Leaf Senescence In Plants Research Articles

Improved Wheat Growth and Yield by Delayed Leaf Senescence Using Developmentally Regulated Expression of a Cytokinin Biosynthesis Gene.

Delaying leaf senescence in plants, especially under water stress conditions, can help to maintain the remobilization of stored nutrients in source–sink relationships, thus leading to improved crop yields. Leaf senescence can be delayed by plant hormones such as cytokinin. Here, the Isopentenyl transferase (IPT) gene, encoding a cytokinin biosynthesis enzyme, driven by a modified AtMYB32xs promoter was transformed into wheat. Transgenic wheat plants exhibited delayed leaf senescence, retaining chlorophyll for longer under controlled environment conditions. Selected independent transgenic events and their corresponding nulls were grown under field conditions for two consecutive years under well-watered and water stress treatments using automated rainout shelters. Three independent transgenic events had improved canopy green cover, lower canopy temperatures, and higher leaf water potential than their respective non-transgenic nulls, with no abnormality in morphology and phenology. Increased grain yield was observed in transgenic events under both water treatments, with the yield increase more pronounced under water stress (26–42%). These results have shown that delayed leaf senescence using the chimeric transgene AtMYB32xs-p::IPT can be a useful strategy to achieve grain yield gains in wheat and potentially other crops for sustainable food production.

Ameliorative roles of melatonin and/or zeolite on chromium-induced leaf senescence in marjoram plants by activating antioxidant defense, osmolyte accumulation, and ultrastructural modification

Growing medicinal herbs on chromium-rich soils can pose severe threats to humans, animals, plants, and ecosystems. Leaf senescence is a typical sign of chromium contamination and may be regulated by foliar application (melatonin) or soil amendment (zeolite) treatments. The favorable roles of melatonin and/or zeolite in the alleviation of chromium-induced senescence processes related to growth, some biochemical and ultrastructural attributes, and essential oil yield in marjoram (Origanum marjorana L.) plants were studied.Marjoram biomass, photosynthetic pigment concentration, nutrient percentage, water status, ascorbic acid level, essential oil yield, and catalase activity declined significantly under chromium-pollution. On the other hand, organic and antioxidant solute levels, superoxide dismutase and guiacol peroxidase activity, oxidized biomolecule levels, as well as chromium concentrations were increased in the herb under chromium contamination. Melatonin and/or zeolite application under normal or chromium-polluted conditions may compensate to some extent growth retardation, essential oil yield, and metabolic disturbance processes. The greatest herbal essential oil yield was obtained in the case of the application of melatonin plus zeolite under normal conditions. Under chromium pollution, cell organelles exhibited a noticeable malformation and lysis, but, melatonin plus zeolite application counteracts the malformation observed with chromium-pollution.The current investigation postulated that either soil amendment with 5 g zeolite kg−1 soil and/or foliar application with 100 μM melatonin have found a useful role to mitigate chromium-induced senescence processes in marjoram herb. Their ameliorating effects may be connected with the regulation of reactive oxygen species scavenging systems, stimulation of osmotic adjustment and preservation of plant cell ultrastructure. Melatonin and/or zeolite can make chromium-rich soils appropriate for the cultivation and production of high-quality herbs by minimizing chromium bioavailability and toxicity to plants.

BrTCP7 Transcription Factor Is Associated with MeJA-Promoted Leaf Senescence by Activating the Expression of BrOPR3 and BrRCCR.

