Identification of TIMING OF CAB EXPRESSION 1 as a temperature-sensitive negative regulator of tuberization in potato

Tuber, or not tuber: Molecular and morphological basis of underground storage organ development

Underground storage organs occur in phylogenetically diverse plant taxa and arise from multiple tissue types including roots and stems. Thickening growth allows underground storage organs to accommodate carbohydrates and other nutrients and requires proliferation at various lateral meristems followed by cell expansion. The WOX-CLE module regulates thickening growth via the vascular cambium in several eudicot systems, but the molecular mechanisms of proliferation at other lateral meristems are not well understood. In potato, onion, and other systems, members of the phosphatidylethanolamine-binding protein (PEBP) gene family induce underground storage organ development in response to photoperiod cues. While molecular mechanisms of tuber development in potato are well understood, we lack detailed mechanistic knowledge for the extensive morphological and taxonomic diversity of underground storage organs in plants.

Transcriptomic and metabolomic profiling provide insight into the role of sugars and hormones in leaf senescence of Pinellia ternata

KEY MESSAGE

The interaction network and pathway map uncover the potential crosstalk between sugar and hormone metabolisms as a possible reason for leaf senescence in P. ternata. Pinellia ternata, an environmentally sensitive medicinal plant, undergoes leaf senescence twice a year, affecting its development and yield. Understanding the potential mechanism that delays leaf senescence could theoretically decrease yield losses. In this study, a typical senescent population model was constructed, and an integrated analysis of transcriptomic and metabolomic profiles of P. ternata was conducted using two early leaf senescence populations and two stay-green populations. The result showed that two key gene modules were associated with leaf senescence which were mainly enriched in sugar and hormone signaling pathways, respectively. A network constructed by unigenes and metabolisms related to the obtained two pathways revealed that several compounds such as D-arabitol and 2MeScZR have a higher significance ranking. In addition, a total of 130 hub genes in this network were categorized into 3 classes based on connectivity. Among them, 34 hub genes were further analyzed through a pathway map, the potential crosstalk between sugar and hormone metabolisms might be an underlying reason of leaf senescence in P. ternata. These findings address the knowledge gap regarding leaf senescence in P. ternata, providing candidate germplasms for molecular breeding and laying theoretical basis for the realization of finely regulated cultivation in future.

Construction of heat stress regulation networks based on Illumina and SMRT sequencing data in potato

Potato (Solanum tuberosum L.) is one of the most important tuber food crops in the world; however, the cultivated potatoes are susceptible to high temperature, by which potato production is adversely affected. Understanding the coping mechanism of potato to heat stress is essential to secure yield and expand adaptability under environmental conditions with rising temperature. However, the lack of heat-related information has significantly limited the identification and application of core genes. To gain deeper insights into heat tolerance genes, next-generation sequencing and single-molecule real-time sequencing were used to learn the transcriptional response of potato to heat stress and 13,159 differentially expressed genes (DEGs) were identified in this study. All DEGs were grouped into 12 clusters using the K-means clustering algorithm. Gene Ontology enrichment analysis revealed that they were involved in temperature signaling, phytohormone, and protein modification. Among them, there were 950 differentially expressed transcription factors (DETFs). According to the network analysis of DETFs at the sixth hour under heat stress, we found some genes that were previously reported to be associated with photoperiodic tuberization, StCO (CONSTANS), tuber formation, StBEL11 (BEL1-LIKE 11), and earliness in potato, StCDF1 (CYCLING DOF FACTOR 1) responding to temperature. Furthermore, we verified the relative expression levels using quantitative real-time polymerase chain reaction, and the results were consistent with the inferences from transcriptomes. In addition, there were 22,125 alternative splicing events and 2,048 long non-coding RNAs. The database and network established in this study will extend our understanding of potato response to heat stress. It ultimately provided valuable resources for molecular analysis of heat stress response in potato and cultivation of potato varieties with heat tolerance.

