Increased Leaf Longevity Research Articles

Interplay among ZF-HD and GRF transcription factors during Arabidopsis leaf development.

The growth-regulating factor (GRF) family of transcriptional factors are involved in the control of leaf size and senescence, inflorescence and root growth, grain size, and plant regeneration. However, there is limited information about the genes regulated by these transcriptional factors, which are in turn responsible for their functions. Using a meta-analysis approach, we identified genes encoding Arabidopsis (Arabidopsis thaliana) zinc-finger homeodomain (ZF-HD) transcriptional factors, as potential targets of the GRFs. We further showed that GRF3 binds to the promoter of one of the members of the ZF-HD family, HOMEOBOX PROTEIN 33 (HB33), and activates its transcription. Increased levels of HB33 led to different modifications in leaf cell number and size that were dependent on its expression levels. Furthermore, we found that expression of HB33 for an extended period during leaf development increased leaf longevity. To cope with the functional redundancy among ZF-HD family members, we generated a dominant repressor version of HB33, HB33-SRDX. Expression of HB33-SRDX from HB33 regulatory regions was seedling-lethal, revealing the importance of the ZF-HD family in plant development. Misexpression of HB33-SRDX in early leaf development caused a reduction in both cell size and number. Interestingly, the loss-of-function of HB33 in lines carrying a GRF3 allele insensitive to miR396 reverted the delay in leaf senescence characteristic of these plants. Our results revealed functions for ZF-HDs in leaf development and linked them to the GRF pathway.

Multi-omics approach reveals the contribution of KLU to leaf longevity and drought tolerance.

KLU, encoded by a cytochrome P450 CYP78A family gene, generates an important—albeit unknown—mobile signal that is distinct from the classical phytohormones. Multiple lines of evidence suggest that KLU/KLU-dependent signaling functions in several vital developmental programs, including leaf initiation, leaf/floral organ growth, and megasporocyte cell fate. However, the interactions between KLU/KLU-dependent signaling and the other classical phytohormones, as well as how KLU influences plant physiological responses, remain poorly understood. Here, we applied in-depth, multi-omics analysis to monitor transcriptome and metabolome dynamics in klu-mutant and KLU-overexpressing Arabidopsis plants. By integrating transcriptome sequencing data and primary metabolite profiling alongside phytohormone measurements, our results showed that cytokinin signaling, with its well-established function in delaying leaf senescence, was activated in KLU-overexpressing plants. Consistently, KLU-overexpressing plants exhibited significantly delayed leaf senescence and increased leaf longevity, whereas the klu-mutant plants showed early leaf senescence. In addition, proline biosynthesis and catabolism were enhanced following KLU overexpression owing to increased expression of genes associated with proline metabolism. Furthermore, KLU-overexpressing plants showed enhanced drought-stress tolerance and reduced water loss. Collectively, our work illustrates a role for KLU in positively regulating leaf longevity and drought tolerance by synergistically activating cytokinin signaling and promoting proline metabolism. These data promote KLU as a potential ideal genetic target to improve plant fitness.

Arabidopsis WRKY45 Interacts with the DELLA Protein RGL1 to Positively Regulate Age-Triggered Leaf Senescence

Leaf senescence can be triggered and promoted by various environmental stressors, developmental cues, and endogenous hormone signals. Several lines of evidence have suggested the involvement of WRKY transcription factors in regulating leaf senescence, but the underlying mechanisms and signaling pathways involved remain elusive. In this study, we identified Arabidopsis thaliana WRKY DNA-binding protein 45 (WRKY45) as a positive regulator of age-triggered leaf senescence. Loss of WRKY45 function resulted in increased leaf longevity in age-triggered senescence, whereas overexpression of WRKY45 significantly accelerated age-triggered leaf senescence. Consistently, expression of SENESCENCE-ASSOCIATED GENEs (SAGs) was significantly reduced in wrky45 mutants but markedly enhanced in transgenic plants overexpressing WRKY45. Chromatin immunoprecipitation assays revealed that WRKY45 directly binds the promoters of several SAGs such as SAG12, SAG13, SAG113, and SEN4. Both in vivo and in vitro biochemical analyses demonstrated that WRKY45 interacts with the DELLA protein RGA-LIKE1 (RGL1), a repressor of the gibberellin (GA) signaling pathway. We found that RGL1 repressed the transcription activation function of WRKY45, thereby attenuating the expression of its regulon. Consistent with this finding, overexpression of RGL1 resulted in significantly increased leaf longevity in age-triggered senescence. Taken together, our results provide compelling evidence that WRKY45 functions as a critical component of the GA-mediated signaling pathway to positively regulate age-triggered leaf senescence.

