From model to crop: functional analysis of a STAY-GREEN gene in the model legume Medicago truncatula and effective use of the gene for alfalfa improvement

Transcriptome-Wide Identification of Dark- and Salt-Induced Senescence-Related NAC Gene Family Members in Alfalfa

Leaves are a key forage part for livestock, and the aging of leaves affects forage biomass and quality. Preventing or delaying premature leaf senescence leads to an increase in pasture biomass accumulation and an improvement in alfalfa quality. NAC transcription factors have been reported to affect plant growth and abiotic stress responses. In this study, 48 NAC genes potentially associated with leaf senescence were identified in alfalfa under dark or salt stress conditions. A phylogenetic analysis divided MsNACs into six subgroups based on similar gene structure and conserved motif. These MsNACs were unevenly distributed in 26 alfalfa chromosomes. The results of the collinearity analysis show that all of the MsNACs were involved in gene duplication. Some cis-acting elements related to hormones and stress were screened in the 2-kb promoter regions of MsNACs. Nine of the MsNAC genes were subjected to qRT-PCR to quantify their expression and Agrobacterium-mediated transient expression to verify their functions. The results indicate that Ms.gene031485, Ms.gene032313, Ms.gene08494, and Ms.gene77666 might be key NAC genes involved in alfalfa leaf senescence. Our findings extend the understanding of the regulatory function of MsNACs in leaf senescence.

Integrating Physiology, Cytology, and Transcriptome to Reveal the Leaf Variegation Mechanism in Phalaenopsis Chia E Yenlin Variegata Leaves

Phalaenopsis orchids, with their unique appearance and extended flowering period, are among the most commercially valuable Orchidaceae worldwide. Particularly, the variegation in leaf color of Phalaenopsis significantly enhances the ornamental and economic value and knowledge of the molecular mechanism of leaf-color variegation in Phalaenopsis is lacking. In this study, an integrative analysis of the physiology, cytology, and transcriptome profiles was performed on Phalaenopsis Chia E Yenlin Variegata leaves between the green region (GR) and yellow region (YR) within the same leaf. The total chlorophyll and carotenoid contents in the YR exhibited a marked decrease of 72.18% and 90.21%, respectively, relative to the GR. Examination of the ultrastructure showed that the chloroplasts of the YR were fewer and smaller and exhibited indistinct stromal lamellae, ruptured thylakoids, and irregularly arranged plastoglobuli. The transcriptome sequencing between the GR and YR led to a total of 3793 differentially expressed genes, consisting of 1769 upregulated genes and 2024 downregulated genes. Among these, the chlorophyll-biosynthesis-related genes HEMA, CHLH, CRD, and CAO showed downregulation, while the chlorophyll-degradation-related gene SGR had an upregulated expression in the YR. Plant-hormone-related genes and transcription factors MYBs (37), NACs (21), ERFs (20), bHLH (13), and GLK (2), with a significant difference, were also analyzed. Furthermore, qRT-PCR experiments validated the above results. The present work establishes a genetic foundation for future studies of leaf-pigment mutations and may help to improve the economic and breeding values of Phalaenopsis.

Mutating alfalfa COUMARATE 3-HYDROXYLASE using multiplex CRISPR/Cas9 leads to reduced lignin deposition and improved forage quality

Alfalfa (Medicago sativa L.) forage quality is adversely affected by lignin deposition in cell walls at advanced maturity stages. Reducing lignin content through RNA interference or antisense approaches has been shown to improve alfalfa forage quality and digestibility. We employed a multiplex CRISPR/Cas9-mediated gene-editing system to reduce lignin content and alter lignin composition in alfalfa by targeting the COUMARATE 3-HYDROXYLASE (MsC3H) gene, which encodes a key enzyme in lignin biosynthesis. Four guide RNAs (gRNAs) targeting the first exon of MsC3H were designed and clustered into a tRNA-gRNA polycistronic system and introduced into tetraploid alfalfa via Agrobacterium-mediated transformation. Out of 130 transgenic lines, at least 73 lines were confirmed to contain gene-editing events in one or more alleles of MsC3H. Fifty-five lines were selected for lignin content/composition analysis. Amongst these lines, three independent tetra-allelic homozygous lines (Msc3h-013, Msc3h-121, and Msc3h-158) with different mutation events in MsC3H were characterized in detail. Homozygous mutation of MsC3H in these three lines significantly reduced the lignin content and altered lignin composition in stems. Moreover, these lines had significantly lower levels of acid detergent fiber and neutral detergent fiber as well as higher levels of total digestible nutrients, relative feed values, and in vitro true dry matter digestibility. Taken together, these results showed that CRISPR/Cas9-mediated editing of MsC3H successfully reduced shoot lignin content, improved digestibility, and nutritional values without sacrificing plant growth and biomass yield. These lines could be used in alfalfa breeding programs to generate elite transgene-free alfalfa cultivars with reduced lignin and improved forage quality.

Functional Characterization of the MsFKF1 Gene Reveals Its Dual Role in Regulating the Flowering Time and Plant Height in Medicago sativa L

Alfalfa (M. sativa), a perennial legume forage, is known for its high yield and good quality. As a long-day plant, it is sensitive to changes in the day length, which affects the flowering time and plant growth, and limits alfalfa yield. Photoperiod-mediated delayed flowering in alfalfa helps to extend the vegetative growth period and increase the yield. We isolated a blue-light phytohormone gene from the alfalfa genome that is an ortholog of soybean FKF1 and named it MsFKF1. Gene expression analyses showed that MsFKF1 responds to blue light and the circadian clock in alfalfa. We found that MsFKF1 regulates the flowering time through the plant circadian clock pathway by inhibiting the transcription of E1 and COL, thus suppressing FLOWERING LOCUS T a1 (FTa1) transcription. In addition, transgenic lines exhibited higher plant height and accumulated more biomass in comparison to wild-type plants. However, the increased fiber (NDF and ADF) and lignin content also led to a reduction in the digestibility of the forage. The key genes related to GA biosynthesis, GA20OX1, increased in the transgenic lines, while GA2OX1 decreased for the inactive GA transformation. These findings offer novel insights on the function of MsFKF1 in the regulation of the flowering time and plant height in cultivated M. sativa. These insights into MsFKF1's roles in alfalfa offer potential strategies for molecular breeding aimed at optimizing flowering time and biomass yield.

Transcription factor LcNAC002 coregulates chlorophyll degradation and anthocyanin biosynthesis in litchi

Chlorophyll degradation and anthocyanin biosynthesis, which often occur almost synchronously during fruit ripening, are crucial for vibrant coloration of fruits. However, the interlink point between their regulatory pathways remains largely unknown. Here, 2 litchi (Litchi chinensis Sonn.) cultivars with distinctively different coloration patterns during ripening, i.e. slow-reddening/stay-green "Feizixiao" (FZX) vs rapid-reddening/degreening "Nuomici" (NMC), were selected as the materials to study the key factors determining coloration. Litchi chinensis STAY-GREEN (LcSGR) was confirmed as the critical gene in pericarp chlorophyll loss and chloroplast breakdown during fruit ripening, as LcSGR directly interacted with pheophorbide a oxygenase (PAO), a key enzyme in chlorophyll degradation via the PAO pathway. Litchi chinensis no apical meristem (NAM), Arabidopsis transcription activation factor 1/2, and cup-shaped cotyledon 2 (LcNAC002) was identified as a positive regulator in the coloration of litchi pericarp. The expression of LcNAC002 was significantly higher in NMC than in FZX. Virus-induced gene silencing of LcNAC002 significantly decreased the expression of LcSGR as well as L. chinensis MYELOBLASTOSIS1 (LcMYB1), and inhibited chlorophyll loss and anthocyanin accumulation. A dual-luciferase reporter assay revealed that LcNAC002 significantly activates the expression of both LcSGR and LcMYB1. Furthermore, yeast-one-hybrid and electrophoretic mobility shift assay results showed that LcNAC002 directly binds to the promoters of LcSGR and LcMYB1. These findings suggest that LcNAC002 is an important ripening-related transcription factor that interlinks chlorophyll degradation and anthocyanin biosynthesis by coactivating the expression of both LcSGR and LcMYB1.

