Saturated humidity accelerates lateral root development in rice (Oryza sativa L.) seedlings by increasing phloem-based auxin transport

Meta-analysis of identified genomic regions and candidate genes underlying salinity tolerance in rice (Oryza sativa L.)

Rice output has grown globally, yet abiotic factors are still a key cause for worry. Salinity stress seems to have the more impact on crop production out of all abiotic stresses. Currently one of the most significant challenges in paddy breeding for salinity tolerance with the help of QTLs, is to determine the QTLs having the best chance of improving salinity tolerance with the least amount of background noise from the tolerant parent. Minimizing the size of the QTL confidence interval (CI) is essential in order to primarily include the genes responsible for salinity stress tolerance. By considering that, a genome-wide meta-QTL analysis on 768 QTLs from 35 rice populations published from 2001 to 2022 was conducted to identify consensus regions and the candidate genes underlying those regions responsible for the salinity tolerance, as it reduces the confidence interval (CI) to many folds from the initial QTL studies. In the present investigation, a total of 65 MQTLs were extracted with an average CI reduced from 17.35 to 1.66 cM including the smallest of 0.01 cM. Identification of the MQTLs for individual traits and then classifying the target traits into correlated morphological, physiological and biochemical aspects, resulted in more efficient interpretation of the salinity tolerance, identifying the candidate genes and to understand the salinity tolerance mechanism as a whole. The results of this study have a huge potential to improve the rice genotypes for salinity tolerance with the help of MAS and MABC.

Turning up the volume: How root branching adaptive responses aid water foraging

Access to water is critical for all forms of life. Plants primarily access water through their roots. Root traits such as branching are highly sensitive to water availability, enabling plants to adapt their root architecture to match soil moisture distribution. Lateral root adaptive responses hydropatterning and xerobranching ensure new branches only form when roots are in direct contact with moist soil. Root traits are also strongly influenced by atmospheric humidity, where a rapid drop leads to a promotion of root growth and branching. The plant hormones auxin and/or abscisic acid (ABA) play key roles in regulating these adaptive responses. We discuss how these signals are part of a novel "water-sensing" mechanism that couples hormone movement with hydrodynamics to orchestrate root branching responses.

Integrated Metabolomic and Transcriptomic Analysis of the Quinoa Seedling Response to High Relative Humidity Stress

Quinoa is of great interest because it is cold- and drought-resistant; however, little research has been performed on quinoa under high relative humidity (RH) stress. In this study, quinoa seedlings of a highly HR-resistant variety ("Dianli-439") and a sensitive variety ("Dianli-969") were subjected to morphological and physiological measurements and metabolome and transcriptome analyses to investigate their response to high RH stress. In total, 1060 metabolites were detected, and lipids and flavonoids were the most abundant, with 173 and 167 metabolites, respectively. In total, 13,095 differentially expressed genes were identified, and the results showed that abscisic acid, auxin, and jasmonic-acid-related genes involved in plant hormone signaling may be involved in the response of quinoa seedlings to high RH stress. The analysis of the transcription factors revealed that the AP2/ERF family may also play an important role in the response to high RH stress. We identified the possible regulatory mechanisms of the hormone signaling pathways under high RH stress. Our findings can provide a basis for the selection and identification of highly resistant quinoa varieties and the screening of the metabolite-synthesis- and gene-regulation-related mechanisms in quinoa in response to RH stress.

Effect of Low Light Stress on Distribution of Auxin (Indole-3-acetic Acid) between Shoot and Roots and Development of Lateral Roots in Barley Plants

Depending on their habitat conditions, plants can greatly change the growth rate of their roots. However, the mechanisms of such responses remain insufficiently clear. The influence of a low level of illumination on the content of endogenous auxins, their localization in leaves and transport from shoots to roots were studied and related to the lateral root branching of barley plants. Following two days' reduction in illumination, a 10-fold reduction in the emergence of lateral roots was found. Auxin (IAA, indole-3-acetic acid) content decreased by 84% in roots and by 30% in shoots, and immunolocalization revealed lowered IAA levels in phloem cells of leaf sections. The reduced content of IAA found in the plants under low light suggests an inhibition of production of this hormone under these conditions. At the same time, two-fold downregulation of the LAX3 gene expression, facilitating IAA influx into the cells, was detected in the roots, as well as a decline in auxin diffusion from shoots through the phloem by about 60%. It was suggested that the reduced emergence of lateral roots in barley under a low level of illumination was due to a disturbance of auxin transport through the phloem and down-regulation of the genes responsible for auxin transport in plant roots. The results confirm the importance of the long distance transport of auxins for the control of the growth of roots under conditions of low light. Further study of the mechanisms that control the transport of auxins from shoots to roots in other plant species is required.