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown. Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China. Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production. These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.

HDA6 is required for jasmonate response, senescence and flowering in Arabidopsis

Post-translational modifications of histones, including acetylation, play a key role in modulating dynamic changes in chromatin structure and gene activity. Histone acetylation is modulated through the action of histone acetyltransferases and deacetylases. HDA6 is a RPD3-type histone deacetylase in Arabidopsis. The Arabidopsis HDA6 mutant, axe1-5, and HDA6 RNA-interfering (HDA6-RNAi) plants displayed higher levels of acetylated H3 compared with wild-type, suggesting that HDA6 affects histone acetylation levels globally. The expression of the jasmonate responsive genes, PDF1.2, VSP2, JIN1, and ERF1, was down-regulated in axe1-5 and HDA6-RNAi plants. Furthermore, axe1-5 and HDA6-RNAi plants displayed increased leaf longevity compared with the wild type. The expression of the senescence-associated genes, SAG12 and SEN4, was down-regulated in the axe1-5 and HDA6-RNAi plants. In addition, axe1-5 and HDA6-RNAi plants displayed late-flowering. The expression of FLC was up-regulated and hyperacetylated in axe1-5 and HDA6-RNAi plants, suggesting that HDA6 is required to deacetylate FLC chromatin and thereby repress its expression. Our results suggest that HDA6 is involved in jasmonate response, senescence, and flowering in Arabidopsis.

Rice tillering dwarf mutant dwarf3 has increased leaf longevity during darkness-induced senescence or hydrogen peroxide-induced cell death

Senescence or cell death in plant leaves is known to be inducible by darkness or H(2)O(2). When the Arabidopsis gene MAX2/ORE9 is disrupted, leaf senescence or cell death in response to the above stimuli is delayed. Because the rice (Oryza sativa L.) gene DWARF3 (D3) is orthologous to MAX2/ORE9, we wished to know whether disruption of D3 also results in increased longevity in leaves. We found that darkness-induced senescence or H(2)O(2)-induced cell death in the third leaf [as measured by chlorophyll degradation, membrane ion leakage and expression of senescence-associated genes (SAGs)] in a d3 rice mutant was delayed by 1-3 d compared to that in its reference line Shiokari. Moreover, the mRNA levels of D3, HTD1 and D10, which are orthologs of Arabidopsis MAX2/ORE9, MAX3 and MAX4, respectively, increased during cell death. These results suggest that D3 protein in rice, like MAX2/ORE9 in Arabidopsis, is involved in leaf senescence or cell death.

Ecophysiological determinants of plant performance under flooding: a comparative study of seven plant families

Summary Plant performance of species in river floodplains is negatively affected by submergence, due to severely hampered gas exchange under water. Several individual traits have been shown to determine flooding tolerance, but the interrelationships among these traits and their effects on plant performance still remain largely unknown. Here, we aim for a more coherent understanding of submergence tolerance, by investigating whether different traits are alternative strategies to enhance survival under water or whether these traits act in concert. Because responses to submergence may be taxon‐specific, we chose a multiple‐species approach that allowed phylogenetic comparisons. The phenotypic traits investigated in the current study were constitutively different in species originating from frequently flooded habitats than in species from dry habitats: wet species were taller and had a higher specific leaf area (SLA), chlorophyll content, aerenchyma content, and longer longevity of terrestrial and ‘aquatic’ leaves compared with species from dry habitats. Moreover, the frequently flooded species appeared to be more plastic for these traits in response to submergence. Neither the phylogenetic component nor the light climate under water were important for the expression of the submergence‐related traits. Principal component analysis (PCA) showed that these traits were divided in two independent clusters structured either around the petiole (i.e. aerenchyma content and shoot length) or the leaf lamina (i.e. SLA, chlorophyll content and leaf longevity). Shoot length and aerenchyma content of the petiole were also positively correlated, albeit not significantly. A positive correlation between SLA and leaf longevity was observed under water, indicating that thinner leaves may have an increased potential for gas exchange, resulting in increased leaf longevity and plant survival. Path analyses indicated significant positive effects of the two trait clusters on plant performance during full submergence. Our study shows that multiple ecophysiological traits act in concert to fine tune responses to dynamic and unpredictable environments such as river floodplains.

