During the later stages of seed maturation, two key adaptive traits are acquired that contribute to seed lifespan and dispersal, longevity and dormancy. The seed-specific heat shock transcription factor A9 is an important hub gene in the transcriptional network of late seed maturation. Here we demonstrate that HSFA9 plays a role in thermotolerance rather than in ex situ seed conservation. Storage of hsfa9 seeds of Medicago truncatula and Arabidopsis had comparable lifespan at moderate storage relative humidity (RH), whereas at high RH, hsfa9 seeds lost their viability much faster than wild type seeds. Furthermore, we show that in M. truncatula, Mthsfa9 seeds acquired more dormancy during late maturation than wild type. Transient expression of MtHSFA9 in hairy roots and transcriptome analysis of Mthsfa9 Tnt1 insertion mutants identified a deregulation of genes involved in ABA biosynthesis, catabolism and signalling. Consistent with these results, Mthsfa9 seeds exhibited increased ABA levels and higher sensitivity to ABA. These data suggest that in legumes, HSFA9 acts as a negative regulator of the depth of seed dormancy during seed development via the modulation of hormonal balance.

Abstract: At the genus and species level, variation in root anatomy and architecture may interact to affect strategies of drought avoidance. To investigate this idea, root anatomy and architecture of the drought-sensitive common bean (Phaseolus vulgaris) and drought-adapted tepary bean (Phaseolus acutifolius) were analyzed in relation to water use under terminal drought. Intraspecific variation for metaxylem anatomy and axial conductance was found in the roots of both species. Genotypes with high-conductance root metaxylem phenotypes acquired and transpired more water per unit leaf area, shoot mass, and root mass than genotypes with low-conductance metaxylem phenotypes. Interspecific variation in root architecture and root depth was observed where P. acutifolius has a deeper distribution of root length than P. vulgaris. In the deeper-rooted P. acutifolius, genotypes with high root conductance were better able to exploit deep soil water than genotypes with low root axial conductance. Contrastingly, in the shallower-rooted P. vulgaris, genotypes with low root axial conductance had improved water status through conservation of soil moisture for sustained water capture later in the season. These results indicate that metaxylem morphology interacts with root system depth to determine a strategy of drought avoidance and illustrate synergism among architectural and anatomical phenotypes for root function.

Commentary based on Paniagua Voirol et al. (2020) Plant responses to insect eggs are not induced by egg-associated microbes, but by a secretion attached to the eggs

In habitats with low water availability, a fundamental challenge for plants will be to maximize photosynthetic C-gain whilst minimizing transpirational water-loss. This tradeoff between C-gain and water-loss can in part be achieved through the coordination of leaf-level photosynthetic and hydraulic traits. To test the relationship of photosynthetic C-gain and transpirational water-loss we grew under common growth conditions 18 C4 grasses adapted to habitats with different mean annual precipitation (MAP) and measured leaf-level structural and anatomical traits associated with mesophyll conductance (gm) and leaf hydraulic conductance (Kleaf). The C4 grasses adapted to lower MAP showed greater mesophyll surface area exposed to intercellular air spaces (Smes) and adaxial stomatal density (SDada) which supported greater gm. These grasses also showed greater leaf thickness and vein-to-epidermis distance which may lead to lower Kleaf. Collectively, these leaf traits associated with gm and Kleaf scaled positively with photosynthetic rates (Anet) and leaf-level water-use efficiency (WUE) with low MAP adapted grasses exhibiting greater Anet and WUE. In summary, we identify a suite of leaf-level traits that appear important for adaptation of C4 grasses to habitats with low MAP and may be useful to identify C4 species showing greater Anet and WUE in drier conditions.

Aphids often carry facultative symbionts to achieve diverse advantages. Serratia symbiotica, one of facultative endosymbionts, increases aphid tolerance to heat. However, whether it benefits aphid colonization on host plants is yet to be determined. In the current study, we found that Acyrthosiphon pisum harboring S. symbiotica had longer feeding duration on Medicago truncatula than Serratia-free aphids. Contrastingly, Serratia-free aphids triggered higher accumulation of ROS, jasmonic acid and salicylic acid responsive genes and cytosolic Ca2+ elevations than Serratia-infected aphids. Transcriptomic analysis of salivary glands indicated that a histidine-rich Ca2+-binding protein-like gene (ApHRC) was expressed more highly in the salivary gland of Serratia-infected aphids than that of Serratia-free aphids. Once ApHRC was silenced, Serratia-infected aphids also displayed shorter phloem-feeding duration and caused Ca2+ elevation and ROS accumulation in plants. Our results suggest that ApHRC, a potential effector up-regulated by S. symbiotica in the salivary glands, evaded plant defense response by suppressing Ca2+ elevation and ROS accumulation, allowing colonization of aphids. This study has provided a revolutionary insight into how facultative symbionts facilitate aphid colonization and adaption to host plants.

