Comparative venation costs of monocotyledon and dicotyledon species in the eastern Colorado steppe.

MAIN CONCLUSION: Leaf vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles did not differ between monocot and dicot species in 21 species from the eastern Colorado steppe. Dicots possessed significantly larger minor vein networks than monocots. Across the tree of life, there is evidence that dendritic vascular transport networks are optimized, balancing maximum speed and integrity of resource delivery with minimal resource investment in transport and infrastructure. Monocot venation, however, is not dendritic, and remains parallel down to the smallest vein orders with no space-filling capillary networks. Given this departure from the "optimized" dendritic network, one would assume that monocots are operating at a significant energetic disadvantage. In this study, we investigate whether monocot venation networks bear significantly greater carbon/construction costs per leaf volume than co-occurring dicots in the same ecosystem, and if so, what physiological or ecological advantage the monocot life form possesses to compensate for this deficit. Given that venation networks could also be optimized for leaf mechanical support or provide herbivory defense, we measured the vascular system of both monocot and dicots at three scales to distinguish between leaf investment in mechanical support (macroscopic vein), total transport and capacitance (vascular bundle), or exclusively water transport (xylem) for both parallel and dendritic venation networks. We observed that vein network cost (total vein surface area per leaf volume) for major veins and vascular bundles was not significantly different between monocot species and dicot species. Dicots, however, possess significantly larger minor vein networks than monocots. The 19 species subjected to gas-exchange measurement in the field displayed a broad range of Amax and but demonstrated no significant relationships with any metric of vascular network size in major or minor vein classes. Given that monocots do not seem to display any leaf hydraulic disadvantage relative to dicots, it remains an important research question why parallel venation (truly parallel, down to the smallest vessels) has not arisen more than once in the history of plant evolution.

Anatomical insights into the vascular layout of the barley rachis: implications for transport and spikelet connection.

BACKGROUND AND AIMS: Vascular patterning is intimately related to plant form and function. Here, using barley (Hordeum vulgare) as a model, we studied the vascular anatomy of the spike-type inflorescence. The main aim of the present work was to clarify the relationship between rachis (spike axis) vasculature and spike size, to define vascular dynamics and to discuss the implications for transport capacity and its interaction with the spikelets. METHODS: We used serial transverse internode sections to determine the internode area, vascular area and number of veins along the rachis of several barley lines. KEY RESULTS: Internode area and total vascular area show a clear positive correlation with spike size, whereas the number of veins is only weakly correlated. The lateral periphery of the rachis contains large mature veins of constant size, whereas the central part is occupied by small immature veins. Spikelet-derived veins entering the rachis often merge with the immature rachis veins but never merge with the mature veins. An increase in floret fertility through the conversion of a two-rowed barley into an isogenic six-rowed line, in addition to a decrease in floret fertility owing to enhanced pre-anthesis tip degeneration caused by the mutation tip sterile 2.b (tst2.b), significantly affected vein size but had limited to no effects on the number of veins or internode area. CONCLUSIONS: The rachis vasculature is the result of a two-step process involving an initial layout followed by size adjustment according to floret fertility/spike size. The restriction of large mature vessels to the periphery and that of small immature vessels to the centre of the rachis suggests that long-distance transport and local supply to spikelets are spatially separated processes. The identification of spikelet-derived veins entering the rachis without fusing with its vasculature indicates that a vascular continuity between rachis and spikelets might be non-essential.

Photosynthesis-independent production of reactive oxygen species in the rice bundle sheath during high light is mediated by NADPH oxidase.