Physiological mechanisms contributing to the increased N stress tolerance of tropical maize selected for drought tolerance

An improved response of crop varieties to various stress factors may be associated to constitutive stress tolerance mechanisms that increase yield and yield stability. Increased leaf longevity, increased water and nutrient uptake, greater assimilate supply during grain filling, and increased grain and ear set have been associated with constitutive stress tolerance mechanisms in maize ( Zea mays L.). We examined tropical maize for adaptive changes associated with drought tolerance that are sustained under N stress and therefore may indicate constitutive stress tolerance mechanisms. Original and drought-tolerant selection cycles of four populations were evaluated in five experiments differing in N supply at Poza Rica, Mexico between 1992 and 1994. Selection for tolerance to mid-season drought stress consistently increased grain yield across N levels due to an increase in both the number of ears per plant and kernel weight. The number of ears per plant was associated with a shorter anthesis–silking interval (ASI) of drought-tolerant cycles. Reduction in ASI due to selection was greater under N stress as compared to well-fertilized conditions, however, it was not associated with either biomass or N accumulation of plants and ears around flowering. The N content of individual kernels did not change with selection and grain N concentration decreased. Greater kernel weights were likely the result of delayed leaf senescence and increased assimilate supply during grain filling. We conclude that decreased ear abortion and increased assimilate supply during grain filling of maize selected for tolerance to mid-season drought also provide tolerance to N stress and therefore may contribute to increased yield and yield stability.

Yield Improvement in Temperate Maize is Attributable to Greater Stress Tolerance

ABSTRACTA retrospective analysis of the physiological basis of genetic yield improvement may provide an understanding of yield potential and may indicate avenues for future yield improvement. Rate of yield improvement of maize (Zea mays L.) hybrids in Ontario, Canada has been ≈1.5% yr−1 during the last five decades. Comparison of short‐season hybrids representing yield improvement from the late 1950s to the late 1980s showed that genetic yield improvement was 2.5% per year and that most of the genetic yield improvement could be attributed to increased stress tolerance. Differences in stress tolerance between older and more recent hybrids have been shown for high plant population density, weed interference, low night temperatures during the grain‐filling period, low soil moisture, low soil N, and a number of herbicides. Yield improvement is the result of more efficient capture and use of resources, and the improved efficiency in resource capture and use of newer hybrids is frequently only evident under stress. Improved resource capture has resulted from increased interception of seasonal incident radiation and greater uptake of nutrients and water. The improved resource capture is associated with increased leaf longevity, a more active root system, and a higher ratio of assimilate supply by the leaf canopy (source) and assimilate demand by the grain (sink) during the grain‐filling period. Improvements of resource use under optimum conditions have been small, as leaf photosynthesis, leaf‐angle distribution of the canopy, grain chemical composition, and the proportion of dry matter allocated to the grain at maturity (i.e., harvest index) have remained virtually constant. Genetic improvement of maize has been accompanied by a decrease in plant‐to‐plant variability. Results of our studies indicate that increased stress tolerance is associated with lower plant‐to‐plant variability and that increased plant‐to‐plant variability results in lower stress tolerance.