Long-distance hormone transport via the phloem

Several key plant hormones are synthesised in the shoot and are advected within the phloem to the root tip. In the root tip, these hormones regulate growth and developmental processes, and responses to environmental cues. However, we lack understanding of how environmental factors and biological parameters affect the delivery of hormones to the root tip. In this study, we build on existing models of phloem flow to develop a mathematical model of sugar transport alongside the transport of a generic hormone. We derive the equations for osmotically driven flow in a long, thin pipe with spatially varying membrane properties to capture the phloem loading and unloading zones. Motivated by experimental findings, we formulate solute membrane transport in terms of passive and active components, and incorporate solute unloading via bulk flow (i.e. advection with the water efflux) by including the Staverman reflection coefficient. We use the model to investigate the coupling between the sugar and hormone dynamics. The model predicts that environmental cues that lead to an increase in active sugar loading, an increase in bulk flow sugar unloading or a decrease in the relative root sugar concentration result in an increase in phloem transport velocity. Furthermore, the model reveals that such increases in phloem transport velocity result in an increase in hormone delivery to the root tip for passively loaded hormones.

Whole-genome resequencing identified QTLs, candidate genes and Kompetitive Allele-Specific PCR markers associated with the large fruit of Atlantic Giant (Cucurbita maxima)

Atlantic Giant (AG) pumpkin (Cucurbita maxima) produces the world's largest fruit. Elucidating the molecular mechanism of AG fruit formation is of scientific and practical importance. In this research, genome-wide resequencing of an F₂ population produced by a cross between AG and its small-fruit ancestor Hubbard was used to identify quantitative trait loci (QTLs) and candidate genes. Transgressive segregation of fruit size-related traits was observed in the F₂ population, suggesting that fruit size was a quantitative trait controlled by multiple genes. A genetic map with an average physical distance of 154 kb per marker was constructed, and 13 QTLs related to fruit size were identified using bin-map construction. RNA sequencing analysis revealed that pathways associated with assimilate accumulation into the fruit, including carbohydrate metabolism, were significantly enriched in differentially expressed genes. According to the predicted impact of mutation on the biological function of certain proteins, 13 genes were selected as candidate genes associated with fruit size, among which two phytohormone-related genes, CmaCh17G011340 (a flavin-containing monooxygenase) and CmaCh04G029660 (a leucine-rich repeat protein kinase) were chosen for further investigation. Finally, one insertion-deletion (inDel) and three single nucleotide polymorphisms (SNPs) were successfully transformed to Kompetitive Allele-Specific PCR (KASP) markers. The novel QTLs and candidate genes identified provide insights into the genetic mechanism of large fruit formation of AG, and the genetic map and tightly linked KASP markers developed in this study can be employed for marker-assisted breeding to alter fruit size of C. maxima.

Review primary and secondary metabolites in phloem sap collected with aphid stylectomy

Phloem plays a central role in assimilate transport as well as in the transport of several secondary compounds. In order to study the chemical composition of phloem sap, different methods have been used for its collection, including stem incisions, EDTA-facilitated exudation or aphid stylectomy. Each collection method has several advantages and disadvantages and, unfortunately, the reported metabolite profiles and concentrations depend on the method used for exudate collection. This review therefore primarily focusses on sugars, amino acids, inorganic ions and further transported compounds like organic acids, nucleotides, phytohormons, defense signals, and lipophilic substances in the phloem sap obtained by aphid stylectomy to facilitate comparability of the data.

