Transcriptomic and metabolomic analyses reveal the importance of ethylene networks in mulberry fruit ripening

Mulberry (Morus alba L.) is a climacteric and highly perishable fruit. Ethylene has been considered to be an important trigger of fruit ripening process. However, the role of ethylene in the mulberry fruit ripening process remains unclear. In this study, we performed a comprehensive analysis of metabolomic and transcriptomic data of mulberry fruit and the physiological changes accompanying the fruit ripening process. Our study revealed that changes in the accumulation of specific metabolites at different stages of fruit development and ripening were closely correlated to transcriptional changes as well as underlying physiological changes and the development of taste biomolecules. The ripening of mulberry fruits was highly associated with the production of endogenous ethylene, and further application of exogenous ethylene assisted the ripening process. Transcriptomic analysis revealed that differential expression of diverse ripening-related genes was involved in sugar metabolism, anthocyanin biosynthesis, and cell wall modification pathways. Network analysis of transcriptomics and metabolomics data revealed that many transcription factors and ripening-related genes were involved, among which ethylene-responsive transcription factor 3 (MaERF3) plays a crucial role in the ripening process. The role of MaERF3 in ripening was experimentally proven in a transient overexpression assay in apples. Our study indicates that ethylene plays a vital role in modulating mulberry fruit ripening. The results provide a basis for guiding the genetic manipulation of mulberry fruits towards sustainable agricultural practices and improve post-harvest management, potentially enhancing the quality and shelf life of mulberry fruits for sustainable agriculture and forestry.

Direct evidence of drought stress memory in mulberry from a physiological perspective: Antioxidative, osmotic and phytohormonal regulations

Drought stress commonly happens more than once during the life cycle of perennial trees. Stress memory endows better capacity to cope with repeated stresses for plants, while the underlying mechanisms are not fully elucidated. In this study, 2-month-old saplings of two mulberry cultivars (Husang32 and 7307 of Morus multicaulis) with or without an early soil water deficit were subjected to subsequent drought for 9 days. The shoot height growth, biomass production, stable carbon isotope discrimination, phytohormones, reactive oxygen species (ROS), osmotic substances and antioxidant enzymes were analyzed after the first and the second drought, respectively. Drought priming saplings sustained comparable or slightly higher biomass accumulation under the second drought than those non-priming. They also exhibited decreased levels of soluble sugars, free proline and soluble proteins, lower accumulation of malonaldehyde (MDA) and superoxide anion (O₂^•-), reduced activities of superoxide dismutase (SOD) and peroxidase (POD) compared to non-priming plants. Moreover, cultivar Husang32 exhibited elevated abscisic acid (ABA) and jasmonic acid (JA) where 7307 displayed opposite changes. PCA suggests that MDA, H₂O₂, free proline, SOD and POD in roots, and ROS, soluble sugars and glutamate reductase in leaves are dominant factors influenced by stress memory. ABA and JA in leaves also play important roles in exerting drought imprints. Collectively, stress memory can confer mulberry resistance to recurrent drought via combined regulations of antioxidative protection, osmotic adjustment and phytohormonal responses.

Molecular mechanism of mulberry response to drought stress revealed by complementary transcriptomic and iTRAQ analyses

BACKGROUND

The use of mulberry leaves has long been limited to raising silkworms, but with the continuous improvement of mulberry (Morus alba) resource development and utilization, various mulberry leaf extension products have emerged. However, the fresh leaves of mulberry trees have a specific window of time for picking and are susceptible to adverse factors, such as drought stress. Therefore, exploring the molecular mechanism by which mulberry trees resist drought stress and clarifying the regulatory network of the mulberry drought response is the focus of the current work.

RESULTS

In this study, natural and drought-treated mulberry grafted seedlings were used for transcriptomic and proteomic analyses (CK vs. DS9), aiming to clarify the molecular mechanism of the mulberry drought stress response. Through transcriptome and proteome sequencing, we identified 9889 DEGs and 1893 DEPs enriched in stress-responsive GO functional categories, such as signal transducer activity, antioxidant activity, and transcription regulator activity. KEGG enrichment analysis showed that a large number of codifferentially expressed genes were enriched in flavonoid biosynthesis pathways, hormone signalling pathways, lignin metabolism and other pathways. Through subsequent cooperation analysis, we identified 818 codifferentially expressed genes in the CK vs. DS9 comparison group, including peroxidase (POD), superoxide dismutase (SOD), aldehyde dehydrogenase (ALDHs), glutathione s-transferase (GST) and other genes closely related to the stress response. In addition, we determined that the mulberry gene MaWRKYIII8 (XP_010104968.1) underwent drought- and abscisic acid (ABA)-induced expression, indicating that it may play an important role in the mulberry response to drought stress.

CONCLUSIONS

Our research shows that mulberry can activate proline and ABA biosynthesis pathways and produce a large amount of proline and ABA, which improves the drought resistance of mulberry. MaWRKYIII8 was up-regulated and induced by drought and exogenous ABA, indicating that MaWRKYIII8 may be involved in the mulberry response to drought stress. These studies will help us to analyse the molecular mechanism underlying mulberry drought tolerance and provide important gene information and a theoretical basis for improving mulberry drought tolerance through molecular breeding in the future.

A comparative study on the circadian rhythm of the electrical signals of Broussonetia papyrifera and Morus alba

The circadian clock regulates a wide range of physiological processes in plants. Here we showed the circadian variations of the electrical signals in Broussonetia papyrifera L. and Morus alba L. in a natural state, which were analyzed using the day-night cycle method. The circadian characteristics of different plant electrical signals were compared by constructing a coupling model for the circadian rhythm of plant electrical signals. The electrical signal sensor had two electrode plates, which were fixed on the two ends of the splint, leaves could then be clamped and measured. The clamping force between the two electrode plates was uniform, which enabled continuous and nondestructive measurements. The results showed that an electric cyclic behavior was observed (circadian cycle) with the circadian variation in the plants within 24 h. Both the resistance (R) and the impedance (Z) increased firstly in the early morning and then decreased subsequently, while the capacitance (C) showed an opposite variation. Under different weather conditions, plant electrical signals showed periodic changes when the temperature and light intensity in the environment slightly changed within the physiological tolerance of plant. This indicated that the circadian clock of plant electrical signals could be maintained endogenously. The variation curves of plant electrical signals as time increased were fitted using the sine equation. The characteristic parameters of circadian rhythm of plant electrical signals were obtained. We found that although all plant electrical signals exhibited electric cyclic behavior, but the characteristics of circadian rhythms of electrical signals were different. This study provided a scientific basic for precisely monitoring plant electrical signals, and a reference for revealing circadian rhythms of plant electrical signals and their occurrence rules.

A salicylic acid inducible mulberry WRKY transcription factor, MiWRKY53 is involved in plant defence response

MiWRKY53 is expressed in response to various stresses and hormones. Although it is localized in the nucleus, it shows no transcriptional activation. Role of SA-mediated plant defence response is demonstrated. WRKY transcription factors are one the largest gene families in plants involved in almost every process in plants including development, physiological processes, and stress response. Salicylic acid (SA) is key regulator of biotic stress against various pathogens in plants acting via its multiple mechanisms to induce defence response. Herein, we have identified and functionally validated WRKY53 from mulberry (Morus indica var. K2). MiWRKY53 expressed differentially in response to different stress and hormonal treatments. MiWRKY53 belongs to group III of WKRY gene family, localized in nucleus, and lacks transcriptional activation activity in yeast. Hormone responsive behaviour of MiWRKY53 Arabidopsis overexpression (OE) transgenics preferentially was noted in root growth assay in response to Salicylic acid (SA). Arabidopsis overexpression plants also displayed alteration in leaf phenotype having wider leaves than the wild-type plants. PR-1 transcripts were higher in MiWRKY53 Arabidopsis OE plants and they displayed resistance towards biotrophic pathogen Pseudomonas syringae PstDC3000. MiWRKY53 Mulberry OE transgenics also depicted SA-responsive behaviour. Several hormones and stress-related cis-acting elements were also identified in the 1.2-Kb upstream regulatory region (URR) of MiWRKY53. Functional characterization of full-length promoter region revealed that it is induced by SA and further analysis of deletion constructs helped in the identification of minimal promoter responsible for its inducibility by SA. Altogether, the findings from this study point towards the SA preferential behaviour of MiWRKY53 and its function as regulator of plant defence response through SA-mediated mechanisms.

