Advances in Environmental Biology, 5(6): 1058-1063, 2011 ISSN 1995-0756 1058 This is a refereed journal and all articles are professionally screened and reviewed ORIGINAL ARTICLE Assessment of Auxin and Cytokinin Levels at Different Grain Type and Position Within a Spike of Wheat Alireza Houshmandfar and Davood Eradatmand Asli Department of Agronomy and Plant Breeding, Islamic Azad University, Saveh Branch, Saveh, Iran. Alireza Houshmandfar and Davood Eradatmand Asli: Assessment of Auxin and Cytokinin Levels at Different Grain Type and Position Within a Spike of Wheat ABSTRACT Grain growth rate (GGR), cytokinins, and indolyl-3-acetic acid (IAA) levels were studied at different grain types and positions within developing grains of wheat (Triticum aestivum L. var. PBW-343). Main spikes were divided into three grain positions included proximal, middle, and distal regions, and further into two grain types included basal and apical grains. Grain dry weight, cytokinins including zeatin (Z) and zeatin riboside (ZR), and IAA levels were determined in ten labelled spikes which sampled five times, seven-day intervals started from seventh day after anthesis (DAA) up to 28th DAA, and at maturity. Both cytokinins and IAA contents increased until 14th DAA. The maximum level of grain growth rate (GGR) was observed at 14th- 21st DAA. Furthermore, the differences in both cytokinins and IAA contents, among spikelets in different regions of the spike, and also among grains within a spikelet were positively correlated with the differences in dry matter accumulation. The results suggest that cytokinins and IAA levels play an important role in regulating grain filling pattern. Furthermore, it could be possible to increase grain weight potential by improving cytokinins and IAA levels in grain, especially during the early filling stage either through breeding or crop management. Key words: Cytokinins; IAA; spike; grain development; wheat. Introduction Wheat is the most important cereal crop which ubiquitous in the food culture of both developed and developing countries in the world. Total world wheat production must increase by approximately 1.5% annually to meet the growing demand for food that will result from population growth and economic development (Rosegrant et al., 1995). A substantial increase in grain yield potential, along with better use of water and fertilizer is required to ensure food security in the future decades. For improvements in photosynthetic capacity to result in additional wheat yield, extra assimilates must be partitioned to developing grains and/or potential grain weight increased to accommodate the extra assimilates [11]. The position of grain within a spike to some extend determines its final grain weight which can range from 20 to 60 mg. The grains from spikelets in the middle region of the spike and from the basal region within each spikelet are more towards the upper level of this range [5]. Various explanations such as assimilate availability and/or transport capacity [21] or the possible role of plant growth regulators [15,25] are offered to explain these differences. Cytokinins play important role in regulating plant growth and development such as intensity and direction of assimilate flow, cell elongation, and cell division rate [10]. In cereals, high levels of cytokinins are generally found in the endosperm of developing seeds, which may be required for active cell division during the early phase of grain setting [17]. Corresponding Author Alireza Houshmandfar, Department of Agronomy and Plant Breeding, Islamic Azad University, Saveh Branch, Saveh, Iran. E-mail: houshmandfar@iau-saveh.ac.ir
Michael and Seiler-Kelbitsch [16], reported that transient grain cytokinin content is correlated with final grain yield of barley. The findings of Alizadeh et al. [1] showed an increase in grain yield of wheat cultivars by the application of exogenous cytokinins. IAA is the major auxin involved in regulating grain development [9,2]. Awan and Alizai [4] observed that the application of IAA significantly reduced spikelet sterility in rice. Wang et al. [23] suggested that the poor grain filling of rice was associated with low grain doses of both IAA and ABA (abscisic acid). Evaluation the relation of dry matter accumulation, cytokinins and IAA levels at different grain type and position could be important to identifying the role of plant growth regulators on differences in dry matter accumulation of grains in a spike, which could be the key in developing wheat with higher grain yield potential. Hence, the objective of this study was to evaluate the cytokinins and IAA levels along with dry matter accumulation at different grain type and position within a spike of PBW-343 wheat. Materials and methods Experimental Setup and Plant Sampling: Single plants of the wheat (Triticum aestivum L. var. PBW-343) were grown in plastic containers with a diameter of 4.5 cm and depth of 20 cm. The pots were filled with a pasteurized soil which classified as a clay loam with 28.1% Sand, 25.7% Clay and 46.2% Silt, an electrical conductivity (ECe) of 1.2 ds m-1, a ph of 7.1 (saturated paste), and organic C of 0.62%. The plants were grown in a screen covered hall under otherwise natural conditions. The pots were watered as described by Houshmandfar et al. [14] and fertilized once a week with half strength Peter's solution (NPK = 10:10:10) [6]. The secondary tillers were removed as they appeared. Ten labelled spikes were sampled five times, seven-day intervals started from seventh day after anthesis (DAA) up to 28th DAA, and at maturity. Spikes were divided into three grain positions included proximal (spikelet number 1 to 5), middle (spikelet number 6 to 15), and distal (spikelet number 16 to 20) regions, and further into two grain types included basal (bold) (grain No. 1 and 2) and apical (small) (grain No. 3 upward). All samples were divided into two parts; one was dried in an oven at 70 C for 72 h, then weighed for dry matter accumulation, and another was frozen in liquid N2 for one min and kept in a freezer at -70 C for zeatin (Z), zeatin riboside (ZR), and IAA (indolyl-3-acetic acid) analysis. Grain growth rate (GGR) was calculated using the