When exposed to high light, plants produce reactive oxygen species (ROS). In Arabidopsis thaliana, local stress such as excess heat or light initiates a systemic ROS wave in phloem and xylem cells dependent on NADPH oxidase/respiratory burst oxidase homolog (RBOH) proteins. In the case of excess light, although the initial local accumulation of ROS preferentially takes place in bundle-sheath strands, little is known about how this response takes place. Using rice and the ROS probes diaminobenzidine and 2',7'-dichlorodihydrofluorescein diacetate, we found that, after exposure to high light, ROS were produced more rapidly in bundle-sheath strands than mesophyll cells. This response was not affected either by CO2 supply or photorespiration. Consistent with these findings, deep sequencing of messenger RNA (mRNA) isolated from mesophyll or bundle-sheath strands indicated balanced accumulation of transcripts encoding all major components of the photosynthetic apparatus. However, transcripts encoding several isoforms of the superoxide/H2O2-producing enzyme NADPH oxidase were more abundant in bundle-sheath strands than mesophyll cells. ROS production in bundle-sheath strands was decreased in mutant alleles of the bundle-sheath strand preferential isoform of OsRBOHA and increased when it was overexpressed. Despite the plethora of pathways able to generate ROS in response to excess light, NADPH oxidase-mediated accumulation of ROS in the rice bundle-sheath strand was detected in etiolated leaves lacking chlorophyll. We conclude that photosynthesis is not necessary for the local ROS response to high light but is in part mediated by NADPH oxidase activity.

Persistent decay of fresh xylem hydraulic conductivity varies with pressure gradient and marks plant responses to injury.

Defining plant hydraulic traits is central to the quantification of ecohydrological processes ranging from land-atmosphere interactions, to tree mortality and water-carbon budgets. A key plant trait is the xylem specific hydraulic conductivity (Kx ), that describes the plant's vascular system capacity to transport water. While xylem's vessels and tracheids are dead upon maturity, the xylem is neither inert nor deadwood, various components of the sapwood and surrounding tissue remaining alive and functional. Moreover, the established definition of Kx assumes linear relations between water flux and pressure gradient by tacitly considering the xylem as a "passive conduit". Here, we re-examine this notion of an inert xylem by systematically characterizing xylem flow in several woody plants using Kx measurements under constant and cyclic pressure gradients. Results show a temporal and pressure gradient dependence of Kx . Additionally, microscopic features in "living branches" are irreversibly modified upon drying of the xylem, thus differentiating the macroscopic definition of Kx for living and dead xylem. The findings highlight the picture of the xylem as a complex and delicate conductive system whose hydraulic behaviour transcends a passive gradient-based flow. The study sheds new light on xylem conceptualization, conductivity measurement protocols, in situ long-distance water transport and ecosystem modelling.

The AGCVIII kinase Dw2 modulates cell proliferation, endomembrane trafficking, and MLG/xylan cell wall localization in elongating stem internodes of Sorghum bicolor.

Stems of bioenergy sorghum (Sorghum bicolor L. Moench.), a drought-tolerant C4 grass, contain up to 50 nodes and internodes of varying length that span 4-5 m and account for approximately 84% of harvested biomass. Stem internode growth impacts plant height and biomass accumulation and is regulated by brassinosteroid signaling, auxin transport, and gibberellin biosynthesis. In addition, an AGCVIII kinase (Dw2) regulates sorghum stem internode growth, but the underlying mechanism and signaling network are unknown. Here we provide evidence that mutation of Dw2 reduces cell proliferation in internode intercalary meristems, inhibits endocytosis, and alters the distribution of heteroxylan and mixed linkage glucan in cell walls. Phosphoproteomic analysis showed that Dw2 signaling influences the phosphorylation of proteins involved in lipid signaling (PLDδ), endomembrane trafficking, hormone, light, and receptor signaling, and photosynthesis. Together, our results show that Dw2 modulates endomembrane function and cell division during sorghum internode growth, providing insight into the regulation of monocot stem development.

Transcription-mediated tissue-specific lignification of vascular bundle causes trade-offs between growth and defence capacity during invasion of Solidago canadensis.

Allocation of more resources to growth but less to defense causing growth vigor of invasive alien plant populations contributes to successful invasion. However, few studies has addressed to relationship between vascular development variation and this mechanism. In this study, a common garden experimentwas established to compare the growth and vascular bundle development between native and introduced populations of Solidago canadensis, which is a wide-distributed invasive species in China. Our results suggested that the rapid growth of introduced populations could be explained by the well-developed and highly lignified xylem; while native populations present more developed and highly lignified phloem, which contributed more resistance to the infection of Sclerotiun rofsii compared with introduced populations. This difference was resulted from tissue-specific tradeoff distribution of lignification related gene expression between xylem and phloem, which is regulated by upstream MYB transcription factors. Our study gives a novel insight of mechanism that explain invasion success: lignin-related gene transcription-mediated tissue-specific lignification of vascular bundle contributes tradeoffs in resource allocation between growth and defence capacity during successful invasion of S. canadensis.