Uncharted routes: exploring the relevance of auxin movement via plasmodesmata

Auxin is an endogenous small molecule with an incredibly large impact on growth and development in plants. Movement of auxin between cells, due to its negative charge at most physiological pHs, strongly relies on families of active transporters. These proteins import auxin from the extracellular space or export it into the same. Mutations in these components have profound impacts on biological processes. Another transport route available to auxin, once the substance is inside the cell, are plasmodesmata connections. These small channels connect the cytoplasms of neighbouring plant cells and enable flow between them. Interestingly, the biological significance of this latter mode of transport is only recently starting to emerge with examples from roots, hypocotyls and leaves. The existence of two transport systems provides opportunities for reciprocal cross-regulation. Indeed, auxin levels influence proteins controlling plasmodesmata permeability, while cell-cell communication affects auxin biosynthesis and transport. In an evolutionary context, transporter driven cell-cell auxin movement and plasmodesmata seem to have evolved around the same time in the green lineage. This highlights a co-existence from early on and a likely functional specificity of the systems. Exploring more situations where auxin movement via plasmodesmata has relevance for plant growth and development, and clarifying the regulation of such transport, will be key aspects in coming years.This article has an associated Future Leader to Watch interview with the author of the paper.

Stomatal behavior following mid- or long-term exposure to high relative air humidity: A review

High relative air humidity (RH ≥ 85%) is frequent in controlled environments, and not uncommon in nature. In this review, we examine the high RH effects on plants with a special focus on stomatal characters. All aspects of stomatal physiology are attenuated by elevated RH during leaf expansion (long-term) in C₃ species. These include impaired opening and closing response, as well as weak diel oscillations. Consequently, the high RH-grown plants are not only vulnerable to biotic and abiotic stress, but also undergo a deregulation between CO₂ uptake and water loss. Stomatal behavior of a single leaf is determined by the local microclimate during expansion, and may be different than the remaining leaves of the same plant. No effect of high RH is apparent in C₄ and CAM species, while the same is expected for species with hydropassive stomatal closure. Formation of bigger stomata with larger pores is a universal response to high RH during leaf expansion, whereas the effect on stomatal density appears to be species- and leaf side-specific. Compelling evidence suggests that ABA mediates the high RH-induced stomatal malfunction, as well as the stomatal size increase. Although high RH stimulates leaf ethylene evolution, it remains elusive whether or not this contributes to stomatal malfunction. Most species lose stomatal function following mid-term (4-7 d) exposure to high RH following leaf expansion. Consequently, the regulatory role of ambient humidity on stomatal functionality is not limited to the period of leaf expansion, but holds throughout the leaf life span.

Deregulation of the OsmiR160 Target Gene OsARF18 Causes Growth and Developmental Defects with an Alteration of Auxin Signaling in Rice

MicroRNAs (miRNAs) control gene expression as key negative regulators at the post-transcriptional level. MiR160 plays a pivotal role in Arabidopsis growth and development through repressing expression of its target AUXIN RESPONSE FACTOR (ARF) genes; however, the function of miR160 in monocots remains elusive. In this study, we found that the mature rice miR160 (OsmiR160) was mainly derived from OsMIR160a and OsMIR160b genes. Among four potential OsmiR160 target OsARF genes, the OsARF18 transcript was cleaved at the OsmiR160 target site. Rice transgenic plants (named mOsARF18) expressing an OsmiR160-resistant version of OsARF18 exhibited pleiotropic defects in growth and development, including dwarf stature, rolled leaves, and small seeds. mOsARF18 leaves were abnormal in bulliform cell differentiation and epidermal cell division. Starch accumulation in mOsARF18 seeds was also reduced. Moreover, auxin induced expression of OsMIR160a, OsMIR160b, and OsARF18, whereas expression of OsMIR160a and OsMIR160b as well as genes involved in auxin signaling was altered in mOsARF18 plants. Our results show that negative regulation of OsARF18 expression by OsmiR160 is critical for rice growth and development via affecting auxin signaling, which will advance future studies on the molecular mechanism by which miR160 fine-tunes auxin signaling in plants.