Leaf morphology and chlorophyll fluorescence characteristics of mulberry seedlings under waterlogging stress

Because of its high flooding tolerance, in recent years, mulberry has become a tree species that is used in plant restoration in impact zones in reservoir areas. Therefore, 3-year-old potted forage mulberry seedlings were used to investigate the mechanism of mulberry adaptation to flooding stress. An indoor simulated flooding method was adopted to study the morphology of mulberry seedling leaves and the changes in leaf chlorophyll fluorescence parameters and fluorescence imaging under different flooding times and depths. The results showed that the leaves of mulberry seedlings treated with shallow submergence remained healthy during the flooding period, while the leaves of mulberry seedlings treated with half submergence and full submergence showed different degrees of waterlogging symptoms in the middle and late flooding periods and formed adventitious roots at the base of the stem. Most of the chlorophyll fluorescence parameters decreased at the beginning of flooding, but the steady-state degree of closure of PS II reaction centres (1-qP_Lss) increased significantly. In the later stage of flooding, the fluorescence parameters showed relatively stable trends. Based on these results, we conclude that mulberry has high flooding tolerance due to a combination of morphological and physiological responses.

Natural population re-sequencing detects the genetic basis of local adaptation to low temperature in a woody plant

Total of 14 SNPs associated with overwintering-related traits and 75 selective regions were detected. Important candidate genes were identified and a possible network of cold-stress responses in woody plants was proposed. Local adaptation to low temperature is essential for woody plants to against changeable climate and safely survive the winter. To uncover the specific molecular mechanism of low temperature adaptation in woody plants, we sequenced 134 core individuals selected from 494 paper mulberry (Broussonetia papyrifera), which naturally distributed in different climate zones and latitudes. The population structure analysis, PCA analysis and neighbor-joining tree analysis indicated that the individuals were classified into three clusters, which showed forceful geographic distribution patterns because of the adaptation to local climate. Using two overwintering phenotypic data collected at high latitudes of 40°N and one bioclimatic variable, genome-phenotype and genome-environment associations, and genome-wide scans were performed. We detected 75 selective regions which possibly undergone temperature selection and identified 14 trait-associated SNPs that corresponded to 16 candidate genes (including LRR-RLK, PP2A, BCS1, etc.). Meanwhile, low temperature adaptation was also supported by other three trait-associated SNPs which exhibiting significant differences in overwintering traits between alleles within three geographic groups. To sum up, a possible network of cold signal perception and responses in woody plants were proposed, including important genes that have been confirmed in previous studies while others could be key potential candidates of woody plants. Overall, our results highlighted the specific and complex molecular mechanism of low temperature adaptation and overwintering of woody plants.

Accumulation and translocation of food chain in soil-mulberry (Morus alba L.)-silkworm (Bombyx mori) under single and combined stress of lead and cadmium

In recent years, heavy metal pollution has caused immeasurable harm to the environment. As an emerging technology, phytoremediation technology has gained a place in the treatment of heavy metal pollution with its unique advantages. This study analyzes the toxic effects of mulberry (Morus alba) seeds, seedling growth and silkworm under heavy metal stress of lead (Pb) and cadmium (Cd), and explore the accumulation and migration of Pb and Cd in the soil-mulberry tree-silkworm system. The main results were as follows: (1) Seed germination and potted seedling experiments were conducted under heavy metal Pb and Cd stresses, and it was found that Pb and Cd had inhibitory effects on mulberry seed germination, growth and photosynthesis of mulberry seedlings, and as the concentration of heavy metals increased, the stronger the inhibitory effect. Moreover, Pb and Cd have a synergistic effect under compound stress. (2) The accumulation and transfer rules of Pb and Cd ions in mulberry were different. The content of Pb in mulberry was root > leaf > stem and the content of Cd was root > stem > leaf. The combined stress promoted the transfer of Pb and Cd from the underground part to the aerial portion of mulberry. (3) The silkworm feeds on mulberry leaves contaminated with heavy metals in this experiment and found that: with the increase of silkworm feeding, the heavy metal content in the silkworm body increased significantly, but the content remained in the silkworm body was less, most of it was excreted with silkworm excrement. Combined stress has no significant effect on the detoxification mechanism of silkworm. It is indispensable to think of the synergistic effect of heavy metals on plants germination when seeds are used for phytoremediation.

Whole genome bisulfite sequencing methylome analysis of mulberry (Morus alba) reveals epigenome modifications in response to drought stress

DNA methylation plays a significant role in many biological processes. Although some studies of DNA methylation have been performed in woody plant, none is known about the methylation patterns of mulberry (Morus alba). In this study, we performed whole genome bisulfite sequencing under drought stress to generate a methylated cytosines map and assessed the effects of the changes on gene expression combined with transcriptomics. We found that the percentage of methylated cytosines varied depending on the local sequence context (CG, CHG and CHH) and external treatment (control, CK; drought stress, DS). The methylation levels under DS were 8.64% higher than that of CK, and differences that were mainly due to the contribution of mCG (6.24%). Additionally, there were 3,243 different methylation and expression associated genes. In addition, methylated genes were enriched within GO subcategories including catalytic activity, cellular process, metabolic process, response to stimulus and regulation of biological process. This is the first study to comprehensively present methylation patterns in mulberry and reveal widespread DNA methylation changes in response to drought stress, which has the potential to enhance our understanding of links between DNA methylation and the modulation of gene expression in plants subjected to abiotic stresses.

Physiological responses of Morus alba L. in heavy metal(loid)-contaminated soil and its associated improvement of the microbial diversity

Woody plants have considerable application potential in the phytoremediation schemes, owing to their long-lived large biomass and prosperous root systems in heavy metal(loid)-contaminated soil. Under greenhouse conditions, the physiological response characteristics and phytoremediation possibility of Morus alba L. and its associated improvement of the bacterial and arbuscular mycorrhizal fungal (AMF) diversities in heavy metal(loid) co-contaminated soils were investigated. The results showed that the cultivated M. alba L. plant exhibited significant tolerance against the heavy metal(loid)s in co-contaminated soil and that the microbial diversities were improved notably. The contents of malondialdehyde (MDA) in M. alba L. leaves decreased with cultivation from 90 to 270 days, while the superoxide dismutase, peroxidase and catalase activities were maintained at normal levels to eliminate the production of lipid peroxides. The chemical compositions (e.g. amino acids, carbohydrates and proteins) in the root of M. alba L. fluctuated slightly throughout the cultivation period. Meanwhile, Cd, Pb and Zn were majorly concentrated in the M. alba L. roots, and the maximum contents were 23.4, 7.40 and 615.5 mg/kg, respectively. According to the polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis results, the influence of M. alba L. on the rhizosphere AMF community was greater than that on the bacteria community. Meanwhile, the bacterial and AMF Shannon diversity indexes in the contaminated soil were enhanced by 18.7-22.0% and 7.14-16.4%, respectively, with the presence of M. alba L. Furthermore, the correlations between the availability of As, Cd, Pb, and Zn and Shannon diversity indexes of the bacterial and AMF communities were significantly (p < 0.05) positive with the phytoremediation of M. alba L. Therefore, M. alba L. can be suggested as a potential plant candidate for ecological remediation and for simultaneously improving the activity and diversity of microorganisms in contaminated soils.