A coherent feed-forward loop drives vascular regeneration in damaged aerial organs of plants growing in a normal developmental context.

Aerial organs of plants, being highly prone to local injuries, require tissue restoration to ensure their survival. However, knowledge of the underlying mechanism is sparse. In this study, we mimicked natural injuries in growing leaves and stems to study the reunion between mechanically disconnected tissues. We show that PLETHORA (PLT) and AINTEGUMENTA (ANT) genes, which encode stem cell-promoting factors, are activated and contribute to vascular regeneration in response to these injuries. PLT proteins bind to and activate the CUC2 promoter. PLT proteins and CUC2 regulate the transcription of the local auxin biosynthesis gene YUC4 in a coherent feed-forward loop, and this process is necessary to drive vascular regeneration. In the absence of this PLT-mediated regeneration response, leaf ground tissue cells can neither acquire the early vascular identity marker ATHB8, nor properly polarise auxin transporters to specify new venation paths. The PLT-CUC2 module is required for vascular regeneration, but is dispensable for midvein formation in leaves. We reveal the mechanisms of vascular regeneration in plants and distinguish between the wound-repair ability of the tissue and its formation during normal development.

Vascular Cambium-Localized AtSPDT Mediates Xylem-to-Phloem Transfer of Phosphorus for Its Preferential Distribution in Arabidopsis.

During plant growth and development mineral elements are preferentially delivered to different organs and tissues to meet the differential demand. It has been shown that the preferential distribution of mineral nutrients in gramineous plants is mediated by node-based transporters, but the mechanisms of preferential distribution in dicots are poorly understood. Here, we report a distinct mechanism for the preferential distribution of phosphorus (P) in Arabidopsis plants, revealed by detailed functional analysis of AtSPDT/AtSULTR3;4 (SULTR-like P Distribution Transporter), a homolog of rice OsSPDT. Like OsSPDT, AtSPDT is localized at the plasma membrane and showed proton-dependent transport activity for P. Interestingly, we found that AtSPDT is mainly expressed in the rosette basal region and leaf petiole, and its expression is up-regulated by P deficiency. Tissue-specific analysis showed that AtSPDT is mainly located in the vascular cambium of different organs, as well as in the parenchyma tissues of both xylem and phloem regions. Knockout of AtSPDT inhibited the growth of new leaves under low P due to decreased P distribution to those organs. The seed yields of the wild-type and atspdt mutant plants are similar, but the seeds of mutant plants contain - less P. These results indicate that AtSPDT localized in the vascular cambium is involved in preferential distribution of P to the developing tissues, through xylem-to-phloem transfer mainly at the rosette basal region and leaf petiole.

Vascular bundle sheath and mesophyll cells modulate leaf water balance in response to chitin.

Plants can detect pathogen invasion by sensing microbe-associated molecular patterns (MAMPs). This sensing process leads to the induction of defense responses. Numerous MAMP mechanisms of action have been described in and outside the guard cells. Here, we describe the effects of chitin, a MAMP found in fungal cell walls and insects, on the cellular osmotic water permeability (Pf ) of the leaf vascular bundle-sheath (BS) and mesophyll cells (MCs), and its subsequent effect on leaf hydraulic conductance (Kleaf ). BS is a parenchymatic tissue that tightly encases the vascular system. BS cells (BSCs) have been shown to influence Kleaf through changes in their Pf , for example, after sensing the abiotic stress response-regulating hormone abscisic acid. It was recently reported that, in Arabidopsis, the chitin receptors-like kinases, chitin elicitor receptor kinase 1 (CERK1) and LYSINE MOTIF RECEPTOR KINASE 5 (LYK5) are highly expressed in the BS as well as the neighboring mesophyll. Therefore, we studied the possible impact of chitin on these cells. Our results revealed that BSCs and MCs exhibit a sharp decrease in Pf in response to chitin treatment. In addition, xylem-fed chitin decreased Kleaf and led to stomatal closure. However, Atlyk5 mutant showed none of these responses. Complementing AtLYK5 in the BSCs (using the SCARECROW promoter) resulted in the response to chitin that was similar to that observed in the wild-type. These results suggest that BS play a role in the perception of apoplastic chitin and in initiating chitin-triggered immunity.