The plant hormone jasmonic acid (JA) has been recognized as an important promoter of leaf senescence in plants. However, upstream transcription factors (TFs) that control JA biosynthesis during JA-promoted leaf senescence remain unknown. In this study, we report the possible involvement of a TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) TF BrTCP7 in methyl jasmonate (MeJA)-promoted leaf senescence in Chinese flowering cabbage. Exogenous MeJA treatment reduced maximum quantum yield (Fv/Fm) and total chlorophyll content, accompanied by the increased expression of senescence marker and chlorophyll catabolic genes, and accelerated leaf senescence. To further understand the transcriptional regulation of MeJA-promoted leaf senescence, a class I member of TCP TFs BrTCP7 was examined. BrTCP7 is a nuclear protein and possesses trans-activation ability through subcellular localization and transcriptional activity assays. A higher level of BrTCP7 transcript was detected in senescing leaves, and its expression was up-regulated by MeJA. The electrophoretic mobility shift assay and transient expression assay showed that BrTCP7 binds to the promoter regions of a JA biosynthetic gene BrOPR3 encoding OPDA reductase3 (OPR3) and a chlorophyll catabolic gene BrRCCR encoding red chlorophyll catabolite reductase (RCCR), activating their transcriptions. Taken together, these findings reveal that BrTCP7 is associated with MeJA-promoted leaf senescence at least partly by activating JA biosynthesis and chlorophyll catabolism, thus expanding our knowledge of the transcriptional mechanism of JA-mediated leaf senescence.

Activation of the Transcription of BrGA20ox3 by a BrTCP21 Transcription Factor Is Associated with Gibberellin-Delayed Leaf Senescence in Chinese Flowering Cabbage during Storage.

Several lines of evidence have implicated the involvement of the phytohormone gibberellin (GA) in modulating leaf senescence in plants. However, upstream transcription factors (TFs) that regulate GA biosynthesis in association with GA-mediated leaf senescence remain elusive. In the current study, we report the possible involvement of a TEOSINTE BRANCHED1/CYCLOIDEA/PCF (TCP) TF BrTCP21 in GA-delayed leaf senescence in Chinese flowering cabbage. Exogenous GA3 treatment maintained a higher value of maximum PSII quantum yield (Fv/Fm) and total chlorophyll content, accompanied by the repression of the expression of senescence-associated genes and chlorophyll catabolic genes, which led to the delay of leaf senescence. A class I member of TCP TFs BrTCP21, was further isolated and characterized. The transcript level of BrTCP21 was low in senescing leaves, and decreased following leaf senescence, while GA3 could keep a higher expression level of BrTCP21. BrTCP21 was further found to be a nuclear protein and exhibit trans-activation ability through transient-expression analysis in tobacco leaves. Intriguingly, the electrophoretic mobility shift assay (EMSA) and transient expression assay illustrated that BrTCP21 bound to the promoter region of a GA biosynthetic gene BrGA20ox3, and activated its transcription. Collectively, these observations reveal that BrTCP21 is associated with GA-delayed leaf senescence, at least partly through the activation of the GA biosynthetic pathway. These findings expand our knowledge on the transcriptional mechanism of GA-mediated leaf senescence.

A 3-bp deletion of WLS5 gene leads to weak growth and early leaf senescence in rice

BackgroundIn rice (Oryza sativa) and other grains, weak growth (dwarfism, short panicle length, and low seed-setting rate) and early senescence lead to reduced yield. The molecular mechanisms behind these processes have been widely studied; however, the complex genetic regulatory networks controlling growth and senescence require further elucidation.ResultsWe isolated a mutant exhibiting weak growth throughout development and early senescence of leaf tips, and designated this mutant weakness and leaf senescence5 (wls5). Histological analysis showed that the poor growth of wls5 plants involved a reduction in cell length and number. Physiological analysis and transmission electron microscopy revealed that the wls5 cells had abnormal chloroplasts, and the mutants underwent chlorophyll degradation triggered by accumulation of reactive oxygen species. Consistent with this, RNA sequencing revealed changes in senescence-related gene expression in wls5 plants. The wls5 mutants also exhibited significantly higher stomatal density and altered phytohormone contents compared with wild-type plants. Fine mapping delimited WLS5 to a 29-kb region on chromosome 5. DNA sequencing of wls5 identified a 3-bp deletion in the first exon of LOC_Os05g04900, resulting in a deletion of a lysine in the predicted protein. Knockout of LOC_Os05g04900 in Nipponbare plants caused leaf senescence, confirming this locus as the causal gene for WLS5.ConclusionsWe identified a novel mutant (wls5) that affects plant development and leaf senescence in rice. LOC_Os05g04900, encoding a protein of unknown function, is the causal gene for wls5. Further molecular study of WLS5 will uncover the roles of this gene in plant growth and leaf senescence.

