Advancing understanding of oat phenology for crop adaptation

Oat (Avena sativa) is an annual cereal grown for forage, fodder and grain. Seasonal flowering behaviour, or phenology, is a key contributor to the success of oat as a crop. As a species, oat is a vernalization-responsive long-day plant that flowers after winter as days lengthen in spring. Variation in both vernalization and daylength requirements broadens adaptation of oat and has been used to breed modern cultivars with seasonal flowering behaviours suited to different regions, sowing dates and farming practices. This review examines the importance of variation in oat phenology for crop adaptation. Strategies to advance understanding of the genetic basis of oat phenology are then outlined. These include the potential to transfer knowledge from related temperate cereals, particularly wheat (Triticum aestivum) and barley (Hordeum vulgare), to provide insights into the potential molecular basis of variation in oat phenology. Approaches that use emerging genomic resources to directly investigate the molecular basis of oat phenology are also described, including application of high-resolution genome-wide diversity surveys to map genes linked to variation in flowering behaviour. The need to resolve the contribution of individual phenology genes to crop performance by developing oat genetic resources, such as near-isogenic lines, is emphasised. Finally, ways that deeper knowledge of oat phenology can be applied to breed improved varieties and to inform on-farm decision-making are outlined.

A Vernalization Response in a Winter Safflower (Carthamus tinctorius) Involves the Upregulation of Homologs of FT, FUL, and MAF

Safflower (Carthamus tinctorius) is a member of the Asteraceae family that is grown in temperate climates as an oil seed crop. Most commercially grown safflower varieties can be sown in late winter or early spring and flower rapidly in the absence of overwintering. There are winter-hardy safflower accessions that can be sown in autumn and survive over-wintering. Here, we show that a winter-hardy safflower possesses a vernalization response, whereby flowering is accelerated by exposing germinating seeds to prolonged cold. The impact of vernalization was quantitative, such that increasing the duration of cold treatment accelerated flowering to a greater extent, until the response was saturated after 2 weeks exposure to low-temperatures. To investigate the molecular-basis of the vernalization-response in safflower, transcriptome activity was compared and contrasted between vernalized versus non-vernalized plants, in both 'winter hardy' and 'spring' cultivars. These genome-wide expression analyses identified a small set of transcripts that are both differentially expressed following vernalization and that also have different expression levels in the spring versus winter safflowers. Four of these transcripts were quantitatively induced by vernalization in a winter hardy safflower but show high basal levels in spring safflower. Phylogenetic analyses confidently assigned that the nucleotide sequences of the four differentially expressed transcripts are related to FLOWERING LOCUS T (FT), FRUITFUL (FUL), and two genes within the MADS-like clade genes. Gene models were built for each of these sequences by assembling an improved safflower reference genome using PacBio-based long-read sequencing, covering 85% of the genome, with N50 at 594,000 bp in 3000 contigs. Possible evolutionary relationships between the vernalization response of safflower and those of other plants are discussed.

Increased above-ground resource allocation is a likely precursor for independent evolutionary origins of annuality in the Pooideae grass subfamily

Semelparous annual plants flower a single time during their 1-yr life cycle, investing much of their energy into rapid reproduction. By contrast, iteroparous perennial plants flower multiple times over several years, and partition their resources between reproduction and persistence. To which extent evolutionary transitions between life-cycle strategies are internally constrained at the developmental, genetic and phylogenetic level is unknown. Here we study the evolution of life-cycle strategies in the grass subfamily Pooideae and test if transitions between them are facilitated by evolutionary precursors. We integrate ecological, life-cycle strategy and growth data in a phylogenetic framework. We investigate if growth traits are candidates for a precursor. Species in certain Pooideae clades are predisposed to evolve annuality from perenniality, potentially due to the shared inheritance of specific evolutionary precursors. Seasonal dry climates, which have been linked to annuality, were only able to select for transitions to annuality when the precursor was present. Allocation of more resources to above-ground rather than below-ground growth is a candidate for the precursor. Our findings support the hypothesis that only certain lineages can respond quickly to changing external conditions by switching their life-cycle strategy, likely due to the presence of evolutionary precursors.

A roadmap for gene functional characterisation in crops with large genomes: Lessons from polyploid wheat

Understanding the function of genes within staple crops will accelerate crop improvement by allowing targeted breeding approaches. Despite their importance, a lack of genomic information and resources has hindered the functional characterisation of genes in major crops. The recent release of high-quality reference sequences for these crops underpins a suite of genetic and genomic resources that support basic research and breeding. For wheat, these include gene model annotations, expression atlases and gene networks that provide information about putative function. Sequenced mutant populations, improved transformation protocols and structured natural populations provide rapid methods to study gene function directly. We highlight a case study exemplifying how to integrate these resources. This review provides a helpful guide for plant scientists, especially those expanding into crop research, to capitalise on the discoveries made in Arabidopsis and other plants. This will accelerate the improvement of crops of vital importance for food and nutrition security.

Developmental Pathways Are Blueprints for Designing Successful Crops

Genes controlling plant development have been studied in multiple plant systems. This has provided deep insights into conserved genetic pathways controlling core developmental processes including meristem identity, phase transitions, determinacy, stem elongation, and branching. These pathways control plant growth patterns and are fundamentally important to crop biology and agriculture. This review describes the conserved pathways that control plant development, using Arabidopsis as a model. Historical examples of how plant development has been altered through selection to improve crop performance are then presented. These examples, drawn from diverse crops, show how the genetic pathways controlling development have been modified to increase yield or tailor growth patterns to suit local growing environments or specialized crop management practices. Strategies to apply current progress in genomics and developmental biology to future crop improvement are then discussed within the broader context of emerging trends in plant breeding. The ways that knowledge of developmental processes and understanding of gene function can contribute to crop improvement, beyond what can be achieved by selection alone, are emphasized. These include using genome re-sequencing, mutagenesis, and gene editing to identify or generate novel variation in developmental genes. The expanding scope for comparative genomics, the possibility to engineer new developmental traits and new approaches to resolve gene-gene or gene-environment interactions are also discussed. Finally, opportunities to integrate fundamental research and crop breeding are highlighted.