The flowering hormone florigen accelerates secondary cell wall biogenesis to harmonize vascular maturation with reproductive development.

Florigen, a proteinaceous hormone, functions as a universal long-range promoter of flowering and concurrently as a generic growth-attenuating hormone across leaf and stem meristems. In flowering plants, the transition from the vegetative phase to the reproductive phase entails the orchestration of new growth coordinates and a global redistribution of resources, signals, and mechanical loads among organs. However, the ultimate cellular processes governing the adaptation of the shoot system to reproduction remain unknown. We hypothesized that if the mechanism for floral induction is universal, then the cellular metabolic mechanisms underlying the conditioning of the shoot system for reproduction would also be universal and may be best regulated by florigen itself. To understand the cellular basis for the vegetative functions of florigen, we explored the radial expansion of tomato stems. RNA-Seq and complementary genetic and histological studies revealed that florigen of endogenous, mobile, or induced origins accelerates the transcription network navigating secondary cell wall biogenesis as a unit, promoting vascular maturation and thereby adapting the shoot system to the developmental needs of the ensuing reproductive phase it had originally set into motion. We then demonstrated that a remarkably stable and broadly distributed florigen promotes MADS and MIF genes, which in turn regulate the rate of vascular maturation and radial expansion of stems irrespective of flowering or florigen level. The dual acceleration of flowering and vascular maturation by florigen provides a paradigm for coordinated regulation of independent global developmental programs.

Differential regulation of auxin and cytokinin during the secondary vascular tissue regeneration in Populus trees.

Tissue regeneration upon wounding in plants highlights the developmental plasticity of plants. Previous studies have described the morphological and molecular changes of secondary vascular tissue (SVT) regeneration after large-scale bark girdling in trees. However, how phytohormones regulate SVT regeneration is still unknown. Here, we established a novel in vitro SVT regeneration system in the hybrid aspen (Populus tremula × Populus tremuloides) clone T89 to bypass the limitation of using field-grown trees. The effects of phytohormones on SVT regeneration were investigated by applying exogenous hormones and utilizing various transgenic trees. Vascular tissue-specific markers and hormonal response factors were also examined during SVT regeneration. Using this in vitro regeneration system, we demonstrated that auxin and cytokinin differentially regulate phloem and cambium regeneration. Whereas auxin is sufficient to induce regeneration of phloem prior to continuous cambium restoration, cytokinin only promotes the formation of new phloem, not cambium. The positive role of cytokinin on phloem regeneration was further confirmed in cytokinin overexpression trees. Analysis of a DR5 reporter transgenic line further suggested that cytokinin blocks the re-establishment of auxin gradients, which is required for the cambium formation. Investigation on the auxin and cytokinin signalling genes indicated these two hormones interact to regulate SVT regeneration. Taken together, the in vitro SVT regeneration system allows us to make use of various molecular and genetic tools to investigate SVT regeneration. Our results confirmed that complementary auxin and cytokinin domains are required for phloem and cambium reconstruction.

A new interpretation on vascular architecture of the cauline system in Commelinaceae (Commelinales).

The vascular system of monocotyledons, including Commelinaceae, has been studied since the 19th century, but to date, the proposed vascular architecture models consist of schematic representations partially based on the authors' interpretation. One of the greatest difficulties in studying these systems is the large number of vascular bundles and the complexity of their connections, especially in the monocotyledons which have a nodal vascular plexus. In this study, shoot apex samples of 14 species of Commelinaceae were submitted to three-dimensional analyses (confocal microscopy, X-ray microtomography, graphic vectorization, and whole-mount diaphanization), as well as conventional techniques in plant anatomy. Based on the results, a previously unreported category of bundles is described in Commelinaceae for the first time, as well as the fact that peripheral bundles are not interrupted or end blindly in the periphery of the pith, as previously thought. With this new interpretation of the vascular architecture, three patterns of nodal vascular plexus are proposed: 1) in the first pattern the internal nodal vascular plexus (IVP) forms a continuous cylinder and does not connect to the external nodal vascular plexus (EVP); 2) the IVP forms a cylinder divided into two columns and does not connect to the EVP and 3) the IVP forms a cylinder connected to the EVP. The first description of central bundles in the Commelinaceae might suggests their existence in closely related groups, such as the remaining four families of Commelinales (i.e., Haemodoraceae, Hanguanaceae, Philydraceae, and Pontederiaceae), and even in other distantly related groups of monocotyledons.