Multiplex CRISPR/Cas9-mediated mutagenesis of alfalfa FLOWERING LOCUS Ta1 (MsFTa1) leads to delayed flowering time with improved forage biomass yield and quality

Alfalfa (Medicago sativa L.) is a perennial flowering plant in the legume family that is widely cultivated as a forage crop for its high yield, forage quality and related agricultural and economic benefits. Alfalfa is a photoperiod sensitive long-day (LD) plant that can accomplish its vegetative and reproductive phases in a short period of time. However, rapid flowering can compromise forage biomass yield and quality. Here, we attempted to delay flowering in alfalfa using multiplex CRISPR/Cas9-mediated mutagenesis of FLOWERING LOCUS Ta1 (MsFTa1), a key floral integrator and activator gene. Four guide RNAs (gRNAs) were designed and clustered in a polycistronic tRNA-gRNA system and introduced into alfalfa by Agrobacterium-mediated transformation. Ninety-six putative mutant lines were identified by gene sequencing and characterized for delayed flowering time and related desirable agronomic traits. Phenotype assessment of flowering time under LD conditions identified 22 independent mutant lines with delayed flowering compared to the control. Six independent Msfta1 lines containing mutations in all four copies of MsFTa1 accumulated significantly higher forage biomass yield, with increases of up to 78% in fresh weight and 76% in dry weight compared to controls. Depending on the harvesting schemes, many of these lines also had reduced lignin, acid detergent fibre (ADF) and neutral detergent fibre (NDF) content and significantly higher crude protein (CP) and mineral contents compared to control plants, especially in the stems. These CRISPR/Cas9-edited Msfta1 mutants could be introduced in alfalfa breeding programmes to generate elite transgene-free alfalfa cultivars with improved forage biomass yield and quality.

Receptor for Activated C Kinase1B (RACK1B) Delays Salinity-Induced Senescence in Rice Leaves by Regulating Chlorophyll Degradation

The widely conserved Receptor for Activated C Kinase1 (RACK1) protein is a WD-40 type scaffold protein that regulates diverse environmental stress signal transduction pathways. Arabidopsis RACK1A has been reported to interact with various proteins in salt stress and Light-Harvesting Complex (LHC) pathways. However, the mechanism of how RACK1 contributes to the photosystem and chlorophyll metabolism in stress conditions remains elusive. In this study, using T-DNA-mediated activation tagging transgenic rice (Oryza sativa L.) lines, we show that leaves from rice RACK1B gene (OsRACK1B) gain-of-function (RACK1B-OX) plants exhibit the stay-green phenotype under salinity stress. In contrast, leaves from down-regulated OsRACK1B (RACK1B-UX) plants display an accelerated yellowing. qRT-PCR analysis revealed that several genes which encode chlorophyll catabolic enzymes (CCEs) are differentially expressed in both RACK1B-OX and RACK1B-UX rice plants. In addition to CCEs, stay-green (SGR) is a key component that forms the SGR-CCE complex in senescing chloroplasts, and which causes LHCII complex instability. Transcript and protein profiling revealed a significant upregulation of OsSGR in RACK1B-UX plants compared to that in RACK1B-OX rice plants during salt treatment. The results imply that senescence-associated transcription factors (TFs) are altered following altered OsRACK1B expression, indicating a transcriptional reprogramming by OsRACK1B and a novel regulatory mechanism involving the OsRACK1B-OsSGR-TFs complex. Our findings suggest that the ectopic expression of OsRACK1B negatively regulates chlorophyll degradation, leads to a steady level of LHC-II isoform Lhcb1, an essential prerequisite for the state transition of photosynthesis for adaptation, and delays salinity-induced senescence. Taken together, these results provide important insights into the molecular mechanisms of salinity-induced senescence, which can be useful in circumventing the effect of salt on photosynthesis and in reducing the yield penalty of important cereal crops, such as rice, in global climate change conditions.

The NAC transcription factor ZmNAC132 regulates leaf senescence and male fertility in maize

Leaf senescence is an integral step in the final stages of plant development, as nutrient remobilization from leaves to sink organs is accomplished during this process. NACs compose a large superfamily of plant-specific TFs involved in multiple plant development processes. Here, we identified a maize NAC TF, ZmNAC132, involved in leaf senescence and male fertility. ZmNAC132 expression was tightly linked to leaf senescence in an age-dependent manner. Knockout of ZmNAC132 led to delays in chlorophyll degradation and leaf senescence, whereas overexpression of ZmNAC132 had the opposite effects. ZmNAC132 could bind to and transactivate the promoter of ZmNYE1, a major chlorophyll catabolic gene, to accelerate chlorophyll degradation during leaf senescence. Moreover, ZmNAC132 affected male fertility through the upregulation of ZmEXPB1, an expansin-encoding gene associated with sexual reproduction and other related genes. Together, the results show that ZmNAC132 participates in the regulation of leaf senescence and male fertility through the targeting of different downstream genes in maize.

Homolog of Pea SGR Controls Stay-Green in Faba Bean (Vicia faba L.)

Faba bean is an important legume crop consumed as a vegetable or snack food, and its green cotyledons could present an attractive color for consumers. A mutation in SGR causes stay-green in plants. In this study, vfsgr was identified from a green-cotyledon-mutant faba bean, SNB7, by homologous blast between the SGR of pea and the transcriptome of faba bean. Sequence analysis revealed that a SNP at position 513 of the CDS of VfSGR caused a pre-stop codon, resulting in a shorter protein in the green-cotyledon faba bean SNB7. A dCaps marker was developed according to the SNP that caused the pre-stop, and this marker was completely associated with the color of the cotyledon of faba bean. SNB7 stayed green during dark treatment, while the expression level of VfSGR increased during dark-induced senescence in the yellow-cotyledon faba bean HST. Transient expression of VfSGR in Nicotiana. benthamiana leaves resulted in chlorophyll degradation. These results indicate that vfsgr is the gene responsible for the stay-green of faba bean, and the dCaps marker developed in this study provides a molecular tool for the breeding of green-cotyledon faba beans.

ABSCISIC ACID INSENSITIVE 4 coordinates eoplast formation to ensure acquisition of seed longevity during maturation in Medicago truncatula

Seed longevity, the capacity to remain alive during dry storage, is pivotal to germination performance and is essential for preserving genetic diversity. It is acquired during late maturation concomitantly with seed degreening and the de-differentiation of chloroplasts into colorless, non-photosynthetic plastids, called eoplasts. As chlorophyll retention leads to poor seed performance upon sowing, these processes are important for seed vigor. However, how these processes are regulated and connected to the acquisition of seed longevity remains poorly understood. Here, we show that such a role is at least provided by ABSCISIC ACID INSENSITIVE 4 (ABI4) in the legume Medicago truncatula. Mature seeds of Mtabi4 mutants contained more chlorophyll than wild-type seeds and exhibited a 75% reduction in longevity and reduced dormancy. MtABI4 was necessary to stimulate eoplast formation, as evidenced by the significant delay in the dismantlement of photosystem II during the maturation of mutant seeds. Mtabi4 seeds also exhibited transcriptional deregulation of genes associated with retrograde signaling and transcriptional control of plastid-encoded genes. Longevity was restored when Mtabi4 seeds developed in darkness, suggesting that the shutdown of photosynthesis during maturation, rather than chlorophyll degradation per se, is a requisite for the acquisition of longevity. Indeed, the shelf life of stay green mutant seeds that retained chlorophyll was not affected. Thus, ABI4 plays a role in coordinating the dismantlement of chloroplasts during seed development to avoid damage that compromises the acquisition of seed longevity. Analysis of Mtabi4 Mtabi5 double mutants showed synergistic effects on chlorophyll retention and longevity, suggesting that they act via parallel pathways.

The transcription factor VviNAC60 regulates senescence- and ripening-related processes in grapevine

Grapevine (Vitis vinifera L.) is one of the most widely cultivated fruit crops because the winemaking industry has huge economic relevance worldwide. Uncovering the molecular mechanisms controlling the developmental progression of plant organs will prove essential for maintaining high-quality grapes, expressly in the context of climate change, which impairs the ripening process. Through a deep inspection of transcriptomic data, we identified VviNAC60, a member of the NAC transcription factor family, as a putative regulator of grapevine organ maturation. We explored VviNAC60 binding landscapes through DNA affinity purification followed by sequencing and compared bound genes with transcriptomics datasets from grapevine plants stably and transiently overexpressing VviNAC60 to define a set of high-confidence targets. Among these, we identified key molecular markers associated with organ senescence and fruit ripening. Physiological, metabolic, and promoter activation analyses showed that VviNAC60 induces chlorophyll degradation and anthocyanin accumulation through the up-regulation of STAY-GREEN PROTEIN 1 (VviSGR1) and VviMYBA1, respectively, with the latter being up-regulated through a VviNAC60-VviNAC03 regulatory complex. Despite sharing a closer phylogenetic relationship with senescence-related homologues to the NAC transcription factor AtNAP, VviNAC60 complemented the non-ripening(nor) mutant phenotype in tomato (Solanum lycopersicum), suggesting a dual role as an orchestrator of both ripening- and senescence-related processes. Our data support VviNAC60 as a regulator of processes initiated in the grapevine vegetative- to mature-phase organ transition and therefore as a potential target for enhancing the environmental resilience of grapevine by fine-tuning the duration of the vegetative phase.

