Leaf Cytokinin Research Articles

Improving dryland maize productivity and water efficiency with heterotrophic ammonia-oxidizing bacteria via nitrification and cytokinin activity

A two-year field experiment conducted under dryland conditions in semi-humid and drought-prone regions of China aimed to assess the effect of ammonia-oxidizing bacterial on maize water use efficiency and yield. A heterotrophic ammonia-oxidizing bacteria (HAOB) strain S2_8_1 was used. Six treatments were applied: (1) no irrigation + HAOB strain (DI), (2) no irrigation + blank culture medium (DM), (3) no irrigation control (DCK), (4) irrigation + HAOB (WI), (5) irrigation + blank culture medium (WM), and (6) irrigation control (WCK). Results revealed that HAOB treatment increased maize growth, yield, and water use efficiency over controls, regardless of whether the year was wet or dry. This improvement was attributed to the accelerated nitrification in the rhizosphere soil due to HAOB inoculation, which subsequently led to increased levels of leaf cytokinins. Overall, these findings suggest that HAOB inoculation holds promise as a strategy to boost water use efficiency and maize productivity in dryland agriculture.

Enhanced Post-Drought Compensatory Growth and Water Utilization in Maize via Rhizosphere Soil Nitrification by Heterotrophic Ammonia-Oxidizing Bacteria

Heterotrophic ammonia-oxidizing bacteria (HAOB), crucial for soil nitrification, have unclear benefits for crop water use. This study explored the impact of a novel HAOB strain, S2_8_1, on maize drought resilience via pot culturing. The experiment included various treatments: control with sufficient water (CK), sufficient water + HAOB strain (WI), limited rewatering (DL), sufficient rewatering (DH), sufficient rewatering + HAOB strain (DHI), and limited rewatering + HAOB strain (DLI). The results revealed below-compensatory growth with DL compared to CK. Interestingly, the HAOB strain displayed survival resilience with a 96% increase in its copy numbers in the rhizosphere soils compared to CK during rewatering. The DLI treatment exhibited equal to compensatory growth, showing a remarkable 169% surge in the water use efficiency versus CK. This improvement was attributed to heightened rhizosphere soil nitrification by HAOB, enhancing the cytokinin production in roots and its transference to leaves, leading to a 25% higher leaf cytokinin concentration with DLI compared to CK during rewatering. Additionally, HAOB DHI prompted overcompensatory growth after sufficient rewatering, boosting nitrification and facilitating cytokinin root-to-leaf transport. However, its water use efficiency was 39% lower than DLI. The study highlights HAOB’s importance in optimizing crop water use, particularly in scenarios of limited rewatering in cropland soils.

Unveiling the dual role of heterotrophic ammonia-oxidizing bacteria: enhancing plant regrowth through modulating cytokinin delivery.

This study aims to investigate the dual impacts of heterotrophic ammonia-oxidizing bacteria (HAOB) strains on the regrowth of Italian ryegrass by studying cytokinin delivery from roots to leaves. The dual impacts encompass both the "soil-inside-role" and "soil-outside-role," which refer to the HAOB operating inside and outside the rhizosphere soil within the rhizosphere microenvironment. The experimental design consisted of two sets of experiments, Exp-1 and Exp-2, involving different treatments. In Exp-1, various concentrations of NO3- were added to the roots to observe the soil-inside-role on cytokinin delivery from roots to leaves. In Exp-2, NO3- addition was combined with HAOB inoculation to observe the combined effects of the root-outside-role and root-inside-role on cytokinin synthesis and transport. The results indicated that NO3- concentrations ranging from 30 to 40 mmol L-1 had the most optimal effect on increasing leaf cytokinin content and delivery from roots to leaves, consequently promoting greater leaf regrowth biomass. When inoculated, the HAOB strain significantly increased rhizosphere soil nitrification rates under the soil-inside-role, leading to increased NO3- release from the soil and a subsequent boost in cytokinin delivery from roots to leaves. Additionally, the HAOB strain independently enhanced cytokinin delivery from roots to leaves outside the rhizosphere soil within the rhizosphere microenvironment, demonstrating its soil-outside-role. The combined effects of the soil-inside-role and soil-outside-role substantially increased leaf cytokinin content, playing a crucial role in promoting Italian ryegrass regrowth. The study's findings shed light on the mechanisms through which HAOB can enhance plant growth by performing dual roles in the rhizosphere, offering potential applications in agriculture. Understanding the interaction between HAOB, cytokinins, and plant growth could lead to more effective strategies for improving crop productivity and promoting sustainable agriculture.