following equation [12]: Where, W1 = Total dry matter of grain at time t1 W2 = Total dry matter of grain at time t2 T1 = Time of first observation T2 = Time of second observation 1059 Linear regression was used to evaluate the relationships between traits. The data were analysed statistically using analysis of variance and critical differences (CD) at 5 percent level were computed. Cytokinins and IAA Analysis: The methods for extraction and purification of Z, ZR, and IAA were modified from those described by Bollmark et al. [8] and Yang et al. (2002). In brief, samples consisting of approximately 50 frozen grains were ground in an ice-cold mortar in 10 ml 80% (v/v) methanol extraction medium containing 1 mmol L-1 butylated hydroxytoluence (BHT) as an antioxidant. The methanolic extract was incubated at 4 C for 4 h and centrifuged at 10,000 rpm for 10 min at the same temperature (4 C). The supernatants were passed through Chromosep C18 columns (C18 Sep-Park Cartridge, Waters Corp, Millford, MA, USA), prewashed with 10 ml 100% (v/v) and 5 ml 80% (v/v) methanol, respectively. About 5 ml of the purified fraction including cytokinins and IAA was collected and dried under N2 and then dissolved in 2 ml phosphate buffer saline (PBS) containing 0.1% (v/v) Tween 20 and 0.1% (w/v) gelatin (ph 7.5) for the determination by an enzyme-linked immunosorbent assay (ELISA). The method for quantification of Z, ZR, and IAA was modified from that described by Zhang et al. [26]. To test the recoveries, 500 ng of each synthetic compound of Z, ZR, and IAA was added to 1 g of fresh grains before purification. Recovery percentages of Z, ZR, and IAA were 78.2 ± 7.2, 79.1 ± 5.9, and 69.4 ± 4.7, respectively. Results: Table 1 demonstrates the grain dry matter accumulation at different grain types and positions within developing grains of wheat. The grain dry weight was positively correlated with the age of the plant from seventh DAA to maturity (r2=0.9462). Grain dry weight was diversely affected due to different grain positions. Grain dry weight of middle region of spike as compare with proximal and distal regions, and grain dry weight of proximal region as compared distal region, improved at all sampled DAA. Furthermore, grain dry weight was positively influenced by grain types. Hence, the maximum levels of grain dry weight were obtained in basal grains in the direction of all sampled DAA. The maximum quantum of disparities between basal and apical grains was observed at distal position, which was to the tune of 38.3, 34.0, 28.3, 27.1 and 26.0 percent in 7th, 14th, 21st, 28th and maturity, respectively.
Generally, grain growth rate (GGR) was high during 7th to 28th DAA. GGR improved from 7th DAA to 14th- 21st DAA then decreased from 14th- 21st DAA to maturity. The lowest amount of GGR was observed during 28th DAA to maturity. Furthermore, the differences in GGR levels, were proportionally depends on both grain type and position dry weights. The only exceptions were during 28th DAA to maturity, basal grains of middle and proximal positions, which the GGR were lower as compared with apical grains of middle and proximal positions (Figure 1). Table 2 indicates the zeatin content at different grain types and positions at various DAA. The Z levels increased from seventh DAA until 14th DAA, and then decreased from 14th DAA until maturity. Therefore, the maximum levels of grain Z for all different grain types and positions were observed at 14th DAA. According to grain type and position, the maximum Z levels of grain were obtained in middle region of spike and in basal grains at all the sampled stages. Furthermore, the differences in Z concentration of grains in different regions positively correlated to differences in related GGR levels (r2=0.8563). The correlation hold true both for comparisons between spikelets in various regions of the spike, and also between florets within spikelets. The maximum quantum of disparities between basal and apical grains was observed at distal position, which was to the tune of 32.9, 28.0, 23.0, 17.2 and 19.0 percent in 7th, 14th, 21st, 28th and maturity, respectively. Table 3 presents the zeatin riboside content of different grain types and positions at various DAA. The ZR levels also increased from seventh DAA 1060 until 14th DAA, and then decreased from 14th DAA until maturity. Therefore, the maximum levels of grain ZR for all different grain types and positions were observed at 14th DAA. The maximum ZR level of grains was obtained in middle region of spike and in basal grains at all sampled DAA. Furthermore, the differences in ZR concentration of grains in different regions positively correlated to differences in related GGR levels (r2=0.8682). The correlation hold true both for comparisons between spikelets in various positions of the spike, and also between basal and apical grains within spikelets. The maximum quantum of disparities between basal and apical grains was also observed at distal position, which was to the tune of 33.9, 28.1, 22.9, 18.4 and 17.2 percent in 7th, 14th, 21st, 28th and maturity, respectively. Table 4 presents the IAA content at different grain types and positions within developing grains of PBW-343 wheat. The IAA levels increased from 7th DAA until 14th DAA, and then decreased from 14th DAA until maturity. Hence, the highest levels of grain IAA for all different grain types and positions were observed at 14th DAA. According to grain position, the maximum level of grains IAA was observed at middle region of spike which produced higher level of IAA as compare with both proximal and distal regions. The IAA level of grain was positively affected by grain type. Therefore, the maximum level of grains IAA was obtained in basal grains for all determined growth stages. The increase in IAA concentration was nearly paralleled to dry matter accumulation. Furthermore, the differences in IAA concentration of grains positively correlated to GGR (r2=0.9553). The correlation also hold true both for comparisons between different grain types and positions. Fig. 1: Grain growth rate (GGR) at different grain type and position within developing grains of wheat. B and A are basal and apical grains, respectively.