In vivo photoprotection mechanisms observed from leaf spectral absorbance changes showing VIS-NIR slow-induced conformational pigment bed changes

Regulated heat dissipation under excessive light comprises a complexity of mechanisms, whereby the supramolecular light-harvesting pigment-protein complex (LHC) shifts state from light harvesting towards heat dissipation, quenching the excess of photo-induced excitation energy in a non-photochemical way. Based on whole-leaf spectroscopy measuring upward and downward spectral radiance fluxes, we studied spectrally contiguous (hyperspectral) transient time series of absorbance A(λ,t) and passively induced chlorophyll fluorescence F(λ,t) dynamics of intact leaves in the visible and near-infrared wavelengths (VIS-NIR, 400-800 nm) after sudden strong natural-like illumination exposure. Besides light avoidance mechanism, we observed on absorbance signatures, calculated from simultaneous reflectance R(λ,t) and transmittance T(λ,t) measurements as A(λ,t) = 1 - R(λ,t) - T(λ,t), major dynamic events with specific onsets and kinetical behaviour. A consistent well-known fast carotenoid absorbance feature (500-570 nm) appears within the first seconds to minutes, seen from both the reflected (backscattered) and transmitted (forward scattered) radiance differences. Simultaneous fast Chl features are observed, either as an increased or decreased scattering behaviour during quick light adjustment consistent with re-organizations of the membrane. The carotenoid absorbance feature shows up simultaneously with a major F decrease and corresponds to the xanthophyll conversion, as quick response to the proton gradient build-up. After xanthophyll conversion (t = 3 min), a kinetically slower but major and smooth absorbance increase was occasionally observed from the transmitted radiance measurements as wide peaks in the green (~ 550 nm) and the near-infrared (~ 750 nm) wavelengths, involving no further F quenching. Surprisingly, in relation to the response to high light, this broad and consistent VIS-NIR feature indicates a slowly induced absorbance increase with a sigmoid kinetical behaviour. In analogy to sub-leaf-level observations, we suggest that this mechanism can be explained by a structure-induced low-energy-shifted energy redistribution involving both Car and Chl. These findings might pave the way towards a further non-invasive spectral investigation of antenna conformations and their relations with energy quenching at the intact leaf level, which is, in combination with F measurements, of a high importance for assessing plant photosynthesis in vivo and in addition from remote observations.

Photochemistry and proteomics of mulberry (Morus alba L.) seedlings under NaCl and NaHCO3 stress

In order to explore the response and adaptation mechanisms of photosynthesis of the leaves of mulberry (Morus alba L.) seedlings to saline-alkali stress. Photosynthetic activity, and the response of related proteomics of M. alba seedling leaves under NaCl and NaHCO₃ stress were studied by using chlorophyll fluorescence and gas exchange technique combined with TMT proteomics. The results showed that NaCl stress had no significant effect on photosystem II (PSII) activity in M. alba seedling leaves. In addition, the expressions of proteins of the PSII oxygen-evolving complex (OEE3-1 and PPD4) and the LHCII antenna (CP24 10A, CP26, and CP29) were increased, and the photosystem I (PSI) activity in the leaves of M. alba seedlings was increased, as well as expressions of proteins, such as PsaF, PsaG, PsaH, PsaL, PsaN, and Ycf4. Under NaHCO₃ stress, the activity of PSII and PSI and the expression of their protein complexes and the electron transfer-related proteins significantly decreased. NaCl stress had little effect on RuBP regeneration during dark reaction in the leaves and the expressions of glucose synthesis related proteins and net photosynthetic rate (P_n) did not decrease significantly. The leaves could adapt to NaCl stress by reducing stomatal conductance (G_s) and increasing water use efficiency (WUE). Under NaHCO₃ stress, the expression of dark reaction-related proteins was mostly down-regulated, while G_s was reduced, which indicated that non-stomatal factors can be responsible for inhibition of carbon assimilation.

Mulberry RGS negatively regulates salt stress response and tolerance

Regulator of G-protein signaling (RGS) protein, the best-characterized accelerating GTPase protein in plants, regulates G-protein signaling and plays important role in abiotic stress tolerance. However, the detailed molecular mechanism of RGS involved in G-protein signaling mediated abiotic stress responses remains unclear. In this study, a mulberry (Morus alba L.) RGS gene (MaRGS) was transformed into tobacco, and the ectopic expression of MaRGS in tobacco decreased the tolerance to salt stress. The transgenic tobacco plants had lower proline content, higher malonaldehyde and H2O2 contents than wild type plants under salt stress condition. Meanwhile, MaRGS overexpression in mulberry seedlings enhances the sensitivity to salt stress and RNAi-silenced expression of MaRGS improves the salt stress response and tolerance. These results suggested that MaRGS negatively regulates salt stress tolerance. Further analysis suggested that D-glucose and autophagy may involve in the response of RGS to salt stress. This study revealed the role of MaRGS in salt stress tolerance and provides a proposed model for RGS regulates G-protein signaling in response to salt stress.

Phytoextraction potential of Pteris vittata L. co-planted with woody species for As, Cd, Pb and Zn in contaminated soil

The objective of this study was to determine the phytoextraction potential of a hyperaccumulator co-planted with a large biomass of woody plant in metal(loid)-contaminated soil. A pot experiment was conducted for 270 days (d) to study the growth, physiological responses, and metal(loid)s accumulation characteristics of plants, which included a shade-requiring, As-hyperaccumulator perennial herb, Pteris vittata L., co-planted with a woody tree, namely Morus alba L. or Broussonetia papyrifera L., for soil contaminated with arsenic (As), cadmium (Cd), lead (Pb), and zinc (Zn). The results showed that the biomass, photosynthetic pigment contents, antioxidant enzyme activity, and uptake of As in P. vittata L. were significantly enhanced by co-planting with M. alba L. or B. papyrifera L. Especially, the uptake of As by P. vittata L. was significantly (p < 0.05) increased by 80.0% and 64.2% when it was co-planted with M. alba L. or B. papyrifera L., respectively, while the As, Cd, Pb, and Zn contents of both M. alba L. and B. papyrifera L. were not significantly promoted by the co-planting. The comprehensive phytoextraction of metal(loid)s could be optimized by the co-planting of P. vittata L. with M. alba L. or B. papyrifera L. The total amount of As in the shoots from co-planting species was significantly (p < 0.05) higher than that of the monoculture with M. alba L. or B. papyrifera L., and that of Cd and Zn in the shoots was significantly (p < 0.05) higher than that of the monoculture with P. vittata L. The results showed that the co-planting of P. vittata L. with M. alba L. or B. papyrifera L. can alleviate the toxic effects of metal(loid)s on plant growth and improve the comprehensive phytoextraction amounts of metal(loid)s, and is a promising strategy for remediation of metal(loid)-contaminated soil.

Mulberry (Morus alba) MmSK gene enhances tolerance to drought stress in transgenic mulberry

Shaggy-like protein kinase (SK) plays important roles in the plant growth development, signal transduction, abiotic stress and biotic stress and substance metabolism regulation. However, the exact function of the response to drought stress in mulberry with SK remains unclear. In this study, a new SK gene that was designated as MmSK (GenBank accession NO: KY348867) was isolated and cloned from mulberry (Morus alba). MmSK contains two SK conservation domains of ATP domain and Serine/Threonine protein kinases active-site signature, and belonged to GSK3/shaggy protein kinase family. The expression of MmSK in mulberry was up-regulated under various abiotic stress treatments. Meanwhile, we observed higher expression levels in the phloem contrasted with other tissues. Mulberry MmSK gene was successfully silenced by virus induced gene silencing (VIGS), and after MmSK was silenced, the expression of MmSK in pTRV2-MmSK-VIGS plant (transgenic mulberry) dropped to 34.02% compared with the negative control inoculated with empty vector pTRV2-00 (CK). Under drought stress, the soluble protein content, proline content, superoxide dismutase (SOD) and peroxidase (POD) activities in transgenic mulberry decreased in different degree compared with the CK. In contrast, the accumulation of malondialdehyde (MDA) increased significantly in transgenic mulberry. With the extension of drought stress treatment time, the soluble protein content, proline content and MDA content gradually increased. The SOD activity and POD activity under drought stress gradually rose to the maximum on the fifth day and then decreased, which consistent with the change trend of MmSK gene expression. These results suggested that MmSK gene could function as a positive regulator of drought stress in mulberry.

