Summer dormancy and winter growth: root survival strategy in a perennial monocotyledon

Physiology, genomics, and evolutionary aspects of desert plants

BACKGROUND

Despite the exposure to arid environmental conditions across the globe ultimately hampering the sustainability of the living organism, few plant species are equipped with several unique genotypic, biochemical, and physiological features to counter such harsh conditions. Physiologically, they have evolved with reduced leaf size, spines, waxy cuticles, thick leaves, succulent hydrenchyma, sclerophyll, chloroembryo, and photosynthesis in nonfoliar and other parts. At the biochemical level, they are evolved to perform efficient photosynthesis through Crassulacean acid metabolism (CAM) and C4 pathways with the formation of oxaloacetic acid (Hatch-Slack pathway) instead of the C3 pathway. Additionally, comparative genomics with existing data provides ample evidence of the xerophytic plants' positive selection to adapt to the arid environment. However, adding more high-throughput sequencing of xerophyte plant species is further required for a comparative genomic study toward trait discovery related to survival. Learning from the mechanism to survive in harsh conditions could pave the way to engineer crops for future sustainable agriculture.

AIM OF THE REVIEW

The distinct physiology of desert plants allows them to survive in harsh environments. However, the genomic composition also contributes significantly to this and requires great attention. This review emphasizes the physiological and genomic adaptation of desert plants. Other important parameters, such as desert biodiversity and photosynthetic strategy, are also discussed with recent progress in the field. Overall, this review discusses the different features of desert plants, which prepares them for harsh conditions intending to translate knowledge to engineer plant species for sustainable agriculture.

KEY SCIENTIFIC CONCEPTS OF REVIEW

This review comprehensively presents the physiology, molecular mechanism, and genomics of desert plants aimed towards engineering a sustainable crop.

NADES Compounds Identified in Hypoxis hemerocallidea Corms during Dormancy

Soaking Hypoxis hemerocallidea corms in distilled water improved the propagation and development of cormlets, suggesting the potential leaching-out of inhibitory chemical compounds. To investigate the presence of inhibitory compounds, nuclear magnetic resonance (NMR) spectral data of the leachate from dormant H. hemerocallidea corms were obtained using a 600 MHz ¹H-NMR spectrometer. The ¹H-NMR analysis led to the identification of choline, succinate, propylene glycol, and lactose, as inhibitory compounds. These four chemical compounds are part of the “Natural Deep Eutectic Solvents” (NADES) that protect plant cells during stress periods, each of which has the potential to inhibit bud growth and development. These compounds are supposedly leached out of the corms during the first rain under natural conditions, possibly accompanied by changes in the ratios of dormancy-breaking phytohormones and inhibitory compounds, to release bud dormancy. The identified chemical compounds heralded a novel frontier in the vegetative propagation of H. hemerocallidea as a medicinal plant, and for its enhanced sustainable uses.

Isolation of rhizosheath and analysis of microbial community structure around roots of Stipa grandis

Root zone microbial structure is particularly complex in plants with rhizosheaths, and greater understanding of the rhizosheath may play an important role in the future development of sustainable agricultural practices. However, one important reason to focus study on rhizosheath microbial structure is that there is no definite method for rhizosheath separation. The aim of this study was to explore rhizosheath isolation methods and the diversity characteristics of microorganisms around the rhizosphere. In this study, we isolated the rhizosheath of Stipa grandis, a dominant species in desert steppe, and the microorganisms in the roots, root epidermis, rhizosheath and rhizosphere soil were extracted and sequenced by 16S rRNA and ITS. The alpha diversity index of bacteria in Stipa grandis rhizosphere soil was the greatest, followed by rhizosheath, and the alpha diversity index of endophytic bacteria in root system was the smallest. The alpha diversity index of fungi in the rhizosheath and rhizosphere soil were significantly higher than that in the root epidermis and root system. There were significant differences in bacterial community structure between the root epidermis, endophytic bacteria, rhizosheath and rhizosphere soil. Unlike bacterial community structure, the community structure of fungi in the root epidermis was similar that of endophytic fungi, but significantly different from those in rhizosheath and rhizosphere soil. This study demonstrated a feasible method for separating plant rhizosheath and root epidermis. We suggest that the root epidermis can act as the interface between the host plant root and the external soil environment. We will have to re-examine the biological and ecological significance of rhizosheath and microorganisms in rhizosheath, as well as the mechanism explaining the close relationship of the rhizosheath and the plant root epidermis. This study provides theoretical and technical guidance for the isolation of the plant rhizosheath and the study of microorganisms in plant rhizosheath.