Ethylene is involved in high air humidity promoted stomatal opening of tomato (Lycopersicon esculentum) leaves

High relative air humidity (RH) promotes stomatal opening in tomato leaves. This study examined the role of the plant hormones abscisic acid (ABA) and ethylene in high RH induced stomatal opening. Plants were grown in high (90%) and moderate (60%) RH or transferred from moderate to high RH. ABA levels were only slightly, but significantly decreased during darkness by increasing RH. However, a significantly higher ethylene evolution was found in high RH compared with moderate RH. Ethephon increased conductance and stomatal aperture in moderate RH. Treatment with amino-ethoxyvinylglycine (AVG) suppressed stomatal opening when plants were transferred from moderate to high RH. Similarly, blocking the ethylene receptor or using an ethylene-insensitive mutant (NR) reduced the response to high RH. These results demonstrate that both ethylene production and sensitivity play a role in high RH-induced stomatal opening in tomato leaves. The increased conductance found when plants were transferred to high RH could be counteracted by exogenous ABA spray. The ABA deficient mutant 'Flacca' produced high levels of ethylene irrespective of the RH and the difference in water loss and conductance between high and moderate grown 'Flacca' plants was attenuated compared with WT. The results indicate that both ABA and ethylene play a role in air humidity control of stomatal movement in tomato.

The interaction between strigolactones and other plant hormones in the regulation of plant development

Plant hormones are small molecules derived from various metabolic pathways and are important regulators of plant development. The most recently discovered phytohormone class comprises the carotenoid-derived strigolactones (SLs). For a long time these compounds were only known to be secreted into the rhizosphere where they act as signaling compounds, but now we know they are also active as endogenous plant hormones and they have been in the spotlight ever since. The initial discovery that SLs are involved in the inhibition of axillary bud outgrowth, initiated a multitude of other studies showing that SLs also play a role in defining root architecture, secondary growth, hypocotyl elongation, and seed germination, mostly in interaction with other hormones. Their coordinated action enables the plant to respond in an appropriate manner to environmental factors such as temperature, shading, day length, and nutrient availability. Here, we will review the current knowledge on the crosstalk between SLs and other plant hormones-such as auxin, cytokinin, abscisic acid (ABA), ethylene (ET), and gibberellins (GA)-during different physiological processes. We will furthermore take a bird's eye view of how this hormonal crosstalk enables plants to respond to their ever changing environments.

Phloem-mobile Aux/IAA transcripts target to the root tip and modify root architecture

In plants, the phloem is the component of the vascular system that delivers nutrients and transmits signals from mature leaves to developing sink tissues. Recent studies have identified proteins, mRNA, and small RNA within the phloem sap of several plant species. It is now of considerable interest to elucidate the biological functions of these potential long-distance signal agents, to further our understanding of how plants coordinate their developmental programs at the whole-plant level. In this study, we developed a strategy for the functional analysis of phloem-mobile mRNA by focusing on IAA transcripts, whose mobility has previously been reported in melon (Cucumis melo cv. Hale's Best Jumbo). Indoleacetic acid (IAA) proteins are key transcriptional regulators of auxin signaling, and are involved in a broad range of developmental processes including root development. We used a combination of vasculature-enriched sampling and hetero-grafting techniques to identify IAA18 and IAA28 as phloem-mobile transcripts in the model plant Arabidopsis thaliana. Micro-grafting experiments were used to confirm that these IAA transcripts, which are generated in vascular tissues of mature leaves, are then transported into the root system where they negatively regulate lateral root formation. Based on these findings, we present a model in which auxin distribution, in combination with phloem-mobile Aux/IAA transcripts, can determine the sites of auxin action.

Efficacy of microarray profiling data combined with QTL mapping for the identification of a QTL gene controlling the initial growth rate in rice