Linking New Alleles at the Oscillator Loci to Flowering and Expansion of Asian Rice

The central oscillator is believed to be the key mechanism by which plants adapt to new environments. However, impacts from hybridization, the natural environment, and human selection have rarely been assessed on the oscillator of a crop. Here, from clearly identified alleles at oscillator loci (OsCCA1/LHY, OsPRR95, OsPRR37, OsPRR59, and OsPRR1) in ten diverse genomes of Oryza sativa, additional accessions, and functional analysis, we show that rice's oscillator was rebuilt primarily by new alleles from recombining parental sequences and subsequent 5' or/and coding mutations. New alleles may exhibit altered transcript levels from that of a parental allele and are transcribed variably among genetic backgrounds and natural environments in RIL lines. Plants carrying more expressed OsCCA1_a and less transcribed OsPRR1_e flower early in the paddy field. 5' mutations are instrumental in varied transcription, as shown by EMSA tests on one deletion at the 5' region of highly transcribed OsPRR1_a. Compared to relatively balanced mutations at oscillator loci of Arabidopsis thaliana, 5' mutations of OsPRR37 (and OsCCA1 to a less degree) were under negative selection while those of OsPRR1 alleles were under strong positive selection. Together, range expansion of Asian rice can be elucidated by human selection on OsPRR1 alleles via local flowering time-yield relationships.

Functional Modules in the Meristems: "Tinkering" in Action

BACKGROUND

A feature of higher plants is the modular principle of body organisation. One of these conservative morphological modules that regulate plant growth, histogenesis and organogenesis is meristems-structures that contain pools of stem cells and are generally organised according to a common principle. Basic content: The development of meristems is under the regulation of molecular modules that contain conservative interacting components and modulate the expression of target genes depending on the developmental context. In this review, we focus on two molecular modules that act in different types of meristems. The WOX-CLAVATA module, which includes the peptide ligand, its receptor and the target transcription factor, is responsible for the formation and control of the activity of all meristem types studied, but it has its own peculiarities in different meristems. Another regulatory module is the so-called florigen-activated complex, which is responsible for the phase transition in the shoot vegetative meristem (e.g., from the vegetative shoot apical meristem to the inflorescence meristem).

CONCLUSIONS

The review considers the composition and functions of these two functional modules in different developmental programmes, as well as their appearance, evolution and use in plant breeding.

Heat, drought, and combined stress effect on transgenic potato plants overexpressing the StERF94 transcription factor

Despite their economic importance worldwide, potato plants are sensitive to various abiotic constraints, such as drought and high temperatures, which cause significant losses in yields and tuber quality. Moreover, because of the climate change phenomenon, plants are frequently subjected to combined stresses, mainly high temperatures and drought. In this context, breeding for tolerant varieties should consider not only plant response to drought or high temperature but also to combined stresses. In the current study, we studied transgenic potato plants overexpressing an ethylene response transcription factor (TF; StERF94) involved in abiotic stress response signaling pathways. Our previous results showed that these transgenic plants display tolerance to salt stress more than wildtype (WT). In this work, we aimed to investigate the effects of drought, heat, and combined stresses on transgenic potato plants overexpressing StERF94 TF under in vitro culture conditions. The obtained results revealed that StERF94 overexpression improved the tolerance of the transgenic plants to drought, heat, and combined stresses through better control of the leaf water and chlorophyll contents, activation of antioxidant enzymes, and an accumulation of proline, especially in the leaves. Indeed, the expression level of antioxidant enzyme-encoding genes (CuZnSOD, FeSOD, CAT1, and CAT2) was significantly induced by the different stress conditions in the transgenic potato plants compared with the WT plants. This study further confirms that StERF94 TF may be implicated in regulating the expression of target genes encoding antioxidant enzymes.