The molecular chaperon AKR2A increases the mulberry chilling-tolerant capacity by maintaining SOD activity and unsaturated fatty acids composition

Chilling is common in nature and can damage most plant species, particularly young leaves and buds. Mulberry (Morus spp.) is an economically important food source for the domesticated silkworm (Bombyx mori). However, weather and climatic extremes, such as "late spring coldness", seriously damage mulberry buds and young leaves. The molecular mechanism involved in the differing mulberry chilling tolerance is unclear. In the present study, we found that mSOD1, mFADII, and mKCS1 interacted with mAKR2A and that the expression of mAKR2A, mSOD, mFAD, and mKCS1 in the chilling-tolerant mulberry variety was higher than that in the chilling-sensitive variety. Unsaturated fatty acids content and superoxide dismutase (SOD) activity in the chilling-tolerant variety was higher than that in the chilling-sensitive variety. After chilling treatment, mSOD1, mKCS1 and mAKR2A expression in the chilling-tolerant variety was reduced to lower than that in the chilling-sensitive variety, whereas mFADII expression increased in the chilling-tolerant variety compared with that in the chilling-sensitive variety, suggesting that the increased expression of the molecular chaperon mAKR2A helped to maintain or prompted the chilling-related proteins in the chilling-tolerant variety.

Intra-specific trends of lumen and wall resistivities of vessels within the stem xylem vary among three woody plants

Water flow through xylem vessels encounters hydraulic resistance when passing through the vessel lumen and end wall. Comparative studies have reported that lumen and end wall resistivities co-limit water flow through stem xylem in several angiosperm woody species that have vessels of different average diameter and length. This study examined the intra-specific relationship between the lumen and end wall resistivities (Rlumen and Rwall) for vessels within the stem xylem using three deciduous angiosperm woody species found in temperate forest. Morus australis Poir. and Acer rufinerve Siebold et Zucc. are early- and late-successional species, and Vitis coignetiae Pulliat ex Planch is a woody liana. According to the Hagen-Poiseuille equation, Rlumen is proportional to the fourth power of vessel diameter (D), whereas vessel length (L) and inter-vessel pit area (Apit) determine Rwall. To estimate Rlumen and Rwall, the scaling relationships between the L and D and between Apit and D were measured. The scaling exponents between L and D were 1.47, 3.19 and 2.86 for A. rufinerve, M. australis and V. coignetiae, respectively, whereas those between Apit and D were 0.242, 2.11 and 2.68, respectively. Unlike the inter-specific relationships, the wall resistivity fraction (Rwall/(Rlumen + Rwall)) within xylem changed depending on D. In M. australis and V. coignetiae, this fraction decreased with increasing D, while in A. rufinerve, it increased with D. Vessels with a high wall resistivity fraction have high Rwall and total resistivity but are expected to have low susceptibility to xylem cavitation due to a small cumulative Apit. In contrast, vessels with a low wall resistivity fraction have low Rwall and total resistivity but high susceptibility to xylem cavitation. Because the wall resistivity fraction varies with D, the stem xylem contains vessels with different hydraulic efficiencies and safety to xylem cavitation. These features produce differences in the hydraulic properties of plants with different life forms.

High throughput deep degradome sequencing reveals microRNAs and their targets in response to drought stress in mulberry (Morus alba)

MicroRNAs (miRNAs) play important regulatory roles by targeting mRNAs for cleavage or translational repression. Identification of miRNA targets is essential to better understanding the roles of miRNAs. miRNA targets have not been well characterized in mulberry (Morus alba). To anatomize miRNA guided gene regulation under drought stress, transcriptome-wide high throughput degradome sequencing was used in this study to directly detect drought stress responsive miRNA targets in mulberry. A drought library (DL) and a contrast library (CL) were constructed to capture the cleaved mRNAs for sequencing. In CL, 409 target genes of 30 conserved miRNA families and 990 target genes of 199 novel miRNAs were identified. In DL, 373 target genes of 30 conserved miRNA families and 950 target genes of 195 novel miRNAs were identified. Of the conserved miRNA families in DL, mno-miR156, mno-miR172, and mno-miR396 had the highest number of targets with 54, 52 and 41 transcripts, respectively, indicating that these three miRNA families and their target genes might play important functions in response to drought stress in mulberry. Additionally, we found that many of the target genes were transcription factors. By analyzing the miRNA-target molecular network, we found that the DL independent networks consisted of 838 miRNA-mRNA pairs (63.34%). The expression patterns of 11 target genes and 12 correspondent miRNAs were detected using qRT-PCR. Six miRNA targets were further verified by RNA ligase-mediated 5' rapid amplification of cDNA ends (RLM-5' RACE). Gene Ontology (GO) annotations and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis revealed that these target transcripts were implicated in a broad range of biological processes and various metabolic pathways. This is the first study to comprehensively characterize target genes and their associated miRNAs in response to drought stress by degradome sequencing in mulberry. This study provides a framework for understanding the molecular mechanisms of drought resistance in mulberry.

Transcription activation activity of ERD15 protein from Morus indica

Early Responsive to Dehydration (ERD) genes are described as rapid response mediators of dehydration stress. Recently, ERD15 has emerged as a novel stress induced transcription factor which might be involved in mediating distinct stress responses in plants. In order to determine whether mulberry ERD15 can act as functional transcription factor, yeast-based assays were performed. Mulberry ERD15 was found to drive high level reporter gene expression in yeast which suggests it may function as a transcription factor. However, due to lack of an identifiable DNA binding domain, deletion analysis was carried out to determine the putative region of the protein involved in mediating protein-DNA interaction. Our results indicate that the region between 70 and 100 amino acids is critical in conferring transcription activation activity and might harbor the DNA binding region of ERD15.

Structures of Bordered Pits Potentially Contributing to Isolation of a Refilled Vessel from Negative Xylem Pressure in Stems of Morus australis Poir.: Testing of the Pit Membrane Osmosis and Pit Valve Hypotheses

Two hypotheses have been proposed to explain the mechanism preventing the refilling vessel water from being drained to the neighboring functional vessels under negative pressure. The pit membrane osmosis hypothesis proposes that the xylem parenchyma cells release polysaccharides that are impermeable to the intervessel pit membranes into the refilling vessel; this osmotically counteracts the negative pressure, thereby allowing the vessel to refill. The pit valve hypothesis proposes that gas trapped within intervessel bordered pits isolates the refilling vessel water from the surrounding functional vessels. Here, using the single-vessel method, we assessed these hypotheses in shoots of mulberry (Morus australis Poir.). First, we confirmed the occurrence of xylem refilling under negative pressure in the potted mulberry saplings. To examine the pit membrane osmosis hypothesis, we estimated the semi-permeability of pit membranes for molecules of various sizes and found that the pit membranes were not semi-permeable to polyethylene glycol of molecular mass <20,000. For the pit valve hypothesis, we formed pit valves in the intervessel pits in the short stem segments and measured the maximum liquid pressure up to which gases in bordered pits were retained. The threshold pressure ranged from 0.025 to 0.10 MPa. These values matched the theoretical value calculated from the geometry of the pit chamber (0.0692-0.101 MPa). Our results suggest that gas in the pits is retained by surface tension, even under substantial positive pressure to resolve gases in the refilling vessel, whereas the molecule size required for the pit membrane osmosis mechanism in mulberry would be unrealistically large.

Intercropped Pteris vittata L. and Morus alba L. presents a safe utilization mode for arsenic-contaminated soil

Intercropping technology provides income for owners of contaminated soil without increasing environmental risk. Therefore, intercropping of arsenic (As) hyperaccumulator Pteris vittata L. with economic crops is now widely utilized in slightly or moderately As-contaminated farmlands. However, the mechanisms for As mobilization and absorption within the intercropping system are still unclear. To clarify As mobilization and absorption within an intercropping system, portable X-ray fluorescence spectrometry and sequential extraction were utilized to detect the spatial distribution and speciation of As in an intercropped system of P. vittata and cash crop mulberry (Morus alba L.). Compared with the P. vittata monoculture, P. vittata intercropping had higher As concentration, which may have been caused by the efficient exploitation of a greater As source in soil. Compared with the M. alba monoculture, M. alba intercropping had lower As concentration, which may have been caused by the As depletion by P. vittata roots. Spatial distribution of As in the soil indicated a "valley" around the P. vittata roots in both monocultured and intercropped systems, implying that As was depleted around the P. vittata roots. Continuous As extraction confirmed that both P. vittata monoculture and P. vittata and M. alba intercropping can efficiently control the risk of As soil contamination. Moreover, the properties of M. alba leaves were further studied. Mulberry leaves in the intercropping system satisfied the national feed standards. Therefore, intercropping presents a safe utilization mode for As-contaminated soil and can increase the income from silkworm-rearing M. alba leaves, without extra environmental risk.