Zebularine treatment is associated with deletion of FT-B1 leading to an increase in spikelet number in bread wheat

The number of rachis nodes (spikelets) on a wheat spike is a component of grain yield that correlates with flowering time. The genetic basis regulating flowering in cereals is well understood, but there are reports that flowering time can be modified at a high frequency by selective breeding, suggesting that it may be regulated by both epigenetic and genetic mechanisms. We investigated the role of DNA methylation in regulating spikelet number and flowering time by treating a semi-spring wheat with the demethylating agent, Zebularine. Three lines with a heritable increase in spikelet number were identified. The molecular basis for increased spikelet number was not determined in 2 lines, but the phenotype showed non-Mendelian inheritance, suggesting that it could have an epigenetic basis. In the remaining line, the increased spikelet phenotype behaved as a Mendelian recessive trait and late flowering was associated with a deletion encompassing the floral promoter, FT-B1. Deletion of FT-B1 delayed the transition to reproductive growth, extended the duration of spike development, and increased spikelet number under different temperature regimes and photoperiod. Transiently disrupting DNA methylation can generate novel flowering behaviour in wheat, but these changes may not be sufficiently stable for use in breeding programs.

New alleles of the wheat domestication gene Q reveal multiple roles in growth and reproductive development

The advantages of free threshing in wheat led to the selection of the domesticated Q allele, which is now present in almost all modern wheat varieties. Q and the pre-domestication allele, q, encode an AP2 transcription factor, with the domesticated allele conferring a free-threshing character and a subcompact (i.e. partially compact) inflorescence (spike). We demonstrate that mutations in the miR172 binding site of the Q gene are sufficient to increase transcript levels via a reduction in miRNA-dependent degradation, consistent with the conclusion that a single nucleotide polymorphism in the miRNA binding site of Q relative to q was essential in defining the modern Q allele. We describe novel gain- and loss-of-function alleles of Q and use these to define new roles for this gene in spike development. Q is required for the suppression of 'sham ramification', and increased Q expression can lead to the formation of ectopic florets and spikelets (specialized inflorescence branches that bear florets and grains), resulting in a deviation from the canonical spike and spikelet structures of domesticated wheat.

Barley (Hordeum vulgare) circadian clock genes can respond rapidly to temperature in an EARLY FLOWERING 3-dependent manner

An increase in global temperatures will impact future crop yields. In the cereal crops wheat and barley, high temperatures accelerate reproductive development, reducing the number of grains per plant and final grain yield. Despite this relationship between temperature and cereal yield, it is not clear what genes and molecular pathways mediate the developmental response to increased temperatures. The plant circadian clock can respond to changes in temperature and is important for photoperiod-dependent flowering, and so is a potential mechanism controlling temperature responses in cereal crops. This study examines the relationship between temperature, the circadian clock, and the expression of flowering-time genes in barley (Hordeum vulgare), a crop model for temperate cereals. Transcript levels of barley core circadian clock genes were assayed over a range of temperatures. Transcript levels of core clock genes CCA1, GI, PRR59, PRR73, PRR95, and LUX are increased at higher temperatures. CCA1 and PRR73 respond rapidly to a decrease in temperature whereas GI and PRR59 respond rapidly to an increase in temperature. The response of GI and the PRR genes to changes in temperature is lost in the elf3 mutant indicating that their response to temperature may be dependent on a functional ELF3 gene.

The Relationships between Development and Low Temperature Tolerance in Barley Near Isogenic Lines Differing for Flowering Behavior

Flowering time, vernalization requirement, photoperiod sensitivity and low temperature tolerance are key traits in the Triticeae. We characterized a set of isogenic genetic stocks-representing single and pairwise substitutions of spring alleles at the VRN-H1, VRN-H2 and VRN-H3 loci in a winter barley background-at the structural, functional and phenotypic levels. High density mapping with reference to the barley genome sequence confirmed that in all cases target VRN alleles were present in the near isogenic lines (NILs) and allowed estimates of introgression size (at the genetic and physical levels) and gene content. Expression data corroborated the structural and phenotypic results. The latter confirmed that substitution of a spring allele at any of the VRN loci is sufficient to eliminate vernalization requirement. There was no significant change in low temperature tolerance with substitution of a spring allele at VRN-H2, but there were significant losses in cold tolerance with substitutions at VRN-H1 and VRN-H3. Reductions in cold tolerance are ascribed to an accelerated transition from the vegetative to reproductive state. The set of NILs will be a rich resource for understanding the genetics of vernalization, low temperature tolerance and other traits encoded/regulated by genes within the introgressed intervals.

Genetic variation in the flowering and yield formation of timothy (Phleum pratense L.) accessions after different photoperiod and vernalization treatments

Timothy is a perennial forage grass grown commonly in Boreal regions. This study explored the effect of vernalization and photoperiod (PP) on flowering and growth characteristics and how this related to changes in expression of three flowering related genes in accessions from different geographic origin. Large variation was found in accessions in their vernalization and PP responses. In southern accessions vernalization response or requirement was not observed, the heading date remained unchanged, and plants flowered without vernalization. On the contrary, northern types had obligatory requirement for vernalization and long PP, but the tiller elongation did not require vernalization at 16-h PP. Longer vernalization or PP treatments reduced the genotypical differences in flowering. Moreover, the vernalization saturation progressed stepwise from main tiller to lateral tillers, and this process was more synchronized in southern accessions. The expression of PpVRN1 was associated with vernalization while PpVRN3 accumulated at long PP. A crucial role for PpVRN3 in the transition to flowering was supported as in southern accession the transcript accumulated in non-vernalized plants after transfer to 16-h PP, and the apices transformed to generative stage. Differences in vernalization requirements were associated with variation in expression levels of PpVRN1 and PpVRN3, with higher expression levels in southern type. Most divergent transcript accumulation of PpMADS10 was found under different vernalization conditions. These differences between accessions can be translated into agronomic traits, such as the tiller composition of canopy, which affects the forage yield. The southern types, with minimal vernalization response, have fast re-growth ability and rapidly decreasing nutritive value, whereas northern types grow slowly and have better quality. This information can be utilized in breeding for new cultivars for longer growing seasons at high latitudes.

