Do elevations in temperature,CO2, and nutrient availability modify belowground carbon gain and root morphology in artificially defoliated silver birch seedlings?

Climate warming increases the risk of insect defoliation in boreal forests. Losses in photosynthetically active surfaces cause reduction in net primary productivity and often compromise carbon reserves of trees. The concurrent effects of climate change and removal of foliage on root growth responses and carbohydrate dynamics are poorly understood, especially in tree seedlings. We investigated if exposures to different combinations of elevated temperature, CO₂, and nutrient availability modify belowground carbon gain and root morphology in artificially defoliated 1‐year‐old silver birches (Betula pendula). We quantified nonstructural carbohydrates (insoluble starch as a storage compound; soluble sucrose, fructose, and glucose) singly and in combination in fine roots of plants under winter dormancy. Also the total mass, fine root proportion, water content, and length of roots were defined. We hypothesized that the measured properties are lower in defoliated birch seedlings that grow with ample resources than with scarce resources. On average, fertilization markedly decreased both the proportion and the carbohydrate concentrations of fine roots in all seedlings, whereas the effect of fertilization on root water content and dry mass was the opposite. However, defoliation mitigated the effect of fertilization on the root water content, as well as on the proportion of fine roots and their carbohydrate concentrations by reversing the outcomes. Elevation in temperature decreased and elevation in CO₂ increased the absolute contents of total nonstructural carbohydrates, whereas fertilization alleviated both these effects. Also the root length and mass increased by CO₂ elevation. This confirms that surplus carbon in birch tissues is used as a substrate for storage compounds and for cell wall synthesis. To conclude, our results indicate that some, but not all elements of climate change alter belowground carbon gain and root morphology in defoliated silver birch seedlings.     

Improving rice tolerance to potassium deficiency by enhancing OsHAK16p:WOX11‐controlled root development

Potassium (K) deficiency in plants confines root growth and decreases root‐to‐shoot ratio, thus limiting root K acquisition in culture medium. A WUSCHEL‐related homeobox (WOX) gene, WOX11, has been reported as an integrator of auxin and cytokinin signalling that regulates root cell proliferation. Here, we report that ectopic expression of WOX11 gene driven by the promoter of OsHAK16 encoding a low‐K‐enhanced K transporter led to an extensive root system and adventitious roots and more effective tiller numbers in rice. The WOX11‐regulated root and shoot phenotypes in the OsHAK16p:WOX11 transgenic lines were supported by K‐deficiency‐enhanced expression of several RR genes encoding type‐A cytokinin‐responsive regulators, PIN genes encoding auxin transporters and Aux/IAA genes. In comparison with WT, the transgenic lines showed increases in root biomass, root activity and K concentrations in the whole plants, and higher soluble sugar concentrations in roots particularly under low K supply condition. The improvement of sugar partitioning to the roots by the expression of OsHAK16p:WOX11 was further indicated by increasing the expression of OsSUT1 and OsSUT4 genes in leaf blades and several OsMSTs genes in roots. Expression of OsHAK16p:WOX11 in the rice grown in moderate K‐deficient soil increased total K uptake by 72% and grain yield by 24%–32%. The results suggest that enlarging root growth and development by the expression of WOX11 in roots could provide a useful option for increasing K acquisition efficiency and cereal crop productivity in low K soil.     

Patterns of rhizosphere carbon flux in sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) saplings

Despite its importance in the terrestrial C cycle rhizosphere carbon flux (RCF) has rarely been measured for intact root–soil systems. We measured RCF for 8‐year‐old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from the Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. Five saplings of each species were pulse labeled with ¹³CO₂ at ambient CO₂ concentrations for 4–6 h, and the ¹³C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We hypothesized yellow birch roots would supply more labile C to the rhizosphere than sugar maple roots based on the presumed greater C requirements of ectomycorrhizal roots. We observed appearance of the label in rhizosphere soil of both species within the first 24 h, and a striking difference between species in the timing of ¹³C release to soil. In sugar maple, peak concentration of the label appeared 1 day after labeling and declined over time whereas in birch the label increased in concentration over the 7‐day chase period. The sum of root and rhizomicrobial respiration in the pots was 19% and 26% of total soil respiration in sugar maple and yellow birch, respectively. Our estimate of the total amount of RCF released by roots was 6.9–7.1% of assimilated C in sugar maple and 11.2–13.0% of assimilated C in yellow birch. These fluxes extrapolate to 55–57 and 90–104 g C m^?2 yr^?1 from sugar maple and yellow birch roots, respectively. These results suggest RCF from both arbuscular mycorrhizal and ectomycorrhizal roots represents a substantial flux of C to soil in northern hardwood forests with important implications for soil microbial activity, nutrient availability and C storage.     