Effect of high salinity and of priming of non-germinated seeds by UV-C light on photosynthesis of lettuce plants grown in a controlled soilless system.

High salinity results in a decrease in plant photosynthesis and crop productivity. The aim of the present study was to evaluate the effect of UV-C priming treatments of lettuce seeds on photosynthesis of plants grown at high salinity. Non-primed and primed seeds were grown in an hydroponic system, with a standard nutrient solution, either supplemented with 100 mM NaCl (high salinity), or not (control). Considering that leaf and root K+ concentrations remained constant and that chlorophyll fluorescence parameters and root growth were not affected negatively in the high salinity treatment, we conclude that the latter was at the origin of a moderate stress only. A substantial decrease in leaf net photosynthetic assimilation (Anet) was however observed as a consequence of stomatal and non-stomatal limitations in the high salinity treatment. This decrease in Anet translated into a decrease in growth parameters; it may be attributed partially to the high salinity-associated increase in leaf concentration in abscisic acid and decrease in stomatal conductance. Priming by UV-C light resulted in an increase in total photosynthetic electron transport rate and Anet in the leaves of plants grown at high salinity. The increase of the latter translated into a moderate increase in growth parameters. It is hypothesized that the positive effect of UV-C priming on Anet and growth of the aerial part of lettuce plants grown at high salinity, is mainly due to its stimulating effect on leaf concentration in salicylic acid. Even though leaf cytokinins' concentration was higher in plants from primed seeds, maintenance of the cytokinins-to-abscisic acid ratio also supports the idea that UV-C priming resulted in protection of plants exposed to high salinity.

Increasing Italian ryegrass (Lolium multiflorum Lam.) regrowth via inoculation with an ammonia‐oxidizing bacterial strain

AbstractThis study aimed to investigate the effect of a soil ammonia‐oxidizing bacterial (AOB) strain on Italian ryegrass (Lolium multiflorum Lam.) regrowth. Potted Italian ryegrass was used as test material. We isolated and screened an AOB strain from the soil for use in inoculation of other soils to increase nitrification. The sequence of the two isolated and screened AOB strains (S2_8_1 and S2_7_25) were 100% similar to that of Ensifer sesbaniae and Acinetobacter pittii. The nitrification inhibitor 3,4‐dimethylpyrazole phosphate (DMPP) was also added to the soil to inhibit nitrification. The experimental design involving the inoculation of the soils with the AOB strain included seven treatments: (1) regrowth without any additives, (2) regrowth with blank enrichment medium added to the soil, (3) regrowth with S2_8_1 strain inoculation, (4) regrowth with S2_7_25 strain inoculation, (5) regrowth with DMPP added to the soil, (6) regrowth with DMPP added to the soil and with S2_8_1 strain inoculation, and (7) regrowth with DMPP added to the soil and with S2_7_25 strain inoculation. Compared with regrowth treatment without any additives, the regrowth treatment with S2_8_1 and S2_7_25 inoculations significantly increased leaf biomass by 23.5–48.6%, the leaf net photosynthetic rate by 27.9–48.5%, the rhizosphere soil nitrification rate by 43.0–144.2%, leaf cytokinin concentrations by 23.7–24.9%, and xylem sap cytokinin concentrations by 32.5–43.2%. The increase of nitrate nitrogen content in rhizosphere soil induced by S2_8_1 and S2_7_25 promoted the transfers of cytokinin from roots to leaves, resulting in the increase of cytokinin concentration in leaves. This increase in leaf cytokinin concentrations improved Italian ryegrass regrowth. However, the S2_8_1 and S2_7_25‐induced increases in the nonrhizosphere soil nitrification rate were not closely related to Italian ryegrass regrowth. The tested AOB strains have considerable potential to increase Italian ryegrass regrowth.