Transcriptome and Metabolite Profiling of Tomato SGR-Knockout Null Lines Using the CRISPR/Cas9 System

Stay-green 1 (SGR1) protein is a critical regulator of chlorophyll degradation and senescence in plant leaves; however, the functions of tomato SGR1 remain ambiguous. Here, we generated an SGR1-knockout (KO) null line via clustered regularly interspaced palindromic repeat (CRISPR)/CRISPR-associated protein 9-mediated gene editing and conducted RNA sequencing and gas chromatography−tandem mass spectrometry analysis to identify the differentially expressed genes (DEGs). Solanum lycopersicum SGR1 (SlSGR1) knockout null line clearly showed a turbid brown color with significantly higher chlorophyll and carotenoid levels than those in the wild-type (WT) fruit. Differential gene expression analysis revealed 728 DEGs between WT and sgr#1-6 line, including 263 and 465 downregulated and upregulated genes, respectively, with fold-change >2 and adjusted p-value < 0.05. Most of the DEGs have functions related to photosynthesis, chloroplasts, and carotenoid biosynthesis. The strong changes in pigment and carotenoid content resulted in the accumulation of key primary metabolites, such as sucrose and its derivatives (fructose, galactinol, and raffinose), glycolytic intermediates (glucose, glucose-6-phosphate, and fructose-6-phosphate), and tricarboxylic acid cycle intermediates (malate and fumarate) in the leaves and fruit of the SGR-KO null lines. Overall, the SGR1-KO null lines developed here provide new evidence for the mechanisms underlying the roles of SGR1 as well as the molecular pathways involved in photosynthesis, chloroplasts, and carotenoid biosynthesis.

Progresses of CRISPR/Cas9 genome editing in forage crops

The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) mediated-genome editing has evolved into a powerful tool that is widely used in plant species to induce editing in the genome for analyzing gene function and crop improvement. CRISPR/Cas9 is an RNA-guided genome editing tool consisting of a Cas9 nuclease and a single-guide RNA (sgRNA). The CRISPR/Cas9 system enables more accurate and efficient genome editing in crops. In this review, we summarized the advances of the CRISPR/Cas9 technology in plant genome editing and its applications in forage crops. We described briefly about the development of CRISPR/Cas9 technology in plant genome editing. We assessed the progress of CRISPR/Cas9-mediated targeted-mutagenesis in various forage crops, including alfalfa, Medicago truncatula, Hordeum vulgare, Sorghum bicolor, Setaria italica and Panicum virgatum. The potentials and challenges of CRISPR/Cas9 in forage breeding were discussed.

STRONG STAYGREEN inhibits DNA binding of PvNAP transcription factors during leaf senescence in switchgrass

Fine tuning the progression of leaf senescence is important for plant fitness in nature, while the "staygreen" phenotype with delayed leaf senescence has been considered a valuable agronomic trait in crop genetic improvement. In this study, a switchgrass (Panicum virgatum L.) CCCH-type Zinc finger gene, Strong Staygreen (PvSSG), was characterized as a suppressor of leaf senescence as overexpression or suppression of the gene led to delayed or accelerated leaf senescence, respectively. Transcriptomic analysis marked that chlorophyll (Chl) catabolic pathway genes were involved in the PvSSG-regulated leaf senescence. PvSSG was identified as a nucleus-localized protein with no transcriptional activity. By yeast two-hybrid screening, we identified its interacting proteins, including a pair of paralogous transcription factors, PvNAP1/2 (NAC-LIKE, ACTIVATED BY AP3/PI). Overexpression of PvNAPs led to precocious leaf senescence at least partially by directly targeting and transactivating Chl catabolic genes to promote Chl degradation. PvSSG, through protein-protein interaction, repressed the DNA-binding efficiency of PvNAPs and alleviated its transactivating effect on downstream genes, thereby functioning as a "brake" in the progression of leaf senescence. Moreover, overexpression of PvSSG resulted in up to 47% higher biomass yield and improved biomass feedstock quality, reiterating the importance of leaf senescence regulation in the genetic improvement of switchgrass and other feedstock crops.

Crystal structure and reaction mechanism of a bacterial Mg-dechelatase homolog from the Chloroflexi Anaerolineae

Chlorophyll degradation plays a myriad of physiological roles in photosynthetic organisms, including acclimation to light environment and nutrient remobilization during senescence. Mg extraction from chlorophyll a is the first and committed step of the chlorophyll degradation pathway. This reaction is catalyzed by the Mg-dechelatase enzyme encoded by Stay-Green (SGR). The reaction mechanism of SGR protein remains elusive since metal ion extraction from organic molecules is not a common enzymatic reaction. Additionally, experimentally derived structural information about SGR or its homologs has not yet been reported. In this study, the crystal structure of the SGR homolog from Anaerolineae bacterium was determined using the molecular replacement method at 1.85 Å resolution. Our previous study showed that three residues-H32, D34, and D62 are essential for the catalytic activity of the enzyme. Biochemical analysis involving mutants of D34 residue further strengthened its importance in the functioning of the dechelatase. Docking simulation also revealed the interaction between the D34 side chain and central Mg ion of chlorophyll a. Structural analysis showed the arrangement of D34/H32/D62 in the form of a catalytic triad that is generally found in hydrolases. The probable reaction mechanism suggests that deprotonated D34 side chain coordinates and destabilizes Mg, resulting in Mg extraction. Besides, H32 possibly acts as a general base catalyst and D62 facilitates H32 to be a better proton acceptor. Taken together, the reaction mechanism of SGR partially mirrors the one observed in hydrolases.

Intraspecific Variation for Leaf Physiological and Root Morphological Adaptation to Drought Stress in Alfalfa (Medicago sativa L.)

Drought stress reduces crop biomass yield and the profitability of rainfed agricultural systems. Evaluation of populations or accessions adapted to diverse geographical and agro-climatic environments sheds light on beneficial plant responses to enhance and optimize yield in resource-limited environments. This study used the morphological and physiological characteristics of leaves and roots from two different alfalfa subspecies during progressive drought stress imposed on controlled and field conditions. Two different soils (Experiments 1 and 2) imposed water stress at different stress intensities and crop stages in the controlled environment. Algorithm-based image analysis of leaves and root systems revealed key morphological and physiological traits associated with biomass yield under stress. The Medicago sativa subspecies (ssp.) sativa population, PI478573, had smaller leaves and maintained higher chlorophyll content (CC), leaf water potential, and osmotic potential under water stress. In contrast, M. sativa ssp. varia, PI502521, had larger leaves, a robust root system, and more biomass yield. In the field study, an unmanned aerial vehicle survey revealed PI502521 to have a higher normalized difference vegetation index (vegetation cover and plant health characteristics) throughout the cropping season, whereas PI478573 values were low during the hot summer and yielded low biomass in both irrigated and rainfed treatments. RhizoVision Explorer image analysis of excavated roots revealed a smaller diameter and a narrow root angle as target traits to increase alfalfa biomass yield irrespective of water availability. Root architectural traits such as network area, solidity, volume, surface area, and maximum radius exhibited significant variation at the genotype level only under limited water availability. Different drought-adaptive strategies identified across subspecies populations will benefit the plant under varying levels of water limitation and facilitate the development of alfalfa cultivars suitable across a broad range of growing conditions. The alleles from both subspecies will enable the development of drought-tolerant alfalfa with enhanced productivity under limited water availability.

STAYGREEN-mediated chlorophyll a catabolism is critical for photosystem stability during heat-induced leaf senescence in perennial ryegrass

Suppression of the chlorophyll a (Chl a) Mg-dechelatase gene, SGR/NYE1, blocks the degradation of Chl a, resulting in a 'stay-green' trait. In this study, we investigated the effect of Chl a catabolism on plant heat-induced leaf senescence in perennial ryegrass (Lolium perenne L.). Under heat stress, the LpSGR-RNAi lines not only lost the stay-green phenotype but also showed accelerated leaf senescence with increased chloroplast disruption, more loss of photosystem (PS) proteins, lower PSⅡ quantum yields, higher levels of energy dissipation, increased accumulation of reactive oxygen species (ROS) and lower ROS-scavenging enzyme activities. Transcriptome analysis revealed that the suppression of LpSGR downregulated genes encoding PS proteins and ROS-scavenging enzymes and upregulated those encoding ROS-generation enzymes under heat stress. To account for the possible side-effects resulting from constitutive suppression of LpSGR on plant growth and heat tolerance, we constructed an ethanol-inducible RNAi vector to suppress LpSGR functions. In the absence of ethanol induction, these lines exhibited the same growth and heat tolerance as the wildtype (WT). Upon ethanol induction, the transgenic lines showed compromised heat tolerance and a postharvest stay-green phenotype. Taken together, SGR-mediated Chl a catabolism is required for plant heat tolerance.