Auxin biosynthetic gene TAR2 is involved in low nitrogen‐mediated reprogramming of root architecture in Arabidopsis

In plants, the plasticity of root architecture in response to nitrogen availability largely determines nitrogen acquisition efficiency. One poorly understood root growth response to low nitrogen availability is an observed increase in the number and length of lateral roots (LRs). Here, we show that low nitrogen‐induced Arabidopsis LR growth depends on the function of the auxin biosynthesis gene TAR2 (tryptophan aminotransferase related 2). TAR2 was expressed in the pericycle and the vasculature of the mature root zone near the root tip, and was induced under low nitrogen conditions. In wild type plants, low nitrogen stimulated auxin accumulation in the non‐emerged LR primordia with more than three cell layers and LR emergence. Conversely, these low nitrogen‐mediated auxin accumulation and root growth responses were impaired in the tar2‐c null mutant. Overexpression of TAR2 increased LR numbers under both high and low nitrogen conditions. Our results suggested that TAR2 is required for reprogramming root architecture in response to low nitrogen conditions. This finding suggests a new strategy for improving nitrogen use efficiency through the engineering of TAR2 expression in roots.     

Exaggerated root respiration accounts for growth retardation in a starchless mutant of Arabidopsis thaliana

The knock‐out mutation of plastidial phosphoglucomutase (pgm) causes a starchless phenotype in Arabidopsis thaliana, and results in a severe growth reduction of plants cultivated under diurnal conditions. It has been speculated that high soluble sugar levels accumulating during the light phase in leaf mesophyll might cause a reduction of photosynthetic activity or that shortage of reduced carbon during the night is the reason for the slow biomass gain of pgm. Separate simultaneous measurements of leaf net photosynthesis and root respiration demonstrate that photosynthetic activity per unit fresh weight is not reduced in pgm, whereas root respiration is strongly elevated. Comparison with a mutant defective in the dominating vacuolar invertase (AtβFruct4) revealed that high sucrose concentration in the cytosol, but not in the vacuole, of leaf cells is responsible for elevated assimilate transport to the root. Increased sugar supply to the root, as observed in pgm mutants, forces substantial respiratory losses. Because root respiration accounts for 80% of total plant respiration under long‐day conditions, this gives rise to retarded biomass formation. In contrast, reduced vacuolar invertase activity leads to reduced net photosynthesis in the shoot and lowered root respiration, and affords an increased root/shoot ratio. The results demonstrate that roots have very limited capacity for carbon storage but exert rigid control of supply for their maintenance metabolism.     

The microRNA miR171h modulates arbuscular mycorrhizal colonization of Medicago truncatula by targeting NSP2

Most land plants live symbiotically with arbuscular mycorrhizal fungi. Establishment of this symbiosis requires signals produced by both partners: strigolactones in root exudates stimulate pre‐symbiotic growth of the fungus, which releases lipochito‐oligosaccharides (Myc‐LCOs) that prepare the plant for symbiosis. Here, we have investigated the events downstream of this early signaling in the roots. We report that expression of miR171h, a microRNA that targets NSP2, is up‐regulated in the elongation zone of the root during colonization by Rhizophagus irregularis (formerly Glomus intraradices) and in response to Myc‐LCOs. Fungal colonization was much reduced by over‐expressing miR171h in roots, mimicking the phenotype of nsp2 mutants. Conversely, in plants expressing an NSP2 mRNA resistant to miR171h cleavage, fungal colonization was much increased and extended into the elongation zone of the roots. Finally, phylogenetic analyses revealed that miR171h regulation of NSP2 is probably conserved among mycotrophic plants. Our findings suggest a regulatory mechanism, triggered by Myc‐LCOs, that prevents over‐colonization of roots by arbuscular mycorrhizal fungi by a mechanism involving miRNA‐mediated negative regulation of NSP2.     