Effect of ammonia-oxidizing bacterial strain that survives drought stress on corn compensatory growth upon post-drought rewatering.

A pot experiment was performed under rain–shelter conditions to explore the effects of drought stress and post-drought rewatering on the abundance of an ammonia-oxidizing bacteria (AOB) strain in corn (Zea mays L.) rhizosphere soils and the relationship between the AOB strain and corn (Zea mays L.) compensatory growth after drought stress rewatering. Corn seedlings were used as test materials, and one AOB strain was isolated and screened from the soil. The experimental design included six treatments: (1) wet (WT), (2) wet with AOB strain inoculation during wetness (WI), (3) wet with AOB strain inoculation during rewatering (WR), (4) post-drought rewatering (DT), (5) post-drought rewatering with AOB strain inoculation during wetness (DI), and (6) post-drought rewatering with AOB strain inoculation during rewatering (DR). Wetness and drought stress were obtained by keeping the soil water content at 75–80% and 50–55% of the field capacities, respectively. The results showed that the isolated and screened AOB strain (S2_8_1) had 100% similarity to Ensifer sesbaniae. The inoculation of S2_8_1 during the wet period in the DI treatment caused it to colonize the rhizosphere soil. Drought stress decreased its abundance, but rewatering resulted in a great increase. The S2_8_1 in the DI treatment increased the total biomass, water use efficiencies, net photosynthetic rates, rhizosphere soil nitrification rates, leaf cytokinin concentrations, xylem sap cytokinin concentrations, copy number of S2_8_1 in rhizosphere soils, and organic carbon contents in rhizosphere soils by 23, 104, 35, 30, 18, 29, 104, and 23% on day 10 after rewatering compared with WT treatment. In the DI treatment, the increase in rhizosphere soil nitrification rates caused by S2_8_1 during wetness was closely related to the cytokinin delivery from roots to leaves and increased leaf cytokinin concentrations. The increase in leaf cytokinin concentrations improved rewatering corn growth, which caused compensatory growth and increased water use. Compensatory and over-compensatory growths occurred in DT and DR treatments, respectively. Therefore, the coexistence of the strain of AOB with corn in rhizosphere soil increased the corn compensatory growth by regulating soil nitrification and root-induced leaf cytokinin.

Leaf cytokinin accumulation promotes potato growth in mixed nitrogen supply by coordination of nitrogen and carbon metabolism

The source and sink balance determines crop growth, which is largely modulated by nitrogen (N) supplies. The use of mixed ammonium and nitrate as N supply can improve plant growth, however mechanisms involving the coordination of carbon and N metabolism are not well understood. Here, we investigated potato plants responding to N forms and confirmed that, compared with sole nitrate supply, mixed N (75 %/25 % nitrate/ammonium) enhanced leaf area, photosynthetic activity and N metabolism and accordingly resulted in outgrowth of stolons and shoot axillary buds. Cytokinin transportation in xylem sap and local cytokinin synthesis in leaves were up-regulated in mixed-N-treated potato plants relative to sole nitrate provision; and exogenous application of 6-benzylaminopurine in addition to sole nitrate restored leaf area, photosynthetic capacity and N content in leaves to the similar as those under mixed-N treatment. Partial defoliation, an effective method to enhance the sink strength, induced more cytokinin content in leaflets under two treatments relative to their respective controls and ultimately resulted in larger photosynthesis capacity and leaf area. These results suggest that mixed-N-enhanced plant growth through the coordination of carbon and N metabolism largely depends on the signal molecule cytokinin modulated by N supplies.