Dawn and Dusk Set States of the Circadian Oscillator in Sprouting Barley (Hordeum vulgare) Seedlings

The plant circadian clock is an internal timekeeper that coordinates biological processes with daily changes in the external environment. The transcript levels of clock genes, which oscillate to control circadian outputs, were examined during early seedling development in barley (Hordeum vulgare), a model for temperate cereal crops. Oscillations of clock gene transcript levels do not occur in barley seedlings grown in darkness or constant light but were observed with day-night cycles. A dark-to-light transition influenced transcript levels of some clock genes but triggered only weak oscillations of gene expression, whereas a light-to-dark transition triggered robust oscillations. Single light pulses of 6, 12 or 18 hours induced robust oscillations. The light-to-dark transition was the primary determinant of the timing of subsequent peaks of clock gene expression. After the light-to-dark transition the timing of peak transcript levels of clock gene also varied depending on the length of the preceding light pulse. Thus, a single photoperiod can trigger initiation of photoperiod-dependent circadian rhythms in barley seedlings. Photoperiod-specific rhythms of clock gene expression were observed in two week old barley plants. Changing the timing of dusk altered clock gene expression patterns within a single day, showing that alteration of circadian oscillator behaviour is amongst the most rapid molecular responses to changing photoperiod in barley. A barley EARLY FLOWERING3 mutant, which exhibits rapid photoperiod-insensitive flowering behaviour, does not establish clock rhythms in response to a single photoperiod. The data presented show that dawn and dusk cues are important signals for setting the state of the circadian oscillator during early development of barley and that the circadian oscillator of barley exhibits photoperiod-dependent oscillation states.

Ppd-1 is a key regulator of inflorescence architecture and paired spikelet development in wheat

The domestication of cereal crops such as wheat, maize, rice and barley has included the modification of inflorescence architecture to improve grain yield and ease harvesting(1). Yield increases have often been achieved through modifying the number and arrangement of spikelets, which are specialized reproductive branches that form part of the inflorescence. Multiple genes that control spikelet development have been identified in maize, rice and barley(2-5). However, little is known about the genetic underpinnings of this process in wheat. Here, we describe a modified spikelet arrangement in wheat, termed paired spikelets. Combining comprehensive QTL and mutant analyses, we show that Photoperiod-1 (Ppd-1), a pseudo-response regulator gene that controls photoperiod-dependent floral induction, has a major inhibitory effect on paired spikelet formation by regulating the expression of FLOWERING LOCUS T (FT)(6,7). These findings show that modulated expression of the two important flowering genes, Ppd-1 and FT, can be used to form a wheat inflorescence with a more elaborate arrangement and increased number of grain producing spikelets.

Direct links between the vernalization response and other key traits of cereal crops

Transcription of the vernalization1 gene (VRN1) is induced by prolonged cold (vernalization) to trigger flowering of cereal crops, such as wheat and barley. VRN1 encodes a MADS box transcription factor that promotes flowering by regulating the expression of other genes. Here we use transcriptome sequencing (RNA-seq) and chromatin immunoprecipitation sequencing (ChIP-seq) to identify direct targets of VRN1. Over 500 genomic regions were identified as potential VRN1-binding targets by ChIP-seq. VRN1 binds the promoter of flowering locus T-like 1, a promoter of flowering in vernalized plants. VRN1 also targets vernalization2 and ODDSOC2, repressors of flowering that are downregulated in vernalized plants. RNA-seq identified additional VRN1 targets that might play roles in triggering flowering. Other targets of VRN1 include genes that play central roles in low-temperature-induced freezing tolerance, spike architecture and hormone metabolism. This provides evidence for direct regulatory links between the vernalization response pathway and other important traits in cereal crops.

The role of seasonal flowering responses in adaptation of grasses to temperate climates

Grasses of the subfamily Pooideae, including important cereal crops and pasture grasses, are widespread in temperate zones. Seasonal regulation of developmental transitions coordinates the life cycles of Pooideae with the passing seasons so that flowering and seed production coincide with favorable conditions in spring. This review examines the molecular pathways that control the seasonal flowering responses of Pooideae and how variation in the activity of genes controlling these pathways can adapt cereals or grasses to different climates and geographical regions. The possible evolutionary origins of the seasonal flowering responses of the Pooideae are discussed and key questions for future research highlighted. These include the need to develop a better understanding of the molecular basis for seasonal flowering in perennial Pooideae and in temperate grasses outside the core Pooideae group.

EARLY FLOWERING3 Regulates Flowering in Spring Barley by Mediating Gibberellin Production and FLOWERING LOCUS T Expression

EARLY FLOWERING3 (ELF3) is a circadian clock gene that contributes to photoperiod-dependent flowering in plants, with loss-of-function mutants in barley (Hordeum vulgare), legumes, and Arabidopsis thaliana flowering early under noninductive short-day (SD) photoperiods. The barley elf3 mutant displays increased expression of FLOWERING LOCUS T1 (FT1); however, it remains unclear whether this is the only factor responsible for the early flowering phenotype. We show that the early flowering and vegetative growth phenotypes of the barley elf3 mutant are strongly dependent on gibberellin (GA) biosynthesis. Expression of the central GA biosynthesis gene, GA20oxidase2, and production of the bioactive GA, GA₁, were significantly increased in elf3 leaves under SDs, relative to the wild type. Inhibition of GA biosynthesis suppressed the early flowering of elf3 under SDs independently of FT1 and was associated with altered expression of floral identity genes at the developing apex. GA is also required for normal flowering of spring barley under inductive photoperiods, with chemical and genetic attenuation of the GA biosynthesis and signaling pathways suppressing inflorescence development under long-day conditions. These findings illustrate that GA is an important floral promoting signal in barley and that ELF3 suppresses flowering under noninductive photoperiods by blocking GA production and FT1 expression.

Low temperatures induce rapid changes in chromatin state and transcript levels of the cereal VERNALIZATION1 gene

Transcriptional activation of the VERNALIZATION1 gene mediates the acceleration of flowering by prolonged cold (vernalization) in temperate cereals. This study examined the earliest stages of the transcriptional response of VRN1 to low temperatures. Time-course analyses, using a sensitive quantitative PCR assay, showed that in sprouting barley seedlings VRN1 transcripts begin to accumulate within 24 hours of the onset of cold. The kinetics of the initial transcriptional response of VRN1 to cold was similar to the cold-induced genes DEHYDRIN5 (DHN5) and COLD REGULATED 14B (COR14B), but occurred at lower levels compared to cold acclimation genes or the response to longer cold treatments. Temperatures between 15 and -2 °C induced expression of VRN1 within 24 hours, with a maximal response observed between 2 and -2 °C. Transcriptional induction was also observed in undifferentiated callus cells. There were significant increases in histone acetylation levels at the VRN1 locus in response to 24-hour cold treatment. Sodium butyrate, a histone deacetylation inhibitor, triggered an increase in histone acetylation at VRN1 chromatin and elevated VRN1 transcript levels. The transcriptional response of VRN1 to short-term cold treatment was examined in near-isogenic lines that have different VRN1 genotypes, showing that an allele of the barley VRN1 gene with an insertion in the first intron and high basal expression levels has a reduced transcriptional response to short term cold treatment. This study suggests that low-temperature induction of VRN1 is a cellular response to cold triggered by the same mechanisms that mediate low-temperature induction of cold acclimation genes.