Tomato fruit ripening factor NOR controls leaf senescence.

NAC transcription factors (TFs) are important regulators of expressional reprogramming during plant development, stress responses, and leaf senescence. NAC TFs also play important roles in fruit ripening. In tomato (Solanum lycopersicum), one of the best characterized NACs involved in fruit ripening is NON-RIPENING (NOR), and the non-ripening (nor) mutation has been widely used to extend fruit shelf life in elite varieties. Here, we show that NOR additionally controls leaf senescence. Expression of NOR increases with leaf age, and developmental as well as dark-induced senescence are delayed in the nor mutant, while overexpression of NOR promotes leaf senescence. Genes associated with chlorophyll degradation as well as senescence-associated genes (SAGs) show reduced and elevated expression, respectively, in nor mutants and NOR overexpressors. Overexpression of NOR also stimulates leaf senescence in Arabidopsis thaliana. In tomato, NOR supports senescence by directly and positively regulating the expression of several senescence-associated genes including, besides others, SlSAG15 and SlSAG113, SlSGR1, and SlYLS4. Finally, we find that another senescence control NAC TF, namely SlNAP2, acts upstream of NOR to regulate its expression. Our data support a model whereby NAC TFs have often been recruited by higher plants for both the control of leaf senescence and fruit ripening.

Melatonin delays leaf senescence of Chinese flowering cabbage by suppressing ABFs-mediated abscisic acid biosynthesis and chlorophyll degradation.

Melatonin and abscisic acid (ABA) play contrasting roles in regulating leaf senescence in plants. The molecular mechanism underlying the interaction between melatonin and ABA involved in leaf senescence, however, remains poorly defined. Herein, we found that exogenous application of melatonin delayed the senescence of Chinese flowering cabbage, accompanied by reduced expression of chlorophyll catabolic and ABA biosynthetic genes, and a lower endogenous ABA level. Significantly, three nucleus-localized transcriptional activators BrABF1, BrABF4, and BrABI5 were identified, and their expressions were repressed by melatonin. In vitro and in vivo binding experiments revealed that BrABF1, BrABF4, and BrABI5 activated the transcription of a series of ABA biosynthetic and chlorophyll catabolic genes by physically binding to their promoters. Moreover, transient over-expression of BrABF1, BrABF4, and BrABI5 in tobacco leaves induced ABA accumulation and promoted chlorophyll degradation by upregulating tobacco ABA biosynthetic and chlorophyll catabolic genes, resulting in the accelerated leaf senescence. These effects were significantly attenuated by melatonin treatment. Our findings suggest that melatonin-mediated inhibition of leaf senescence involves suppression of ABFs-mediated ABA biosynthesis and chlorophyll degradation. Unraveling of the molecular regulatory mechanism of leaf senescence controlled by ABA and melatonin expands our understanding of the regulation of this phenomenon and offers potentially more effective molecular breeding strategies for extending the shelf-life of Chinese flowering cabbage.

Senescence and Defense Pathways Contribute to Heterosis.

Hybrids are used extensively in agriculture due to their superior performance in seed yield and plant growth, yet the molecular mechanisms underpinning hybrid performance are not well understood. Recent evidence has suggested that a decrease in basal defense response gene expression regulated by reduced levels of salicylic acid (SA) may be important for vigor in certain hybrid combinations. Decreasing levels of SA in the Arabidopsis (Arabidopsis thaliana) accession C24 through the introduction of the SA catabolic enzyme salicylate1 hydroxylase (NahG) increases plant size, phenocopying the large-sized C24/Landsberg erecta (Ler) F1 hybrids. C24♀ × Ler♂ F1 hybrids and C24 NahG lines shared differentially expressed genes and pathways associated with plant defense and leaf senescence including decreased expression of SA biosynthetic genes and SA response genes. The expression of TL1 BINDING TRANSCRIPTION FACTOR1, a key regulator in resource allocation between growth and defense, was decreased in both the F1 hybrid and the C24 NahG lines, which may promote growth. Both C24 NahG lines and the F1 hybrids showed decreased expression of the key senescence-associated transcription factors WRKY53, NAC-CONTAINING PROTEIN29, and ORESARA1 with a delayed onset of senescence compared to C24 plants. The delay in senescence resulted in an extension of the photosynthetic period in the leaves of F1 hybrids compared to the parental lines, potentially allowing each leaf to contribute more resources toward growth.