Gastrodia elata, a fully mycoheterotrophic orchid without photosynthetic ability, only grows symbiotically with the fungus Armillaria. The mechanism of carbon distribution in this mycoheterotrophy is unknown. We detected high sucrose concentrations in all stages of Gastrodia tubers, suggesting sucrose may be the major sugar transported between fungus and orchid. Thick symplasm-isolated wall interfaces in colonized and adjacent large cells implied involvement of sucrose importers. Two sucrose transporter (SUT)-like genes, GeSUT4 and GeSUT3, were identified that are highly expressed in young Armillaria-colonized tubers. Yeast complementation and radio tracer experiments confirmed that GeSUT4 functioned as a high-affinity sucrose-specific proton-dependent importer. Plasma-membrane/tonoplast localization of GeSUT4-GFP fusions, and high RNA expression of GeSUT4 in symbiotic and large cells indicated that GeSUT4 likely functions in active sucrose transport for intercellular allocation and intracellular homeostasis. Transgenic Arabidopsis overexpressing GeSUT4 had larger leaves but sensitive to excess sucrose and roots were colonized with fewer mutualistic Bacillus, supporting the role of GeSUT4 in regulating sugar allocation. This is not only the first documented carbon import system in a mycoheterotrophic interaction, but also highlights the evolutionary importance of sucrose transporters for regulation of carbon flow in all types of plant-microbe interactions.

The polysaccharidic mucilage is a widespread plant trait with diverse features, often present around plant structures in contact with the environment, providing numerous functions including protection and adhesion. In myxodiasporous species, a mucilage is released upon the imbibition of the seed (myxospermy) or the fruit (myxocarpy), and therefore can play roles in the early seedling stages. It is unclear whether myxodiaspory has one or multiple evolutionary origins and why it disappeared in several species. Here, we summarize the recent advances on (i) the mucilage and mucilage secretory cell diversity, (ii) the evolution of the molecular actors involved in myxospermy underlying the observed inter- and intra-species natural diversity and (iii) the recently identified ecological functions. At the intra-species level, a high polymorphism was detected for a few genes in relation to the observed morphological diversity. Well characterized transcriptions factors interact in master regulatory complexes to balance carbon partitioning in Arabidopsis thaliana seeds. These transcription factors were sequentially recruited during seed plant evolution to control diverse traits including myxospermy, and their functions in seeds seem to be conserved across Rosids. Historically, the ecological functions of seed mucilage were mostly related to germination and seed dissemination but recently some exosystemic functions were uncovered such as soil micro-organism control and plant establishment support.

Knowing how switchgrass (Panicum virgatum L.) responds and adapts to phosphorus (P)-limitation will aid efforts to optimize P-efficiency in this species for sustainable biomass production. This integrative study investigated the impacts of mild, moderate, and severe P-stress on genome transcription and whole-plant metabolism, physiology and development in an unprecedented way for this species. P-limitation reduced overall plant growth, increased root/shoot ratio, increased root branching at moderate P-stress, and decreased root diameter with increased density and length of root hairs at severe P-stress. RNA-seq analysis revealed thousands of genes that were differentially expressed under moderate and severe P-stress in roots and/or shoots compared to P-replete plants, with many stress-induced genes involved in transcriptional and other forms of regulation, primary and secondary metabolism, transport, and other processes involved in P-acquisition and homeostasis. Amongst the latter were multiple miRNA399 genes and putative targets of these. Metabolite profiling showed that levels of most sugars and sugar alcohols decreased with increasing P stress, while organic and amino acids increased under mild and moderate P-stress in shoots and roots, although this trend reversed under severe P-stress, especially in shoots.

This article comments on: “Melatonin improves rice salinity stress tolerance by NADPH oxidase-dependent control of the plasma membrane K+ transporters and K+ homeostasis”.

Theories attempting to explain species coexistence in plant communities have argued in favor of species’ capacities to occupy a multidimensional niche with spatial, temporal and biotic axes. We used the concept of hydrological niche segregation to learn how ecological niches are structured both spatially and temporally and whether small scale humidity gradients between adjacent niches are the main factor explaining water partitioning among tree species in a highly water-limited semiarid forest ecosystem. By combining geophysical methods, isotopic ecology, plant ecophysiology and anatomical measurements, we show how coexisting pine and oak species share, use and temporally switch between diverse spatially distinct niches by employing a set of functionally coupled plant traits in response to changing environmental signals. We identified four geospatial niches that turned into nine, when considering the temporal dynamics of the wetting/drying cycles in the substrate and the particular plant species adaptations to garner, transfer, store and use water. Under water scarcity, pine and oak exhibited water use segregation from different niches, yet under maximum drought when oak trees crossed physiological thresholds, niche overlap occurred. The identification of niches and mechanistic understanding of when and how species use them will help unify theories of plant coexistence and competition.