Transcriptome Analysis of Leaf Senescence Regulation Under Alkaline Stress in Medicago truncatula

In plants, the leaf is an essential photosynthetic organ, and is the primary harvest in forage crops such as alfalfa (Medicago sativa). Premature leaf senescence caused by environmental stress can result in significant yield loss and quality reduction. Therefore, the stay-green trait is important for improving the economic value of forage crops. Alkaline stress can severely damage leaf cells and, consequently, cause leaf senescence. To understand the molecular regulatory mechanisms and identify vital senescence-associated genes under alkaline stress, we used high-throughput sequencing to study transcriptional changes in Medicago truncatula, a model plant for forage crops. We identified 2,165 differentially expressed genes, 985 of which were identical to those in the dark-induced leaf senescence group. Gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed that the 985 genes were mainly enriched in nutrient cycling processes such as cellular amino acid metabolic processes and organic substance catabolic processes, indicating nutrient redistribution. The other 1,180 differentially expressed genes were significantly enriched in the oxidoreductase complex, aerobic respiration, and ion transport. Our analysis showed the two gene sets guiding the coupled physiological and biochemical alterations play different roles under alkaline stress with a coordinated and integrated way. Many transcription factor families were identified from these differentially expressed genes, including MYB, WRKY, bHLH, and NAC which have particular preference involved in stress resistance and regulation of senescence. Our results contribute to the exploration of the molecular regulatory mechanisms of leaf senescence in M. truncatula under alkaline stress and provide new candidate genes for future breeding to improve the biomass and quality of forage crops.

Identification of a gene responsible for seedpod spine formation and other phenotypic alterations using whole-genome sequencing analysis in Medicago truncatula

In nature, some plant species produce seedpods with spines, which is an adaptive biological trait for protecting the seed and helping seed dispersal. However, the molecular mechanism of spine formation is still unclear. While conducting routine tissue culture and transformation in the model legume Medicago truncatula, we identified a smooth seedpod (ssp1) mutant with a suite of other phenotypic changes. Preliminary analysis showed that the mutation was derived from the tissue culture process. Genetic segregation analysis suggested that ssp1 is a recessive mutant. By combining whole-genome sequencing and bioinformatics analysis, we found that the mutant phenotype was caused by a single nucleotide polymorphism and a 30 bp deletion in the gene locus Medtr4g039430, named SSP1. Complementation of the M. truncatula ssp1 and Arabidopsis twd1 mutants showed complete restoration, indicating that SSP1 is an ortholog of Arabidopsis TWD1 which encodes an immunophilin-like FK506-binding protein 42. The formation of spines on seedpods is associated with auxin transport. The method used in this study offers an effective way for detecting genes responsible for somaclonal variations. The results demonstrate, for the first time, that SSP1 plays a crucial role in the determination of spine formation on seedpods.

Changes in fruit pigment accumulation, chloroplast development, and transcriptome analysis in the CRISPR/Cas9-mediated knockout of Stay-green 1 (slsgr1) mutant

The green-flesh (gf) mutant of the tomato fruit ripen to a muddy brown color and has been demonstrated previously to be a loss-of-function mutant. Here, we provide more evidence to support this view that SlSGR1 is involved in color change in ripening tomato fruits. Knocking out SlSGR1 expression using a clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 genome editing strategy showed obviously a muddy brown color with significantly higher chlorophyll and carotenoid content compared with wild-type (WT) fruits. To further verify the role of SlSGR1 in fruit color change, we performed transcriptome deep sequencing (RNA-seq) analysis, where a total of 354 differentially expressed genes (124/230 downregulated/upregulated) were identified between WT and slsgr1. Additionally, the expression of numerous genes associated with photosynthesis and chloroplast function changed significantly when SlSGR1 was knocked out. Taken together, these results indicate that SlSGR1 is involved in color change in ripening fruit via chlorophyll degradation and carotenoid biosynthesis.

Primary metabolic processes as drivers of leaf ageing

Ageing in plants is a highly coordinated and complex process that starts with the birth of the plant or plant organ and ends with its death. A vivid manifestation of the final stage of leaf ageing is exemplified by the autumn colours of deciduous trees. Over the past decades, technological advances have allowed plant ageing to be studied on a systems biology level, by means of multi-omics approaches. Here, we review some of these studies and argue that these provide strong support for basic metabolic processes as drivers for ageing. In particular, core cellular processes that control the metabolism of chlorophyll, amino acids, sugars, DNA and reactive oxygen species correlate with leaf ageing. However, while multi-omics studies excel at identifying correlative processes and pathways, molecular genetic approaches can provide proof that such processes and pathways control ageing, by means of knock-out and ectopic expression of predicted regulatory genes. Therefore, we also review historic and current molecular evidence to directly test the hypotheses unveiled by the systems biology approaches. We found that the molecular genetic approaches, by and large, confirm the multi-omics-derived hypotheses with notable exceptions, where there is scant evidence that chlorophyll and DNA metabolism are important drivers of leaf ageing. We present a model that summarises the core cellular processes that drive leaf ageing and propose that developmental processes are tightly linked to primary metabolism to inevitably lead to ageing and death.

Genome-Wide Analysis of Glycoside Hydrolase Family 35 Genes and Their Potential Roles in Cell Wall Development in Medicago truncatula

Plant β-galactosidases (BGAL) function in various cell wall biogeneses and modifications, and they belong to the glycoside hydrolase family. However, the roles of BGAL family members in Medicago truncatula cell wall remodeling remain unclear. In this study, a total of 25 MtBGAL members of the glycoside hydrolase gene family 35 were identified, and they were clustered into nine sub-families. Many cis-acting elements possibly related to MeJA and abscisic acid responses were identified in the promoter region of the MtBGAL genes. Transcript analyses showed that these MtBGAL genes exhibited distinct expression patterns in various tissues and developing stem internodes. Furthermore, a stem-specific expression module associated with cell wall metabolic pathways was identified by weighted correlation network analysis (WGCNA). In particular, MtBGAL1 and MtBGAL23 within the stem-specific expression module were highly expressed in mature stems. In addition, several genes involved in lignin, cellulose, hemicellulose and pectin pathways were co-expressed with MtBGAL1 and MtBGAL23. It was also found that MtBGAL1 and MtBGAL23 were localized to the cell wall at the subcellular level, indicating their roles in the modification of cell wall metabolites in Medicago. As a whole, these results will be useful for further functional characterization and utilization of BGAL genes in cell wall modifications aiming to improve the quality of legume forage crops.

VviNAC33 promotes organ de-greening and represses vegetative growth during the vegetative-to-mature phase transition in grapevine

Plants undergo several developmental transitions during their life cycle. In grapevine, a perennial woody fruit crop, the transition from vegetative/green-to-mature/woody growth involves transcriptomic reprogramming orchestrated by a small group of genes encoding regulators, but the underlying molecular mechanisms are not fully understood. We investigated the function of the transcriptional regulator VviNAC33 by generating and characterizing transgenic overexpressing grapevine lines and a chimeric repressor, and by exploring its putative targets through a DNA affinity purification sequencing (DAP-seq) approach combined with transcriptomic data. We demonstrated that VviNAC33 induces leaf de-greening, inhibits organ growth and directly activates the expression of STAY-GREEN PROTEIN 1 (SGR1), which is involved in Chl and photosystem degradation, and AUTOPHAGY 8f (ATG8f), which is involved in the maturation of autophagosomes. Furthermore, we show that VviNAC33 directly inhibits AUXIN EFFLUX FACILITATOR PIN1, RopGEF1 and ATP SYNTHASE GAMMA CHAIN 1T (ATPC1), which are involved in photosystem II integrity and activity. Our results show that VviNAC33 plays a major role in terminating photosynthetic activity and organ growth as part of a regulatory network governing the vegetative-to-mature phase transition.

