Abscission is an active process by which plants shed vegetative and reproductive parts during various developmental stages of their life cycle though a narrow zone of anatomically distinct cells that constitute AZ (Estornell et al., 2013). Besides developmental signals and environmental stresses, commercially available compounds also cause abscission in plant organs (Kossuth et al., 1978; Taylor and Whitelaw, 2001). The peel of citrus fruit consists of the colored outer flavedo and the inner white spongy albedo. However, calyx AZ is situated deeper within the albedo tissue and could be visually identified (Brown and Burns, 1998).
CMNP is a pyrazole compound and demonstrated as a potential nontoxic abscission agent for the late season ‘Valencia’ sweet orange crop (Li et al., 2008). It has also been shown to aid in mechanical harvesting of citrus in Florida (Yuan and Burns, 2004). Several studies have been conducted to understand the mechanism of CMNP-induced loosening in mature citrus fruit (Kossuth et al., 1978; Li et al., 2008; Yuan and Burns, 2004). Mechanisms of abscission vary among particular plant parts (Lewis et al., 2006). These studies were focused on CMNP-induced metabolic changes in FT of citrus fruit instead of calyx AZ (Alferez et al., 2005; Alferez et al., 2006; Alferez et al., 2007; Kumar and Ebel, 2015). It has been shown that following CMNP application, there is a gradual decline in chlorophyll content with concomitant rise in total carotenoid in FT of citrus (Alferez et al., 2005). These authors also showed an increase in electrolyte leakage upon CMNP treatment, which suggests membrane degradation could be the possible mechanism for CMNP-induced abscission in citrus (Alferez et al., 2005). It has been proposed that CMNP-induced lipid signaling in FT promotes abscission in calyx AZ (Alferez et al., 2005). CMNP-mediated higher levels of phospholipase A2 (PLA2) and LOX activities were observed in FT (Alferez et al., 2005). However, inhibitory studies using aristolochic acid, which is a potent inhibitor of PLA2 and LOX revealed that lipid signaling partly (≈70%) accounts for the CMNP-induced abscission (Alferez et al., 2005). This evidence suggests that in addition to lipid signaling, other unknown mechanisms also operate in CMNP-mediated abscission process. We showed that other signaling molecules are reactive oxygen species (ROS), such as O·−, H2O2, and hydroxyl radicals (OH·), in CMNP-mediated abscission cascade (Kumar and Ebel, 2015; Sakamoto et al., 2008). Our earlier work on FT showed the generation of H2O2 at 1 DAT, which is a mobile signaling molecule and has been shown to induce abscission (Kumar and Ebel, 2015; Sakamoto et al., 2008).
Several other pyrazole compounds [pyrazole pyrimidines, pyrazole hydrazone (anticancerous), methyl pyrazole (pesticide), and pyrazolo triazine (herbicides)] also share a common mode of action in treated cells by generating ROS (Graillot et al., 2012; Hassan et al., 2011; Vicentini et al., 2004).
There are reports that showed a direct relationship between ROS and the abscission process (Djanaguiraman et al., 2004; Sakamoto et al., 2008). Sakamoto et al. (2008) observed that H2O2 is continuously produced by the cell of AZ in vitro following ethylene exposure. In addition, ethephon-induced abscission was suppressed by inhibitors of H2O2 in Capsicum baccatum plants (Sakamoto et al., 2008). However, higher activity of antioxidant enzymes has been shown to delay abscission in tomato (Lycopersicon esculentum Mill.) fruit (Djanaguiraman et al., 2004).
The objective of this study was to explore the role of CMNP-induced oxidative metabolism in AZ of ‘Valencia’ sweet orange [Citrus sinensis (L.) Osbeck] and to understand oxidative association between FT and AZ to further dissect the overall mechanism of abscission. This work provided preliminary data on CMNP-induced H2O2 changes in AZ of ‘Valencia’ sweet orange and also suggests the existence of oxidative cooperation between FT and AZ to mediate CMNP-induced abscission cascade in citrus.
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