Long-distance control of potato storage organ formation by SELF PRUNING 3D and FLOWERING LOCUS T-like 1

Plants program their meristem-associated developmental switches for timely adaptation to a changing environment. Potato (Solanum tuberosum L.) tubers differentiate from specialized belowground branches or stolons through radial expansion of their terminal ends. During this process, the stolon apex and closest axillary buds enter a dormancy state that leads to tuber eyes, which are reactivated the following spring and generate a clonally identical plant. The potato FLOWERING LOCUS T homolog SELF-PRUNING 6A (StSP6A) was previously identified as the major tuber-inducing signal that integrates day-length cues to control the storage switch. However, whether some other long-range signals also act as tuber organogenesis stimuli remains unknown. Here, we show that the florigen SELF PRUNING 3D (StSP3D) and FLOWERING LOCUS T-like 1 (StFTL1) genes are activated by short days, analogously to StSP6A. Overexpression of StSP3D or StFTL1 promotes tuber formation under non-inductive long days, and the tuber-inducing activity of these proteins is graft transmissible. Using the non-tuber-bearing wild species Solanum etuberosum, a natural SP6A null mutant, we show that leaf-expressed SP6A is dispensable for StSP3D long-range activity. StSP3D and StFTL1 mediate secondary activation of StSP6A in stolon tips, leading to amplification of this tuberigen signal. StSP3D and StFTL1 were observed to bind the same protein partners as StSP6A, suggesting that they can also form transcriptionally active complexes. Together, our findings show that additional mobile tuber-inducing signals are regulated by the photoperiodic pathway.

Responses of Aerial and Belowground Parts of Different Potato (Solanum tuberosum L.) Cultivars to Heat Stress

The mechanism of potato (Solanum tuberosum L.) thermotolerance has been the focus of intensive research for many years because plant growth and tuber yield are highly sensitive to heat stress. However, the linkage between the aerial and belowground parts of potato plants in response to high temperatures is not clear. To disentangle this issue, the aerial and belowground parts of the heat-resistant cultivar Dian187 (D187) and the heat-sensitive cultivar Qingshu 9 (Qs9) were independently exposed to high-temperature (30 °C) conditions using a special incubator. The results indicated that when the belowground plant parts were maintained at a normal temperature, the growth of the aerial plant parts was maintained even when independently exposed to heat stress. In contrast, the treatment that independently exposed the belowground plant parts to heat stress promoted premature senescence in the plant's leaves, even when the aerial plant parts were maintained at a normal temperature. When the aerial part of the plant was independently treated with heat stress, tuberization belowground was not delayed, and tuberization suppression was not as severe as when the belowground plant parts independently underwent heat stress. Heat stress on the belowground plant parts alone had virtually no damaging effects on the leaf photosynthetic system but caused distinct tuber deformation, secondary growth, and the loss of tuber skin colour. Transcriptome analysis revealed that the treatment of the belowground plant parts at 30 °C induced 3361 differentially expressed genes in the Qs9 cultivar's expanding tubers, while the D187 cultivar had only 10,148 differentially expressed genes. Conversely, when only the aerial plant parts were treated at 30 °C, there were just 807 DEGs (differentially expressed genes) in the D187 cultivar's expanding tubers compared with 6563 DEGs in the Qs9 cultivar, indicating that the two cultivars with different heat sensitivities have distinct regulatory mechanisms of tuberization when exposed to heat stress. The information provided in this study may be useful for further exploring the genes associated with high-temperature resistance in potato cultivars.

PEBP Signaling Network in Tubers and Tuberous Root Crops

Tubers and tuberous root crops are essential carbohydrate sources and staple foods for humans, second only to cereals. The developmental phase transition, including floral initiation and underground storage organ formation, is controlled by complex signaling processes involving the integration of environmental and endogenous cues. FLOWERING LOCUS T (FT) and TERMINAL FLOWER 1/CENTRORADIALIS (TFL1/CEN), members of the phosphatidylethanolamine-binding protein (PEBP) gene family, play a central role in this developmental phase transition process. FT and FT-like proteins have a function to promote developmental phase transition, while TFL1/CEN act oppositely. The balance between FT and TFL1/CEN is critical to ensure a successful plant life cycle. Here, we present a summarized review of the role and signaling network of PEBP in floral initiation and underground storage organ formation, specifically in tubers and tuberous root crops. Lastly, we point out several questions that need to be answered in order to have a more complete understanding of the PEBP signaling network, which is crucial for the agronomical improvement of tubers and tuberous crops.