Linking phenylpropanoid metabolism, lignin deposition, and plant growth inhibition.

Lignin, a polymer found in the plant secondary cell wall, is a major contributor to biomass' recalcitrance toward saccharification. Because of this negative impact toward the value of lignocellulosic crops, there is a special interest in modifying the content and composition of this important plant biopolymer. For many years this endeavor has been hindered by the plant growth inhibition that is often associated with manipulations to phenylpropanoid metabolism. Although the actual mechanism by which dwarfism arises remains unknown, recent advances in tissue-specific lignin complementation and better understanding of phenylpropanoid transcriptional regulation has made it possible to disentangle lignin modification from perturbations in plant development.

Bundle sheath extensions affect leaf structural and physiological plasticity in response to irradiance.

Coordination between structural and physiological traits is key to plants' responses to environmental fluctuations. In heterobaric leaves, bundle sheath extensions (BSEs) increase photosynthetic performance (light-saturated rates of photosynthesis, Amax ) and water transport capacity (leaf hydraulic conductance, Kleaf ). However, it is not clear how BSEs affect these and other leaf developmental and physiological parameters in response to environmental conditions. The obscuravenosa (obv) mutation, found in many commercial tomato varieties, leads to absence of BSEs. We examined structural and physiological traits of tomato heterobaric and homobaric (obv) near-isogenic lines grown at two different irradiance levels. Kleaf , minor vein density, and stomatal pore area index decreased with shading in heterobaric but not in homobaric leaves, which show similarly lower values in both conditions. Homobaric plants, on the other hand, showed increased Amax , leaf intercellular air spaces, and mesophyll surface area exposed to intercellular airspace (Smes ) in comparison with heterobaric plants when both were grown in the shade. BSEs further affected carbon isotope discrimination, a proxy for long-term water-use efficiency. BSEs confer plasticity in traits related to leaf structure and function in response to irradiance levels and might act as a hub integrating leaf structure, photosynthetic function, and water supply and demand.

Getting closer: vein density in C4 leaves.

Contents Summary 1260 I. Introduction 1260 II. Molecular and genetic mechanisms of C4 leaf venation 1262 III. Conclusions and future perspectives 1266 Acknowledgements 1266 References 1266 SUMMARY: C4 grasses are major contributors to the world's food supply. Their highly efficient method of carbon fixation is a unique adaptation that combines close vein spacing and distinct photosynthetic cell types. Despite its importance, the molecular genetic basis of C4 leaf development is still poorly understood. Here we summarize current knowledge of leaf venation and review recent progress in understanding molecular and genetic regulation of vascular patterning events in C4 plants. Evidence points to the interplay of auxin, brassinosteroids, SHORTROOT/SCARECROW and INDETERMINATE DOMAIN transcription factors. Identification and functional characterization of candidate regulators acting early in vascular development will be essential for further progress in understanding the precise regulation of these processes.

Anther and pollen development in sweet cherry (Prunus avium L.) in relation to winter dormancy.

Anther and pollen development is a highly conserved process in angiosperms, but while pollen formation in annual plants occurs in a few days, in temperate woody perennials, it requires several months. How anther and pollen development is framed in terms of seasonality plays a clear part in reproductive success. In this study, seasonal anther and pollen development is characterized in two sweet cherry cultivars over 2 years, paying special attention to the period of dormancy and unveiling the role of starch in this process. We evaluated starch content from the autumn until bud burst with the help of an image analysis system fitted to a light microscope. Microscope observations allowed the temporal relationship of pollen development to the phenological stages of flower and bud development to be determined. In both cultivars and years, anther and pollen development followed the same pattern. Development was halted by dormancy, when the anthers showed no morphological changes until several weeks after chilling fulfillment, until the milder temperatures reactivated development. After dormancy, starch was accumulated in the connective tissue until tracheary element differentiation. Quantification of starch in the connective tissue of anthers revealed its importance in supporting pollen meiosis and subsequent anther growth.