Isoprene enhances leaf cytokinin metabolism and induces early senescence.

Summary Isoprene, a major biogenic volatile hydrocarbon of climate‐relevance, indisputably mitigates abiotic stresses in emitting plants. However functional relevance of constitutive isoprene emission in unstressed plants remains contested. Isoprene and cytokinins (CKs) are synthesized from a common substrate and pathway in chloroplasts. It was postulated that isoprene emission may affect CK‐metabolism.Using transgenic isoprene‐emitting (IE) Arabidopsis and isoprene nonemitting (NE) RNA‐interference grey poplars (paired with respective NE and IE genotypes), the life of individual IE and NE leaves from emergence to abscission was followed under stress‐free conditions. We monitored plant growth rate, aboveground developmental phenotype, modelled leaf photosynthetic energy status, quantified the abundance of leaf CKs, analysed Arabidopsis and poplar leaf transcriptomes by RNA‐sequencing in presence and absence of isoprene during leaf senescence.Isoprene emission by unstressed leaves enhanced the abundance of CKs (isopentenyl adenine and its precursor) by > 200%, significantly upregulated genes coding for CK‐synthesis, CK‐signalling and CK‐degradation, hastened plant development, increased chloroplast metabolic rate, altered photosynthetic energy status, induced early leaf senescence in both Arabidopsis and poplar. IE leaves senesced sooner even in decapitated poplars where source–sink relationships and hormone homeostasis were perturbed.Constitutive isoprene emission significantly accelerates CK‐led leaf and organismal development and induces early senescence independent of growth constraints. Isoprene emission provides an early‐riser evolutionary advantage and shortens lifecycle duration to assist rapid diversification in unstressed emitters.

Increasing corn compensatory growth upon post-drought rewatering using ammonia-oxidising bacterial strain inoculation

This study explored the effects of ammonia oxidizing bacterial (AOB) strain S2_7_25 on the compensatory growth of corn (Zea mays L.) upon post-drought rewatering using seedlings as test materials. This study consisted of two treatment conditions: isolation and screening of AOB strain and AOB strain inoculated to soils. Isolated and screened from soil, S2_7_25 had 100% similarity to Acinetobacter pittii. The experimental design of S2_7_25 inoculation to soils was composed of five treatments: (1) wetness, (2) post-drought rewatering, (3) post-drought rewatering with S2_7_25 inoculation, (4) post-drought rewatering with nitrification inhibitor addition to soil and (5) post-drought rewatering with nitrification inhibitor addition to soil and S2_7_25 inoculation. Rewatering increased the total biomass and net photosynthetic rate of corn compared with wetness, and S2_7_25 inoculation greatly increased these parameters of rewatering corn. However, nitrification inhibitor addition greatly decreased these parameters of rewatering corn inoculated with or without S2_7_25. Rewatering increased the rhizosphere soil nitrification rate of corn, and rewatering with S2_7_25 inoculation further increased it. However, nitrification inhibitor addition decreased this parameter in rewatering corn with or without S2_7_25 inoculation. The cytokinin delivery rates from the roots to the leaves were regulated by soil nitrification in the rhizosphere, and it determined the leaf cytokinin concentration that is closely related to corn rewatering growth. The highest leaf cytokinin concentration during rewatering caused over-compensatory growth and the highest water use efficiency in rewatering corn with S2_7_25 inoculation among all treatments. Compensatory and under-compensatory growth occurred in rewatering corn and in corn with nitrification inhibitor addition, respectively. In a word, S2_7_25 increased corn compensatory growth by regulating soil nitrification and root-induced leaf cytokinin.