Abiotic Stresses Intervene with ABA Signaling to Induce Destructive Metabolic Pathways Leading to Death: Premature Leaf Senescence in Plants.

Abiotic stresses trigger premature leaf senescence by affecting some endogenous factors, which is an important limitation for plant growth and grain yield. Among these endogenous factors that regulate leaf senescence, abscisic acid (ABA) works as a link between the oxidase damage of cellular structure and signal molecules responding to abiotic stress during leaf senescence. Considering the importance of ABA, we collect the latest findings related to ABA biosynthesis, ABA signaling, and its inhibitory effect on chloroplast structure destruction, chlorophyll (Chl) degradation, and photosynthesis reduction. Post-translational changes in leaf senescence end with the exhaustion of nutrients, yellowing of leaves, and death of senescent tissues. In this article, we review the literature on the ABA-inducing leaf senescence mechanism in rice and Arabidopsis starting from ABA synthesis, transport, signaling receptors, and catabolism. We also predict the future outcomes of investigations related to other plants. Before changes in translation occur, ABA signaling that mediates the expression of NYC, bZIP, and WRKY transcription factors (TFs) has been investigated to explain the inducing effect on senescence-associated genes. Various factors related to calcium signaling, reactive oxygen species (ROS) production, and protein degradation are elaborated, and research gaps and potential prospects are presented. Examples of gene mutation conferring the delay or induction of leaf senescence are also described, and they may be helpful in understanding the inhibitory effect of abiotic stresses and effective measures to tolerate, minimize, or resist their inducing effect on leaf senescence.

Overexpression of salt-induced protein (salT) delays leaf senescence in rice.

Senescence, a highly programmed process, largely determines yield and quality of crops. However, knowledge about the onset and progression of leaf senescence in crop plants is still limited. Here, we report that salt-induced protein (salT), a new gene, may be involved in leaf senescence. Overexpressing salT could prolong the duration of leaves with higher concentrations of chlorophyll compared with the wild type. Moreover, overexpression of salT could delay the senescence of rice leaves though the inhibition of senescence associated genes (SAGs). Overall, the characterization of salT suggested that it is a new gene affecting the leaf senescence induced by natural and dark conditions.

Sedoheptulose-1,7-Bisphosphatase is Involved in Methyl Jasmonate- and Dark-Induced Leaf Senescence in Tomato Plants.

Leaf senescence represents the final stage of leaf development and is regulated by diverse internal and environmental factors. Jasmonates (JAs) have been demonstrated to induce leaf senescence in several species; however, the mechanisms of JA-induced leaf senescence remain largely unknown in tomato plants (Solanum lycopersicum). In the present study, we tested the hypothesis that sedoheptulose-1,7-bisphosphatase (SBPase), an enzyme functioning in the photosynthetic carbon fixation in the Calvin–Benson cycle, was involved in methyl jasmonate (MeJA)- and dark-induced leaf senescence in tomato plants. We found that MeJA and dark induced senescence in detached tomato leaves and concomitantly downregulated the expression of SlSBPASE and reduced SBPase activity. Furthermore, CRISPR/Cas9 (clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9)-mediated mutagenesis of SlSBPASE led to senescence-associated characteristics in slsbpase mutant plants, including loss of chlorophyll, repressed photosynthesis, increased membrane ion leakage, and enhanced transcript abundance of senescence-associated genes. Collectively, our data suggest that repression of SBPase by MeJA and dark treatment plays a role in JA- and dark-induced leaf senescence.

