C3 state is an increase in vein density such that the leaf contains a larger proportion of bundle sheath cells. Recent findings indicate that there may be significant intraspecific variation in traits such as vein density in C4 plants but to use such natural variation for trait-mapping, rapid phenotyping would be required. Here we report a high-throughput method to quantify vein density that leverages the bundle sheath-specific accumulation of starch found in C4 species. Starch staining allowed high-contrast images to be acquired permitting image analysis with MATLAB- and Python-based programmes. The method works for dicotyledons and monocotolydons. We applied this method to Gynandropsis gynandra where significant variation in vein density was detected between natural accessions, and Zea mays where no variation was apparent in the genotypically diverse lines assessed. We anticipate this approach will be useful to map genes controlling vein density in C4 species demonstrating natural variation for this trait.

SHORT-ROOT and SCARECROW homologs regulate patterning of diverse cell types within and between species

The roles of SHORT-ROOT (SHR) and SCARECROW (SCR) in ground tissue patterning and differentiation have been well established in the root of Arabidopsis thaliana. Recently, work in additional organs and species revealed the extensive functional diversification of these genes, including regulation of cortical divisions essential for nodule organogenesis in legume roots, bundle sheath specification in the Arabidopsis leaf, patterning of inner leaf cell layers in maize, and stomatal development in rice. The co-option of distinct functions and cell types is attributed to different mechanisms, including paralog retention, spatiotemporal changes in gene expression, and novel protein functions. Elaborating our knowledge of the SHR-SCR module further unravels the developmental regulation that controls diverse forms and functions within and between species.

Developmental regulation of leaf venation patterns: monocot versus eudicots and the role of auxin

Organisation and patterning of the vascular network in land plants varies in different taxonomic, developmental and environmental contexts. In leaves, the degree of vascular strand connectivity influences both light and CO₂ harvesting capabilities as well as hydraulic capacity. As such, developmental mechanisms that regulate leaf venation patterning have a direct impact on physiological performance. Development of the leaf venation network requires the specification of procambial cells within the ground meristem of the primordium and subsequent proliferation and differentiation of the procambial lineage to form vascular strands. An understanding of how diverse venation patterns are manifest therefore requires mechanistic insight into how procambium is dynamically specified in a growing leaf. A role for auxin in this process was identified many years ago, but questions remain. In this review we first provide an overview of the diverse venation patterns that exist in land plants, providing an evolutionary perspective. We then focus on the developmental regulation of leaf venation patterns in angiosperms, comparing patterning in eudicots and monocots, and the role of auxin in each case. Although common themes emerge, we conclude that the developmental mechanisms elucidated in eudicots are unlikely to fully explain how parallel venation patterns in monocot leaves are elaborated.

Genome-Wide Association Study of Vascular Bundle-Related Traits in Maize Stalk

The vascular bundle plays an important role in nutrient transportation in plants and exerts great influence on crop yield. Maize is widely used for food, feed, and fuel, producing the largest yield in the world. However, genes and molecular mechanism controlling vascular bundle-related traits in maize have largely remained undiscovered. In this study, a natural population containing 248 diverse maize inbred lines genotyped with high-throughput SNP markers was used for genome-wide association study. The results showed that broad variations existed for the vascular bundle-related traits which are subject to genetic structure and it was suitable for association analysis. In this study, we identified 15, 13, 2, 1, and 5 SNPs significantly associated with number of small vascular bundle, number of large vascular bundle, average area of single small vascular bundle, average area of single large vascular bundle, and cross-sectional area, respectively. The 210 candidate genes in the confidence interval can be classified into ten biological processes, three cellular components, and eight molecular functions. As for the Kyoto Encyclopedia of Genes and Genomes analysis of the candidate genes, a total of six pathways were identified. Finally, we found five genes related to vascular development, three genes related to cell wall, and two genes related to the mechanical strength of the stalk. Our results provide the further understanding of the genetic foundation of vascular bundle-related traits in maize stalk.

grasviq: an image analysis framework for automatically quantifying vein number and morphology in grass leaves