Rhynie chert fossils demonstrate the independent origin and gradual evolution of lycophyte roots.

Mapping fossil traits onto the land plant phylogenetic framework indicates that there were at least two independent origins of roots among extant vascular plants - once in lycophytes and independently in euphyllophytes. At least two rooting structural types are found among extinct species preserved in the Rhynie chert. First, species that lacked roots and developed horizontal axes that developed rhizoids. Second, the rooting axes of Asteroxylon mackiei resembled the roots of extant lycopsids but lacked root hairs and root caps. These two rooting structures preceded the evolution of the roots of extant lycophytes comprising axes on which root hairs and root caps developed. These data demonstrate the defining root characters evolved gradually in the lycophyte lineage.

Histochemical observations and gene expression changes related to internal browning in tuberous roots of sweet potato (Ipomea batatas).

The mechanism underlying internal browning (IB), or brown discoloration, of the central region of tuberous roots of sweet potato (Ipomoea batatas) was examined. IB disorder begins in roots from approx. 90 days after transplanting, and the severity increases significantly with time. IB damage initially occurs in cells around the secondary vascular tissue, and the area per cell occupied by starch grains in this region was larger than in the unaffected region. High levels of reducing sugars, polyphenol oxidase (PPO) activities, chlorogenic acid, and hydrogen peroxide (H2O2) were detected in cells from the IB damaged regions. The content of sugar and polyphenols was higher in disks (transverse sections) with larger amounts of damaged tissues than in disks of sound root. The transcript levels of acid invertase (IbAIV) tended to be higher with greater IB severity, whereas fluctuation patterns of ADP-glucose pyrophosphorylase (IbAGPase), granule bound starch synthase (IbGBSS), and starch branching enzyme 1 (IbSBE1) were lower with higher IB severity. These observations suggest that the incidence of IB disorder in sweet potato is largely dependent on the excessive generation of reactive oxygen species (ROS) in cells around the secondary vascular tissues due to the abundant accumulation of sugar and/or starch grains during the root maturation period.

Uncovering C4-like photosynthesis in C3 vascular cells.

In C4 plants, the vascularization of the leaf is extended to include a ring of photosynthetic bundle sheath cells, which have essential and specific functions. In contrast to the substantial knowledge of photosynthesis in C4 plants, relatively little is known about photosynthesis in C3 plant veins, which differs substantially from that in C3 mesophyll cells. In this review we highlight the specific photosynthetic machinery present in C3 vascular cells, which likely evolved prior to the divergence between C3 and C4 plants. The associated primary processes of carbon recapture, nitrogen transport, and antioxidant metabolism are discussed. This review of the basal C4 photosynthesis in C3 plants is significant in the context of promoting the potential for biotechnological development of C4-transgenic rice crops.

In situ warming in the Antarctic: effects on growth and photosynthesis in Antarctic vascular plants.

The Antarctic Peninsula has experienced a rapid warming in the last decades. Although recent climatic evidence supports a new tendency towards stabilization of temperatures, the impacts on the biosphere, and specifically on Antarctic plant species, remain unclear. We evaluated the in situ warming effects on photosynthesis, including the underlying diffusive, biochemical and anatomical determinants, and the relative growth of two Antarctic vascular species, Colobanthus quitensis and Deschampsia antarctica, using open top chambers (OTCs) and gas exchange measurements in the field. In C. quitensis, the photosynthetic response to warming relied on specific adjustments in the anatomical determinants of the leaf CO2 transfer, which enhanced mesophyll conductance and photosynthetic assimilation, thereby promoting higher leaf carbon gain and plant growth. These changes were accompanied by alterations in the leaf chemical composition. By contrast, D. antarctica showed no response to warming, with a lack of significant differences between plants grown inside OTCs and plants grown in the open field. Overall, the present results are the first reporting a contrasting effect of in situ warming on photosynthesis and its underlying determinants, of the two unique Antarctic vascular plant species, which could have direct consequences on their ecological success under future climate conditions.