Source : sink ratio and leaf senescence in maize: : I. Dry matter accumulation and partitioning during grain filling

Whether the stay-green characteristic of recent maize hybrids is related to a larger source : sink ratio and/or a more efficient use of N during reproductive development was studied in a 3-year field experiment. The experiment was conducted with two short-season maize hybrids, Pride 5 (old) and Pioneer 3902 (recent), grown at two soil-N levels and with source : sink treatments during grain filling ranging from partial defoliation to no grain. Results confirmed that greater dry matter accumulation of Pioneer 3902, relative to Pride 5, was associated with greater leaf longevity. Change in stover weight from silking to maturity an indicator of the difference in supply and demand of assimilates during grain filling, varied from −30% with defoliation to +25% for the no sink treatment. The change was always greater in the new hybrid, indicating that the old hybrid was more source limited. Number of green leaves, an indicator of leaf longevity, was greatest when supply and demand of assimilates during grain filling were approximately equal. Leaf longevity was enhanced by an increase in soil N and the effect was larger in the high source : sink treatments, but differences in leaf longevity between the two hybrids were not influenced by soil-N level. In conclusion, increased leaf longevity of a new relative to an old hybrid was associated with a larger source : sink ratio during grain filling.

Effects of water stress on the leaf demography of Agropyron desertorum, Agropyron dasystachyum, Bromus inermis, and Stipa viridula

We examined the effect of water stress on leaf demography in four grass species known to differ in degree of drought resistance. Stress was imposed by growing plants in solution culture and lowering water potential of the solution either slowly, or rapidly, by the addition of polyethylene glycol. Rapid stress reduced the number of leaves by increasing leaf death rate and decreasing birth rate. Gradual stress reduced the number of leaves by temporarily decreasing leaf birth rate; death rate also decreased, partially compensating for the reduction in birth rate. Death rates were age specific; rapid stress had little effect on young leaves, and gradual stress increased leaf longevity to the greatest extent in those leaves that emerged immediately prior to the imposition of stress. As a result, plants subjected to rapid stress had a young leaf age structure, while plants subjected to gradual stress had an older age structure. Differences among species in tolerance to water stress were related in part to differences in leaf age structure. Species with a young leaf age structure exhibited less of an increase in leaf mortality in response to rapid stress, and more of an increase in leaf longevity in response to gradual stress than other species. Key words: drought tolerance, leaf turnover, leaf age structure, polyethylene glycol, leaf senescence.

Photosynthetic Carbon Production and Use by Developing Cotton Leaves and Bolls

Understanding the photosynthetic C contribution of leaves to vegetative and reproductive processes is important in defining yield productivity. However, the morphological complexity of the cotton (Gossypium hirsutum L.) canopy has proven a significant barrier to establishing specific yield relationships. Therefore, a 2‐yr study examined the developmental sequences of C production and utilization by leaves and bolls within the crop canopy. Photosynthesis of sympodial leaves reached a maximum just prior to anthesis of the subtended flower and thereafter declined throughout the boll‐filling period. Carbon budgets for individual sympodial leaves and their subtended bolls at mainstem Nodes 8, 10, and 12 indicated that C production was seldom synchronized with C utilization. Carbon import requirements for the first three fruiting positions at mainstem Node 10 were 50,37, and 21%, respectively. Bolls at mainstem Node 8 collectively required >60% import of C to sustain growth during the season. Only at mainstem Node 12 were leaves capable of supplying the total C needs of their subtended bolls. Carbon deficiencies at mainstem Nodes 8 and 10 suggested that substantial translocation of photosynthate from adjacent leaves and leaves outside the mainstem node was necessary. This was confirmed through short‐term studies of l4C‐assimilate movement from leaves to developing cotton bolls. Breeding goals for improved C production by leaves were simulated to evaluate avenues for increased yields through crop genetics. These simulations indicated an advantage of increased leaf longevity in the maintenance of C production.

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation.

The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.

Modular and Demographic Analysis of Plant Leaf Area in Sward and Woodland Populations of Dactylis Glomerata and Bromus Erectus