Leaf senescence: progression, regulation, and application

Leaf senescence, the last stage of leaf development, is a type of postmitotic senescence and is characterized by the functional transition from nutrient assimilation to nutrient remobilization which is essential for plants' fitness. The initiation and progression of leaf senescence are regulated by a variety of internal and external factors such as age, phytohormones, and environmental stresses. Significant breakthroughs in dissecting the molecular mechanisms underpinning leaf senescence have benefited from the identification of senescence-altered mutants through forward genetic screening and functional assessment of hundreds of senescence-associated genes (SAGs) via reverse genetic research in model plant Arabidopsis thaliana as well as in crop plants. Leaf senescence involves highly complex genetic programs that are tightly tuned by multiple layers of regulation, including chromatin and transcription regulation, post-transcriptional, translational and post-translational regulation. Due to the significant impact of leaf senescence on photosynthesis, nutrient remobilization, stress responses, and productivity, much effort has been made in devising strategies based on known senescence regulatory mechanisms to manipulate the initiation and progression of leaf senescence, aiming for higher yield, better quality, or improved horticultural performance in crop plants. This review aims to provide an overview of leaf senescence and discuss recent advances in multi-dimensional regulation of leaf senescence from genetic and molecular network perspectives. We also put forward the key issues that need to be addressed, including the nature of leaf age, functional stay-green trait, coordination between different regulatory pathways, source-sink relationship and nutrient remobilization, as well as translational researches on leaf senescence.

NOL-mediated functional stay-green traits in perennial ryegrass (Lolium perenne L.) involving multifaceted molecular factors and metabolic pathways regulating leaf senescence

Loss of chlorophyll (Chl) is a hallmark of leaf senescence, which may be regulated by Chl catabolic genes, including NON-YELLOW COLORING 1 (NYC1)-like (NOL). The objective of this study was to determine molecular factors and metabolic pathways underlying NOL regulation of leaf senescence in perennial grass species. LpNOL was cloned from perennial ryegrass (Lolium perenne L.) and found to be highly expressed in senescent leaves. Transient overexpression of LpNOL accelerated leaf senescence and Chl b degradation in Nicotiana benthamiana. LpNOL RNA interference (NOLi) in perennial ryegrass not only significantly blocked Chl degradation in senescent leaves, but also delayed initiation and progression of leaf senescence. This study found that NOL, in addition to functioning as a Chl b reductase, could enact the functional stay-green phenotype in perennial grass species, as manifested by increased photosynthetic activities in NOLi plants. Comparative transcriptomic analysis revealed that NOL-mediated functional stay-green in perennial ryegrass was mainly achieved through the modulation of Chl catabolism, light harvesting for photosynthesis, photorespiration, cytochrome respiration, carbohydrate catabolism, oxidative detoxification, and abscisic acid biosynthesis and signaling pathways.

Comparative Transcriptome Analysis of Salt Stress-Induced Leaf Senescence in Medicago truncatula

Leaves are the most critical portion of forage crops such as alfalfa (Medicago sativa). Leaf senescence caused by environmental stresses significantly impacts the biomass and quality of forages. To understand the molecular mechanisms and identify the key regulator of the salt stress-induced leaf senescence process, we conducted a simple and effective salt stress-induced leaf senescence assay in Medicago truncatula, which was followed by RNA-Seq analysis coupled with physiological and biochemical characterization. By comparing the observed expression data with that derived from dark-induced leaf senescence at different time points, we identified 3,001, 3,787, and 4,419 senescence-associated genes (SAGs) for salt stress-induced leaf senescence on day 2, 4, and 6, respectively. There were 1546 SAGs shared by dark and salt stress treatment across the three time points. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses showed that the 1546 SAGs were mainly related to protein and amino acids metabolism, photosynthesis, chlorophyll metabolism, and hormone signaling during leaf senescence. Strikingly, many different transcription factors (TFs) families out of the 1546 SAGs, including NAC, bHLH, MYB, and ERF, were associated with salt stress-induced leaf senescence processes. Using the transient expression system in Nicotiana benthamiana, we verified that three functional NAC TF genes from the 1546 SAGs were related to leaf senescence. These results clarify SAGs under salt stress in M. truncatula and provide new insights and additional genetic resources for further forage crop breeding.

SlSGRL, a tomato SGR-like protein, promotes chlorophyll degradation downstream of the ABA signaling pathway

Chlorophyll (chl) degradation plays a vital role during green plant growth and development, including nutrient metabolism, fruit and seed maturation, and phototoxic detoxification. STAY-GREEN (SGR) is a plant-specific regulator involved in chl degradation. Previous studies showed that SlSGR1 functioned in chl degradation and lycopene accumulation during fruit ripening of tomato (Solanum lycopersicum). However, little is known about SlSGR-LIKE (SlSGRL) gene, which is a homolog of SlSGR1. We cloned the SlSGRL gene and created transgenic tomato plants overexpressing (OE) SlSGRL. Expression analysis showed that SlSGRL was up-regulated by abscisic acid (ABA). Our data showed that SlSGRL-OE lines exhibited earlier leaf yellowing than wild-type (WT) lines under ABA treatment. Yeast two-hybrid (Y2H) assay revealed that SlSGRL interacted with pheophytin pheophorbide hydrolase (SlPPH) and light-harvesting complex a2 (SlLHCa2) to promote the chl degradation. Further analysis demonstrated that ABA-INSENSITIVE5 (SlABI5) and SlABI5-LIKE regulated SlSGRL expression by directly binding to the sequence (-611 to -582) of the SlSGRL promoter that included an ABRE cis-element. We proposed that SlSGRL, which was regulated by SlABI5/SlABI5-LIKE, mainly acted in ABA-induced chl degradation via interacting with SlPPH and SlLHCa2.

Genetic and Physical Localization of the Gene Controlling Leaf Pigmentation Pattern in Medicago truncatula

In Medicago truncatula, some ecotypes form a black or purple stain in the middle of adaxial leaf surface due to accumulation of anthocyanins. However, this morphological marker is missing in some other ecotypes, although anthocyanin biosynthesis pathway is not disrupted. Genetic analysis indicated that the lack of the leaf spot of anthocyanins accumulation is a dominant trait, which is controlled by a single gene, LPP1. Genetic mapping indicated that the LPP1 gene was delimited to a 280 kb-region on Chromosome 7. A total of 8 protein-coding genes were identified in the LPP1 locus through gene annotation and sequence analysis. Of those, two genes, putatively encoding MYB-transcriptional suppressors, were selected as candidates for functional validation.

Roles of stay-green (SGR) homologs during chlorophyll degradation in green plants

Chlorophyll (Chl) degradation is one of the most obvious signs of leaf senescence and fruit ripening. Stay-green (SGR) homologs that can remove magnesium from Chl a are the most important components in Chl degradation pathway in green plants. SGR homologs are not only universally involved in Chl breakdown during the senescence of green organs, but also play crucial roles in other organs during plant growth and development, such as fruit mature and nodule development. In this review, we focus on the diverse functions of SGR homologs in plant growth and development. A better understanding of SGR would be helpful for providing a theoretical basis for further illustrating the regulatory mechanism of SGR homologs.

Development of a Highly Efficient Multiplex Genome Editing System in Outcrossing Tetraploid Alfalfa (Medicago sativa)

Alfalfa (Medicago sativa) is an outcrossing tetraploid legume species widely cultivated in the world. The clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (CRISPR/Cas9) system has been successfully used for genome editing in many plant species. However, the use of CRISPR/Cas9 for gene knockout in alfalfa is still very challenging. Our initial single gRNA-CRISPR/Cas9 system had very low mutagenesis efficiency in alfalfa with no mutant phenotype. In order to develop an optimized genome editing system in alfalfa, we constructed multiplex gRNA-CRISPR/Cas9 vectors by a polycistronic tRNA-gRNA approach targeting the Medicago sativa stay-green (MsSGR) gene. The replacement of CaMV35S promoter by the Arabidopsis ubiquitin promoter (AtUBQ10) to drive Cas9 expression in the multiplex gRNA system led to a significant improvement in genome editing efficiency, whereas modification of the gRNA scaffold resulted in lower editing efficiency. The most effective multiplex system exhibited 75% genotypic mutagenesis efficiency, which is 30-fold more efficient than the single gRNA vector. Importantly, phenotypic change was easily observed in the mutants, and the phenotypic mutation efficiency reached 68%. This highly efficient multiplex gRNA-CRISPR/Cas9 genome editing system allowed the generation of homozygous mutants with a complete knockout of the four allelic copies in the T0 generation. This optimized system offers an effective way of testing gene functions and overcomes a major barrier in the utilization of genome editing for alfalfa improvement.