Exogenous brassinosteroid application delays senescence and promotes hyponasty in Carica papaya L. leaves

The application of brassinosteroids (BR) is associated with increased tolerance to various stresses such as those induced by pathogens, temperature, salinity and drought. Here, we studied the influence of BRs analogues on the leaf senescence of papaya juvenile plants. The objective was to determine whether BRs affect plant growth and leaf senescence by promoting or inhibiting the degradation of chlorophyll and/or leaf abscission. Two-month-old plants were divided into five treatments: C (control without BR application); BRWC (BR applied in the whole canopy); BRL1 (BR applied in the youngest fully expanded leaf, L1); BRL2 (BR applied in L2 which is the secondly more mature leaf in relation to L1) and BRL3 (BR applied in L3 which is the third more mature leaf in relation to L2). We concluded that BRs do not interfere with the senescence of the youngest leaf (L1) but in older leaves (L3) BRWC maintains the leaves greener and attached to the plant for a longer time when compared to the leaves from the other treatments. BR application in whole-canopy increased the plant height, and it caused a delay in leaf senescence and, consequently, in leaf abscission. In addition, hyponastic leaves developed after BR-application in the whole-canopy. The senescence process and leaf abscission in papaya, stimulated by exogenous BRs, were directly related to leaf age.

Circadian control of ORE1 by PRR9 positively regulates leaf senescence in Arabidopsis

The circadian clock coordinates the daily cyclic rhythm of numerous biological processes by regulating a large portion of the transcriptome. In animals, the circadian clock is involved in aging and senescence, and circadian disruption by mutations in clock genes frequently accelerates aging. Conversely, aging alters circadian rhythmicity, which causes age-associated physiological alterations. However, interactions between the circadian clock and aging have been rarely studied in plants. Here, we investigated potential roles for the circadian clock in the regulation of leaf senescence in plants. Members of the evening complex in Arabidopsis circadian clock, EARLY FLOWERING 3 (ELF3), EARLY FLOWERING 4 (ELF4), and LUX ARRHYTHMO (LUX), as well as the morning component PSEUDO-RESPONSE REGULATOR 9 (PRR9), affect both age-dependent and dark-induced leaf senescence. The circadian clock regulates the expression of several senescence-related transcription factors. In particular, PRR9 binds directly to the promoter of the positive aging regulator ORESARA1 (ORE1) gene to promote its expression. PRR9 also represses miR164, a posttranscriptional repressor of ORE1 Consistently, genetic analysis revealed that delayed leaf senescence of a prr9 mutant was rescued by ORE1 overexpression. Thus, PRR9, a core circadian component, is a key regulator of leaf senescence via positive regulation of ORE1 through a feed-forward pathway involving posttranscriptional regulation by miR164 and direct transcriptional regulation. Our results indicate that, in plants, the circadian clock and leaf senescence are intimately interwoven as are the clock and aging in animals.

Research Progress of Leaf Senescence Related Genes in Tobacco

Tobacco (Nicotiana tabacum L.) is an important model plant that has been widely used in plant biological research. Tobacco is also an economic crop in which mature yellow leaves are harvested. Leaf maturity and the yellowing process have a decisive influence on the quality of tobacco products. The mechanism, influencing factors and regulating genes of plant leaf senescence was briefly summarized in this review.The research progress in genes related to tobacco leaf senescence and the effects of exogenous genes on the senescence of tobacco was also summarized. It is of great importance to study tobacco leaf senescence process and related genes in order to improve the quality of tobacco.

The Direct Involvement of Dark-Induced Tic55 Protein in Chlorophyll Catabolism and Its Indirect Role in the MYB108-NAC Signaling Pathway during Leaf Senescence in Arabidopsis thaliana.

The chloroplast relies on proteins encoded in the nucleus, synthesized in the cytosol and subsequently transported into chloroplast through the protein complexes Toc and Tic (Translocon at the outer/inner membrane of chloroplasts). A Tic complex member, Tic55, contains a redox-related motif essential for protein import into chloroplasts in peas. However, Tic55 is not crucial for protein import in Arabidopsis. Here, a tic55-II-knockout mutant of Arabidopsis thaliana was characterized for Tic55 localization, its relationship with other translocon proteins, and its association with plant leaf senescence when compared to the wild type. Individually darkened leaves (IDLs) obtained through dark-induced leaf senescence were used to demonstrate chlorophyll breakdown and its relationship with plant senescence in the tic55-II-knockout mutant. The IDLs of the tic55-II-knockout mutant contained higher chlorophyll concentrations than those of the wild type. Our microarray analysis of IDLs during leaf senescence identified seven senescence-associated genes (SAGs) that were downregulated in the tic55-II-knockout mutant: ASP3, APG7, DIN2, DIN11, SAG12, SAG13, and YLS9. Real-time quantitative PCR confirmed the reliability of microarray analysis by showing the same expression patterns with those of the microarray data. Thus, Tic55 functions in dark-induced aging in A. thaliana by indirectly regulating downstream SAGs expression. In addition, the expression of four NAC genes, including ANAC003, ANAC010, ANAC042, and ANAC075 of IDL treated tic55-II-knockout mutant appeared to be downregulated. Yeast one hybrid assay revealed that only ANAC003 promoter region can be bound by MYB108, suggesting that a MYB-NAC regulatory network is involved in dark-stressed senescence.

