Effects of 1-Methylcyclopropene and Naphthaleneacetic Acid on Fruit Set and Expression of Genes Related to Ethylene Biosynthesis and Perception and Cell Wall Degradation in Apple

The effects of 1-methylcyclopropene (1-MCP) and naphthaleneacetic acid (NAA) on fruit set and the expression of genes related to ethylene biosynthesis and perception and cell wall degradation in apple (Malus ·domestica Borkh.) were studied when applied during the normal chemical thinning period. 1-MCP at 209 mg L had a small negative effect or no effect on the final fruit set, depending on the experiment, but could cause a transient delay of June drop when applied at petal fall or the 10-mm stage in ‘Pioneer McIntosh’ apple. 1-MCP at 160 mg L had no effect on fruit abscission but induced ethylene production by leaves and fruit of ‘Golden Delicious’ apple. NAA at 6 or 15 mg L effectively increased fruit abscission in both apple cultivars. NAA enhanced the expression of genes related to ethylene biosynthesis (MdACS5A, MdACS5B, and MdACO1) or perception (MdETR1, MdETR1b, MdETR2, MdERS1, and MdERS2) and cell wall degradation (MdPG2). 1-MCP did not affect the expression of MdACS5A and MdACS5B in the fruit abscission zone (FAZ), although it enhanced the expression of these two genes in the fruit cortex (FC) from 6 hours to 1 day after treatment. The expression of MdACO1 in both tissues was increased by 1-MCP by 3 days post-treatment and thereafter. 1-MCP had only a small influence on the expression of most ethylene receptor genes, with the exception of MdETR1, which was upregulated in the FC to a level similar to that observed for NAA treatment. In response to 1-MCP, in the FAZ, the expression of MdCel1 and MdPG2 was upregulated at the beginning and the end, respectively, of the experiment, but otherwise remained at or below control levels. 1-MCP did not inhibit NAA-induced abscission of young apple fruit, suggesting that abscission does not solely depend on ethylene signal transduction, or that the periods of effectiveness for 1-MCP and ethylene were asynchronous. Ethylene is involved in young apple fruit abscission (Curry, 1991; Dal Cin et al., 2005; McArtney, 2002; Zhu et al., 2008). Application of ethephon, an ethylene-releasing compound, effectively promoted the abscission of young fruit in apple (Walsh et al., 1979; Yuan, 2007), while aminoethoxyvinylglycine (AVG), an inhibitor of ethylene biosynthesis, reduced fruit ethylene production and the abscission of young fruit in apple (Williams and Flook, 1980; Zhu et al., 2008). Increased abscission of young apple fruit caused by the chemical thinner naphthaleneacetic acid (NAA) is linked with increased ethylene production; hence, NAA may act in part through ethylene signaling (Curry, 1991; Zhu et al., 2008). The pathway of ethylene synthesis has been well established in higher plants. Ethylene is formed from methionine via S-adenosyl-L-methionine (SAM) and 1-aminocyclopropane1-carboxylic acid (ACC) (Yang and Hoffman, 1984). The conversions of SAM to ACC and ACC to ethylene are the two committed steps in ethylene biosynthesis, and are catalyzed by ACC synthase (ACS) and ACC oxidase (ACO), respectively (Alexander and Grierson, 2002). AVG is a competitive inhibitor of ACS and other members of the class of pyridoxal-5#phosphate-dependent enzymes (Huai et al., 2001). Thus far, five ACS genes and one ACO gene have been isolated and characterized (Dal Cin et al., 2005; Li and Yuan, 2008) in apple. It has been reported that MdACS1 and MdACO1 are related to the increase in fruit ethylene during fruit ripening, whereas MdACS5A, MdACS5B, and MdACO1 are associated with ethylene production by young fruit (Li and Yuan, 2008). Ethylene is perceived by a series of receptors that undergo conformational changes upon ethylene binding and then inactivate the Raf-like serine/threonine kinase CTR1, a negative regulator in ethylene signal transduction. This relieves the repression on downstream signaling components, thus allowing for activation of the EIN3/EIL transcription factors and ethylene-inducible genes, which control a myriad of ethyleneassociated plant growth and development processes (Chang and Received for publication 14 June 2010. Accepted for publication 6 July 2010. We gratefully acknowledge the financial support from the Virginia Agricultural Council and Rohm and Haas Company and the assistance of David H. Carbaugh and Grace Engelman. This paper is dedicated to the memory of Rongcai Yuan. He was a devoted plant scientist, revered colleague and a kind and patient mentor. He is greatly missed by those who were privileged to know him. Corresponding author. E-mail: [email protected]. 