Seedling vigor, which is controlled by many quantitative trait loci (QTLs), is one of several important agronomic traits for direct-seedling rice systems. However, isolating these QTL genes is laborious and expensive. Here, we combined QTL mapping and microarray profiling to identify QTL genes for seedling vigor. By performing QTL mapping using 82 backcross inbred lines (BILs) of the Koshihikari (japonica) and Habataki (indica) cultivars for the rice initial growth, we identified two QTLs, early-stage plant development1/2 (qEPD1 and qEPD2), whose Koshihikari alleles promote plant height and/or leaf sheath length. Phenotypic analysis of the two substituted lines carrying the Habataki qEPD1 or qEPD2 allele revealed that qEPD2 functioned more dominantly for the initial growth of rice. From the microarray experiment, 55 and 45 candidate genes were found in the qEPD1 and qEPD2 genomic regions, which are expressed differentially between each substitution line (SL) and Koshihikari. Gibberellin 20 oxidase-2 (OsGA20ox2), which is identical to Semi Dwarf1 (SD1), was included among the 55 candidate genes of qEPD1, whereas its paralog, OsGA20ox1, was included among the 45 candidate genes of qEPD2. Consistently, introduction of the Koshihikari OsGA20ox1 allele into SL(qEPD2) increaseed its plant height and leaf sheath length significantly relative to the introduction of the Habataki OsGA20ox1 allele. Therefore, microarray profiling could be useful for rapidly screening QTL candidate genes. We concluded that OsGA20ox1 and OsGA20ox2 (SD1) function during the initial growth of rice, but OsGA20ox1 plays a dominant role in increasing plant height and leaf sheath length at the initial growth stage.

Hormonal regulation of lateral root development in Arabidopsis modulated by MIZ1 and requirement of GNOM activity for MIZ1 function

Plant organ development is important for adaptation to a changing environment. Genetic and physiological studies have revealed that plant hormones play key roles in lateral root formation. In this study, we show that MIZU-KUSSEI1 (MIZ1), which was identified originally as a regulator of hydrotropism, functions as a novel regulator of hormonally mediated lateral root development. Overexpression of MIZ1 (MIZ1OE) in roots resulted in a reduced number of lateral roots being formed; however, this defect could be recovered with the application of auxin. Indole-3-acetic acid quantification analyses showed that free indole-3-acetic acid levels decreased in MIZ1OE roots, which indicates that alteration of auxin level is critical for the inhibition of lateral root formation in MIZ1OE plants. In addition, MIZ1 negatively regulates cytokinin sensitivity on root development. Application of cytokinin strongly induced the localization of MIZ1-green fluorescent protein to lateral root primordia, which suggests that the inhibition of lateral root development by MIZ1 occurs downstream of cytokinin signaling. Surprisingly, miz2, a weak allele of gnom, suppressed developmental defects in MIZ1OE plants. Taken together, these results suggest that MIZ1 plays a role in lateral root development by maintaining auxin levels and that its function requires GNOM activity. These data provide a molecular framework for auxin-dependent organ development in Arabidopsis (Arabidopsis thaliana).

Rice develop wavy seminal roots in response to light stimulus

Rice (Oryza sativa L.) seminal roots are the primary roots to emerge from germinated seeds. Here, we demonstrate that the photomorphology of the seminal roots was diverse among rice varieties, and the light-induced wavy roots were found mostly in indica-type rice varieties. The light-induced wavy morphology in rice seminal roots has been different with curling or coiling roots in some other specific conditions, such as high air humidity or high nitrogen nutrient. The efficiency of light-induced root waving was developmental stage dependent. The wavy root phenotype was caused by asymmetric cell growth around the stele. Using the inhibitors to block auxin polar transport and fatty acid oxygenation, the role of auxin and oxylipins in the morphogenesis of light-induced wavy roots was investigated. Expressions of genes encoded in the enzymes involved in fatty acid oxygenation in light-exposed roots were monitored by reverse transcriptase-polymerase chain reaction. Our results suggested that auxin polar transport was essential for inducing wavy seminal roots by light stimulus. In addition, the ketol oxylipins derived from allene oxide synthase (EC 4.2.1.92)-mediated fatty acid oxygenation function as intracellular signals for triggering the light-induced wavy root phenotype.

Root apoplastic barriers block Na+ transport to shoots in rice (Oryza sativa L.)