Effects of rhizosphere soil nitrification on the compensatory growth of Italian ryegrass (Lolium multiflorum Lam) based on root‐produced cytokinin

AbstractCompensatory growth of forage grass can determine its optimal biomass, which is beneficial for increasing its production. The effect of rhizosphere soil nitrification on Italian ryegrass (Lolium multiflorum Lam) regrowth was investigated on the basis of root‐produced cytokinin concentration in leaves to reveal the forage grass compensatory growth mechanism. The nitrification inhibitor 3,4‐dimethylpyrazole phosphate was added to inhibit soil nitrification. Without soil nitrification being inhibited, compared with no clipping one defoliation cycle increased the leaf photosynthetic rate by 43.82%–53.79%, increased the leaf cytokinin content by 33.48%–34.73%, and increased the cytokinin transport from roots to leaves by 28.88%–39.47%. Nitrification inhibitor decreased soil nitrification rates by 23.33%–84.17% in the rhizosphere and by 42.71%–68.29% in the bulk soil during regrowth. In the rhizosphere micro‐environment, nitrification increased soil nitrate concentration that played an important role in the transport of cytokinin from roots to leaves during regrowth, increasing the leaf cytokinin concentration. However, bulk soil nitrification rate and nitrate content had little influence on leaf cytokinin concentration during regrowth. An increase in leaf cytokinin improved the photosynthesis and the regrowth. Without soil nitrification being inhibited, total biomass at the end of regrowth period was 1.09 times higher in plants that were defoliated once than in non‐clipped plants. However, soil nitrification inhibitor and two defoliation cycles decreased Italian ryegrass regrowth. Super compensatory growth occurred in the one defoliation cycle Italian ryegrasses without adding soil nitrification inhibitor. Compensatory growth occurred in once‐defoliated Italian ryegrass with adding soil nitrification inhibitor and in twice‐defoliated Italian ryegrass without adding soil nitrification inhibitor. In conclusion, rhizosphere soil nitrification is the key factor that regulates the compensatory growth of Italian ryegrass.

Corn compensatory growth upon post-drought rewatering based on the effects of rhizosphere soil nitrification on cytokinin

This study aims to investigate the relationship between rhizosphere soil nitrification and compensatory growth of potted corn (Zea mays L.) seedlings upon post-drought rewatering on the basis of leaf cytokinin induced by roots. The nitrification inhibitor 3, 4-dimethylpyrazole phosphate was added to the soil to restrain nitrification in soil. The experimental design consisted of six treatments: (1) wetness, (2) wetness with the nitrification inhibitor addition, (3) wetness with the nitrification inhibitor and nitrate additions, (4) post-drought rewatering, (5) post-drought rewatering with the nitrification inhibitor addition, and (6) post-drought rewatering with the nitrification inhibitor and nitrate additions. The results showed that compared with wetness rewatering alone increased the aboveground and total biomasses and the net photosynthetic rates of corn, but rewatering had little influence on these parameters following the nitrification inhibitor addition or both the inhibitor and nitrate additions. Without the nitrification inhibitor addition, the increase of carbohydrate content in root exudates of rewatering corn caused by drought stress accounted for rhizosphere soil nitrification rate augment during rewatering that were closely related to the delivery rate of cytokinin from the roots to the leaves; however, increased nitrification rates in non-rhizosphere soil were not recorded in rewatering treatment. Without the addition of the nitrification inhibitor, the net leaf photosynthetic rate of corn increased due to the elevated cytokinin levels in leaves, leading to the occurrence of compensatory growth and high water use efficiency of corn. By contrast, nitrification inhibitor application decreased the rhizosphere soil nitrification rate, followed by the consequent under-compensatory growth and low water use efficiency of corn; and the under-compensatory growth and low water use efficiency of corn also occurred in corns with the addition of both nitrification inhibitor and nitrate. Therefore, rhizosphere nitrification plays a key role in corn compensatory growth.