Identification of high-temperature-responsive genes in cereals

High temperature influences plant development and can reduce crop yields. We examined how ambient temperature influences reproductive development in the temperate cereals wheat (Triticum aestivum) and barley (Hordeum vulgare). High temperature resulted in rapid progression through reproductive development in long days, but inhibited early stages of reproductive development in short days. Activation of the long-day flowering response pathway through day-length-insensitive alleles of the PHOTOPERIOD1 gene, which result in high FLOWERING LOCUS T-like1 transcript levels, did not allow rapid early reproductive development at high temperature in short days. Furthermore, high temperature did not increase transcript levels of FLOWERING LOCUS T-like genes. These data suggest that genes or pathways other than the long-day response pathway mediate developmental responses to high temperature in cereals. Transcriptome analyses suggested a possible role for vernalization-responsive genes in the developmental response to high temperature. The MADS-box floral repressor HvODDSOC2 is expressed at elevated levels at high temperature in short days, and might contribute to the inhibition of early reproductive development under these conditions. FLOWERING PROMOTING FACTOR1-like, RNase-S-like genes, and VER2-like genes were also identified as candidates for high-temperature-responsive developmental regulators. Overall, these data suggest that rising temperatures might elicit different developmental responses in cereal crops at different latitudes or times of year, due to the interaction between temperature and day length. Additionally, we suggest that different developmental regulators might mediate the response to high temperature in cereals compared to Arabidopsis (Arabidopsis thaliana).

The promoter of the cereal VERNALIZATION1 gene is sufficient for transcriptional induction by prolonged cold

The VERNALIZATION1 (VRN1) gene of temperate cereals is transcriptionally activated by prolonged cold during winter (vernalization) to promote flowering. To investigate the mechanisms controlling induction of VRN1 by prolonged cold, different regions of the VRN1 gene were fused to the GREEN FLUORESCENT PROTEIN (GFP) reporter and expression of the resulting gene constructs was assayed in transgenic barley (Hordeum vulgare). A 2 kb segment of the promoter of VRN1 was sufficient for GFP expression in the leaves and shoot apex of transgenic barley plants. Fluorescence increased at the shoot apex prior to inflorescence initiation and was subsequently maintained in the developing inflorescence. The promoter was also sufficient for low-temperature induction of GFP expression. A naturally occurring insertion in the proximal promoter, which is associated with elevated VRN1 expression and early flowering in some spring wheats, did not abolish induction of VRN1 transcription by prolonged cold, however. A translational fusion of the promoter and transcribed regions of VRN1 to GFP, VRN1::GFP, was localised to nuclei of cells at the shoot apex of transgenic barley plants. The distribution of VRN1::GFP at the shoot apex was similar to the expression pattern of the VRN1 promoter-GFP reporter gene. Fluorescence from the VRN1::GFP fusion protein increased in the developing leaves after prolonged cold treatment. These observations suggest that the promoter of VRN1 is targeted by mechanisms that trigger vernalization-induced flowering in economically important temperate cereal crops.

Transcriptome analysis of the vernalization response in barley (Hordeum vulgare) seedlings

Temperate cereals, such as wheat (Triticum spp.) and barley (Hordeum vulgare), respond to prolonged cold by becoming more tolerant of freezing (cold acclimation) and by becoming competent to flower (vernalization). These responses occur concomitantly during winter, but vernalization continues to influence development during spring. Previous studies identified VERNALIZATION1 (VRN1) as a master regulator of the vernalization response in cereals. The extent to which other genes contribute to this process is unclear. In this study the Barley1 Affymetrix chip was used to assay gene expression in barley seedlings during short or prolonged cold treatment. Gene expression was also assayed in the leaves of plants after prolonged cold treatment, in order to identify genes that show lasting responses to prolonged cold, which might contribute to vernalization-induced flowering. Many genes showed altered expression in response to short or prolonged cold treatment, but these responses differed markedly. A limited number of genes showed lasting responses to prolonged cold treatment. These include genes known to be regulated by vernalization, such as VRN1 and ODDSOC2, and also contigs encoding a calcium binding protein, 23-KD jasmonate induced proteins, an RNase S-like protein, a PR17d secretory protein and a serine acetyltransferase. Some contigs that were up-regulated by short term cold also showed lasting changes in expression after prolonged cold treatment. These include COLD REGULATED 14B (COR14B) and the barley homologue of WHEAT COLD SPECIFIC 19 (WSC19), which were expressed at elevated levels after prolonged cold. Conversely, two C-REPEAT BINDING FACTOR (CBF) genes showed reduced expression after prolonged cold. Overall, these data show that a limited number of barley genes exhibit lasting changes in expression after prolonged cold treatment, highlighting the central role of VRN1 in the vernalization response in cereals.

Make hay when the sun shines: the role of MADS-box genes in temperature-dependant seasonal flowering responses

MADS-box transcription factors specify plant meristem identity. In doing so, they determine when floral organs are produced at the shoot apex and control the timing of flowering. The transcriptional activity of key MADS-box genes is controlled by temperature in many plants, and this synchronises flowering with changing seasons. Here we review how seasonal temperature variation influences the developmental programme of plants via transcriptional regulation of MADS-box genes. In particular we examine the role of MADS-box genes in regulating the acceleration of flowering by vernalization (prolonged periods of cold), using FLOWERING LOCUS C of Arabidopsis and VERNALIZATION1 of cereals as examples. A potential role for SHORT VEGETATIVE PHASE-like genes in controlling winter bud dormancy is also examined, as are potential roles for MADS-box genes in regulating developmental responses to elevated growth temperatures. We conclude that understanding how temperature regulates the transcription of MADS-box genes provides insight into how seasonal fluctuations in temperature influence plant development. Plant breeders may be able to use natural variation in temperature-responsive MADS-box genes to breed future crop varieties.