Rice is an important crop that is very sensitive to salinity. However, some varieties differ greatly in this feature, making investigations of salinity tolerance mechanisms possible. The cultivar Pokkali is salinity tolerant and is known to have more extensive hydrophobic barriers in its roots than does IR20, a more sensitive cultivar. These barriers located in the root endodermis and exodermis prevent the direct entry of external fluid into the stele. However, it is known that in the case of rice, these barriers are bypassed by most of the Na(+) that enters the shoot. Exposing plants to a moderate stress of 100 mM NaCl resulted in deposition of additional hydrophobic aliphatic suberin in both cultivars. The present study demonstrated that Pokkali roots have a lower permeability to water (measured using a pressure chamber) than those of IR20. Conditioning plants with 100 mM NaCl effectively reduced Na(+) accumulation in the shoot and improved survival of the plants when they were subsequently subjected to a lethal stress of 200 mM NaCl. The Na(+) accumulated during the conditioning period was rapidly released when the plants were returned to the control medium. It has been suggested that the location of the bypass flow is around young lateral roots, the early development of which disrupts the continuity of the endodermal and exodermal Casparian bands. However, in the present study, the observed increase in lateral root densities during stress in both cultivars did not correlate with bypass flow. Overall the data suggest that in rice roots Na(+) bypass flow is reduced by the deposition of apoplastic barriers, leading to improved plant survival under salt stress.

Auxin control of root development

A plant's roots system determines both the capacity of a sessile organism to acquire nutrients and water, as well as providing a means to monitor the soil for a range of environmental conditions. Since auxins were first described, there has been a tight connection between this class of hormones and root development. Here we review some of the latest genetic, molecular, and cellular experiments that demonstrate the importance of generating and maintaining auxin gradients during root development. Refinements in the ability to monitor and measure auxin levels in root cells coupled with advances in our understanding of the sources of auxin that contribute to these pools represent important contributions to our understanding of how this class of hormones participates in the control of root development. In addition, we review the role of identified molecular components that convert auxin gradients into local differentiation events, which ultimately defines the root architecture.

Phloem sap intricacy and interplay with aphid feeding

Aphididae feed upon the plant sieve elements (SE), where they ingest sugars, nitrogen compounds and other nutrients. For ingestion, aphid stylets penetrate SE, and because of the high hydrostatic pressure in SE, phloem sap exudes out into the stylets. Severing stylets to sample phloem exudates (i.e. stylectomy) has been used extensively for the study of phloem contents. Alternative sampling techniques are spontaneous exudation upon wounding that only works in a few plant species, and the popular EDTA-facilitated exudation technique. These approaches have allowed fundamental advances on the understanding of phloem sap composition and sieve tube physiology, which are surveyed in this review. A more complete picture of metabolites, ions, proteins and RNAs present in phloem sap is now available, which has provided large evidence for the phloem role as a signalling network in addition to its primary role in partitioning of photo-assimilates. Thus, phloem sap sampling methods can have remarkable applications to analyse plant nutrition, physiology and defence responses. Since aphid behaviour is suspected to be affected by phloem sap quality, attempts to manipulate phloem sap content were recently undertaken based on deregulation in mutant plants of genes controlling amino acid or sugar content of phloem sap. This opens up new strategies to control aphid settlement on a plant host.

Studies on sodium bypass flow in lateral rootless mutants lrt1 and lrt2, and crown rootless mutant crl1 of rice (Oryza sativa L.)

An apoplastic pathway, the so-called bypass flow, is important for Na+ uptake in rice (Oryza sativa L.) under saline conditions; however, the precise site of entry is not yet known. We report the results of our test of the hypothesis that bypass flow of Na+ in rice occurs at the site where lateral roots emerge from the main roots. We investigated Na+ uptake and bypass flow in lateral rootless mutants (lrt1, lrt2), a crown rootless mutant (crl1), their wild types (Oochikara, Nipponbare and Taichung 65, respectively) and in seedlings of rice cv. IR36. The results showed that shoot Na+ concentration in lrt1, lrt2 and crl1 was lower (by 20-23%) than that of their wild types. In contrast, the bypass flow quantified using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS) was significantly increased in the mutants, from an average of 1.1% in the wild types to 3.2% in the mutants. Similarly, bypass flow in shoots of IR36 where the number of lateral and crown roots had been reduced through physical and hormonal manipulations was dramatically increased (from 5.6 to 12.5%) as compared to the controls. The results suggest that the path of bypass flow in rice is not at the sites of lateral root emergence.