Cytokinin dynamics in xylem sap and leaves of fruiting and de-fruited sunflower hybrids of contrasting post-anthesis canopy senescence patterns during flowering and grain-filling

To determine whether xylem sap and leaf cytokinin levels were associated with live root length density (LRLD) and leaf senescence, these variables were measured in intact (control) plants and those from which florets were removed, in hybrids of contrasting post-anthesis senescence patterns. Field experiments were carried out during two consecutive seasons using sunflower hybrids (Paraiso75, stay-green [SG]; and Paraiso65, fast dry down [FDD]) of contrasting canopy senescence patterns. Crop carbon partitioning was altered by removing all florets (-F treatment) before anthesis. At crop level, the dynamics of LRLD, xylem sap flow rate, cytokinins in the xylem sap, and green leaf area index (GLAI) were measured, while at leaf level, dynamics of total chlorophyll content, trans-Zeatin content, and net photosynthesis were recorded for leaf positions 17, 20, 22, 24 and 27. In both hybrids, the –F treatment had higher LRLD, cytokinins in the xylem sap, and leaf trans-Zeatin concentration at anthesis and during the grain filling period than control plants. All measured variables decreased earlier in the control treatment than the -F treatment (in both hybrids), and in the FDD hybrid than the SG hybrid. The dynamics of xylem sap cytokinins were related to the dynamics of LRLD and that of leaf trans-Zeatin levels, contributing to contrasting canopy senescence patterns and underlying the responses to defruiting in both hybrids.

Effects of crown height on the compensatory growth of Italian ryegrass based on combined effects of stored organic matter and cytokinin

AbstractThe interaction between stored organic matters in crowns and leaf cytokinin induced by roots was determined to investigate the compensatory growth of Italian ryegrass (Lolium multiflorum. Lam). The experiment comprised five treatments: (a) clipping once with 5 cm crown height, (b) clipping once with 2 cm crown height, (c) clipping twice with 5 cm crown height, (d) clipping twice with 2 cm crown height and (e) no clipping. Results showed that low crowns and single clipping promoted regrowth and the sum total of all previously clipping and final harvest of Italian ryegrass. Super compensatory growth and compensatory growth, which were assessed based in total biomasses, respectively, occurred in Italian ryegrasses with high crowns and single defoliation and those with high crowns only. Increased leaf net photosynthetic rates in Italian ryegrasses with high crown were promoted by stored organic matter in crown and leaf cytokinin in the first and later 7‐day regrowth periods, respectively. Compared with single clipping, frequent clipping decreased the stored organic matter in roots; causing decreases in leaf cytokinin concentration and photosynthetic rate. The delivery rate of cytokinin from the roots to the leaves determined the leaf cytokinin concentration in Italian ryegrass during the regrowth period. Thus, organic matters from crowns and leaf cytokinin induced by roots played a combined effect on aboveground compensatory growth in the Italian ryegrasses.

Methyl jasmonate enhances salt tolerance of almond rootstocks by regulating endogenous phytohormones, antioxidant activity and gas-exchange

The effects of methyl jasmonate (MeJA) foliar application (0, 0.025, 0.050 and 0.075 mM) on the growth and physiological responses of two almond rootstocks (GF677 and bitter almond) exposed to various concentrations of NaCl in irrigation water (0, 50, 100 and 150 mM) were evaluated. 60 days after salt stress exposure, the mitotic index of root apical meristem cells as well as shoot and root growth, activity of main antioxidant enzymes, gas exchange parameters and contents of cytokinins and ABA were determined. Salt stress decreased the plants’ growth, particularly at higher levels. Application of MeJA in optimal concentrations of 0.025 to 0.05 mM alleviated the adverse effect of salt stress by increasing the photosynthetic rate, activity of antioxidant enzymes (APX, SOD and POX), root and shoot dry mass, as well as cell membrane integrity. Furthermore, MeJA application brought about a two-fold increase in the concentration of leaf cytokinins. This reposition of cytokinins was due to restriction of both the activity of cytokinin oxidase and gene expression of this enzyme. The MeJA mitigating effect on the growth of salt-stressed plants could be a result of the inhibition of cytokinin decline under salt stress. The results revealed the effective impact of endogenous cytokinins in protective and growth improvement effects of MeJA on almond rootstocks under salt stress.