The role of arabinokinase in arabinose toxicity in plants

Plant cell wall polymers are synthesized by glycosyltransferases using nucleotide sugars as substrates. Most UDP‐sugars are synthesized from UDP‐glucose via de novo pathways but salvage pathways work in parallel to recycle sugars, which have been released during cell wall polymer and glycoprotein turnover. Here we report on the cloning and biochemical analysis of two arabinokinases in Arabidopsis. Arabinokinase is a 100 kDa protein located in the cytosol with a putative N‐terminal glycosyltransferase domain and a C‐terminal sugar‐1‐kinase domain. This unique structure is highly conserved in the plant kingdom. Arabinokinase has a high affinity for l ‐arabinose, which is the only sugar substrate of this GHMP (galactose; homoserine; mevalonate; phosphomevalonate) kinase. Plants that were knocked‐out for arabinokinase and the previously described ara1‐1 mutant were characterized. The ARA1‐1 mutant form of the enzyme carries a point mutation in an α‐helix. The mutation is close to the substrate binding site and changes the K_m value for arabinose from 80 μm in the wild type to 17 000 μm in ARA1‐1. The previous arabinose toxicity explanation is challenged by knockout plants in arabinokinase that accumulate higher levels of arabinose but do not show signs of arabinose toxicity. Analysis of marker genes from sugar signalling pathways (SnRK1 and Tor) suggest that ara1‐1 misinterprets its carbon energy status. Although glucose is present in ara1‐1 similar to wild type levels, it constitutively changes gene expression as typically found in wild type plants only under starvation conditions. Furthermore, ara1‐1 shows increased expression of marker genes for programmed cell death as found in other lesion mimic mutants.     

The Snf1‐related protein kinases SnRK2.4 and SnRK2.10 are involved in maintenance of root system architecture during salt stress

The sucrose non‐fermenting‐1‐related protein kinase 2 (SnRK2) family represents a unique family of plant‐specific protein kinases implicated in cellular signalling in response to osmotic stress. In our studies, we observed that two class 1 SnRK2 kinases, SnRK2.4 and SnRK2.10, are rapidly and transiently activated in Arabidopsis roots after exposure to salt. Under saline conditions, snrk2.4 knockout mutants had a reduced primary root length, while snrk2.10 mutants exhibited a reduction in the number of lateral roots. The reduced lateral root density was found to be a combinatory effect of a decrease in the number of lateral root primordia and an increase in the number of arrested lateral root primordia. The phenotypes were in agreement with the observed expression patterns of genomic yellow fluorescent protein (YFP) fusions of SnRK2.10 and ‐2.4, under control of their native promoter sequences. SnRK2.10 was found to be expressed in the vascular tissue at the base of a developing lateral root, whereas SnRK2.4 was expressed throughout the root, with higher expression in the vascular system. Salt stress triggered a rapid re‐localization of SnRK2.4–YFP from the cytosol to punctate structures in root epidermal cells. Differential centrifugation experiments of isolated Arabidopsis root proteins confirmed recruitment of endogenous SnRK2.4/2.10 to membranes upon exposure to salt, supporting their observed binding affinity for the phospholipid phosphatidic acid. Together, our results reveal a role for SnRK2.4 and ‐2.10 in root growth and architecture in saline conditions.     

Diurnal periodicity of assimilate transport shapes resource allocation and whole‐plant carbon balance

Whole‐plant carbon balance comprises diurnal fluctuations of photosynthetic carbon gain and respiratory losses, as well as partitioning of assimilates between phototrophic and heterotrophic organs. Because it is difficult to access, the root system is frequently neglected in growth models, or its metabolism is rated based on generalizations from other organs. Here, whole‐plant cuvettes were used for investigating total‐plant carbon exchange with the environment over full diurnal cycles. Dynamics of primary metabolism and diurnally resolved phloem exudation profiles, as proxy of assimilate transport, were combined to obtain a full picture of resource allocation. This uncovered a strong impact of periodicity of inter‐organ transport on the efficiency of carbon gain. While a sinusoidal fluctuation of the transport rate, with minor diel deflections, minimized respiratory losses in Arabidopsis wild‐type plants, triangular or rectangular patterns of transport, found in mutants defective in either starch or sucrose metabolism, increased root respiration at the end or beginning of the day, respectively. Power spectral density and cross‐correlation analysis revealed that only the rate of starch synthesis was strictly correlated to the rate of net photosynthesis in wild‐type, while in a sucrose‐phosphate synthase mutant (spsa1), this applied also to carboxylate synthesis, serving as an alternative carbon pool. In the starchless mutant of plastidial phospho‐gluco mutase (pgm), none of these rates, but concentrations of sucrose and glucose in the root, followed the pattern of photosynthesis, indicating direct transduction of shoot sugar levels to the root. The results demonstrate that starch metabolism alone is insufficient to buffer diurnal fluctuations of carbon exchange.     

What controls variation in carbon use efficiency among Amazonian tropical forests?