Leaves and stolons transcriptomic analysis provide insight into the role of phytochrome F in potato flowering and tuberization

Photoperiod plays a critical role in controlling the formation of sexual or vegetative reproductive organs in potato. Although StPHYF-silenced plants overcome day-length limitations to tuberize through a systemic effect on tuberigen StSP6A expression in the stolon, the comprehensive regulatory network of StPHYF remains obscure. Therefore, the present study investigated the transcriptomes of StPHYF-silenced plants and observed that, in addition to known components of the photoperiodic tuberization pathway, florigen StSP3D and other flowering-related genes were activated in StPHYF-silenced plants, exhibiting an early flowering response. Additionally, grafting experiments uncovered the long-distance effect of StPHYF silencing on gene expression in the stolon, including the circadian clock components, flowering-associated MADSs, and tuberization-related regulatory genes. Similar to the AtFT-AtAP1 regulatory module in Arabidopsis, the present study established that the AP1-like StMADS1 functions downstream of the tuberigen activation complex (TAC) and that suppressing StMADS1 inhibits tuberization in vitro and delays tuberization in vivo. Moreover, the expression of StSP6A was downregulated in StMADS1-silenced plants, implying the expression of StSP6A may be feedback-regulated by StMADS1. Overall, these results reveal that the regulatory network of StPHYF controls flowering and tuberization and targets the crucial tuberization factor StMADS1 through TAC, thereby providing a better understanding of StPHYF-mediated day-length perception during potato reproduction.

Keeping time in the dark: Potato diel and circadian rhythmic gene expression reveals tissue-specific circadian clocks

The circadian clock is an internal molecular oscillator and coordinates numerous physiological processes through regulation of molecular pathways. Tissue-specific clocks connected by mobile signals have previously been found to run at different speeds in Arabidopsis thaliana tissues. However, tissue variation in circadian clocks in crop species is unknown. In this study, leaf and tuber global gene expression in cultivated potato under cycling and constant environmental conditions was profiled. In addition, we used a circadian-regulated luciferase reporter construct to study tuber gene expression rhythms. Diel and circadian expression patterns were present among 17.9% and 5.6% of the expressed genes in the tuber. Over 500 genes displayed differential tissue specific diel phases. Intriguingly, few core circadian clock genes had circadian expression patterns, while all such genes were circadian rhythmic in cultivated tomato leaves. Furthermore, robust diel and circadian transcriptional rhythms were observed among detached tubers. Our results suggest alternative regulatory mechanisms and/or clock composition is present in potato, as well as the presence of tissue-specific independent circadian clocks. We have provided the first evidence of a functional circadian clock in below-ground storage organs, holding important implications for other storage root and tuberous crops.

Temporally distinct regulatory pathways coordinate thermo-responsive storage organ formation in potato

Tuberization is an important developmental process in potatoes, but it is highly affected by environmental conditions. Temperature is a major environmental factor affecting tuberization, with high temperatures suppressing tuber development. However, the temporal aspects of thermo-responsive tuberization remain elusive. In this study, we show that FT homolog StSP6A is suppressed by temporally distinct regulatory pathways. Experiments using StSP6A-overexpressing plants show that post-transcriptional regulation plays a major role at the early stage, while transcriptional regulation is an important late-stage factor, suppressing StSP6A at high temperatures in leaves. Overexpression of StSP6A in leaves restores tuber formation but does not recover tuber yield at the late stage, possibly because of suppressed sugar transport at high temperatures. Transcriptome analyses lead to the identification of potential regulators that may be involved in thermo-responsive tuberization at different stages. Our work shows that potato has temporally distinct molecular mechanisms that finely control tuber development at high temperatures.