Life span and structure of ephemeral root modules of different functional groups from a desert system

The terminal branch orders of plant root systems have been proposed as short-lived 'ephemeral' modules specialized for resource absorption. The occurrence of ephemeral root modules has so far only been reported for a temperate tree species and it is unclear if the concept also applies to other woody (shrub, tree) and herb species. Fine roots of 12 perennial dicotyledonous herb, shrub and tree species were monitored for two growing seasons using a branch-order classification, sequential sampling and rhizotrons in the Taklamakan desert. Two root modules existed in all three plant functional groups. Among the first five branch orders, the first two (perennial herbs, shrubs) or three (trees) root orders were ephemeral and had a primary anatomical structure, high nitrogen (N) concentrations, high respiration rates and very short life spans of 1-4 months, whereas the last two branch orders in all functional groups were perennial, with thicker diameters, no or collapsed cortex, distinct secondary growth, low N concentrations, low respiration rates, but much longer life spans. Ephemeral, short-lived root modules and long-lived, persistent root modules seem to be a general feature across many plant functional groups and could represent a basic root system design.

Phenotypic and Transcriptomic Analyses of Autotetraploid and Diploid Mulberry (Morus alba L.)

Autopolyploid plants and their organs are often larger than their diploid counterparts, which makes them attractive to plant breeders. Mulberry (Morus alba L.) is an important commercial woody plant in many tropical and subtropical areas. In this study, we obtained a series of autotetraploid mulberry plants resulting from a colchicine treatment. To evaluate the effects of genome duplications in mulberry, we compared the phenotypes and transcriptomes of autotetraploid and diploid mulberry trees. In the autotetraploids, the height, breast-height diameter, leaf size, and fruit size were larger than those of diploids. Transcriptome data revealed that of 21,229 expressed genes only 609 (2.87%) were differentially expressed between diploids and autotetraploids. Among them, 30 genes were associated with the biosynthesis and signal transduction of plant hormones, including cytokinin, gibberellins, ethylene, and auxin. In addition, 41 differentially expressed genes were involved in photosynthesis. These results enhance our understanding of the variations that occur in mulberry autotetraploids and will benefit future breeding work.

Characterization and expression of genes involved in the ethylene biosynthesis and signal transduction during ripening of mulberry fruit

Although ethylene is well known as an essential regulator of fruit development, little work has examined the role ethylene plays in the development and maturation of mulberry (Morus L.) fruit. To study the mechanism of ethylene action during fruit development in this species, we measured the ethylene production, fruit firmness, and soluble solids content (SSC) during fruit development and harvest. By comparing the results with those from other climacteric fruit, we concluded that Morus fruit are probably climacteric. Genes associated with the ethylene signal transduction pathway of Morus were characterized from M. notabilis Genome Database, including four ethylene receptor genes, a EIN2-like gene, a CTR1-like gene, four EIN3-like genes, and a RTE1-like gene. The expression patterns of these genes were analyzed in the fruit of M. atropurpurea cv. Jialing No.40. During fruit development, transcript levels of MaETR2, MaERS, MaEIN4, MaRTE, and MaCTR1 were lower at the early stages and higher after 26 days after full bloom (DAF), while MaETR1, MaEIL1, MaEIL2, and MaEIL3 remained constant. In ripening fruit, the transcripts of MaACO1 and MaACS3 increased, while MaACS1 and MaACO2 decreased after harvest. The transcripts of MaACO1, MaACO2, and MaACS3 were inhibited by ethylene, and 1-MCP (1–methylcyclopropene) upregulated MaACS3. The transcripts of the MaETR-like genes, MaRTE, and MaCTR1 were inhibited by ethylene and 1-MCP, suggesting that ethylene may accelerate the decline of MaETRs transcripts. No significant changes in the expression of MaEIN2, MaEIL1, and MaEIL3 were observed during ripening or in response to ethylene, while the expressions of MaEIL2 and MaEIL4 increased rapidly after 24 h after harvest (HAH) and were upregulated by ethylene. The present study provides insights into ethylene biosynthesis and signal transduction in Morus plants and lays a foundation for the further understanding of the mechanisms underlying Morus fruit development and ripening.

Beyond seasonal climate: statistical estimation of phenological responses to weather

Phenological events, such as the timing of flowering or insect emergence, are influenced by a complex combination of climatic and non-climatic factors. Although temperature is generally considered most important, other weather events such as frosts and precipitation events can also influence many species' phenology. Non-climatic variables such as photoperiod and site-specific habitat characteristics can also have important effects on phenology. Forecasting phenological shifts due to climate change requires understanding and quantifying how these multiple factors combine to affect phenology. However, current approaches to analyzing phenological data have a limited ability for quantifying multiple drivers simultaneously. Here, we use a novel statistical approach to estimate the combined effects of multiple variables, including local weather events, on the phenology of several taxa (a tree, an insect, and a fungus). We found that thermal forcing had a significant positive effect on each species, frost events delayed the phenology of the tree and butterfly, and precipitation had a positive effect on fungal fruiting. Using data from sites across latitudinal gradients, we found that these effects are remarkably consistent across sites once latitude and other site effects are accounted for. This consistency suggests an underlying biological response to these variables that is not commonly estimated using data from field observations. This approach's flexibility will be useful for forecasting ongoing phenological responses to changes in climate variability in addition to seasonal trends.

The response of mulberry trees after seedling hardening to summer drought in the hydro-fluctuation belt of Three Gorges Reservoir Areas

Interest has developed in the potential of mulberry (Morus alba), a woody perennial, for revegetating the hydro-fluctuation belt of the Three Gorges Reservoir due to its resistance to water-logging stress. To be useful, the trees must also be able to withstand dry conditions in summer when temperatures can be very high and droughts become severe quickly. Here, we report a study in which mulberry seedlings were grown in a greenhouse under a variety of simulated soil water conditions reflecting potential summer scenarios in the hydro-fluctuation belt of the Three Gorges Reservoir Area. We compared the responses of two pretreatment groups of mulberry seedlings to different levels of drought stress. The pretreatment groups differed with respect to drought hardening: the daily-managed (DM) group had relative soil moisture held constant in the range 70-80 %, while the drought-hardened (DH) group had relative soil moisture held constant at 40-50 %. Following the month-long pretreatment of seedlings, the two groups of young trees (DM and DH) were then respectively subjected to three levels of drought stress for a month: normal watering, moderate drought stress, and severe drought stress. A series of measurements comparing the physiological status of the plants in the two groups were then made, and the following results were obtained: (1) As drought stress increased, the heights, base diameters, root surface areas, photosynthetic rates (Pn), stomatal conductances (Gs), and transpiration rates (Tr) of the mulberry trees in both groups (DM and DH) decreased significantly, while the specific root area and abscisic acid (ABA) contents had increasing trends. Root activity and instantaneous water use efficiency of mulberry trees in both groups (DM and DH) were all raised under drought stress conditions than under normal watering, but the root/shoot ratio and leaf water potential were lowered. (2) At the same level of soil water content, the heights, base diameters, root/shoot ratios, root surface areas, specific root areas, photosynthetic rates (Pn), stomatal conductances (Gs), and transpiration rates (Tr) of the young mulberry trees in the DH were all significantly higher than those of the control group (DM). Leaf water potential, instantaneous water use efficiency, and abscisic acid content of DH were all significantly lower than DM. Under different degrees of drought stress, the growth of mulberry trees will be inhibited, but the trees can respond to the stress by increasing the root absorptive area and enhancing capacity for water retention. Mulberry trees demonstrate strong resistance to drought stress, and furthermore drought resistance can be improved by drought hardening during the seedling stage.

Roles of cell walls and intracellular contents in supercooling capability of xylem parenchyma cells of boreal trees

The supercooling capability of xylem parenchyma cells (XPCs) in boreal hardwood species differs depending not only on species, but also season. In this study, the roles of cell walls and intracellular contents in supercooling capability of XPCs were examined in three boreal hardwood species, Japanese beech, katsura tree and mulberry, whose supercooling capability differs largely depending on species and season. XPCs in these species harvested in winter and summer were treated by rapid freezing and thawing (RFT samples) or by RFT with further washing (RFTW samples) to remove intracellular contents from XPCs in order to examine the roles of cell walls in supercooling. RFT samples were also treated with glucose solution (RFTG samples) to examine roles of intracellular contents in supercooling. The supercooling capabilities of these samples were examined by differential thermal analysis after ultrastructural observation of XPCs by a cryo-scanning electron microscope to confirm effects of the above treatments. XPCs in RFTW samples showed a large reduction in supercooling capability to similar temperatures regardless of species or season. On the other hand, XPCs in RFTG samples showed a large increase in supercooling capability to similar temperatures regardless of species or season. These results indicate that although cell walls have an important role in maintenance of supercooling, change in supercooling capability of XPCs is induced by change in intracellular contents, but not by change in cell wall properties.