O3-Induced Leaf Senescence in Tomato Plants Is Ethylene Signaling-Dependent and Enhances the Population Abundance of Bemisia tabaci.

Elevated ozone (O3) can alter the phenotypes of host plants particularly in induction of leaf senescence, but few reports examine the involvement of phytohormone in O3-induced changes in host phenotypes that influence the foraging quality for insects. Here, we used an ethylene (ET) receptor mutant Nr and its wild-type to determine the function of the ET signaling pathway in O3-induced leaf senescence, and bottom-up effects on the performance of Bemisia tabaci in field open-top chambers (OTCs). Our results showed that elevated O3 reduced photosynthetic efficiency and chlorophyll content and induced leaf senescence of plant regardless of plant genotype. Leaf senescence in Nr plants was alleviated relative to wild-type under elevated O3. Further analyses of foliar quality showed that elevated O3 had little effect on phytohormone-mediated defenses, but significantly increased the concentration of amino acids in two plant genotypes. Furthermore, Nr plants had lower amino acid content relative to wild-type under elevated O3. These results provided an explanation of O3-induced increase in abundance of B. tabaci. We concluded that O3-induced senescence of plant was ET signal-dependent, and positive effects of O3-induced leaf senescence on the performance of B. tabaci largely resulted from changes of nutritional quality of host plants.

Climate, plant organs and species control dissolved nitrogen and phosphorus in fresh litter in a subalpine forest on the eastern Tibetan Plateau

Fresh litter contains a higher concentration of dissolved phosphorus (DP) than dissolved nitrogen (DN), which implies a more efficient DN transformation or reabsorption in the subalpine forest on the eastern Tibetan Plateau. Both DN and DP concentrations increased with the increase of mean monthly temperature, although the concentrations were also regulated by plant organs and species. The dissolved nitrogen (DN) and dissolved phosphorus (DP) released from fresh litter are important pathways by which total nitrogen and phosphorus are transferred from the vegetation to soil in forest ecosystems. However, few studies have paid attention to the DN and DP in fresh litter, which affects our understanding of the nitrogen and phosphorus cycles. The objectives of this study were to elucidate the dynamic characteristics of the concentrations and storage of DN and DP, and to analyze how DN and DP are affected by different plant species and organs, and climate factors. Fresh litter was collected in three plots in a spruce-fir forest and classified by different plant species and organs. Concentration and storage of DN and DP in fresh litter were determined and related to the climatic variables that were monthly recorded. The concentration of DP was higher than that of DN in fresh litter, and the concentrations of both elements were determined by plant organs and species. Moreover, The DN and DP concentration was positively related to mean monthly temperature, while DN and DP storage was negatively correlated with mean monthly temperature and monthly precipitation. The storage of DN and DP was determined by litter biomass, which the order in litter from different plant organs was leaves>twigs>miscellaneous>flowers and fruits. The storage of DN and DP in leaves showed two peaks in April and October, but that in twigs and the miscellaneous showed only one peak in October. Our results indicated that dissolved nitrogen (DN) is transferred and reabsorbed more than dissolved phosphorus (DP) before plant leaf senescence and other organs fall. Furthermore, DN and DP were associated with climate, plant organs and species in a subalpine forest on the eastern Tibetan Plateau.