Effects of plant population density and root-induced cytokinin on the corn compensatory growth during post-drought rewatering

The effect of plant population density (PPD) and root-induced leaf cytokinin on the compensatory growth of potted corn seedlings during post-drought rewatering was investigated. The study design comprised four treatments: (1) wetness with low PPD, (2) wetness with high PPD, (3) rewatering with low PPD, and (4) rewatering with high PPD. Results showed that drought stress restrained the growth of corns. By contrast, rewatering enhanced the net photosynthetic rate and growth of corns. During the 8 days of rewatering, compensatory growth during post-drought rewatering occurred in corns with high PPD; however, such compensatory growth did not occur in corns with low PPD. Zeatin riboside concentrations in leaves and xylem saps were significantly higher under rewatering treatment than those under wet treatment. High leaf cytokinin concentration accelerated corn growth. The coefficients of variation and Gini-coefficient of wet treatment were significantly higher than those of rewatering treatment under high PPD, demonstrating that intense intraspecific competition occurred in the wet treatment. Extreme intraspecific competition negatively affected net photosynthetic rate. In brief, the interactions between root-induced leaf cytokinin and weak intraspecific competition promoted the compensatory growth under high PPD.

No post-drought compensatory growth of corns with root cutting based on cytokinin induced by roots

The effect of shallow and deep roots on compensatory growth of potted corn seedlings during post-drought rewatering was investigated on the basis of leaf cytokinin induced by roots. Two treatment conditions, namely, root cutting and nitrate (NO3−) addition, were used in the study. Results showed that drought stress retained corn growth. In the absence of root cutting, rewatering increased the net photosynthetic rates of corn and induced compensatory growth. However, under root cutting, no compensatory growth occurred in the rewatered corns. In the absence of root cutting, significantly higher zeatin riboside (ZR) concentrations in the leaves and xylem saps were recorded under rewatering treatment than under wet treatment. However, these results were not observed under root cutting owing to loss of deep roots. Furthermore, corns with deep roots added with NO3− showed significantly higher ZR concentration in the leaves and xylem saps than those with shallow roots added with NO3−. High leaf cytokinin concentration is known to induce rapid corn growth. Thus, no compensatory growth occurred during post-drought rewatering in corns with few deep roots, likely due to lack of leaf cytokinin enhancement induced by deep roots.

Cytokinins: A key player in determining differences in patterns of canopy senescence in Stay-Green and Fast Dry-Down sunflower ( Helianthus annuus L.) hybrids

Leaf senescence during grain filling can reduce crop yield. We studied, under field conditions and during grain-filling, the association between leaf cytokinin levels and the onset of leaf senescence in sunflower hybrids of contrasting canopy senescence patterns (Paraiso75, stay-green [SG] and Paraiso65, fast dry down [FDD]). At crop level, dynamics of live root length density (LRLD) and green leaf area index (GLAI) were followed, while at leaf level dynamics of total chlorophyll content, trans-Zeatin content, net photosynthesis and PSII quantum yield, were followed in leaf positions 17, 20, 22 and 24. Responses of these leaf variables to exogenous cytokinin applications to leaves at position 17 were also followed. SG exhibited greater (p < 0.05) LRLD and GLAI values at anthesis. In both hybrids, LRLD began to fall before GLAI. All variables decreased earlier (p < 0.05) in FDD. Initial leaf levels of trans-Zeatin were three times higher (p < 0.05) in SG. Exogenous cytokinin applications maintained leaf-level variables. These are the first results showing associations between LRLD dynamics with the dynamics of leaf cytokinin levels and changes in indicators of leaf functionality. Also, this is the first study in which estimates are made of cytokinin thresholds below which leaf senescence begins in two hybrids of contrasting canopy senescence patterns. These advances in the understanding, at both crop and leaf levels, of the controls and consequences of SG during grain filling, a trait known to improve crop water uptake under drought and increase biomass accumulation during grain filling, provide support for breeding efforts aimed at profiting from this trait to increase crop yields.