Biotechnological Perspectives of Omics and Genetic Engineering Methods in Alfalfa

For several decades, researchers are working to develop improved major crops with better adaptability and tolerance to environmental stresses. Forage legumes have been widely spread in the world due to their great ecological and economic values. Abiotic and biotic stresses are main factors limiting legume production, however, alfalfa (Medicago sativa L.) shows relatively high level of tolerance to drought and salt stress. Efforts focused on alfalfa improvements have led to the release of cultivars with new traits of agronomic importance such as high yield, better stress tolerance or forage quality. Alfalfa has very high nutritional value due to its efficient symbiotic association with nitrogen-fixing bacteria, while deep root system can help to prevent soil water loss in dry lands. The use of modern biotechnology tools is challenging in alfalfa since full genome, unlike to its close relative barrel medic (Medicago truncatula Gaertn.), was not released yet. Identification, isolation, and improvement of genes involved in abiotic or biotic stress response significantly contributed to the progress of our understanding how crop plants cope with these environmental challenges. In this review, we provide an overview of the progress that has been made in high-throughput sequencing, characterization of genes for abiotic or biotic stress tolerance, gene editing, as well as proteomic and metabolomics techniques bearing biotechnological potential for alfalfa improvement.

Genome-Wide Identification, Expression Analysis and Functional Study of CCT Gene Family in Medicago truncatula

The control of flowering time has an important impact on biomass and the environmental adaption of legumes. The CCT (CO, COL and TOC1) gene family was elucidated to participate in the molecular regulation of flowering in plants. We identified 36 CCT genes in the M. truncatula genome and they were classified into three distinct subfamilies, PRR (7), COL (11) and CMF (18). Synteny and phylogenetic analyses revealed that CCT genes occurred before the differentiation of monocot and dicot, and CCT orthologous genes might have diversified among plants. The diverse spatial-temporal expression profiles indicated that MtCCT genes could be key regulators in flowering time, as well as in the development of seeds and nodules in M. truncatula. Notably, 22 MtCCT genes with typical circadian rhythmic variations suggested their different responses to light. The response to various hormones of MtCCT genes demonstrated that they participate in plant growth and development via varied hormones dependent pathways. Moreover, six MtCCT genes were dramatically induced by salinity and dehydration treatments, illustrating their vital roles in the prevention of abiotic injury. Collectively, our study provides valuable information for the in-depth investigation of the molecular mechanism of flowering time in M. truncatula, and it also provides candidate genes for alfalfa molecular breeding with ideal flowering time.

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

Since 1999, various forward- and reverse-genetic approaches have uncovered nearly 200 genes required for symbiotic nitrogen fixation (SNF) in legumes. These discoveries advanced our understanding of the evolution of SNF in plants and its relationship to other beneficial endosymbioses, signaling between plants and microbes, the control of microbial infection of plant cells, the control of plant cell division leading to nodule development, autoregulation of nodulation, intracellular accommodation of bacteria, nodule oxygen homeostasis, the control of bacteroid differentiation, metabolism and transport supporting symbiosis, and the control of nodule senescence. This review catalogs and contextualizes all of the plant genes currently known to be required for SNF in two model legume species, Medicago truncatula and Lotus japonicus, and two crop species, Glycine max (soybean) and Phaseolus vulgaris (common bean). We also briefly consider the future of SNF genetics in the era of pan-genomics and genome editing.

Stay-Green Trait: A Prospective Approach for Yield Potential, and Drought and Heat Stress Adaptation in Globally Important Cereals

The yield losses in cereal crops because of abiotic stress and the expected huge losses from climate change indicate our urgent need for useful traits to achieve food security. The stay-green (SG) is a secondary trait that enables crop plants to maintain their green leaves and photosynthesis capacity for a longer time after anthesis, especially under drought and heat stress conditions. Thus, SG plants have longer grain-filling period and subsequently higher yield than non-SG. SG trait was recognized as a superior characteristic for commercially bred cereal selection to overcome the current yield stagnation in alliance with yield adaptability and stability. Breeding for functional SG has contributed in improving crop yields, particularly when it is combined with other useful traits. Thus, elucidating the molecular and physiological mechanisms associated with SG trait is maybe the key to defeating the stagnation in productivity associated with adaptation to environmental stress. This review discusses the recent advances in SG as a crucial trait for genetic improvement of the five major cereal crops, sorghum, wheat, rice, maize, and barley with particular emphasis on the physiological consequences of SG trait. Finally, we provided perspectives on future directions for SG research that addresses present and future global challenges.

Functional Dissection of the Chickpea (Cicer arietinum L.) Stay-Green Phenotype Associated with Molecular Variation at an Ortholog of Mendel's I Gene for Cotyledon Color: Implications for Crop Production and Carotenoid Biofortification

"Stay-green" crop phenotypes have been shown to impact drought tolerance and nutritional content of several crops. We aimed to genetically describe and functionally dissect the particular stay-green phenomenon found in chickpeas with a green cotyledon color of mature dry seed and investigate its potential use for improvement of chickpea environmental adaptations and nutritional value. We examined 40 stay-green accessions and a set of 29 BC2F4-5 stay-green introgression lines using a stay-green donor parent ICC 16340 and two Indian elite cultivars (KAK2, JGK1) as recurrent parents. Genetic studies of segregating populations indicated that the green cotyledon trait is controlled by a single recessive gene that is invariantly associated with the delayed degreening (extended chlorophyll retention). We found that the chickpea ortholog of Mendel's I locus of garden pea, encoding a SGR protein as very likely to underlie the persistently green cotyledon color phenotype of chickpea. Further sequence characterization of this chickpea ortholog CaStGR1 (CaStGR1, for carietinum stay-green gene 1) revealed the presence of five different molecular variants (alleles), each of which is likely a loss-of-function of the chickpea protein (CaStGR1) involved in chlorophyll catabolism. We tested the wild type and green cotyledon lines for components of adaptations to dry environments and traits linked to agronomic performance in different experimental systems and different levels of water availability. We found that the plant processes linked to disrupted CaStGR1 gene did not functionality affect transpiration efficiency or water usage. Photosynthetic pigments in grains, including provitaminogenic carotenoids important for human nutrition, were 2-3-fold higher in the stay-green type. Agronomic performance did not appear to be correlated with the presence/absence of the stay-green allele. We conclude that allelic variation in chickpea CaStGR1 does not compromise traits linked to environmental adaptation and agronomic performance, and is a promising genetic technology for biofortification of provitaminogenic carotenoids in chickpea.

Transforming compound leaf patterning by manipulating REVOLUTA in Medicago truncatula

Leaves are derived from the shoot apical meristem with three distinct axes: dorsoventral, proximodistal and mediolateral. Different regulators are involved in the establishment of leaf polarity. Members of the class III homeodomain-leucine zipper (HD-ZIPIII) gene family are critical players in the determination of leaf adaxial identity mediated by microRNA165/166. However, their roles in compound leaf development are still unclear. By screening of a retrotransposon-tagged mutant population of the model legume plant Medicago truncatula, a mutant line with altered leaflet numbers was isolated and characterized. Mutant leaves partially lost their adaxial identity. Leaflet numbers in the mutant were increased along the proximodistal axis, showing pinnate pentafoliate leaves in most cases, in contrast to the trifoliate leaves of the wild type. Detailed characterization revealed that a lesion in a HD-ZIPIII gene, REVOLUTA (MtREV1), resulted in the defects of the mutant. Overexpression of MtMIR166-insensitive MtREV1 led to adaxialized leaves and ectopic leaflets along the dorsoventral axis. Accompanying the abnormal leaf patterning, the free auxin content was affected. Our results demonstrate that MtREV1 plays a key role in determination of leaf adaxial-abaxial polarity and compound leaf patterning, which is associated with proper auxin homeostasis.

The C-terminal cysteine-rich motif of NYE1/SGR1 is indispensable for its function in chlorophyll degradation in Arabidopsis

The C-terminal cysteine-rich motif of NYE1/SGR1 affects chlorophyll degradation likely by mediating its self-interaction and conformational change, and somehow altering its Mg-dechelating activity in response to the changing redox potential. During green organ senescence in plants, the most prominent phenomenon is the degreening caused by net chlorophyll (Chl) loss. NON-YELLOWING1/STAY-GREEN1 (NYE1/SGR1) was recently reported to be able to dechelates magnesium (Mg) from Chl a to initiate its degradation, but little is known about the domain/motif basis of its functionality. In this study, we carried out a protein truncation assay and identified a conserved cysteine-rich motif (CRM, P-X3-C-X3-C-X-C2-F-P-X5-P) at its C terminus, which is essential for its function. Genetic analysis showed that all four cysteines in the CRM were irreplaceable, and enzymatic assays demonstrated that the mutation of each of the four cysteines affected its Mg-dechelating activity. The CRM plays a critical role in the conformational change and self-interaction of NYE1 via the formation of inter- and intra-molecular disulfide bonds. Our results may provide insight into how NYE1 responds to rapid redox changes during leaf senescence and in response to various environmental stresses.