SUMMARY (1) Dactylis glomerata and Bromus erectus, two perennial grasses, were studied in two contrasted habitats: an open sward where D. glomerata is more frequent, and an adjacent Quercus pubescens woodland where B. erectus is more frequent. (2) A modular demographic analysis of the growth of above-ground plant parts was used as a non-destructive method to follow the seasonal evolution of leaf photosynthetic area and its age structure. (3) The four populations had similar seasonal changes in total photosynthetic area per plant, with two production peaks occurring in autumn and spring. The increase in tiller and leaf numbers occurred before the peaks of total leaf area per plant which resulted from leaf elongation. As the mean temperature of the air decreased, the rate of leaf emergence decreased but the longevity of the leaves increased, resulting in an increased proportion of older age-classes of photosynthetic area in winter. A similar shift towards older age-classes occurred during the summer drought. (4) The tree canopy affected the water status of the plants, as measured by leaf water potential, leading to increased leaf longevity and leaf length during the summer in the woodland compared to the sward. Except during summer, plants of each species had a similar total photosynthetic area in both habitats, but with a different combination of shoot and leaf components: in the woodland, individuals had fewer tillers but a higher photosynthetic area per tiller than in the sward. (5) B. erectus had a larger leaf area per plant than D. glomerata in both habitats from a higher number of tillers per plant, an increased number of leaves per tiller and a larger leaf size. Leaf longevity was longer in B. erectus. The decrease in the rate of leaf emergence in winter was more pronounced for B. erectus than for D. glomerata. (6) The effect of the environment on the size and components of the leaf area was similar for D. glomerata and B. erectus despite the contrasted frequency of the species in open and shady habitats.

Gas exchange and SO2 fumigation studies with irrigated and unirrigated field grown Diplacus aurantiacus and Heteromeles arbutifolia.

Experiments were performed on an evergreen (Heteromeles arbutifolia) and a drought deciduous shrub (Diplacus aurantiacus) to determine, 1) whether approaches for evaluating SO2 absorption by leaves in laboratory studies could be extended to field studies, 2) the effects of irrigation on metabolism and SO2 responses of the study species during a season when water was limiting, 3) to interpret SO2 responses on the basis of SO2 flux rates. Laboratory-developed approaches for evaluating SO2 absorption by leaves were found to be suitable for use with field plants, despite field plants having lower gas exchange rates. Supplementing water during times of deficit did not override all the biological and environmental factors that limited photosynthesis (A). Irrigation increased leaf longevity of D. aurantiacus, and stomatal conductance to water vapour (g); g was also shown to increase with H. arbutifolia on irrigation. Irrigation profoundly influenced plant response to SO2. Unwatered D. aurantiacus had only a small g and therefore a reduced capacity to absorb SO2 and respond to SO2; which resulted in apparent SO2 avoidance. Water availability and SO2 both affect g and therefore, SO2 flux rates into the mesophyll. Different ambient SO2 concentrations of 8.3 and 26.2 μmol m-3 (0.2 and 0.6 ppm) were both found to result in similar SO2 flux rates into the leaf, due to variations in g in response to water availability. Changes in g did not always result in changes in A, implying that carbon fixation may be little affected by some SO2 exposures, although still potentially affecting such processes as maintenance of leaf water potential, transpirational cooling and nutrient uptake.

Effects of Abscisic Acid on Growth of Wheat (Triticum aestivum L)

Daily application of abscisic acid (ABA) to growing wheat plants, although initially inhibiting growth, resulted, after a short lag, in an increase in the number of leaves and tillers. This may have been due to reduced apical dominance. At 84 days the total dry weight and area of all leaves produced up to this time was less for the plants treated with ABA than for the control plants. However, the area of green, living leaves and the dry weight were not significantly affected by the ABA treatment. Further effects of the daily ABA treatment were the inhibition of transpiration, especially on the abaxial surface, the reduction of leaf size, the promotion of flowering and the stimulation of trichome formation on the leaf surfaces. ABA did not promote leaf senescence in whole plants and actually increased leaf longevity.

Top-removal in single stem plants of Vicia faba L.

Summary Indeterminate growth habit in Vicia faba is characterised by only a few flowers on each inflorescence forming pods. Removal of the tops of the plants was shown, under some circumstances, markedly to increase the proportion of flowers setting and pods maturing on the remaining inflorescences. A consequence of de-topping was increased leaf longevity, in some cases well beyond ripening of the pods. The consequences of this longevity for leaf efficiency toward the pods they subtended were examined in treatments combining de-topping with partial defoliation. Evidence obtained suggested that leaf longevity does not result in increased efficiency, neither in its influence upon pod set nor pod fill. Where top removal enhances the performance of an inflorescence to or near maximum, the authors conclude that available photosynthetic sources are not in excess of pod requirements.