ODDSOC2 is a MADS box floral repressor that is down-regulated by vernalization in temperate cereals

In temperate cereals, such as wheat (Triticum aestivum) and barley (Hordeum vulgare), the transition to reproductive development can be accelerated by prolonged exposure to cold (vernalization). We examined the role of the grass-specific MADS box gene ODDSOC2 (OS2) in the vernalization response in cereals. The barley OS2 gene (HvOS2) is expressed in leaves and shoot apices but is repressed by vernalization. Vernalization represses OS2 independently of VERNALIZATION1 (VRN1) in a VRN1 deletion mutant of einkorn wheat (Triticum monococcum), but VRN1 is required to maintain down-regulation of OS2 in vernalized plants. Furthermore, barleys that carry active alleles of the VRN1 gene (HvVRN1) have reduced expression of HvOS2, suggesting that HvVRN1 down-regulates HvOS2 during development. Overexpression of HvOS2 delayed flowering and reduced spike, stem, and leaf length in transgenic barley plants. Plants overexpressing HvOS2 showed reduced expression of barley homologs of the Arabidopsis (Arabidopsis thaliana) gene FLOWERING PROMOTING FACTOR1 (FPF1) and increased expression of RNase-S-like genes. FPF1 promotes floral development and enhances cell elongation, so down-regulation of FPF1-like genes might explain the phenotypes of HvOS2 overexpression lines. We present an extended model of the genetic pathways controlling vernalization-induced flowering in cereals, which describes the regulatory relationships between VRN1, OS2, and FPF1-like genes. Overall, these findings highlight differences and similarities between the vernalization responses of temperate cereals and the model plant Arabidopsis.

Differentiation of plant cells during symbiotic nitrogen fixation

Nitrogen-fixing symbioses between legumes and bacteria of the family Rhizobiaceae involve differentiation of both plant and bacterial cells. Differentiation of plant root cells is required to build an organ, the nodule, which can feed and accommodate a large population of bacteria under conditions conducive to nitrogen fixation. An efficient vascular system is built to connect the nodule to the root, which delivers sugars and other nutrients to the nodule and removes the products of nitrogen fixation for use in the rest of the plant. Cells in the outer cortex differentiate to form a barrier to oxygen diffusion into nodules, which helps to produce the micro-aerobic environment necessary for bacterial nitrogenase activity. Cells of the central, infected zone of nodules undergo multiple rounds of endoreduplication, which may be necessary for colonisation by rhizobia and may enable enlargement and greater metabolic activity of these cells. Infected cells of the nodule contain rhizobia within a unique plant membrane called the peribacteroid or symbiosome membrane, which separates the bacteria from the host cell cytoplasm and mediates nutrient and signal exchanges between the partners. Rhizobia also undergo differentiation during nodule development. Not surprisingly, perhaps, differentiation of each partner is dependent upon interactions with the other. High-throughput methods to assay gene transcripts, proteins, and metabolites are now being used to explore further the different aspects of plant and bacterial differentiation. In this review, we highlight recent advances in our understanding of plant cell differentiation during nodulation that have been made, at least in part, using high-throughput methods.

The molecular biology of seasonal flowering-responses in Arabidopsis and the cereals

BACKGROUND

In arabidopsis (Arabidopsis thaliana), FLOWERING LOCUS T (FT) and FLOWERING LOCUS C (FLC) play key roles in regulating seasonal flowering-responses to synchronize flowering with optimal conditions. FT is a promoter of flowering activated by long days and by warm conditions. FLC represses FT to delay flowering until plants experience winter.

SCOPE

The identification of genes controlling flowering in cereals allows comparison of the molecular pathways controlling seasonal flowering-responses in cereals with those of arabidopsis. The role of FT has been conserved between arabidopsis and cereals; FT-like genes trigger flowering in response to short days in rice or long days in temperate cereals, such as wheat (Triticum aestivum) and barley (Hordeum vulgare). Many varieties of wheat and barley require vernalization to flower but FLC-like genes have not been identified in cereals. Instead, VERNALIZATION2 (VRN2) inhibits long-day induction of FT-like1 (FT1) prior to winter. VERNALIZATION1 (VRN1) is activated by low-temperatures during winter to repress VRN2 and to allow the long-day response to occur in spring. In rice (Oryza sativa) a VRN2-like gene Ghd7, which influences grain number, plant height and heading date, represses the FT-like gene Heading date 3a (Hd3a) in long days, suggesting a broader role for VRN2-like genes in regulating day-length responses in cereals. Other genes, including Early heading date (Ehd1), Oryza sativa MADS51 (OsMADS51) and INDETERMINATE1 (OsID1) up-regulate Hd3a in short days. These genes might account for the different day-length response of rice compared with the temperate cereals. No genes homologous to VRN2, Ehd1, Ehd2 or OsMADS51 occur in arabidopsis.

CONCLUSIONS

It seems that different genes regulate FT orthologues to elicit seasonal flowering-responses in arabidopsis and the cereals. This highlights the need for more detailed study into the molecular basis of seasonal flowering-responses in cereal crops or in closely related model plants such as Brachypodium distachyon.

Regions associated with repression of the barley (Hordeum vulgare) VERNALIZATION1 gene are not required for cold induction

Activity of the VERNALIZATION1 (VRN1) gene is required for flowering in temperate cereals such as wheat and barley. In varieties that require prolonged exposure to cold to flower (vernalization), VRN1 is expressed at low levels and is induced by vernalization to trigger flowering. In other varieties, deletions or insertions in the first intron of the VRN1 gene are associated with increased VRN1 expression in the absence of cold treatment, reducing or eliminating the requirement for vernalization. To characterize natural variation in VRN1, the first intron of the barley (Hordeum vulgare) VRN1 gene (HvVRN1) was assayed for deletions or insertions in a collection of 1,000 barleys from diverse geographical regions. Ten alleles of HvVRN1 containing deletions or insertions in the first intron were identified, including three alleles that have not been described previously. Different HvVRN1 alleles were associated with differing levels of HvVRN1 expression in non-vernalized plants and with different flowering behaviour. Using overlapping deletions, we delineated regions in the HvVRN1 first intron that are associated with low levels of HvVRN1 expression in non-vernalized plants. Deletion of these intronic regions does not prevent induction of HvVRN1 by cold or the maintenance of increased HvVRN1 expression following cold treatment. We suggest that regions within the first intron of HvVRN1 are required to maintain low levels of HvVRN1 expression prior to winter but act independently of the regulatory mechanisms that mediate induction of HvVRN1 by cold during winter.