A Novel Stay-Green Mutant of Rice with Delayed Leaf Senescence and Better Harvest Index Confers Drought Tolerance

Three Ethyl methansulphonate (EMS)-induced stay-green mutants (SGM-1, SGM-2 and SGM-3) and their wild-type (WT), were tested for their Stay-Green (SG) and drought tolerance nature as the relation between these two attributes is not yet established in rice. In the dark induced senescence assay, SGM-3 showed delayed senescence while SGM-1 and SGM-2 showed complete lack of senescence. Mutants showed stable transcript abundance over time, for 15 candidate genes (CGs) associated with senescence, compared to the WT. SGM-3 however showed moderately increasing transcript abundance over time for ATG6a, ATG4a, NYC1, NOL and NYC3. Only SGM-3 performed better than the WT for yield and harvest index under well irrigated as well as drought conditions, though all the mutants showed better performance for other agronomic traits under both the conditions and ascorbate peroxidase activity under drought. Thus, SG trait showed positive correlation with drought tolerance though only SGM-3 could convert this into higher harvest index. Sequence analysis of 80 senescence-associated genes including the 15 CGs showed non-synonymous mutations in four and six genes in SGM-1 and SGM-2 respectively, while no SNPs were found in SGM-3. Analysis of the earlier reported Quantitative Trait Loci (QTL) regions in SGM-3 revealed negligible variations from WT, suggesting it to be a novel SG mutant.

Fine mapping and molecular characterization of the virescent gene vsp in Upland cotton (Gossypium hirsutum)

The vsp gene was fine mapped to a 353.7-kb region, and a 201-bp deletion that affected chloroplast development and chlorophyll biosynthesis was found in the candidate gene GhPUR4. Virescent mutations can be used as marker traits in heterosis breeding and can also be used to research chloroplast development, chlorophyll biosynthesis and photosynthesis mechanisms. Here, we obtained a light-sensitive virescent mutant, vsp, that has reduced chlorophyll (Chl) content and abnormal chloroplast development. Then, the virescent space (vsp) gene in the vsp mutant was preliminarily mapped to a 38.32-Mb region of chromosome D04 using a high-density SNP genetic map with a total length of 5384.33 cM and 4472 bin markers. Furthermore, the vsp gene was narrowed down to a 353.7-kb region that contains 15 candidate genes using 484 virescent individuals from an F₂ population. Sequence analysis of genes in this region showed that a 201-bp deletion was present in the Gh_D04G1108 (GhPUR4) gene in the vsp mutant. The 201-bp deletion of Gh_D04G1108 caused the deletion of 67 AAs in the GhPUR4 protein. Virus-induced gene silencing (VIGS) of GhPUR4 in normal plants caused reduced GhPUR4 gene expression levels, reduced Chl content, abnormal chloroplast development and virescent true leaves. This study could help us unravel the function of GhPUR4 in chloroplast development and Chl biosynthesis at the early developmental stages of the true leaves in cotton, which could promote the research and application of virescent mutations in cotton heterosis breeding.

Characterization of Soybean STAY-GREEN Genes in Susceptibility to Foliar Chlorosis of Sudden Death Syndrome

Genetic mappings for soybean sudden death syndrome foliar chlorosis suggested that STAY-GREEN genes with loss-of-susceptibility mechanism may have different breeding merits for disease resistance.

The H3K27me3 demethylase REF6 promotes leaf senescence through directly activating major senescence regulatory and functional genes in Arabidopsis

The roles of histone demethylation in the regulation of plant flowering, disease resistance, rhythmical response, and seed germination have been elucidated recently; however, how histone demethylation affects leaf senescence remains largely unclear. In this study, we exploited yeast one-hybrid (Y1H) to screen for the upstream regulators of NONYELLOWING1 (NYE1), and identified RELATIVE OF EARLY FLOWERING6 (REF6), a histone H3 lysine 27 tri-methylation (H3K27me3) demethylase, as a putative binding protein of NYE1 promoter. By in vivo and in vitro analyses, we demonstrated that REF6 directly binds to the motif CTCGYTY in NYE1/2 promoters through its zinc finger domain and positively regulates their expression. Loss-of-function of REF6 delayed chlorophyll (Chl) degradation, whereas overexpression of REF6 accelerated Chl degradation. Subsequently, we revealed that REF6 positively regulates the general senescence process by directly up-regulating ETHYLENE INSENSITIVE 2 (EIN2), ORESARA1 (ORE1), NAC-LIKE, ACTIVATED BY AP3/PI (NAP), PYRUVATE ORTHOPHOSPHATE DIKINASE (PPDK), PHYTOALEXIN DEFICIENT 4 (PAD4), LIPOXYGENASE 1 (LOX1), NAC DOMAIN CONTAINING PROTEIN 3 (AtNAC3), and NAC TRANSCRIPTION FACTOR-LIKE 9 (NTL9), the key regulatory and functional genes predominantly involved in the regulation of developmental leaf senescence. Importantly, loss-of-function of REF6 increased H3K27me3 levels at all the target Senescence associated genes (SAGs). We therefore conclusively demonstrate that H3K27me3 methylation represents an epigenetic mechanism prohibiting the premature transcriptional activation of key developmentally up-regulated senescence regulatory as well as functional genes in Arabidopsis.

Leaves of higher plants start yellowing and subsequently die (senescence) at particular developmental stages as a result of both internal and external regulations. Leaf senescence is evolved to facilitate nutrient remobilization to young/important organs to meet their rapid development, and a large number of genes (Senescence associated genes, SAGs) are activated to regulate/facilitate the process. It has been intriguing how these genes are kept transcriptionally inactive to ensure an effective photosynthesis before the initiation of leaf senescence. Here, we reveal an epigenetic mechanism responsible for the prohibition of their premature transcription. We found that an H3K27me3 demethylase, RELATIVE OF EARLY FLOWERING 6 (REF6), directly promotes the expression of its ten target senescence regulatory and functional genes (EIN2, ORE1, NAP, AtNAC3, NTL9, NYE1/2, LOX1, PAD4, and PPDK), which are involved in major phytohormones’ signaling, biosynthesis, and chlorophyll degradation. Crucially, REF6 is substantially involved in promoting the H3K27me3 demethylation of both their promoter and/or coding regions during the aging process of leaves. We therefore provide conclusive evidence that H3K27me3 methylation is an epigenetic mechanism hindering the premature transcriptional activation of key SAGs, which helps to explain the “aging effect” of senescence initiation.

Mg-dechelatase is involved in the formation of photosystem II but not in chlorophyll degradation in Chlamydomonas reinhardtii

The STAY-GREEN (SGR) gene encodes Mg-dechelatase which catalyzes the conversion of chlorophyll (Chl) a to pheophytin (Pheo) a. This reaction is the first and most important regulatory step in the Chl degradation pathway. Conversely, Pheo a is an indispensable molecule in photosystem (PS) II, suggesting the involvement of SGR in the formation of PSII. To investigate the physiological functions of SGR, we isolated Chlamydomonas sgr mutants by screening an insertion-mutant library. The sgr mutants had reduced maximum quantum efficiency of PSII (F_v /F_m ) and reduced Pheo a levels. These phenotypes were complemented by the introduction of the Chlamydomonas SGR gene. Blue Native polyacrylamide gel electrophoresis and immunoblotting analysis showed that although PSII levels were reduced in the sgr mutants, PSI and light-harvesting Chl a/b complex levels were unaffected. Under nitrogen starvation conditions, Chl degradation proceeded in the sgr mutants as in the wild type, indicating that ChlamydomonasSGR is not required for Chl degradation and primarily contributes to the formation of PSII. In contrast, in the Arabidopsis sgr triple mutant (sgr1 sgr2 sgrL), which completely lacks SGR activity, PSII was synthesized normally. These results suggest that the Arabidopsis SGR participates in Chl degradation while the ChlamydomonasSGR participates in PSII formation despite having the same catalytic property.