Changes in cytokinins are sufficient to alter developmental patterns of defense metabolites in Nicotiana attenuata.

Plant defense metabolites are well known to be regulated developmentally. The optimal defense (OD) theory posits that a tssue's fitness values and probability of attack should determine defense metabolite allocations. Young leaves are expected to provide a larger fitness value to the plant, and therefore their defense allocations should be higher when compared with older leaves. The mechanisms that coordinate development with defense remain unknown and frequently confound tests of the OD theory predictions. Here we demonstrate that cytokinins (CKs) modulate ontogeny-dependent defenses in Nicotiana attenuata. We found that leaf CK levels highly correlate with inducible defense expressions with high levels in young and low levels in older leaves. We genetically manipulated the developmental patterns of two different CK classes by using senescence- and chemically inducible expression of CK biosynthesis genes. Genetically modifying the levels of different CKs in leaves was sufficient to alter ontogenic patterns of defense metabolites. We conclude that the developmental regulation of growth hormones that include CKs plays central roles in connecting development with defense and therefore in establishing optimal patterns of defense allocation in plants.

Correlation of the corn compensatory growth mechanism after post-drought rewatering with cytokinin induced by root nitrate absorption

The mechanism of compensatory growth in corn after post-drought rewatering at the seedling stage was explored by investigating the levels and effects of several plant hormones. This study consisted of two treatment conditions: nitrate (NO3−) addition to the roots and cytokinin addition to the leaves. Results showed that drought stress reduced the biomass of aboveground parts and the whole plant, but increased root soluble carbohydrate concentration and root activity. Post-drought rewatering under the addition of NO3− to the roots increased corn growth. Biomass values of the aboveground parts and the whole plant were similar between the rewatering and wetness at 10 days after rewatering conditions. Upon addition of NO3− to roots, post- drought rewatering increased the cytokinin contents of leaves and its delivery rate from roots to leaves. Addition of cytokinin to leaves without the addition of NO3− to roots increased the growth rate of corn seedlings upon post-drought rewatering and simultaneously caused high leaf cytokinin concentration. Thus, increase in concentration of root-derived cytokinin concentration in leaves was closely related to compensatory growth in corn upon post-drought rewatering at the seedling stage.

Basipetal auxin versus acropetal cytokinin transport, and their interaction with NO3 fertilisation in cotyledon senescence and sink:source relationships in cucumber (Cucumis sativus L.).

The paramount role of cytokinins (CKs) in initiation, as well as prevention, of senescence is well established. In recent years, experimental methods have become available to raise and lower the CK concentration and experimentally manipulate senescence. Decapitating the apical shoot and adding the synthetic auxin naphthylacetic acid to the cut stem reduced endogenous CKs to low levels. Conversely, if no auxin was applied, xylem and leaf CK levels increased dramatically, indicating that basipolar auxin transport is a key determinant in the synthesis of CKs and is potentially more important than NO(3). Manipulating the concentration of applied NO(3) caused considerable variation in leaf CK levels and concomitant changes in senescence. These and other results suggest that the frequently discussed decrease in nitrogen use efficiency (NUE) may be more highly regulated by CKs than by NO(3). Analysis of the re-metabolisation and re-allocation of chlorophyll, proteins, amino acids and starch in three different cucumber cultivars indirectly showed that these metabolites were significantly affected by the concentration of CKs in the leaves. Further research in this area may allow leaf senescence and plant yield to be more efficiently regulated by manipulating CKs and/or basipolar auxin transport instead of nitrate.