Landsat time series-based multiyear spectral angle clustering (MSAC) model to monitor the inter-annual leaf senescence of exotic saltcedar

In the western United States, the rapid expansion of exotic saltcedar along riparian corridors has drastically altered landscape structures and ecosystem functions. Monitoring the geographical distribution and spatio-temporal dynamics of this invasive species is essentially critical to conduct the systematic restoration of affected riparian ecosystems. Previous studies indicated that the leaf senescence stage is the optimal time window to remotely monitor saltcedar distributions. Yet due to climate variability and anthropogenic forcing, the timing of saltcedar leaf senescence varies over space and time. Given that the saltcedar leaf senescence stage only lasts for a short temporal window (i.e., three or four weeks), pinpointing the appropriate Landsat image across years and locations without the expert knowledge is challenging. Remotely sensed phenological time series analysis provides a practical means to locate the leaf senescence date on a per-pixel basis. However, affected by temporal revisit rates and cloud contamination, Landsat time series can only capture very limited temporal segments of vegetation phenology. The lack of Landsat imagery throughout the year makes the conventional time series analysis difficult. In this study, we developed a multiyear spectral angle clustering (MSAC) model to monitor the inter-annual leaf senescence of saltcedar with limited Landsat imagery on a per-pixel basis. The MSAC model leverages the Landsat images across years to temporally predict the fall phenology of plant species in a single year, and constructs the synthesized time series of spectral signatures to estimate critical phenological transition dates. Results indicated that the MSAC model could guide the construction of the composite Landsat surface reflectance image to accommodate spatial and inter-annual variation in the timing of plant leaf senescence. The phenology-guided composite image from the MSAC model achieved a greater saltcedar mapping accuracy than any single Landsat image in 2004. The proposed MSAC model provides new insights in estimating phenological transition dates of plant species with limited Landsat imagery. It opens up unique opportunities to guide the selection of representative remotely sensed imagery on a per-pixel basis for repetitive saltcedar mapping over wide geographical regions.

CIRCADIAN CLOCK-ASSOCIATED 1 Inhibits Leaf Senescence in Arabidopsis.

Leaf senescence is an integral part of plant development, and the timing and progressing rate of senescence could substantially affect the yield and quality of crops. It has been known that a circadian rhythm synchronized with external environmental cues is critical for the optimal coordination of various physiological and metabolic processes. However, the reciprocal interactions between the circadian clock and leaf senescence in plants remain unknown. Here, through measuring the physiological and molecular senescence related markers of several circadian components mutants, we found that CIRCADIAN CLOCK-ASSOCIATED 1 inhibits leaf senescence. Further molecular and genetic studies revealed that CCA1 directly activates GLK2 and suppresses ORE1 expression to counteract leaf senescence. As plants age, the expression and periodic amplitude of CCA1 declines and thus weakens the inhibition of senescence. Our findings reveal an age-dependent circadian clock component of the process of leaf senescence.

Identification of the group IIa WRKY subfamily and the functional analysis of GhWRKY17 in upland cotton (Gossypium hirsutum L.).

WRKY transcription factors play important roles in plant defense, stress response, leaf senescence, and plant growth and development. Previous studies have revealed the important roles of the group IIa GhWRKY genes in cotton. To comprehensively analyze the group IIa GhWRKY genes in upland cotton, we identified 15 candidate group IIa GhWRKY genes in the Gossypium hirsutum genome. The phylogenetic tree, intron-exon structure, motif prediction and Ka/Ks analyses indicated that most group IIa GhWRKY genes shared high similarity and conservation and underwent purifying selection during evolution. In addition, we detected the expression patterns of several group IIa GhWRKY genes in individual tissues as well as during leaf senescence using public RNA sequencing data and real-time quantitative PCR. To better understand the functions of group IIa GhWRKYs in cotton, GhWRKY17 (KF669857) was isolated from upland cotton, and its sequence alignment, promoter cis-acting elements and subcellular localization were characterized. Moreover, the over-expression of GhWRKY17 in Arabidopsis up-regulated the senescence-associated genes AtWRKY53, AtSAG12 and AtSAG13, enhancing the plant’s susceptibility to leaf senescence. These findings lay the foundation for further analysis and study of the functions of WRKY genes in cotton.