The influence of vernalization and daylength on expression of flowering-time genes in the shoot apex and leaves of barley (Hordeum vulgare)

Responses to prolonged low-temperature treatment of imbibed seeds (vernalization) were examined in barley (Hordeum vulgare). These occurred in two phases: the perception of prolonged cold, which occurred gradually at low temperatures, and the acceleration of reproductive development, which occurred after vernalization. Expression of the VERNALIZATION1 gene (HvVRN1) increased gradually in germinating seedlings during vernalization, both at the shoot apex and in the developing leaves. This occurred in darkness, independently of VERNALIZATION2 (HvVRN2), consistent with the hypothesis that expression of HvVRN1 is induced by prolonged cold independently of daylength flowering-response pathways. After vernalization, expression of HvVRN1 was maintained in the shoot apex and leaves. This was associated with accelerated inflorescence initiation and with down-regulation of HvVRN2 in the leaves. The largest determinant of HvVRN1 expression levels in vernalized plants was the length of seed vernalization treatment. Daylength did not influence HvVRN1 expression levels in shoot apices and typically did not affect expression in leaves. In the leaves of plants that had experienced a saturating seed vernalization treatment, expression of HvVRN1 was higher in long days, however. HvFT1 was expressed in the leaves of these plants in long days, which might account for the elevated HvVRN1 expression. Long-day up-regulation of HvVRN1 was not required for inflorescence initiation, but might accelerate subsequent stages of inflorescence development. Similar responses to seed vernalization were also observed in wheat (Triticum aestivum). These data support the hypothesis that VRN1 is induced by cold during winter to promote spring flowering in vernalization-responsive cereals.

Integration of seasonal flowering time responses in temperate cereals

Our paper describes how the genetic pathways which regulate vernalization and long-day flowering responses are integrated to promote spring flowering in cereals. This process is mediated by the VERNALIZATION1 (VRN1) and VRN2 genes. VRN2 encodes a CONSTANS-like protein that represses FLOWERING LOCUS T (FT1) to block the long-day flowering response until plants are vernalized. When plants are vernalized VRN1, a FRUITFUL-like MADS box transcription factor, is induced. This downregulates VRN2, allowing long-day induction of FT1 to occur post-vernalization. A comparison of the pathways regulating seasonal induction of flowering in cereals with those of Arabidopsis shows the vernalization response pathway has evolved convergently to regulate the activity of a conserved daylength response pathway in these divergent plant lineages.

Low-temperature and daylength cues are integrated to regulate FLOWERING LOCUS T in barley

Interactions between flowering time genes were examined in a doubled haploid barley (Hordeum vulgare) population segregating for H. vulgare VERNALIZATION1 (HvVRN1), HvVRN2, and PHOTOPERIOD1 (PPD-H1). A deletion allele of HvVRN2 was associated with rapid inflorescence initiation and early flowering, but only in lines with an active allele of PPD-H1. In these lines, the floral promoter FLOWERING LOCUS T (HvFT1) was expressed at high levels without vernalization, and this preceded induction of HvVRN1. Lines with the deletion allele of HvVRN2 and the inactive ppd-H1 allele did not undergo rapid inflorescence initiation and were late flowering. These data suggest that HvVRN2 counteracts PPD-H1 to prevent flowering prior to vernalization. An allele of HvVRN1 that is expressed at high basal levels (HvVRN1-1) was associated with rapid inflorescence initiation regardless of HvVRN2 or PPD-H1 genotype. HvFT1 was expressed without vernalization in lines with the HvVRN1-1 allele and HvFT1 transcript levels were highest in lines with the active PPD-H1 allele; this correlated with rapid apex development postinflorescence initiation. Thus, expression of HvVRN1 promotes inflorescence initiation and up-regulates HvFT1. Analysis of HvVRN1 expression in different genetic backgrounds postvernalization showed that HvVRN2, HvFT1, and PPD-H1 are unlikely to play a role in low-temperature induction of HvVRN1. In a vernalization responsive barley, HvFT1 is not induced by low temperatures alone, but can be induced by long days following prolonged low-temperature treatment. We conclude that low-temperature and daylength flowering-response pathways are integrated to control expression of HvFT1 in barley, and that this might occur through regulation of HvVRN2 activity.

The molecular basis of vernalization-induced flowering in cereals

Genetic analyses have identified three genes that control the vernalization requirement in wheat and barley; VRN1, VRN2 and FT (VRN3). These genes have now been isolated and shown to regulate not only the vernalization response but also the promotion of flowering by long days. VRN1 is induced by vernalization and accelerates the transition to reproductive development at the shoot apex. FT is induced by long days and further accelerates reproductive apex development. VRN2, a floral repressor, integrates vernalization and day-length responses by repressing FT until plants are vernalized. A comparison of flowering time pathways in cereals and Arabidopsis shows that the vernalization response is controlled by different MADS box genes, but integration of vernalization and long-day responses occurs through similar mechanisms.

Short vegetative phase-like MADS-box genes inhibit floral meristem identity in barley

Analysis of the functions of Short Vegetative Phase (SVP)-like MADS-box genes in barley (Hordeum vulgare) indicated a role in determining meristem identity. Three SVP-like genes are expressed in vegetative tissues of barley: Barley MADS1 (BM1), BM10, and Vegetative to Reproductive Transition gene 2. These genes are induced by cold but are repressed during floral development. Ectopic expression of BM1 inhibited spike development and caused floral reversion in barley, with florets at the base of the spike replaced by tillers. Head emergence was delayed in plants that ectopically express BM1, primarily by delayed development after the floral transition, but expression levels of the barley VRN1 gene (HvVRN1) were not affected. Ectopic expression of BM10 inhibited spike development and caused partial floral reversion, where florets at the base of the spike were replaced by inflorescence-like structures, but did not affect heading date. Floral reversion occurred more frequently when BM1 and BM10 ectopic expression lines were grown in short-day conditions. BM1 and BM10 also inhibited floral development and caused floral reversion when expressed in Arabidopsis (Arabidopsis thaliana). We conclude that SVP-like genes function to suppress floral meristem identity in winter cereals.