Why do some forests produce biomass more efficiently than others? Variations in Carbon Use Efficiency (CUE: total Net Primary Production (NPP)/ Gross Primary Production (GPP)) may be due to changes in wood residence time (Biomass/NPP_wood), temperature, or soil nutrient status. We tested these hypotheses in 14, one ha plots across Amazonian and Andean forests where we measured most key components of net primary production (NPP: wood, fine roots, and leaves) and autotrophic respiration (R_a; wood, rhizosphere, and leaf respiration). We found that lower fertility sites were less efficient at producing biomass and had higher rhizosphere respiration, indicating increased carbon allocation to belowground components. We then compared wood respiration to wood growth and rhizosphere respiration to fine root growth and found that forests with residence times <40 yrs had significantly lower maintenance respiration for both wood and fine roots than forests with residence times >40 yrs. A comparison of rhizosphere respiration to fine root growth showed that rhizosphere growth respiration was significantly greater at low fertility sites. Overall, we found that Amazonian forests produce biomass less efficiently in stands with residence times >40 yrs and in stands with lower fertility, but changes to long‐term mean annual temperatures do not impact CUE.     

Phytophthora and Pythium Associated with Feeder Root Rot of Citrus in the Transvaal Province of South Africa

Mild to extensive feeder root rot was present in all 23 orchards, with trees showing symptoms of citrus decline from nine areas in the Transvaal Province of South Africa. Phytophthora nicotianae and Pythium spp. were isolated from diseased roots and rhizosphere soils in all areas sampled. Isolations from diseased feeder roots showed P. nicotianae present in 26% of orchards during Spring and 61% of orchards during Autumn, while Pythium spp. were present in 56% of orchards during Spring and 65% of orchards during Autumn. In isolations from baited rhizosphere soils, P. nicotianae was present in 56% of orchards during Spring and 52% of orchards during Autumn, while Pythium spp. were present in 69% of orchards during Spring and 82% of orchards during Autumn. In rhizosphere soils, the mean population density of Pythium spp. was higher than that of P. nicotianae throughout the season. Only P. nicotianae was consistently isolated during thesurvey. Different Pythium spp. were isolated of which two were tentatively identified as P. paroecandrum and Pythium‘Gp.G’.     

Flowering‐mediated root‐fungus symbiosis loss is related to jasmonate‐dependent root soluble sugar deprivation

The role of flowering in root‐fungal symbiosis is not well understood. Because flowering and fungal symbionts are supported by carbohydrates, we hypothesized that flowering modulates root‐beneficial fungal associations through alterations in carbohydrate metabolism and transport. We monitored fungal colonization and soluble sugars in the roots of Arabidopsis thaliana following inoculation with a mutualistic fungus Phomopsis liquidambari across different plant developmental stages. Jasmonate signalling of wild‐type plants, sugar transport, and root invertase of wild‐type and jasmonate‐insensitive plants were exploited to assess whether and how jasmonate‐dependent sugar dynamics are involved in flowering‐mediated fungal colonization alterations. We found that flowering restricts root‐fungal colonization and activates root jasmonate signalling upon fungal inoculation. Jasmonates reduce the constitutive and fungus‐induced accumulation of root glucose and fructose at the flowering stage. Further experiments with sugar transport and metabolism mutant lines revealed that root glucose and fructose positively influence fungal colonization. Diurnal, jasmonate‐dependent inhibitions of sugar transport and soluble invertase activity were identified as likely mechanisms for flowering‐mediated root sugar depletion upon fungal inoculation. Collectively, our results reveal that flowering drives root‐fungus cooperation loss, which is related to jasmonate‐dependent root soluble sugar depletion. Limiting the spread of root‐fungal colonization may direct more resources to flower development.     

Analogous wheat root rhizosphere microbial successions in field and greenhouse trials in the presence of biocontrol agents Paenibacillus peoriae SP9 and Streptomyces fulvissimus FU14

Two Pythium-infested soils were used to compare the wheat root and rhizosphere soil microbial communities from plants grown in the field or in greenhouse trials and their stability in the presence of biocontrol agents. Bacteria showed the highest diversity at early stages of wheat growth in both field and greenhouse trials, while fungal diversity increased later on, at 12 weeks of the crop cycle. The microbial communities were stable in roots and rhizosphere samples across both soil types used in this study. Such stability was also observed irrespective of the cultivation system (field or greenhouse) or addition of biocontrol coatings to wheat seeds to control Pythium disease (in this study soil infected with Pythium sp. clade F was tested). In greenhouse plant roots, Archaeorhizomyces, Debaryomyces, Delftia, and unclassified Pseudeurotiaceae were significantly reduced when compared to plant roots obtained from the field trials. Some operational taxonomic units (OTUs) represented genetic determinants clearly transmitted vertically by seed endophytes (specific OTUs were found in plant roots) and the plant microbiota was enriched over time by OTUs from the rhizosphere soil. This study provided key information regarding the microbial communities associated with wheat roots and rhizosphere soils at different stages of plant growth and the role that Paenibacillus and Streptomyces strains play as biocontrol agents in supporting plant growth in infested soils.     