The mechanisms by which herbivores induce plant defenses are well studied. However, how specialized herbivores suppress plant resistance is still poorly understood. Here, we discovered a rice (Oryza sativa) leucine-rich repeat receptor-like kinase, OsLRR-RLK2, which is induced upon attack by gravid females of a specialist piercing-sucking herbivore, the brown planthopper (BPH, Nilaparvata lugens). Silencing OsLRR-RLK2 decreases the constitutive activity of mitogen-activated protein kinase (OsMPK6) and alters BPH-induced transcript levels of several defense-related WRKY transcription factors. Moreover, silencing OsLRR-RLK2 reduces BPH-induction of jasmonic acid and ethylene but promotes the biosynthesis of both elicited salicylic acid and H2O2; silencing also enhances the production of volatiles emitted from rice plants infested with gravid BPH females. These changes decrease BPH preference and performance in the glasshouse and the field. Our study identifies OsLRR-RLK2 as a major susceptibility factor of rice against BPH. It is likely to be employed by BPH to suppress host plant defenses for its own benefit via signaling crosstalk and/or changing the plant’s defense-related signaling profile.

Current crop protection strategies against the fungal pathogen Botrytis cinerea rely on a combination of conventional fungicides and host genetic resistance. However, due to pathogen evolution and legislation in the use of fungicides, these strategies are not sufficient to protect plants against this pathogen. Defence elicitors can stimulate plant defence mechanisms through a phenomenon known as priming. Priming results in a faster and/or stronger expression of resistance upon pathogen recognition by the host. This work aims to study priming of a commercial formulation of the elicitor chitosan. Treatments with chitosan result in induced resistance in solanaceous and brassicaceous plants. In tomato plants, enhanced resistance has been linked with priming of callose deposition and accumulation of the plant hormone jasmonic acid (JA). Large-scale transcriptomic analysis revealed that chitosan primes gene expression at early time-points after infection. In addition, two novel tomato genes with a characteristic priming profile were identified, Avr9/Cf-9 rapidly-elicited protein 75 (ACRE75) and 180 (ACRE180). Transient and stable overexpression of ACRE75, ACRE180 and their Nicotiana benthamiana homologs, revealed that they are positive regulators of plant resistance against B. cinerea. This provides valuable information in the search for strategies to protect Solanaceae plants against B. cinerea.

To understand the growth response to drought, we performed a proteomics study in the leaf growth zone of maize (Zea mays L.) seedlings and functionally characterized the role of starch biosynthesis in the regulation of growth, photosynthesis and antioxidant capacity, using the shrunken2 mutant (sh2), defective in ADP-glucose pyrophosphorylase. Drought induced differential expression of 284 proteins overrepresented for photosynthesis, amino acids, sugar and starch metabolism, and redox-regulation. Changes in protein levels correlated with enzyme activities (increased ATP synthase, cysteine synthase, starch synthase, RuBisCo, peroxiredoxin, glutaredoxin, thioredoxin and decreased triosephosphate isomerase, ferredoxin, cellulose synthase activities, respectively) and metabolite concentrations (increased ATP, cysteine, glycine, serine, starch, proline and decreased cellulose levels). The sh2 mutant had a reduced ability to increase starch levels under drought conditions, causing soluble sugar starvation at the end of the night and impaired leaf growth. Increased RuBisCo activity and pigment concentrations observed in WT in response to drought were lacking in the mutant, which suffered more oxidative damage and recovered more slowly after re-watering. These results demonstrate that starch biosynthesis plays a crucial role in maintaining leaf growth under drought stress and facilitates enhanced carbon acquisition upon recovery.

Although cell wall polymers play important roles in the tolerance of plants to abiotic stress, the effects of salinity on cell wall composition and metabolism in grasses remain largely unexplored. Here, we conducted an in-depth study of changes in cell wall composition and phenolic metabolism induced upon salinity in maize seedlings and plants. Cell wall characterization revealed that salt stress modulated the deposition of cellulose, matrix polysaccharides and lignin. The extraction and analysis of arabinoxylans by size-exclusion chromatography, two-dimensional NMR spectroscopy and carbohydrate gel electrophoresis showed a reduction of arabinoxylan content in salt-stressed roots, with no changes in xylose/arabinose ratios. Saponification and mild acid hydrolysis followed by RP-HPLC analysis revealed that salt stress also reduced the feruloylation of arabinoxylans. Determination of lignin content and composition by nitrobenzene oxidation and two-dimensional NMR confirmed the increased incorporation of syringyl units in lignin polymer. Our data also revealed the induction of the expression of genes and enzymes enrolled in phenylpropanoid biosynthesis under salinity. The UPLC-MS-based metabolite profiling confirmed the modulation of phenolic profiling by salinity and the accumulation of ferulate and its derivatives 3- and 4-O-feruloyl quinate. In conclusion, we present a model for explaining cell wall remodeling in response to salinity.