Response of the Morus bombycis growing season to temperature and its latitudinal pattern in Japan

Changes in leaf phenology lengthen the growing season length (GSL, the days between leaf budburst and leaf fall) under the global warming. GSL and the leaf phenology response to climate change is one of the most important predictors of climate change effect on plants. Empirical evidence of climatic effects on GSL remains scarce, especially at a regional scale and the latitudinal pattern. This study analyzed the datasets of leaf budburst and fall phenology in Morus bombycis (Urticales), which were observed by the agency of the Japan Meteorological Agency (JMA) from 1953 to 2005 over a wide range of latitudes in Japan (31 to 44° N). In the present study, single regression slopes of leaf phenological timing and air temperature across Japan were calculated and their spatial patterns using general linear models were tested. The results showed that the GSL extension was caused mainly by a delay in leaf fall phenology. Relationships between latitude and leaf phenological and GSL responses against air temperature were significantly negative. The response of leaf phenology and GSL to air temperature at lower latitudes was larger than that at higher latitudes. The findings indicate that GSL extension should be considered with regards to latitude and climate change.

[Effects of mulberry-soybean intercropping on carbon-metabolic microbial diversity in saline-alkaline soil]

Aiming at the characteristics that mulberry-soybean intercropping could alleviate the damage of saline-alkaline soil, Biolog technique was adopted to study the effects of this intercropping on the diversity of carbon-metabolic microbial community in the rhizosphere of saline-alkaline soil. Under mulberry-soybean intercropping, the average well color development (AWCD) symbolizing the metabolic activity of soil microbes was obviously higher, as compared with that under mulberry or soybean monocropping, being the lowest under mulberry monocropping. The McIntosh index was also higher under intercropping than under monocropping, but the Shannon index and Simpson index had less difference between intercropping and monocropping, indicating that intercropping changed the composition and enhanced the diversity of the microbial community in the rhizosphere of saline-alkaline soil. Principal component analysis (PCA) showed that the carbon source utilization mode of the soil microbial community differed between intercropping and monocropping, and the main carbon sources were carbohydrate, carboxylic acid, and polymers. Soil pH and salinity were the main factors limiting the diversity of the microbial community in saline-alkaline soil, and intercropping could effectively decrease the soil pH and salinity and promote the improvement of soil microbial community diversity.

[Effects of Morus alba and Setaria italica intercropping on their plant growth and diurnal variation of photosynthesis]

A field investigation was conducted to study the effects of intercropping Morus aIba and Setaria italica on their dry matter production, land use efficiency, and diurnal variation of leaf photosynthesis. Under intercropping, the plant height, basal diameter, root length, and branch number of M. alba increased by 6.0%, 13.7%, 6.8%, and 14.8%, respectively, and the leaf yield of M. alba was increased by 31.3%, as compared with monoculture M. alba. In contrast, the plant height and root length of intercropped S. italica had no significant difference with those of monoculture S. italica. Intercropping enhanced the equivalent ratio and use efficiency of arable land. For both M. alba and S. italica in monoculture or intercropping, their leaf photosynthetic depression all occurred at midday (12 :00), but the leaf photosynthetic depression of monoculture M. alba was heavier than that of intercropped M. alba. Intercropping promoted the leaf stomatal conductance (g(s)) and water use efficiency (WUE) of M. alba at midday, increased the photosynthetic carbon assimilation of M. alba, and inhibited the decline of M. alba leaf actual photochemical efficiency of PS II (phi(PS II)), photosynthetic electron transport rate (ETR), and the maximal photochemical of PS II (F(v)/F(m)) , which might contribute to alleviate the leaf photosynthetic depression of M. alba at midday. It was concluded that M. alba and S. italica intercropping could obviously improve the leaf photosynthetic capacity of M. alba.

[Effects of NaCl and Na2CO3 stresses on the growth and photosynthesis characteristics of Morus alba seedlings]

Taking 1-year old Morus alba variety 'Qinglong' seedlings as test materials, this paper studied their growth and photosynthetic characteristics under the stresses of different concentration neutral salt NaCl and alkali salt Na2CO3. Salt stresses decreased the plant height and the leaf number, biomass, and photosynthetic capacity of the seedlings markedly. With increasing concentration Na+, the leaf stomatal conductance, transpiration rate, net photosynthetic rate, actual photochemical efficiency, electron transport rate, and photochemical quenching (qP) decreased obviously, the energy dissipation rate increased, and the light use efficiency and photosynthetic capacity dropped down. At low concentrations Na+ (< 150 mmol x L(-1)), the seedlings growth and leaf photosynthetic capacity were slightly inhibited, and the adaptability of the seedlings to the salt stresses increased via the increase of root/shoot ratio. However, this protection mechanism was impaired by increasing salt concentration. Na2CO3 stress (Na+ concentration > 50 mmol x L(-)) had stronger inhibitory effects on the seedlings growth and leaf photosynthetic capacity, and the effect increased with increasing Na+ concentration. It was concluded that at Na+ concentration < 150 mmol x L(-1), the photosynthetic adaptability of M. alba to neutral salt stress was mainly dependent on the plant morphology and photosynthetic metabolism, but at Na+ concentration > 150 mmol x L(-1), the photosynthetic adaptability of M. alba to alkali salt stress was mainly dependent on the photosynthetic metabolism.

Evidence from individual inference for high-dimensional coexistence: long-term experiments on recruitment response

BACKGROUND

For competing species to coexist, individuals must compete more with others of the same species than with those of other species. Ecologists search for tradeoffs in how species might partition the environment. The negative correlations among competing species that would be indicative of tradeoffs are rarely observed. A recent analysis showed that evidence for partitioning the environment is available when responses are disaggregated to the individual scale, in terms of the covariance structure of responses to environmental variation. That study did not relate that variation to the variables to which individuals were responding. To understand how this pattern of variation is related to niche variables, we analyzed responses to canopy gaps, long viewed as a key variable responsible for species coexistence.

METHODOLOGY/PRINCIPAL FINDINGS

A longitudinal intervention analysis of individual responses to experimental canopy gaps with 12 yr of pre-treatment and 8 yr post-treatment responses showed that species-level responses are positively correlated--species that grow fast on average in the understory also grow fast on average in response to gap formation. In other words, there is no tradeoff. However, the joint distribution of individual responses to understory and gap showed a negative correlation--species having individuals that respond most to gaps when previously growing slowly also have individuals that respond least to gaps when previously growing rapidly (e.g., Morus rubra), and vice versa (e.g., Quercus prinus).

CONCLUSIONS/SIGNIFICANCE

Because competition occurs at the individual scale, not the species scale, aggregated species-level parameters and correlations hide the species-level differences needed for coexistence. By disaggregating models to the scale at which the interaction occurs we show that individual variation provides insight for species differences.

Stress-inducible expression of barley Hva1 gene in transgenic mulberry displays enhanced tolerance against drought, salinity and cold stress

Coping with different kinds of biotic and abiotic stresses is the foundation of sustainable agriculture. Although conventional breeding and marker-assisted selection are being employed in mulberry (Morus indica L.) to develop better varieties, nonetheless the longer time periods required for these approaches necessitates the use of precise biotechnological approaches for sustainable agriculture. In an attempt to improve stress tolerance of mulberry, an important plant of the sericulture industry, an encoding late embryogenesis abundant gene from barley (HVA1) was introduced into mulberry plants by Agrobacterium-mediated transformation. Transgenic mulberry with barley Hva1 under a constitutive promoter actin1 was shown to enhance drought and salinity tolerance. Here, we report that overexpression of barley Hva1 also confers cold tolerance in transgenic mulberry. Further, barley Hva1 gene under control of a stress-inducible promoter rd29A can effectively negate growth retardation under non-stress conditions and confer stress tolerance in transgenic mulberry. Transgenic lines display normal morphology to enhanced growth and an increased tolerance against drought, salt and cold conditions as measured by free proline, membrane stability index and PSII activity. Protein accumulation was detected under stress conditions confirming inductive expression of HVA1 in transgenics. Investigations to assess stress tolerance of these plants under field conditions revealed an overall better performance than the non-transgenic plants. Enhanced expression of stress responsive genes such as Mi dnaJ and Mi 2-cysperoxidin suggests that Hva1 can regulate downstream genes associated with providing abiotic stress tolerance. The investigation of transgenic lines presented here demonstrates the acquisition of tolerance against drought, salt and cold stress in plants overexpressing barley Hva1, indicating that Arabidopsis rd29A promoter can function in mulberry.