HvVRN2 responds to daylength, whereas HvVRN1 is regulated by vernalization and developmental status

Two genetic loci control the vernalization response in winter cereals; VRN1, which encodes an AP1-like MADS-box transcription factor, and VRN2, which has been mapped to a chromosome region containing ZCCT zinc finger transcription factor genes. We examined whether daylength regulates expression of HvVRN1 and HvVRN2. In a vernalization-responsive winter barley (Hordeum vulgare), expression of HvVRN1 is regulated by vernalization and by development, but not by daylength. Daylength affected HvVRN1 expression in only one of six vernalization-insensitive spring barleys examined and so cannot be a general feature of regulation of this gene. In contrast, daylength is the major determinant of expression levels of two ZCCT genes found at the barley VRN2 locus, HvZCCTa and HvZCCTb. In winter barley, high levels of HvZCCTa and HvZCCTb expression were detected only when plants were grown in long days. During vernalization in long-day conditions, HvVRN1 is induced and expression of HvZCCTb is repressed. During vernalization under short days, induction of HvVRN1 occurs without changes in HvZCCTa and HvZCCTb expression. Analysis of HvZCCTa and HvZCCTb expression levels in a doubled haploid population segregating for different vernalization and daylength requirements showed that HvVRN1 genotype determines HvZCCTa and HvZCCTb expression levels. We conclude that the vernalization response is mediated through HvVRN1, whereas HvZCCTa and HvZCCTb respond to daylength cues to repress flowering under long days in nonvernalized plants.

Molecular and cell biology of a family of voltage-dependent anion channel porins in Lotus japonicus

Voltage-dependent anion channels (VDACs) are generally considered as the main pathway for metabolite transport across the mitochondrial outer membrane. Recent proteomic studies on isolated symbiosome membranes from legume nodules indicated that VDACs might also be involved in transport of nutrients between plants and rhizobia. In an attempt to substantiate this, we carried out a detailed molecular and cellular characterization of VDACs in Lotus japonicus and soybean (Glycine max). Database searches revealed at least five genes encoding putative VDACs in each of the legumes L. japonicus, Medicago truncatula, and soybean. We obtained and sequenced cDNA clones from L. japonicus encoding five full-length VDAC proteins (LjVDAC1.1-1.3, LjVDAC2.1, and LjVDAC3.1). Complementation of a yeast (Saccharomyces cerevisiae) mutant impaired in VDAC1, a porin of the mitochondrial outer membrane, showed that LjVDAC1.1, LjVDAC1.2, LjVDAC2.1, and LjVDAC3.1, but not LjVDAC1.3, are functional and targeted to the mitochondrial outer membrane in yeast. Studies of the expression pattern of the five L. japonicus VDAC genes revealed largely constitutive expression of each throughout the plant, including nodules. Antibodies to LjVDAC1.1 of L. japonicus and the related POM36 protein of potato (Solanum tuberosum) recognized several proteins between 30 and 36 kD on western blots, including LjVDAC1.1, LjVDAC1.2, LjVDAC1.3, and LjVDAC2.1. Immunolocalization of VDACs in L. japonicus and soybean root nodules demonstrated their presence on not only mitochondria but also on numerous, small vesicles at the cell periphery. No evidence was found for the presence of VDACs on the symbiosome membrane. Nonetheless, the data indicate that VDACs may play more diverse roles in plants than suspected previously.

MADS box genes control vernalization-induced flowering in cereals

By comparing expression levels of MADS box transcription factor genes between near-isogenic winter and spring lines of bread wheat, Triticum aestivum, we have identified WAP1 as the probable candidate for the Vrn-1 gene, the major locus controlling the vernalization flowering response in wheat. WAP1 is strongly expressed in spring wheats and moderately expressed in semispring wheats, but is not expressed in winter wheat plants that have not been exposed to vernalization treatment. Vernalization promotes flowering in winter wheats and strongly induces expression of WAP1. WAP1 is located on chromosome 5 in wheat and, by synteny with other cereal genomes, is likely to be collocated with Vrn-1. These results in hexaploid bread wheat cultivars extend the conclusion made by Yan et al. [Yan, L., Loukoianov, A., Tranquilli, G., Helguera, M., Fahima, T. & Dubcovsky, J. (2003) Proc. Natl. Acad. Sci. USA 100, 6263-6268] in the diploid wheat progenitor Triticum monococcum that WAP1 (TmAP1) corresponds to the Vrn-1 gene. The barley homologue of WAP1, BM5, shows a similar pattern of expression to WAP1 and TmAP1. BM5 is not expressed in winter barleys that have not been vernalized, but as with WAP1, expression of BM5 is strongly induced by vernalization treatment. In spring barleys, the level of BM5 expression is determined by interactions between the Vrn-H1 locus and a second locus for spring habit, Vrn-H2. There is now evidence that AP1-like genes determine the time of flowering in a range of cereal and grass species.

Increased level of hemoglobin 1 enhances survival of hypoxic stress and promotes early growth in Arabidopsis thaliana

Overexpression of a class 1 Hb (GLB1) protects Arabidopsis thaliana plants from the effects of severe hypoxia. Overexpression of the bifunctional symbiotic Hb (GLB1S) from Parasponia andersonii in A. thaliana also increases survival after hypoxia. Plants overexpressing the Hb 1 protein, mutated to have a low oxygen affinity, are as susceptible to hypoxia as WT plants, suggesting that the protection against hypoxia depends on the ability of the Hb to bind ligands, such as oxygen, with high affinity. A mild hypoxia pretreatment (5%) induces the Hb gene and increases the survival of plants after severe hypoxic treatment (0.1%). These results with Hb 1 show that plant Hbs have a role other than in nitrogen-fixing root nodules. Plants overexpressing the GLB1 protein show early vigorous growth in nonhypoxic conditions and are 50% larger in weight than the controls at 14 days. The constitutive expression of GLB1 also resulted in a reduced number of root hairs and increased number of laterals in the root system.

The soybean GmN6L gene encodes a late nodulin expressed in the infected zone of nitrogen-fixing nodules

Previously, we determined the N-terminal amino acid sequences of a number of putative peribacteroid membrane proteins from soybean. Here, we report the cloning of a gene, GmN6L, that encodes one of these proteins. The protein encoded by GmN6L is similar in sequence to MtN6, an early nodulin expressed in Medicago truncatula roots in response to infection by Sinorhizobium meliloti. The GmN6L gene was strongly expressed in mature nodules but not in other plant organs. GmN6L protein was first detected 2 weeks after inoculation with Bradyrhizobium japonicum and was limited to the infected zone of nodules. GmN6L protein was found in symbiosomes isolated from mature soybean nodules, both as a soluble protein and as a peripheral membrane protein bound to the peribacteroid membrane. These data indicate that GmN6L is a late nodulin, which is not involved in the infection process. Homology between GmN6L and FluG, a protein involved in signaling in Aspergillus nidulans, suggests that GmN6L may play a role in communication between the host and microsymbionts during symbiotic nitrogen fixation.