Phosphorus (P) is an essential mineral nutrient for plants. Nevertheless, excessive P accumulation in leaf mesophyll cells causes necrotic symptoms in land plants; this phenomenon is termed P toxicity. However, the detailed mechanisms underlying P toxicity in plants have not yet been elucidated. This study aimed to investigate the molecular mechanism of P toxicity in rice. We found that under excessive inorganic P (Pi) application, Rubisco activation decreased and photosynthesis was inhibited, leading to lipid peroxidation. Although the defence systems against reactive oxygen species (ROS) accumulation were activated under excessive Pi application conditions, the Cu/Zn-type superoxide dismutase activities were inhibited. A metabolic analysis revealed that excessive Pi application led to an increase in the cytosolic sugar phosphate concentration and the activation of phytic acid synthesis. These conditions induced mRNA expression of genes that are activated under metal-deficient conditions, although metals did accumulate. These results suggest that P toxicity is triggered by the attenuation of both photosynthesis and metal availability within cells mediated by phytic acid accumulation. Here, we discuss the whole phenomenon of P toxicity, beginning from the accumulation of Pi within cells to death in land plants.

The Plumbaginaceae (non-core Caryophyllales) is a family well known for species adapted to a wide range of arid and saline habitats. Of its salt-tolerant species, at least 45 are in the genus Limonium; two in each of Aegialitis, Limoniastum and Myriolimon, and one each in Psylliostachys, Armeria, Ceratostigma, Goniolimon and Plumbago. All the halophytic members of the family have salt glands and salt glands are also common in the closely related Tamaricaceae and Frankeniaceae. The halophytic species of the three families can secrete a range of ions (Na+, K+, Ca2+, Mg2+, Cl-, HCO3-, SO42) and other elements (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn). Salt glands are, however, absent in salt-tolerant members of the sister family Polygonaceae. We describe the structure of the salt glands in the three families and consider whether glands might have arisen as a means to avoid the toxicity of Na+ and/or Cl- or to regulate Ca2+ concentrations with the leaves. We conclude that the establishment of lineages with salt glands took place after the split between the Polygonaceae and its sister group the Plumbaginaceae.

Utilizing phosphate more efficiently is crucial for sustainable crop production. Highly efficient rice (Oryza sativa) cultivars have been identified and this study aims to identify metabolic markers associated with P utilization efficiency. P deficiency generally reduced leaf P concentrations and CO2 assimilation rates but efficient cultivars were reducing leaf P concentrations further than inefficient ones while maintaining similar CO2 assimilation rates. Adaptive changes in carbon metabolism were detected but equally in efficient and inefficient cultivar groups. Groups furthermore did not differ with respect to partial substitutions of phospholipids by sulfo- and galactolipids. Metabolites significantly more abundant in the efficient group, such as sinapate, benzoate and glucoronate, were related to antioxidant defense and may help alleviating oxidative stress caused by P deficiency. Sugar alcohols ribitol and threitol were another marker metabolite for higher phosphate efficiency as were several amino acids, especially threonine. Since these metabolites are not known to be associated with P deficiency, they may provide novel clues for the selection of more P efficient genotypes. In conclusion, metabolite signatures detected here were not related to phosphate metabolism but rather helped P efficient lines to keep vital processes functional under the adverse conditions of P starvation.

Norway spruce is a conifer storing large amounts of terpenoids in resin ducts of various tissues. Parts of the terpenoids stored in needles can be emitted together with de novo synthesized terpenoids. Since previous studies provided hints on xylem transported terpenoids as a third emission source, we tested if terpenoids are transported in xylem sap of Norway spruce. We further aimed at understanding if they might contribute to terpenoid emission from needles. We determined terpenoid content and composition in xylem sap, needles, bark, wood and roots of field grown trees, as well as terpenoid emissions from needles. We found considerable amounts of terpenoids – mainly oxygenated compounds - in xylem sap. The terpenoid concentration in xylem sap was relatively low compared to the content in other tissues where terpenoids are stored in resin ducts. Importantly, the terpenoid composition in the xylem sap greatly differed from the composition in wood, bark or roots suggesting that an internal transport of terpenoids takes place at the sites of xylem loading. Our work gives hints that plant internal transport of terpenoids exists within conifers; studies on their functions should be a focus of future research.