[Effects of drought stress on the biomass distribution and photosynthetic characteristics of cluster mulberry]

One-year-old Morus alba seedlings were cultivated in pots containing soil with different water contents to study the biomass, chlorophyll fluorescence parameters, net photosynthesis rate, stomatal conductance, transpiration rate and water use efficiency of the seedlings in response to diverse water stress. The results showed that the aboveground biomass and total biomass decreased with increasing water stress. However, more photosynthetic products were transferred to the roots in relation to the aboveground parts to make the root-shoot ratio increase. The fluorescence parameters, such as F(o), F(v) and F(v)/F(m), and stomatal conductance, transpiration rate and water use efficiency decreased with increasing water stress.

Physiological optimality, allocation trade-offs and antioxidant protection linked to better leaf yield performance in drought exposed mulberry

BACKGROUND

Mulberry (Morus spp. L.), usually linked to silkworm rearing, is now considered as a potential forage for livestock feeding and has great potential in world agriculture. Trait-based investigations for leaf yield stability in mulberry under water stress have not been studied extensively. The present study aims to identify candidate traits conferring leaf yield stability in mulberry under drought.

RESULTS

Four popular, indigenous mulberry cultivars (Morus indica L. cvs AR-12, K-2, M. Local and V-1) were investigated. Low leaf temperature (T(l)), higher internal/ambient CO(2) ratios (C(i)/C(a)), greater stomatal conductance to CO(2) (g(s)) and stability in photosystem II efficiency were associated with better net photosynthetic rates (P(n)) in V-1, generating maximum leaf yield when compared to other drought-exposed cultivars. Increased accumulation of foliar α-tocopherol and ascorbic acid-glutathione pool, associated with higher carotenoids, proline and glycine betaine, facilitated lower lipid peroxidation and better leaf yield in V-1 under drought.

CONCLUSION

Minimal plasticity in photosynthetic gas exchange traits and better quantitative growth characteristics were attributed to leaf yield stability under drought. Lower photoinhibition, stabilized photochemistry, effective osmoregulation and enhanced activity of foliar antioxidants extensively contributed to drought tolerance and higher leaf yield in mulberry.

Velocity and pattern of ice propagation and deep supercooling in woody stems of Castanea sativa, Morus nigra and Quercus robur measured by IDTA

Infrared differential thermal analysis (IDTA) was used to monitor the velocity and pattern of ice propagation and deep supercooling of xylem parenchyma cells (XPCs) during freezing of stems of Castanea sativa L., Morus nigra L. and Quercus robur L. that exhibit a macro- and ring-porous xylem. Measurements were conducted on the surface of cross- and longitudinal stem sections. During high-temperature freezing exotherms (HTEs; -2.8 to -9.4°C), initial freezing was mainly observed in the youngest year ring of the sapwood (94%), but occasionally elsewhere (older year rings: 4%; bark: 2%). Initially, ice propagated rapidly in the largest xylem conduits. This resulted in a distinct freezing pattern of concentric circles in C. sativa and M. nigra. During HTEs, supercooling of XPCs became visible in Q. robur stems, but not in the other species that have narrower pith rays. Intracellular freezing of supercooled XPCs of Q. robur became visible by IDTA during low-temperature freezing exotherms (<-17.4 °C). Infrared differential thermal analysis revealed the progress and the two-dimensional pattern of XPC freezing. XPCs did not freeze at once, but rather small cell groups appeared to freeze at random anywhere in the xylem. By IDTA, ice propagation and deep supercooling in stems can be monitored at meaningful spatial and temporal resolutions.

Velocity and pattern of ice propagation and deep supercooling in woody stems of Castanea sativa, Morus nigra and Quercus robur measured by IDTA

Infrared differential thermal analysis (IDTA) was used to monitor the velocity and pattern of ice propagation and deep supercooling of xylem parenchyma cells (XPCs) during freezing of stems of Castanea sativa L., Morus nigra L. and Quercus robur L. that exhibit a macro- and ring-porous xylem. Measurements were conducted on the surface of cross- and longitudinal stem sections. During high-temperature freezing exotherms (HTEs; -2.8 to -9.4°C), initial freezing was mainly observed in the youngest year ring of the sapwood (94%), but occasionally elsewhere (older year rings: 4%; bark: 2%). Initially, ice propagated rapidly in the largest xylem conduits. This resulted in a distinct freezing pattern of concentric circles in C. sativa and M. nigra. During HTEs, supercooling of XPCs became visible in Q. robur stems, but not in the other species that have narrower pith rays. Intracellular freezing of supercooled XPCs of Q. robur became visible by IDTA during low-temperature freezing exotherms (<-17.4 °C). Infrared differential thermal analysis revealed the progress and the two-dimensional pattern of XPC freezing. XPCs did not freeze at once, but rather small cell groups appeared to freeze at random anywhere in the xylem. By IDTA, ice propagation and deep supercooling in stems can be monitored at meaningful spatial and temporal resolutions.

[Effects of irradiance transition on leaf photochemical efficiency of mulberry under different light conditions]

Taking the seedlings of mulberry (Morus alba) variety Menggusang as test materials, and by using chlorophyll fluorescence technique, this paper studied the effects of irradiance transition on the leaf active photochemical efficiency (phi(PS II)), apparent electron transport rate (ETR), and non-photochemical quenching (NPQ) under different light conditions, and analyzed the changes of three components of NPQ. When the irradiance was transferred from dark or weak light to sunlight, the photosynthetic quantum efficiency of full-light leaves was higher than that of weak-light leaves, the inducible time responses of full-light leaves' Psi(PS II) and ETR were faster to reach steady state as compared to those of week-light leaves', and the induction of full-light leaves' NPQ had a trend of rapid increase in the first two minutes and slow decrease subsequently. The state transitional quenching component (q(T)) of full-light leaves under high light condition made up 18% of the total non-photochemical quenching (NPQ), whereas the q(T) of weak-light leaves only amounted to 7% of NPQ, which meant that the mulberry leaves under full-light condition could adapt to the changes of light intensity by means of higher efficiency of photosynthetic quantum and greater contribution to the adjustment of light energy between photosystem I (PS I ) and photosystem II (PS II), compared to the leaves under weak-light condition.

Morphology and oxidative physiology of boron-deficient mulberry plants

The aim of the study was to induce B deficiency symptoms and to relate the generation of reactive oxygen species (ROS) and altered cellular redox environment with the effects of B deficiency in mulberry (Morus alba L.) cv. Kanva-2 plants. Study was undertaken on antioxidant responses, malondialdehyde (MDA) content as an indicator of oxidative damage and ratio of dehydroascorbate (DHA) to ascorbic acid (AsA) as an index of cellular redox environment in B-deficient (0.0 microM) and B-supraoptimal (33 microM) mulberry plants. B deficiency symptoms appeared as upward cupping of the young emerging leaves. Later on, B-deficient plants developed lenticels like cracks on major vein, petiole and stem. B-deficient leaves had higher water potential (Psi) and relative water content (RWC), contained a lower concentration of B, less chloroplastic pigments and high tissue Fe, Mn and Zn concentrations compared to the controls. Hydrogen peroxide was accumulated in leaves of B-deficient and B-supraoptimal plants. B-supraoptimal plants also showed an increased DHA/AsA ratio. The activities of superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), peroxidase (POD, EC 1.11.1.7) and ascorbate peroxidase (APX, EC 1.11.1.11) were increased in B-deficient leaves. The activities of SOD and POD were decreased in B-supraoptimal plants. The results suggest that B deficiency aggravates oxidative stress through enhanced generation of ROS in mulberry plants.