Beyond facilitating transport and providing mechanical support to the leaf, veins have important roles in the performance and productivity of plants and the ecosystem. In recent decades, computational image analysis has accelerated the extraction and quantification of vein traits, benefiting fields of research from agriculture to climatology. However, most of the existing leaf vein image analysis programs have been developed for the reticulate venation found in dicots. Despite the agroeconomic importance of cereal grass crops, like Oryza sativa (rice) and Zea mays (maize), a dedicated image analysis program for the parallel venation found in monocots has yet to be developed. To address the need for an image-based vein phenotyping tool for model and agronomic grass species, we developed the grass vein image quantification (grasviq) framework. Designed specifically for parallel venation, this framework automatically segments and quantifies vein patterns from images of cleared leaf pieces using classical computer vision techniques. Using image data sets from maize inbred lines and auxin biosynthesis and transport mutants in maize, we demonstrate the utility of grasviq for quantifying important vein traits, including vein density, vein width and interveinal distance. Furthermore, we show that the framework can resolve quantitative differences and identify vein patterning defects, which is advantageous for genetic experiments and mutant screens. We report that grasviq can perform high-throughput vein quantification, with precision on a par with that of manual quantification. Therefore, we envision that grasviq will be adopted for vein phenomics in maize and other grass species.

BdERECTA controls vasculature patterning and phloem-xylem organization in Brachypodium distachyon

BACKGROUND

The vascular system of plants consists of two main tissue types, xylem and phloem. These tissues are organized into vascular bundles that are arranged into a complex network running through the plant that is essential for the viability of land plants. Despite their obvious importance, the genes involved in the organization of vascular tissues remain poorly understood in grasses.

RESULTS

We studied in detail the vascular network in stems from the model grass Brachypodium distachyon (Brachypodium) and identified a large set of genes differentially expressed in vascular bundles versus parenchyma tissues. To decipher the underlying molecular mechanisms of vascularization in grasses, we conducted a forward genetic screen for abnormal vasculature. We identified a mutation that severely affected the organization of vascular tissues. This mutant displayed defects in anastomosis of the vascular network and uncommon amphivasal vascular bundles. The causal mutation is a premature stop codon in ERECTA, a LRR receptor-like serine/threonine-protein kinase. Mutations in this gene are pleiotropic indicating that it serves multiple roles during plant development. This mutant also displayed changes in cell wall composition, gene expression and hormone homeostasis.

CONCLUSION

In summary, ERECTA has a pleiotropic role in Brachypodium. We propose a major role of ERECTA in vasculature anastomosis and vascular tissue organization in Brachypodium.

Developmental and biophysical determinants of grass leaf size worldwide

One of the most notable ecological trends-described more than 2,300  years ago by Theophrastus-is the association of small leaves with dry and cold climates, which has recently been recognized for eudicotyledonous plants at a global scale^1-3. For eudicotyledons, this pattern has been attributed to the fact that small leaves have a thinner boundary layer that helps to avoid extreme leaf temperatures⁴ and their leaf development results in vein traits that improve water transport under cold or dry climates^5,6. However, the global distribution of leaf size and its adaptive basis have not been tested in the grasses, which represent a diverse lineage that is distinct in leaf morphology and that contributes 33% of terrestrial primary productivity (including the bulk of crop production)⁷. Here we demonstrate that grasses have shorter and narrower leaves under colder and drier climates worldwide. We show that small grass leaves have thermal advantages and vein development that contrast with those of eudicotyledons, but that also explain the abundance of small leaves in cold and dry climates. The worldwide distribution of leaf size in grasses exemplifies how biophysical and developmental processes result in convergence across major lineages in adaptation to climate globally, and highlights the importance of leaf size and venation architecture for grass performance in past, present and future ecosystems.

Rice Big Grain 1 promotes cell division to enhance organ development, stress tolerance and grain yield

Grain/seed yield and plant stress tolerance are two major traits that determine the yield potential of many crops. In cereals, grain size is one of the key factors affecting grain yield. Here, we identify and characterize a newly discovered gene Rice Big Grain 1 (RBG1) that regulates grain and organ development, as well as abiotic stress tolerance. Ectopic expression of RBG1 leads to significant increases in the size of not only grains but also other major organs such as roots, shoots and panicles. Increased grain size is primarily due to elevated cell numbers rather than cell enlargement. RBG1 is preferentially expressed in meristematic and proliferating tissues. Ectopic expression of RBG1 promotes cell division, and RBG1 co-localizes with microtubules known to be involved in cell division, which may account for the increase in organ size. Ectopic expression of RBG1 also increases auxin accumulation and sensitivity, which facilitates root development, particularly crown roots. Moreover, overexpression of RBG1 up-regulated a large number of heat-shock proteins, leading to enhanced tolerance to heat, osmotic and salt stresses, as well as rapid recovery from water-deficit stress. Ectopic expression of RBG1 regulated by a specific constitutive promoter, GOS2, enhanced harvest index and grain yield in rice. Taken together, we have discovered that RBG1 regulates two distinct and important traits in rice, namely grain yield and stress tolerance, via its effects on cell division, auxin and stress protein induction.