Unexpected diversity and evolutionary lability in root architectural ecomorphs in the rushes of the hyperdiverse Cape flora

Plants use roots to access soil resources, so differences in root traits and their ecological consequences could be a mechanism of species coexistence and niche divergence. Current views of the evolution of root diversity are informed by large-scale evolutionary analyses based on taxonomically coarse sampling and led to the 'root trait phylogenetic conservatism hypothesis'. Here we test this hypothesised conservatism among closely related species, and whether root variation plays an ecological role. We collected root architectural traits for the species-rich Cape rushes (Restionaceae) in the field and from herbaria. We used machine learning to interpolate missing data. Using model-based clustering we classified root syndromes. We modelled the proportion of the syndromes along environmental gradients using assemblages and environmental data of 735 plots. We fitted trait evolutionary models to test for the conservatism hypothesis. We recognised five root syndromes. Responses to environmental gradients are syndrome specific and thus these represent ecomorphs. Trait evolutionary models reveal an evolutionary lability in these ecomorphs. This could present the mechanistic underpinning of the taxonomic radiation of this group which has been linked to repeated habitat shifts. Our results challenge the perspective of strong phylogenetic conservatism and root trait evolution may more generally drive diversification.

The response of fine root morphological and physiological traits to added nitrogen in Schrenk's spruce (Picea schrenkiana) of the Tianshan mountains, China

Fine roots are essential for water and nutrient uptake in plants, but little is known about the variation in fine root traits and the underlying mechanisms that drive it. Understanding the responses of fine root function traits to changing environmental conditions and the role of fine root traits as drivers of forest ecosystem processes are critical for informing physiological and ecological theory as well as ecosystem management. We measured morphological and physiological traits of fine roots from six soil layers and three diameter classes in Schrenk’s spruce (Picea shrenkiana) forests of the Tianshan mountains, China. We found significant effects of nitrogen addition on these morphological and physiological traits, which varied by soil layer and root diameter. Specifically, specific root length (SRL) was higher in medium N addition group (N2) than in control group (N0). Specific root area (SRA) was higher in the control group (N0) than fertilized groups (N1, N2 and N3). Root tissue density (RTD) was higher in low N addition group (N1) than in the other group. Root dry matter content had no significant difference among four treatment groups. SRL, SRA, and RTD of fine roots in different diameter classes were all significantly different between high N addition (N3) and the control (N0) groups. The physiological characteristics of fine roots showed that soluble sugar (SS), fine root vitality (FRV), and tissue water content (TWC) in different soil layers were higher in the control group than in the fertilized groups. While soluble protein (SP), malondialdehyde (MDA) and free proline (FP) were lower in the control group (N0) than in the fertilized groups. In addition, SS, FRV, SP, TWC, FP, and MDA in all N addition treatments groups were significantly different from the control group. Fine root morphological traits were closely related to physiological traits, and added nitrogen inputs change these correlations. Our study confirms that nitrogen addition has specific effects on the morphological and physiological traits of fine roots of Schrenk’s spruce, and the effects of N addition vary according to the amount added.

Are winter and summer dormancy symmetrical seasonal adaptive strategies? The case of temperate herbaceous perennials

BACKGROUND

Dormancy in higher plants is an adaptive response enabling plant survival during the harshest seasons and has been more explored in woody species than in herbaceous species. Nevertheless, winter and summer shoot meristem dormancy are adaptive strategies that could play a major role in enhancing seasonal stress tolerance and resilience of widespread herbaceous plant communities.

SCOPE

This review outlines the symmetrical aspects of winter and summer dormancy in order to better understand plant adaptation to severe stress, and highlight research priorities in a changing climate. Seasonal dormancy is a good model to explore the growth-stress survival trade-off and unravel the relationships between growth potential and stress hardiness. Although photoperiod and temperature are known to play a crucial, though reversed, role in the induction and release of both types of dormancy, the thresholds and combined effects of these environmental factors remain to be identified. The biochemical compounds involved in induction or release in winter dormancy (abscisic acid, ethylene, sugars, cytokinins and gibberellins) could be a priority research focus for summer dormancy. To address these research priorities, herbaceous species, being more tractable than woody species, are excellent model plants for which both summer and winter dormancy have been clearly identified.

CONCLUSIONS

Summer and winter dormancy, although responding to inverse conditions, share many characteristics. This analogous nature can facilitate research as well as lead to insight into plant adaptations to extreme conditions and the evolution of phenological patterns of species and communities under climate change. The development of phenotypes showing reduced winter and/or enhanced summer dormancy may be expected and could improve adaptation to less predictable environmental stresses correlated with future climates. To this end, it is suggested to explore the inter- and intraspecific genotypic variability of dormancy and its plasticity according to environmental conditions to contribute to predicting and mitigating global warming.