An Arabidopsis ATPase gene involved in nematode‐induced syncytium development and abiotic stress responses

The beet cyst nematode Heterodera schachtii induces syncytia in the roots of Arabidopsis thaliana, which are its only nutrient source. One gene, At1g64110, that is strongly up‐regulated in syncytia as shown by RT‐PCR, quantitative RT‐PCR, in situ RT‐PCR and promoter::GUS lines, encodes an AAA+‐type ATPase. Expression of two related genes in syncytia, At4g28000 and At5g52882, was not detected or not different from control root segments. Using amiRNA lines and T‐DNA mutants, we show that At1g64110 is important for syncytium and nematode development. At1g64110 was also inducible by wounding, jasmonic acid, salicylic acid, heat and cold, as well as drought, sodium chloride, abscisic acid and mannitol, indicating involvement of this gene in abiotic stress responses. We confirmed this using two T‐DNA mutants that were more sensitive to abscisic acid and sodium chloride during seed germination and root growth. These mutants also developed significantly smaller roots in response to abscisic acid and sodium chloride. An in silico analysis showed that ATPase At1g64110 (and also At4g28000 and At5g52882) belong to the ‘meiotic clade’ of AAA proteins that includes proteins such as Vps4, katanin, spastin and MSP1.     

Engineering the expression level of cytosolic nucleoside diphosphate kinase in transgenic Solanum tuberosum roots alters growth,respiration and carbon metabolism

Nucleoside diphosphate kinase (NDPK) is a ubiquitous enzyme that catalyzes the transfer of the γ‐phosphate from a donor nucleoside triphosphate to an acceptor nucleoside diphosphate. In this study we used a targeted metabolomic approach and measurement of physiological parameters to report the effects of the genetic manipulation of cytosolic NDPK (NDPK1) expression on physiology and carbon metabolism in potato (Solanum tuberosum) roots. Sense and antisense NDPK1 constructs were introduced in potato using Agrobacterium rhizogenes to generate a population of root clones displaying a 40‐fold difference in NDPK activity. Root growth, O₂ uptake, flux of carbon between sucrose and CO₂, levels of reactive oxygen species and some tricarboxylic acid cycle intermediates were positively correlated with levels of NDPK1 expression. In addition, NDPK1 levels positively affected UDP‐glucose and cellulose contents. The activation state of ADP‐glucose pyrophosphorylase, a key enzyme in starch synthesis, was higher in antisense roots than in roots overexpressing NDPK1. Further analyses demonstrated that ADP‐glucose pyrophosphorylase was more oxidized, and therefore less active, in sense clones than antisense clones. Consequently, antisense NDPK1 roots accumulated more starch and the starch to cellulose ratio was negatively affected by the level of NDPK1. These data support the idea that modulation of NDPK1 affects the distribution of carbon between starch and cellulose biosynthetic pathways.     

A sucrose non‐fermenting‐1‐related protein kinase‐1 gene,IbSnRK1, improves starch content,composition, granule size,degree of crystallinity and gelatinization in transgenic sweet potato

Sucrose non‐fermenting‐1‐related protein kinase‐1 (SnRK1) is an essential energy‐sensing regulator and plays a key role in the global control of carbohydrate metabolism. The SnRK1 gene has been found to increase starch accumulation in several plant species. However, its roles in improving starch quality have not been reported to date. In this study, we found that the IbSnRK1 gene was highly expressed in the storage roots of sweet potato and strongly induced by exogenous sucrose. Its expression followed the circandian rhythm. Its overexpression not only increased starch content, but also decreased proportion of amylose, enlarged granule size and improved degree of crystallinity and gelatinization in transgenic sweet potato, which revealed, for the first time, the important roles of SnRK1 in improving starch quality of plants. The genes involved in starch biosynthesis pathway were systematically up‐regulated, and the content of ADP‐glucose as an important precursor for starch biosynthesis and the activities of key enzymes were significantly increased in transgenic sweet potato. These findings indicate that IbSnRK1 improves starch content and quality through systematical up‐regulation of the genes and the increase in key enzyme activities involved in starch biosynthesis pathway in transgenic sweet potato. This gene has the potential to improve starch content and quality in sweet potato and other plants.