[Responses of mulberry seedlings photosynthesis and antioxidant enzymes to chilling stress after low-temperature acclimation]

Taking the seedlings of mulberry (Morus alba) variety 'Qiuyu' as test materials, this paper studied the changes of their leaf photosynthesis and antioxidant enzymes during low-temperature acclimation, chilling stress, and normal temperature recovery. The results showed that low-temperature acclimation at 12 degrees C for 3 days markedly increased the cold resistance of mulberry seedlings. After the acclimation, the leaf net photosynthetic rate (P(n)), stomatal conductance (G(s)), and maximum photochemical efficiency of photosystem II (F(v)/F(m)) under the chilling stress at 3 degrees C for 3 days increased obviously and recovered rapidly, compared with the control non-acclimation. During the period of the low-temperature acclimation and chilling stress, the leaf proline and soluble sugar contents increased. After the acclimation, the leaf malondialdehyde (MDA) content was obviously lower while the leaf ascorbate peroxidase (APX) activity was obviously higher than the control, illustrating that the increase of osmolytes content and APX activity played important roles in the cold resistance of mulberry seedlings.

Interspecific seed discounting and the fertility cost of hybridization in an endangered species

Hybrid fertilizations can have negative demographic effects on taxa by usurping ovules that would otherwise give rise to nonhybrid offspring. The consequent reduction in conspecific matings may be exaggerated in rare taxa and constitutes a fertility cost that has rarely been quantified. Here, the effect of interspecific mating was estimated on the fecundity of locally rare red mulberry (Morus rubra), which hybridizes with introduced white mulberry (Morus alba) and red yen white hybrids. First, the asymmetry in pollen production among red, white and hybrid mulberry in two sympatric populations was quantified. The fertility cost of hybridization was then assessed experimentally by estimating seed production and rates of interspecific mating in red mulberry trees from plots where white and hybrid mulberry trees were selectively removed. On average, the percentage of mulberry pollen per plot produced by red mulberry (8%) was significantly lower than the mean for white and hybrid mulberry combined (92%). Experimentally removing white and hybrid mulberry increased the siring fertility of red mulberry by 14% but produced no change in seed set. Results indicate that seeds of red mulberry, ordinarily sired by conspecific pollen, are being discounted through fertilization of ovules by heterospecific pollen, which may contribute to local decline of red mulberry.

Induction characteristics and response of photosynthetic quantum conversion to changes in irradiance in mulberry plants

A study was conducted, using rapid time course of chlorophyll (Chl) fluorescence parameters, and light-response curves of Chl fluorescence parameters, to determine the induction requirements and response of photosystem II (PSII) photochemistry and non-photochemical reactions after changes in irradiance in greenhouse mulberry plants. The induction of PSII photochemistry rapidly approached to steady state after leaves were treated from darkness to low irradiance (LI). When irradiance of leaves changed from darkness to high irradiance (HI), a biphasic induction was observed. A slight photoinhibition occurred in the leaves exposed to sunlight coming to the greenhouse, whereas a chronic photoinhibition occurred in the leaves fully exposed to sunlight outside the greenhouse. The chronic photoinhibition was demonstrated by sustained reduction of maximal quantum yield of PSII photochemistry (Fv/Fm). Moreover, the leaves of mulberry plants in greenhouse were sensitive to abrupt changes in irradiance and the sensitivity of leaves suffered in a short-term (1h) high light treatment was reduced, based on the changes in photosynthetic quantum conversion. These results demonstrated an inducible response of photosynthetic quantum conversion to changes in irradiance in mulberry.

Rhodethrin: a novel indole terpenoid ether produced by Rhodobacter sphaeroides has cytotoxic and phytohormonal activities

A novel metabolite was isolated from the culture supernatants of Rhodobacter sphaeroides OU5 when grown on L-tryptophan as sole source of nitrogen under photoheterotrophic conditions. It was identified by IR, NMR ((1)H, (13)C) and MS as an indole terpenoid ether [3-hydroxy-6-(1H-indol-3-yloxy)-4-methylhexanoic acid] and is named as rhodethrin. Rhodethrin at 0.5 microM gave positive test in auxin bioassay and initiated early rooting in tissue-cultured plants than IAA at 5 microM. Rhodethrin has cytotoxic activity against Sup-T(1) lymphoma and Colo-125 cancer cell lines at 10 nM.

Benefit to N2-fixing alder of extending growth period at the cost of leaf nitrogen loss without resorption

This study examines the adaptive role of not resorbing N in N(2)-fixing deciduous trees in terms of their energy balance. The autumnal growth of N(2)-fixing Alnus firma Sieb. et Zucc. (alder) was compared with that of the non-N(2)-fixing Morus bombycis Koizumi (mulberry), which resorbs leaf N. The freezing resistance of leaves of both species was -2 degrees C. Mulberry seedlings lost their photosynthetic ability in mid-October, although the minimum temperature was still above 0 degrees C. Thereafter, their leaves turned yellow and were gradually shed. In contrast, seedlings of the alder maintained their photosynthetic ability until mid-November, when the minimum temperature fell to the freezing resistance limit. Thereafter, their leaves were shed quickly without an autumn tint. The mulberry resorbed 48.9% of leaf N, whereas the alder resorbed hardly any. These results show that, compared with the mulberry tree, the alder extended its growth period for 1 month in return for losing leaf N without resorption. The amount of energy assimilated by the alder in the extended growth period was about six times that required for compensating for the nitrogen loss, if the compensation is dependent only on the tree's own nitrogen fixation. This surplus energy balance has probably allowed N(2)-fixing deciduous trees to evolve their non-N-resorbing trait.

Roles of the plasma membrane and the cell wall in the responses of plant cells to freezing

In an effort to clarify the responses of a wide range of plant cells to freezing, we examined the responses to freezing of the cells of chilling-sensitive and chilling-resistant tropical and subtropical plants. Among the cells of the plants that we examined, those of African violet ( Saintpaulia grotei Engl.) leaves were most chilling-sensitive, those of hypocotyls in mungbean [ Vigna radiata (L.) R. Wilcz.] seedlings were moderately chilling-sensitive, and those of orchid [ Paphiopedilum insigne (Wallich ex Lindl.) Pfitz.] leaves were chilling-resistant, when all were chilled at -2 degrees C. By contrast, all these plant cells were freezing-sensitive and suffered extensive damage when they were frozen at -2 degrees C. Cryo-scanning electron microscopy (Cryo-SEM) confirmed that, upon chilling at -2 degrees C, both chilling-sensitive and chilling-resistant plant cells were supercooled. Upon freezing at -2 degrees C, by contrast, intracellular freezing occurred in Saintpaulia leaf cells, frost plasmolysis followed by intracellular freezing occurred in mungbean seedling cells, and extracellular freezing (cytorrhysis) occurred in orchid leaf cells. We postulate that chilling-related destabilization of membranes might result in the loss of the ability of the plasma membrane to act as a barrier against the propagation of extracellular ice in chilling-sensitive plant cells. We also examined the role of cell walls in the response to freezing using cells in which the plasma membrane had been disrupted by repeated freezing and thawing. In chilling-sensitive Saintpaulia and mungbean cells, the cells with a disrupted plasma membrane responded to freezing at -2 degrees C by intracellular freezing. By contrast, in chilling-resistant orchid cells, as well as in other cells of chilling-resistant and freezing-resistant plant tissues, including leaves of orchard grass ( Dactylis glomerata L.), leaves of Arabidopsis thaliana (L.) Heynh. and cortical tissues of mulberry ( Morus bombycis Koids.), cells with a disrupted plasma membrane responded to freezing by extracellular freezing. Our results indicate that, in the chilling-sensitive plants cells that we examined, not only the plasma membrane but also the cell wall lacked the ability to serve as a barrier against the propagation of extracellular ice, whereas in the chilling-resistant plant cells that we examined, not only the plasma membrane but also the cell wall acted as a barrier against the propagation of extracellular ice. It appears, therefore, that not only the plasma membrane but also the cell wall greatly influences the freezing behavior of plant cells.