Mineral nutrition of campos rupestres plant species on contrasting nutrient-impoverished soil types

In Brazil, the campos rupestres occur over the Brazilian shield, and are characterized by acidic nutrient-impoverished soils, which are particularly low in phosphorus (P). Despite recognition of the campos rupestres as a global biodiversity hotspot, little is known about the diversity of P-acquisition strategies and other aspects of plant mineral nutrition in this region. To explore nutrient-acquisition strategies and assess aspects of plant P nutrition, we measured leaf P and nitrogen (N) concentrations, characterized root morphology and determined the percentage arbuscular mycorrhizal (AM) colonization of 50 dominant species in six communities, representing a gradient of soil P availability. Leaf manganese (Mn) concentration was measured as a proxy for carboxylate-releasing strategies. Communities on the most P-impoverished soils had the highest proportion of nonmycorrhizal (NM) species, the lowest percentage of mycorrhizal colonization, and the greatest diversity of root specializations. The large spectrum of leaf P concentration and variation in root morphologies show high functional diversity for nutritional strategies. Higher leaf Mn concentrations were observed in NM compared with AM species, indicating that carboxylate-releasing P-mobilizing strategies are likely to be present in NM species. The soils of the campos rupestres are similar to the most P-impoverished soils in the world. The prevalence of NM strategies indicates a strong global functional convergence in plant mineral nutrition strategies among severely P-impoverished ecosystems.

Ecological importance of sedges: a survey of the Australasian Cyperaceae genus Lepidosperma

BACKGROUND

Sedges (Cyperaceae) form an important ecological component of many ecosystems around the world. Sword and rapier sedges (genus Lepidosperma) are common and widespread components of the southern Australian and New Zealand floras, also occurring in New Caledonia, West Papua, Borneo, Malaysia and southern China. Sedge ecology is seldom studied and no comprehensive review of sedge ecology exists. Lepidosperma is unusual in the Cyperaceae with the majority of species occurring in dryland habitats.

SCOPE

Extensive review of ecological literature and field observations shows Lepidosperma species to be important components of many ecosystems, often dominating understorey and sedge-rich communities. For the first time, a detailed ecological review of a Cyperaceae genus is presented.

CONCLUSIONS

Lepidosperma species are long-lived perennials with significant abundance and persistence in the landscape. Speciation patterns in the genus are of considerable interest due to complex biogeographical patterns and a high degree of habitat specificity. Potential benefits exist for medicinal products identified from several Lepidosperma species. Over 178 organisms, including 26 mammals, 42 birds, six reptiles, five amphibians, eight arachnids, 75 insects, three crustaceans and 13 fungi, are found to be dependent on, or making use of, Lepidosperma species. A significant relationship exists between Lepidosperma species and the moth genus Elachista. Implications for the conservation and ecology of both sedges and associated species are discussed.

Aluminium tolerance of root hairs underlies genotypic differences in rhizosheath size of wheat (Triticum aestivum) grown on acid soil

We found significant genetic variation in the ability of wheat (Triticum aestivum) to form rhizosheaths on acid soil and assessed whether differences in aluminium (Al(3+) ) tolerance of root hairs between genotypes was the physiological basis for this genetic variation. A method was developed to rapidly screen rhizosheath size in a range of wheat genotypes. Backcrossed populations were generated from cv Fronteira (large rhizosheath) using cv EGA-Burke (small rhizosheath) as the recurrent parent. A positive correlation existed between rhizosheath size on acid soil and root hair length. In hydroponic experiments, root hairs of the backcrossed lines with large rhizosheaths were more tolerant of Al(3+) toxicity than the backcrossed lines with small rhizosheaths. We conclude that greater Al(3+) tolerance of root hairs underlies the larger rhizosheath of wheat grown on acid soil. Tolerance of the root hairs to Al(3+) was largely independent of the TaALMT1 gene which suggests that different genes encode the Al(3+) tolerance of root hairs. The maintenance of longer root hairs in acid soils is important for the efficient uptake of water and nutrients.

Development and persistence of sandsheaths of Lyginia barbata (Restionaceae): relation to root structural development and longevity

BACKGROUND AND AIMS

Strongly coherent sandsheaths that envelop perennial roots of many monocotyledonous species of arid environments have been described for over a century. This study, for the first time, details the roles played by the structural development of the subtending roots in the formation and persistence of the sheaths.