Auxin-responsive SAUR39 gene modulates auxin level in rice

The signaling molecule auxin plays a central role in several aspects of plant growth and developmental processes. Underlying optimization of these processes are complex mechanisms orchestrating the expression of genes involved in controlling auxin level, movement and signalling. The small auxin-up RNA (SAUR) family comprises a large set of genes whose expressions are early auxin-responsive. However, the function of these genes is largely unknown. Loss-of-function mutants in a number of Arabidopsis SAUR genes did not show any marked phenotypic differences to wild-type plants likely due to compensatory functions of conserved members of the SAUR gene family. We have recently shown that a rice SAUR39 gene negatively regulates auxin synthesis and transport in rice. Here we propose a model that constitutive induction of SAUR39 gene expression reduces growth and seed yield in rice plants due to the presence of a lower auxin level, reduced polar auxin transport, less chlorophyll and increased sugar and anthocyanin contents. In wild-type plants, the SAUR39 gene is expressed at a low level and is transiently induced by changes in external auxin or other environmental stimuli, but within hours of this change its expression is reduced to the low constitutive level. This homeostatic mechanism is essential for optimal plant growth and seed yield and its disruption due to the constitutive overexpression of SAUR39 leads to a set of negative pleiotropic phenotypes.

SAUR39, a small auxin-up RNA gene, acts as a negative regulator of auxin synthesis and transport in rice

The phytohormone auxin plays a critical role for plant growth by regulating the expression of a set of genes. One large auxin-responsive gene family of this type is the small auxin-up RNA (SAUR) genes, although their function is largely unknown. The expression of the rice (Oryza sativa) SAUR39 gene showed rapid induction by transient change in different environmental factors, including auxin, nitrogen, salinity, cytokinin, and anoxia. Transgenic rice plants overexpressing the SAUR39 gene resulted in lower shoot and root growth, altered shoot morphology, smaller vascular tissue, and lower yield compared with wild-type plants. The SAUR39 gene was expressed at higher levels in older leaves, unlike auxin biosynthesis, which occurs largely in the meristematic region. The transgenic plants had a lower auxin level and a reduced polar auxin transport as well as the down-regulation of some putative auxin biosynthesis and transporter genes. Biochemical analysis also revealed that transgenic plants had lower chlorophyll content, higher levels of anthocyanin, abscisic acid, sugar, and starch, and faster leaf senescence compared with wild-type plants at the vegetative stage. Most of these phenomena have been shown to be negatively correlated with auxin level and transport. Transcript profiling revealed that metabolic perturbations in overexpresser plants were largely due to transcriptional changes of genes involved in photosynthesis, senescence, chlorophyll production, anthocyanin accumulation, sugar synthesis, and transport. The lower growth and yield of overexpresser plants was largely recovered by exogenous auxin application. Taken together, the results suggest that SAUR39 acts as a negative regulator for auxin synthesis and transport.

Phloem transport: cellular pathways and molecular trafficking

The phloem transports nutrients, defensive compounds, and informational signals throughout vascular plants. Sampling the complex components of mobile phloem sap is difficult because of the damage incurred when the pressurized sieve tubes are breached. In this review we discuss sampling methods, the artifacts that can be introduced by different sampling procedures, the intricate pathways by which materials enter and exit the phloem, and the major types of compounds transported. Loading and unloading patterns are largely determined by the conductivity and number of plasmodesmata and the position-dependent function of solute-specific, plasma membrane transport proteins. Recent evidence indicates that mobile proteins and RNA are part of the plant's long-distance communication signaling system. Evidence also exists for the directed transport and sorting of macromolecules as they pass through plasmodesmata. A future challenge is to dissect the molecular and cellular aspects of long-distance macromolecular trafficking in the signal transduction pathways of the whole plant.

Suppression and promotion of growth by ethylene in rice seedlings depends on ambient humidity

We examined the effect of ethylene on the growth of rice seedlings (Oryza sativa L.) at various degrees of humidity. Ethylene significantly suppressed the growth of shoots when applied to seedlings grown under 30% relative humidity (RH), but promoted the growth of shoots when applied to seedlings grown under 100% RH. The application of gibberellic acid (GA(3)) promoted the elongation of shoots in seedlings grown under 30% and 100% RH. Ethylene inhibited the shoot elongation induced by GA(3) at 30% RH, but enhanced the elongation induced by GA(3) at 100% RH. These results indicate that ethylene can either promote or suppress the growth of rice shoots depending on ambient humidity, and that these actions of ethylene may be mediated through modulating the responsiveness of shoots to gibberellin.