402 J. AMER. SOC. HORT. SCI. 135(5):402–409. 2010. Bleecker, 2004; Chen et al., 2005). Six ethylene receptors have been identified in tomato [Solanum lycopersicum L. (Bleecker, 1999)] and five have been isolated and characterized in apple (Dal Cin et al., 2005; Li and Yuan, 2008). Among the ethyleneinducible transcriptional cascade are the genes for hydrolases such as b-1,4-glucanase (cellulase or EG) and polygalacturonase (PG), which are induced in the fruit abscission zone where they catalyze the breakdown of the middle lamellae and cell walls and promote fruit drop (Bonghi et al., 2000; Ward et al., 1999). 1-Methylcyclopropene (1-MCP), a gaseous inhibitor of ethylene action, has been used to delay postharvest ripening of climacteric fruit such as apple (Fan et al., 1999; Sisler and Serek, 1997), peach [Prunus persica L. (Kluge and Jacomino, 2002)], avocado [Persea americana Mill. (Jeong et al., 2002)], and mango [Mangifera indica L. (Jiang and Joyce, 2000)]. 1-MCP also delayed leaf abscission in sweet orange [Citrus sinensis L. (Zhong et al., 2001)] and mature fruit abscission in apple (Li and Yuan, 2008; Yuan and Carbaugh, 2007; Yuan and Li, 2008), and has been widely used in the cut flower industry (Blankenship and Dole, 2003). 1-MCP has been formulated as a stable powder that releases the active gaseous form when mixed with water. Recently, a sprayable formulation of 1-MCP became available for experimental use in the field. The purposes of the present work were to study the effect of a sprayable formulation of 1-MCP on apple fruit set and the effects of 1-MCP and NAA on the expression of genes related to ethylene biosynthesis, perception, and cell wall degradation. Materials and Methods EXPT. 1: ‘PIONEER MCINTOSH’/M.9, BELCHERTOWN, MA. Twenty 18-year-old ‘Pioneer McIntosh’ apple trees were selected in a non-irrigated block at the University of Massachusetts Horticultural Research Center in 2007. Trees were trained as a central leader and were cared for using normal industry accepted culture and pest management practices. At the pink stage of flower development, two representative limbs per tree were randomly selected and tagged and their circumferences were measured. After counting all blossom clusters on the selected limbs, blossom cluster density was calculated by dividing the number of blossom clusters by the square centimeter of limb cross-sectional area (LCSA). Trees were placed into five groups (replications) based upon similarity in the calculated blossom cluster density. Treatments were randomly assigned among the four trees within each replication. Treatments were sprayable 1-MCP (Rohm and Haas Company, Spring House, PA) applied at three distinct physiological stages: bloom (10 May), petal fall (17 May), and 10-mm-diameter fruit (24 May). One tree in each replication was not sprayed and served as the untreated control. The sprayable formulation of 1-MCP was applied as a dilute handgun application using an 11.4-L backpack sprayer propelled with CO2 at 276 KPa pressure. In the backpack sprayer, 62.5 g of 1-MCP was placed along with 113.5 mL of summer oil (AFxDR-038, Rohm and Hass) and 6 mL of silicone surfactant (Silwet L-77; Helena Chemical Co., Memphis, TN). This gave a final 1-MCP concentration in the tank of 209 mg L with 1% oil and 0.05% Silwet L-77. The sprayable 1-MCP was mixed in the orchard. The sprayer was filled with water and then silicone surfactant and summer oil were added and mixed using a portable drill equipped with an attached paint mixer. The Table 1. Effect of 1-methylcyclopropene (1-MCP) application at different physiological stages on fruit set of ‘Pioneer McIntosh’/