METHODS

The structural development of root tissues associated with persistent sandsheaths was studied in Lyginia barbata, native to the Western Australian sand plains. Cryo-scanning electron microscopy CSEM, optical microscopy and specific staining methods were applied to fresh, field material. The role of root hairs was clarified by monitoring sheath development in roots separated from the sand profile by fine mesh.

KEY RESULTS AND CONCLUSIONS

The formation of the sheaths depends entirely on the numerous living root hairs which extend into the sand and track closely around individual grains enmeshing, by approx. 12 cm from the root tip, a volume of sand more than 14 times that of the subtending root. The longevity of the perennial sheaths depends on the subsequent development of the root hairs and of the epidermis and cortex. Before dying, the root hairs develop cellulosic walls approx. 3 µm thick, incrusted with ferulic acid and lignin, which persist for the life of the sheath. The dead hairs remain in place fused to a persistent platform of sclerified epidermis and outer cortex. The mature cortex comprises this platform, a wide, sclerified inner rim and a lysigenous central region - all dead tissue. We propose that the sandsheath/root hair/epidermis/cortex complex is a structural unit facilitating water and nutrient uptake while the tissues are alive, recycling scarce phosphorus during senescence, and forming, when dead, a persistent essential structure for maintenance of a functional stele in the perennial Lyginia roots.

Comparative proteomic profiles of the soybean (Glycine max) root apex and differentiated root zone

The root apical meristem (RAM) is responsible for the growth of the plant root system. Because of the importance of root architecture in the performance of crop plants, we established a proteome reference map of the soybean root apex and compared this with the proteome of the differentiated root zone. The root apex samples contained the apical 1 mm of the root, comprising the RAM, quiescent center and root cap. We identified 342 protein spots from 550 excised proteins (∼62%) of root apex samples by MALDI-TOF MS/MS analysis. All these proteins were also present in the differentiated root, but differed in abundance. Functional classification showed that the most numerous protein categories represented in the root were those of stress response, glycolysis, redox homeostasis and protein processing. Using DIGE, we identified 73 differentially accumulated proteins between root apex and differentiated root. Proteins overrepresented in the root apex belonged primarily to the pathways for protein synthesis and processing, cell redox homeostasis and flavonoid biosynthesis. Proteins underrepresented in the root apex were those of glycolysis, tricarboxylic acid metabolism and stress response. Our results highlight the importance of stress and defense response, redox control and flavonoid metabolism in the root apex.

Evolution of tree nutrition

Using a broad definition of trees, the evolutionary origins of trees in a nutritional context is considered using data from the fossil record and molecular phylogeny. Trees are first known from the Late Devonian about 380 million years ago, originated polyphyletically at the pteridophyte grade of organization; the earliest gymnosperms were trees, and trees are polyphyletic in the angiosperms. Nutrient transporters, assimilatory pathways, homoiohydry (cuticle, intercellular gas spaces, stomata, endohydric water transport systems including xylem and phloem-like tissue) and arbuscular mycorrhizas preceded the origin of trees. Nutritional innovations that began uniquely in trees were the seed habit and, certainly (but not necessarily uniquely) in trees, ectomycorrhizas, cyanobacterial, actinorhizal and rhizobial (Parasponia, some legumes) diazotrophic symbioses and cluster roots.

Seasonal water relations of Lyginia barbata (Southern rush) in relation to root xylem development and summer dormancy of root apices

*Periods of dormancy in shallow roots allow perennial monocotyledons to establish deep root systems, but we know little about patterns of xylem maturation, water-transport capacities and associated economies in water use of growing and dormant roots. *Xylem development, anatomy, conductance and in situ cellular [K] and [Cl] were investigated in roots of field-grown Lyginia barbata (Restionaceae) in Mediterranean southwestern Australia. Parallel studies of gas exchange, culm relative water loss and soil water content were conducted. *Stomatal conductance and photosynthesis decreased during summer drought as soil profiles dried, but rates recovered when dormant roots became active with the onset of wetter conditions. Anatomical studies identified sites of close juxtaposition of phloem and xylem in dormant and growing roots. Ion data and dye tracing showed mature late metaxylem of growing roots was located >or= 100 mm from the tip, but at only <or= 10 mm for dormant roots. Dormant roots remained hydrated in dry soils (0.001-0.005 g g(-1)). *Effective regulation of growth and water-conserving/obtaining properties permits the survival of shallow roots of L. barbata during summer drought and may represent important strategies for establishing deeper perennial root systems in other monocotyledonous plants adapted to seasonally dry habitats.