Predicting Division Planes of Three-Dimensional Cells by Soap-Film Minimization

One key aspect of cell division in multicellular organisms is the orientation of the division plane. Proper division plane establishment contributes to normal plant body organization. To determine the importance of cell geometry in division plane orientation, we designed a three-dimensional probabilistic mathematical model to directly test the century-old hypothesis that cell divisions mimic soap-film minima. According to this hypothesis, daughter cells have equal volume and the division plane occurs where the surface area is at a minimum. We compared predicted division planes to a plant microtubule array that marks the division site, the preprophase band (PPB). PPB location typically matched one of the predicted divisions. Predicted divisions offset from the PPB occurred when a neighboring cell wall or PPB was directly adjacent to the predicted division site to avoid creating a potentially structurally unfavorable four-way junction. By comparing divisions of differently shaped plant cells (maize [Zea mays] epidermal cells and developing ligule cells and Arabidopsis thaliana guard cells) and animal cells (Caenorhabditis elegans embryonic cells) to divisions simulated in silico, we demonstrate the generality of this model to accurately predict in vivo division. This powerful model can be used to separate the contribution of geometry from mechanical stresses or developmental regulation in predicting division plane orientation.

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.

WUSCHEL-RELATED HOMEOBOX4 acts as a key regulator in early leaf development in rice

Rice (Oryza sativa) has long and narrow leaves with parallel veins, similar to other grasses. Relative to Arabidopsis thaliana which has oval-shaped leaves, our understanding of the mechanism of leaf development is insufficient in grasses. In this study, we show that OsWOX4, a member of the WUSCHEL-RELATED HOMEOBOX gene family, plays important roles in early leaf development in rice. Inducible downregulation of OsWOX4 resulted in severe defects in leaf development, such as an arrest of vascular differentiation, a partial defect in the early cell proliferation required for midrib formation, and a failure to maintain cellular activity in general parenchyma cells. In situ analysis showed that knockdown of OsWOX4 reduced the expression of two LONELY GUY genes, which function in the synthesis of active cytokinin, in developing vascular bundles. Consistent with this, cytokinin levels were downregulated by OsWOX4 knockdown. Transcriptome analysis further showed that OsWOX4 regulates multiple genes, including those responsible for cell cycle progression and hormone action, consistent with the effects of OsWOX4 downregulation on leaf phenotypes. Collectively, these results suggest that OsWOX4 acts as a key regulator at an early stage of leaf development. Our previous work revealed that OsWOX4 is involved in the maintenance of shoot apical meristem in rice, whereas AtWOX4 is specifically associated with the maintenance of vascular stem cells in Arabidopsis. Thus, the function of the two orthologous genes seems to be diversified between rice and Arabidopsis.

Genetic analysis of rice mutants responsible for narrow leaf phenotype and reduced vein number

Leaves are a major site for photosynthesis and a key determinant of plant architecture. Rice produces thin and slender leaves, which consist of the leaf blade and leaf sheath separated by the lamina joint. Two types of vasculature, the large and small vascular bundles, run in parallel, together with a strong structure, the midrib. In this paper, we examined the function of four genes that regulate the width of the leaf blade and the vein number: NARROW LEAF1 (NAL1), NAL2, NAL3 and NAL7. We backcrossed original mutants of these genes with the standard wild-type rice, Taichung 65. We then compared the effect of each mutation on similar genetic backgrounds and examined genetic interactions of these genes. The nal1 single mutation and the nal2 nal3 double mutation showed a severe effect on leaf width, resulting in very narrow leaves. Although vein number was also reduced in the nal1 and nal2 nal3 mutants, the small vein number was more strongly reduced than the large vein number. In contrast, the nal7 mutation showed a milder effect on leaf width and vein number, and both the large and small veins were similarly affected. Thus, the genes responsible for narrow leaf phenotype seem to play distinct roles. The nal7 mutation showed additive effects on both leaf width and vein number, when combined with the nal1 single or the nal2 nal3 double mutation. In addition, observations of inner tissues revealed that cell differentiation was partially compromised in the nal2 nal3 nal7 mutant, consistent with the severe reduction in leaf width in this triple mutant.