1061 Table 1: Grain dry weight (mg grain-1) at different grain types (basal and apical) and positions (proximal, middle and distal) within developing grains of wheat (Triticum aestivum L. var. PBW-343). 7th 6.2 4.0 6.5 4.2 6.0 3.7 (- 35.5) (- 35.4) (- 38.3) 14th 19.5 13.9 20.6 15.3 18.8 12.4 (- 28.7) (- 25.7) (- 34.0) 21st 34.3 26.5 35.8 28.4 33.3 23.9 (- 22.7) (- 20.6) (- 28.3) 28th 46.1 36.4 47.7 38.7 45.0 32.8 (- 21.0) (- 18.9) (- 27.1) Maturity 50.6 41.6 51.6 43.5 49.7 36.8 (- 17.8) (- 15.7) (- 26.0) level; Age: 5.35; Position/Type: 2.62; Age Position/Type: 6.61 Table 2: Cytokinins (zeatin) (Z) contents (ng g-1 fresh weight) at different grain types (basal and apical) and positions (proximal, middle and distal) within developing grains of wheat (Triticum aestivum L. var. PBW-343) 7th 237.1 174.7 258.6 209.2 221.1 148.4 (- 26.3) (- 19.1) (- 32.9) 14th 669.0 552.2 776.7 652.4 636.9 458.3 (- 21.0) (- 16.0) (- 28.0) 21st 591.9 497.2 646.9 566.0 549.9 423.4 (- 16.0) (- 12.5) (- 23.0) 28th 98.9 86.4 107.4 97.9 88.1 72.9 (- 12.6) (- 8.8) (- 17.2) Maturity 2.2 1.8 2.6 2.2 2.1 1.7 (- 18.2) (- 15.4) (- 19.0) level; Age: 24.3; Position/Type: 9.2; Age Position/Type: 31.6 Table 3: Cytokinins (zeatin riboside) (ZR) contents (ng g-1 fresh weight) at different grain types (basal and apical) and positions (proximal, middle and distal) within developing grains of wheat (Triticum aestivum L. var. PBW-343) 7th 293.9 217.7 323.2 267.5 280.7 185.5 (- 25.9) (- 17.8) (- 33.9) 14th 845.8 673.7 932.1 782.9 764.3 549.9 (- 20.3) (- 16.0) (- 28.1) 21st 728.0 615.6 794.8 700.2 675.5 520.8 (- 15.4) (- 11.9) (- 22.9) 28th 128.6 112.8 139.2 125.3 116.2 94.8 (- 12.3) (- 10.0) (- 18.4) Maturity 3.1 2.5 3.7 3.1 2.9 2.4 (- 19.4) (- 16.2) (- 17.2) level; Age: 32.3; Position/Type: 13.7; Age Position/Type: 42.1 Table 4: IAA contents (ng g-1 fresh weight) at different grain types (basal and apical) and positions (proximal, middle and distal) within developing grains of wheat (Triticum aestivum L. var. PBW-343) 7th 295.2 189.2 323.1 219.7 277.8 158.6 (- 35.9) (- 32.0) (- 42.9) 14th 905.7 606.8 994.3 695.1 815.3 489.2 (- 33.0) (- 30.1) (- 40.0) 21st 786.1 566.0 874.9 615.4 699.8 447.9 (- 28.0) (- 29.7) (- 36.0) 28th 98.9 75.3 107.6 79.6 91.5 63.9 (- 23.9) (- 26.0) (- 30.2) Maturity 0.86 0.64 1.02 0.82 0.76 0.49 (- 25.6) (- 19.6) (- 35.3) level; Age: 36.9; Position/Type: 15.1; Age Position/Type: 48.3
Discussion: Plant growth regulators play an important role in regulating plant growth and development. Grain filling period is an influential stage of crop life cycle which strongly effected grain yield. The differences in dry weight per grain are highly flexible [5]. Middle region of spike as compare with proximal and distal regions produces the maximum level of grain dry weight [3]. We have investigated the relation between cytokinins and IAA levels, along with dry matter accumulation and related GGR at different grain types and positions within developing grains of wheat. Both cytokinins and IAA levels increased remarkably in the early phase of grain setting while the GGR was max. Furthermore, the differences in both cytokinins and IAA levels, among spikelets in different regions of the spike, and also among grains within a spikelet were positively correlated with the differences in dry matter accumulation. Yang et al. 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