Novel aspects of symbiotic nitrogen fixation uncovered by transcript profiling with cDNA arrays

An array of 2,304 cDNA clones derived from nitrogen-fixing nodules of Lotus japonicus was produced and used to detect differences in relative gene transcript abundance between nodules and uninfected roots. Transcripts of 83 different genes were found to be more abundant in nodules than in roots. More than 50 of these have never before been identified as nodule-induced in any species. Expression of 36 genes was detected in nodules but not in roots. Several known nodulin genes were included among the nodule-induced genes. Also included were genes involved in sucrose breakdown and glycolysis, CO2 recycling, and amino acid synthesis, processes that are known to be accelerated in nodules compared with roots. Genes involved in membrane transport, hormone metabolism, cell wall and protein synthesis, and signal transduction and regulation of transcription were also induced in nodules. Genes that may subvert normal plant defense responses, including two encoding enzymes involved in detoxification of active oxygen species and one that may prohibit phytoalexin synthesis, were also identified. The data represent a rich source of information for hypothesis building and future exploration of symbiotic nitrogen fixation.

GmZIP1 encodes a symbiosis-specific zinc transporter in soybean

The importance of zinc in organisms is clearly established, and mechanisms involved in zinc acquisition by plants have recently received increased interest. In this report, the identification, characterization and location of GmZIP1, the first soybean member of the ZIP family of metal transporters, are described. GmZIP1 was found to possess eight putative transmembrane domains together with a histidine-rich extra-membrane loop. By functional complementation of zrt1zrt2 yeast cells no longer able to take up zinc, GmZIP1 was found to be highly selective for zinc, with an estimated K(m) value of 13.8 microm. Cadmium was the only other metal tested able to inhibit zinc uptake in yeast. An antibody raised against GmZIP1 specifically localized the protein to the peribacteroid membrane, an endosymbiotic membrane in nodules resulting from the interaction of the plant with its microsymbiont. The specific expression of GmZIP1 in nodules was confirmed by Northern blot, with no expression in roots, stems, or leaves of nodulated soybean plants. Antibodies to GmZIP1 inhibited zinc uptake by symbiosomes, indicating that at least some of the zinc uptake observed in isolated symbiosomes could be attributed to GmZIP1. The orientation of the protein in the membrane and its possible role in the symbiosis are discussed.

Expression and evolution of functionally distinct haemoglobin genes in plants

Haemoglobin genes have been found in a number of plant species, but the number of genes known has been too small to allow effective evolutionary inferences. We present nine new non-symbiotic haemoglobin sequences from a range of plants, including class 1 haemoglobins from cotton, Citrus and tomato, class 2 haemoglobins from cotton, tomato, sugar beet and canola and two haemoglobins from the non-vascular plants, Marchantia polymorpha (a liverwort) and Physcomitrella patens (a moss). Our molecular phylogenetic analysis of all currently known non-symbiotic haemoglobin genes and a selection of symbiotic haemoglobins have confirmed the existence of two distinct classes of haemoglobin genes in the dicots. It is likely that all dicots have both class 1 and class 2 non-symbiotic haemoglobin genes whereas in monocots we have detected only class 1 genes. The symbiotic haemoglobins from legumes and Casuarina are related to the class 2 non-symbiotic haemoglobins, whilst the symbiotic haemoglobin from Parasponia groups with the class 1 non-symbiotic genes. Probably, there have been two independent recruitments of symbiotic haemoglobins. Although the functions of the two non-symbiotic haemoglobins remain unknown, their patterns of expression within plants suggest different functions. We examined the expression in transgenic plants of the two non-symbiotic haemoglobins from Arabidopsis using promoter fusions to a GUS reporter gene. The Arabidopsis GLB1 and GLB2 genes are likely to be functionally distinct. The class 2 haemoglobin gene (GLB2) is expressed in the roots, leaves and inflorescence and can be induced in young plants by cytokinin treatment in contrast to the class 1 gene (GLB1) which is active in germinating seedlings and can be induced by hypoxia and increased sucrose supply, but not by cytokinin treatment.

Identification with proteomics of novel proteins associated with the peribacteroid membrane of soybean root nodules

Soybean peribacteroid membrane (PBM) proteins were isolated from nitrogen-fixing root nodules and subjected to N-terminal sequencing. Sequence data from 17 putative PBM proteins were obtained. Six of these proteins are homologous to proteins of known function. These include three chaperones (HSP60, BiP [HSP70], and PDI) and two proteases (a serine and a thiol protease), all of which are involved in some aspect of protein processing in plants. The PBM homologs of these proteins may play roles in protein translocation, folding, maturation, or degradation in symbiosomes. Two proteins are homologous to known, nodule-specific proteins from soybean, nodulin 53b and nodulin 26B. Although the function of these nodulins is unknown, nodulin 53b has independently been shown to be associated with the PBM. All of the eight proteins with identifiable homologs are likely to be peripheral rather than integral membrane proteins. Possible reasons for this apparent bias are discussed. The identification of homologs of HSP70 and HSP60 associated with the PBM is the first evidence that the molecular machinery for co- or post-translational import of cytoplasmic proteins is present in symbiosomes. This has important implications for the biogenesis of this unique, nitrogen-fixing organelle.

Two hemoglobin genes in Arabidopsis thaliana: the evolutionary origins of leghemoglobins

We cloned two hemoglobin genes from Arabidopsis thaliana. One gene, AHB1, is related in sequence to the family of nonsymbiotic hemoglobin genes previously identified in a number of plant species (class 1). The second hemoglobin gene, AHB2, represents a class of nonsymbiotic hemoglobin (class 2) related in sequence to the symbiotic hemoglobin genes of legumes and Casuarina. The properties of these two hemoglobins suggest that the two families of nonsymbiotic hemoglobins may differ in function from each other and from the symbiotic hemoglobins. AHB1 is induced, in both roots and rosette leaves, by low oxygen levels. Recombinant AHB1 has an oxygen affinity so high as to make it unlikely to function as an oxygen transporter. AHB2 is expressed at a low level in rosette leaves and is low temperature-inducible. AHB2 protein has a lower affinity for oxygen than AHB1 but is similar to AHB1 in having an unusually low, pH-sensitive oxygen off-rate.