Variation in vein density and mesophyll cell architecture in a rice deletion mutant population

There is a need to develop rice plants with improved photosynthetic capacity and efficiency in order to enhance potential grain yield. Alterations in internal leaf morphology may be needed to underpin some of these improvements. One target is the production of a 'Kranz-like' anatomy, commonly considered to be required to achieve the desired levels of photosynthesis seen in C(4) crops. Kranz anatomy typically has two or three mesophyll cells interspersing adjacent veins. As a first step to determining the potential for such anatomical modifications in rice leaves, a population of rice deletion mutants was analysed for alterations in vein patterning and mesophyll cells in the interveinal regions. Significant variation is demonstrated in vein arrangement and the sequential distribution of major and minor veins across the leaf width, although there is a significant correlation between the total number of veins present and the width of the leaf. Thus the potential is demonstrated for modifying rice leaf structure. Six distinct rice mutant lines, termed altered leaf morphology (alm) mutants, were analysed for the architecture of their interveinal mesophyll cell arrangement. It is shown that in these mutant lines, the distance between adjacent minor veins and adjacent minor and major veins is essentially determined by the size of the interveinal mesophyll cells rather than changes in mesophyll cell number across this region, and hence interveinal distance changes as a result of cell expansion rather than cell division. This observation will be important when developing screens for traits relevant for the introduction of Kranz anatomy into rice.

Disruption of signaling in a fungal-grass symbiosis leads to pathogenesis

Symbiotic associations between plants and fungi are a dominant feature of many terrestrial ecosystems, yet relatively little is known about the signaling, and associated transcriptome profiles, that define the symbiotic metabolic state. Using the Epichloë festucae-perennial ryegrass (Lolium perenne) association as a model symbiotic experimental system, we show an essential role for the fungal stress-activated mitogen-activated protein kinase (sakA) in the establishment and maintenance of this mutualistic interaction. Deletion of sakA switches the fungal interaction with the host from mutualistic to pathogenic. Infected plants exhibit loss of apical dominance, premature senescence, and dramatic changes in development, including the formation of bulb-like structures at the base of tillers that lack anthocyanin pigmentation. A comparison of the transcriptome of wild-type and sakA associations using high-throughput mRNA sequencing reveals dramatic changes in fungal gene expression consistent with the transition from restricted to proliferative growth, including a down-regulation of several clusters of secondary metabolite genes and up-regulation of a large set of genes that encode hydrolytic enzymes and transporters. Analysis of the plant transcriptome reveals up-regulation of host genes involved in pathogen defense and transposon activation as well as dramatic changes in anthocyanin and hormone biosynthetic/responsive gene expression. These results highlight the fine balance between mutualism and antagonism in a plant-fungal interaction and the power of deep mRNA sequencing to identify candidate sets of genes underlying the symbiosis.

COE1, an LRR-RLK responsible for commissural vein pattern formation in rice

Leaf veins have a complex network pattern. Formation of this vein pattern has been widely studied as a model of tissue pattern formation in plants. To understand the molecular mechanism governing the vascular patterning process, we isolated the rice mutant, commissural vein excessive1 (coe1). The coe1 mutants had short commissural vein (CV) intervals and produced clustered CVs. Application of 1-N-naphthylphthalamic acid and brefeldin A decreased CV intervals, and application of 1-naphthaleneacetic acid increased CV intervals in wild-type rice; however, coe1 mutants were insensitive to these chemicals. COE1 encodes a leucine-rich repeat receptor-like kinase, whose amino acid sequence is similar to that of brassinosteroid-insensitive 1-associated receptor kinase 1 (BAK1), and which is localized at the plasma membrane. Because of the sequence similarity of COE1 to BAK1, we also examined the involvement of brassinosteroids in CV formation. Brassinolide, an active brassinosteroid, decreased the CV intervals of wild-type rice, and brassinazole, an inhibitor of brassinosteroid biosynthesis, increased the CV intervals of wild-type rice, but coe1 mutants showed insensitivity to these chemicals. These results suggest that auxin and brassinosteroids regulate CV intervals in opposite directions, and COE1 may regulate CV intervals downstream of auxin and brassinosteroid signals.