Toward the Design of Potato Tolerant to Abiotic Stress

Potato is a major global crop that has an important role to play in food security, reducing poverty and improving human nutrition. Productivity in potato however is limited in many environments by its sensitivity to abiotic stresses such as elevated temperature, drought, frost, and salinity. In this chapter we focus on the effects of elevated temperature on potato yields as high temperature is the most important uncontrollable factor affecting growth and yield of potato. We describe some of the physiological impacts of elevated temperature and review recent findings about response mechanisms. We describe genetic approaches that could be used to identify allelic variants of genes that may be useful to breed for increased climate resilience, an approach that could be deployed with recent advances in potato breeding.

Future-Proofing Potato for Drought and Heat Tolerance by Overexpression of Hexokinase and SP6A

Crop yield is largely affected by global climate change. Especially periods of heat and drought limit crop productivity worldwide. According to current models of future climate scenarios, heatwaves and periods of drought are likely to increase. Potato, as an important food crop of temperate latitudes, is very sensitive to heat and drought which impact tuber yield and quality. To improve abiotic stress resilience of potato plants, we aimed at co-expressing hexokinase 1 from Arabidopsis thaliana (AtHXK1) in guard cells and SELF-PRUNING 6A (SP6A) using the leaf/stem-specific StLS1 promoter in order to increase water use efficiency as well as tuberization under drought and heat stress. Guard cell-specific expression of AtHXK1 decreased stomatal conductance and improved water use efficiency of transgenic potato plants as has been shown for other crop plants. Additionally, co-expression with the FT-homolog SP6A stimulated tuberization and improved assimilate allocation to developing tubers under control as well as under single and combined drought and heat stress conditions. Thus, co-expression of both proteins provides a novel strategy to improve abiotic stress tolerance of potato plants.

Physiological, Biochemical, and Transcriptional Responses to Single and Combined Abiotic Stress in Stress-Tolerant and Stress-Sensitive Potato Genotypes

Potato production is often constrained by abiotic stresses such as drought and high temperatures which are often present in combination. In the present work, we aimed to identify key mechanisms and processes underlying single and combined abiotic stress tolerance by comparative analysis of tolerant and susceptible cultivars. Physiological data indicated that the cultivars Desiree and Unica were stress tolerant while Agria and Russett Burbank were stress susceptible. Abiotic stress caused a greater reduction of photosynthetic carbon assimilation in the susceptible cultivars which was associated with a lower leaf transpiration rate. Oxidative stress, as estimated by the accumulation of malondialdehyde was not induced by stress treatments in any of the genotypes with the exception of drought stress in Russett Burbank. Stress treatment resulted in increases in ascorbate peroxidase activity in all cultivars except Agria which increased catalase activity in response to stress. Transcript profiling highlighted a decrease in the abundance of transcripts encoding proteins associated with PSII light harvesting complex in stress tolerant cultivars. Furthermore, stress tolerant cultivars accumulated fewer transcripts encoding a type-1 metacaspase implicated in programmed cell death. Stress tolerant cultivars exhibited stronger expression of genes associated with plant growth and development, hormone metabolism and primary and secondary metabolism than stress susceptible cultivars. Metabolite profiling revealed accumulation of proline in all genotypes following drought stress that was partially suppressed in combined heat and drought. On the contrary, the sugar alcohols inositol and mannitol were strongly accumulated under heat and combined heat and drought stress while galactinol was most strongly accumulated under drought. Combined heat and drought also resulted in the accumulation of Valine, isoleucine, and lysine in all genotypes. These data indicate that single and multiple abiotic stress tolerance in potato is associated with a maintenance of CO₂ assimilation and protection of PSII by a reduction of light harvesting capacity. The data further suggests that stress tolerant cultivars suppress cell death and maintain growth and development via fine tuning of hormone signaling, and primary and secondary metabolism. This study highlights potential targets for the development of stress tolerant potato cultivars.