STAYGREEN, STAY HEALTHY: a loss-of-susceptibility mutation in the STAYGREEN gene provides durable, broad-spectrum disease resistances for over 50 years of US cucumber production

The Gy14 cucumber (Cucumis sativus) is resistant to oomyceteous downy mildew (DM), bacterial angular leaf spot (ALS) and fungal anthracnose (AR) pathogens, but the underlying molecular mechanisms are unknown. Quantitative trait locus (QTL) mapping for the disease resistances in Gy14 and further map-based cloning identified a candidate gene for the resistant loci, which was validated and functionally characterized by spatial-temporal gene expression profiling, allelic diversity and phylogenetic analysis, as well as local association studies. We showed that the triple-disease resistances in Gy14 were controlled by the cucumber STAYGREEN (CsSGR) gene. A single nucleotide polymorphism (SNP) in the coding region resulted in a nonsynonymous amino acid substitution in the CsSGR protein, and thus disease resistance. Genes in the chlorophyll degradation pathway showed differential expression between resistant and susceptible lines in response to pathogen inoculation. The causal SNP was significantly associated with disease resistances in natural and breeding populations. The resistance allele has undergone selection in cucumber breeding. The durable, broad-spectrum disease resistance is caused by a loss-of-susceptibility mutation of CsSGR. Probably, this is achieved through the inhibition of reactive oxygen species over-accumulation and phytotoxic catabolite over-buildup in the chlorophyll degradation pathway. The CsSGR-mediated host resistance represents a novel function of this highly conserved gene in plants.

Knockdown of STAYGREEN in Perennial Ryegrass (Lolium perenne L.) Leads to Transcriptomic Alterations Related to Suppressed Leaf Senescence and Improved Forage Quality

Chl breakdown is a hallmark of leaf senescence. Protein degradation is tightly associated with accelerated Chl catabolism during leaf senescence. Therefore, blocking or reducing Chl breakdown and thereby improving Chl and leaf protein contents is desirable for agronomic improvement in perennial forage grasses. Perennial ryegrass (Lolium perenne L.) is one principle cool-season forage grass in temperate areas throughout the world. In this study, the perennial ryegrass STAY-GREEN gene (LpSGR) was cloned and characterized. LpSGR was highly expressed in developmentally or dark-induced senescent leaves. LpSGR was subcellularly localized in chloroplast and interacted with the other Chl catabolic enzymes. RNA interference (RNAi) of LpSGR in perennial ryegrass blocked the degradation of Chl, resulting in increased Chl content and photochemical efficiency in senescent leaves. The RNAi transgenic plants had significantly improved forage quality, with up to 46.1% increased protein content in the harvested biomass. Transcriptome comparison revealed that suppression of LpSGR led to multiple alterations in metabolic pathways in locations inside the chloroplast. Most transcription factors of senescence-associated hormonal signaling pathways (e.g. ABA, ethylene and jasmonic acid) had decreased expression levels in the RNAi plants. These results provided a foundation for the further study on the regulatory mechanism of LpSGR in perennial ryegrass for the purpose of forage improvement with delayed leaf senescence and higher forage quality.

Molecular improvement of alfalfa for enhanced productivity and adaptability in a changing environment

Due to an expanding world population and increased buying power, the demand for ruminant products such as meat and milk is expected to grow substantially in coming years, and high levels of forage crop production will therefore be a necessity. Unfortunately, urbanization of agricultural land, intensive agricultural practices, and climate change are all predicted to limit crop production in the future, which means that the development of forage cultivars with improved productivity and adaptability will be essential. Because alfalfa (Medicago sativa L.) is one of the most widely cultivated perennial forage crops, it has been the target of much research in this field. In this review, we discuss progress that has been made towards the improvement of productivity, abiotic stress tolerance, and nutrient-use efficiency, as well as disease and pest resistance, in alfalfa using biotechnological techniques. Furthermore, we consider possible future priorities and avenues for attaining further enhancements in this crop as a means of contributing to the realization of food security in a changing environment.

Tomato (Solanum lycopersicum) SlIPT4, encoding an isopentenyltransferase, is involved in leaf senescence and lycopene biosynthesis during fruit ripening

BACKGROUND

Lycopene is an important carotenoid pigment in red fruits and vegetables, especially in tomato. Although lycopene biosynthesis and catabolism have been found to be regulated by multiple factors including phytohormones, little is known about their regulatory mechanism. Cytokinins are crucial to various aspects of plant growth. Isopentenyltransferases (IPTs) catalyze the initial rate-limiting step of cytokinins biosynthesis, however, their roles in fruit ripening remain unclear.

RESULTS

Here, the functions of SlIPT4, encoding an isopentenyltransferase, were characterized via RNAi-mediated gene silencing in tomato. As we expected, silencing of SlIPT4 expression resulted in accelerated leaf senescence. However, down-expression of SlIPT4 generated never-red orange fruits, corresponding with a dramatic reduction of lycopene. Among lycopene biosynthesis-related genes, the fact of remarkable decrease of ZISO transcript and upregulation of other genes, revealed that SlIPT4 regulates positively lycopene biosynthesis via directly affecting ZISO expression, and also supported the existence of regulatory loops in lycopene biosynthesis pathway. Meanwhile, the accumulation of abscisic acid (ABA) was reduced and the transcripts PSY1 were increased in SlIPT4-RNAi fruits, supporting the feedback regulation between ABA and lycopene biosynthesis.

CONCLUSION

The study revealed the crucial roles of SlIPT4 in leaf senescence and the regulatory network of lycopene biosynthesis in tomato, providing a new light on the lycopene biosynthesis and fruit ripening.

Senescence and nitrogen use efficiency in perennial grasses for forage and biofuel production

Organ senescence is an important developmental process in plants that enables recycling of nutrients, such as nitrogen, to maximize reproductive success. Nitrogen is the mineral nutrient required in greatest amount by plants, although soil-N limits plant productivity in many natural and agricultural systems, especially systems that receive little or no fertilizer-N. Use of industrial N-fertilizers in agriculture increased crop yields several fold over the past century, although at substantial cost to fossil energy reserves and the environment. Therefore, it is important to optimize nitrogen use efficiency (NUE) in agricultural systems. Organ senescence contributes to NUE in plants and manipulation of senescence in plant breeding programs is a promising approach to improve NUE in agriculture. Much of what we know about plant senescence comes from research on annual plants, which provide most of the food for humans. Relatively little work has been done on senescence in perennial plants, especially perennial grasses, which provide much of the forage for grazing animals and promise to supply much of the biomass required by the future biofuel industry. Here, we review briefly what is known about senescence from studies of annual plants, before presenting current knowledge about senescence in perennial grasses and its relationship to yield, quality, and NUE. While higher yield is a common target, desired N-content diverges between forage and biofuel crops. We discuss how senescence programs might be altered to produce high-yielding, stress-tolerant perennial grasses with high-N (protein) for forage or low-N for biofuels in systems optimized for NUE.

Functional Genomics in the Study of Metabolic Pathways in Medicago truncatula: An Overview

In addition to its value as a model system for studies on symbiotic nitrogen fixation, Medicago truncatula has recently become an organism of choice for dissection of complex pathways of secondary metabolism. This work has been driven by two main reasons, both with practical implications. First Medicago species possess a wide range of flavonoid and terpenoid natural products, many of which, for example, the isoflavonoids and triterpene saponins, have important biological activities impacting both plant and animal (including human) health. Second, M. truncatula serves as an excellent model for alfalfa, the world's major forage legume, and forage quality is determined in large part by the concentrations of products of secondary metabolism, particularly lignin and condensed tannins. We here review recent progress in understanding the pathways leading to flavonoids, lignin, and triterpene saponins through utilization of genetic resources in M. truncatula.

Large-Scale Investigation of Soybean Gene Functions by Overexpressing a Full-Length Soybean cDNA Library in Arabidopsis

Molecular breeding has become an important approach for crop improvement, and a prerequisite for molecular breeding is elucidation of the functions of genetic loci or genes. Soybean is one of the most important food and oil crops worldwide. However, due to the difficulty of genetic transformation in soybean, studies of its functional genomics lag far behind those of other crops such as rice, which severely impairs the progress of molecular improvement in soybean. Here, we describe an effective large-scale strategy to investigate the functions of soybean genes via overexpression of a full-length soybean cDNA library in Arabidopsis. The overexpression vector pJL12 was modified for use in the construction of a normalized full-length cDNA library. The constructed cDNA library showed good quality; repetitive clones represented approximately 4%, insertion fragments were approximately 2.2 kb, and the full-length rate was approximately 98%. This cDNA library was then overexpressed in Arabidopsis, and approximately 2000 transgenic lines were preliminarily obtained. Phenotypic analyses of the positive T₁ transgenic plants showed that more than 5% of the T₁ transgenic lines displayed abnormal developmental phenotypes, and approximately 1% of the transgenic lines exhibited potentially favorable traits. We randomly amplified 4 genes with obvious phenotypes (enlarged seeds, yellowish leaves, more branches, and dense siliques) and repeated the transgenic analyses in Arabidopsis. Subsequent phenotypic observation demonstrated that these phenotypes were indeed due to the overexpression of soybean genes. We believe our strategy represents an effective large-scale approach to investigate the functions of soybean genes and further reveal genes favorable for molecular improvement in soybean.