SITUACIONES DE PRESCRIPCIÓN Y VENTA DE UNA EFG

2.2 Use of exogenous ethylene for improving seed germination

In agriculture, success or failure in crop production mostly relates to rapid and successful seed germination and healthy and vigorous seedlings. Several physiological changes undergoes within the seeds that prevent seeds from growing under unfavorable conditions (Mahmood et al., 2002, 2010). But some seeds fail to break dormancy even under normal conditions. This leads to late and poor seed germination. This issue is of great concern when off-season vegetables have to grow. Thus, seed dormancy is considered one of the biggest reasons of poor seedling vigor and crop stand. Ethylene is prerequisites for breaking primary dormancy of different seeds (Matilla, 2000). Figures 2.3 and 2.4 explore the ethylene role in seed dormancy. Some seeds do not germinate and become persistent to dormancy whenever their embryonic axis fails to

Figure 2.3 Model for interactions of ethylene with abscisic acid and nitric oxide signaling pathways for regulation of seed germination and dormancy (Arc et al., 2013)

(This scheme is based on genetic analyses, microarray data, and physiological studies on seed responsiveness to ABA, ethylene, or NO. ABA binding to PYR/PYL/RCAR receptor induces the formation of a protein complex with PP2C and the inhibition of phosphatase activity. In the absence of ABA, PP2C dephosphorylate SnRK2. When ABA is present, PP2C binding to the receptor releases inhibition of SnRK2 activity, which can phosphorylate downstream targets, including ABI5-related transcription factors. Interactions between ABI3 and ABI5 mediate transcriptional regulation of ABA-responsive genes. Ethylene positively regulates its own biosynthesis, by acting on ACC synthesis catalyzed by ACS and subsequent conversion to ethylene by ACO. This last step is also subject to ABA inhibition. Ethylene is perceived by receptors (among which ETR1) located in the endoplasmic reticulum; its binding leads to the deactivation of the receptors that become enable to recruit CTR1. Release of CTR1 inhibition allows EIN2 to act as a positive regulator of ethylene signaling pathway. EIN2 acts upstream of nuclear transcription factors, such as EIN3, EILs, and ERBPs/ERFs. Ethylene down-regulates ABA accumulation by both inhibiting its synthesis and promoting its inactivation, and also negatively regulates ABA signaling. In germinating seeds, NO enhances ABA catabolism and may also negatively regulate ABA synthesis and perception. Moreover, NO promotes both ethylene synthesis and signaling pathway. ABA, abscisic acid; ABI3, ABA insensitive3; ABI5, ABA insensitive5; ACC, 1-aminocyclopropane 1-carboxylic acid; ACO, ACC oxidase; ACS, ACC synthase; CTR1, constitutive triple response 1; CYP707A, ABA-8′-hydroxylase; EIL, EIN3-like; EIN, ethylene-insensitive;

EREBP, ethylene-responsive element binding protein; ERF, ethylene response factor; Et, ethylene; ETR1, ethylene receptor1; NCED, 9-cis-epoxycarotenoid dioxygenase; NO, nitric oxide;

PP2C, clade A type 2C protein phosphatases; PYR/PYL/RCAR, pyrabactin resistance1/PYR1-like/regulatory components of ABA receptor; SnRK2, group III sucrose non-fermenting-1-related protein kinase 2; a dashed line is used when regulatory targets are not precisely identified)

Figure 2.4 Working model for the role of ethylene in germination of seeds (modified from Kucera et al., 2005)

Class I ß-1, 3-glucanase (ßGLU I) accumulates just prior to endosperm rupture and is proposed to promote radicle protrusion by weakening of the endosperm. Plant hormones and environmental factors alter the germination process and in strict correlation with this either promote (+) or inhibit (-) ßGLU I induction.

GA = gibberellin(s); ABA = abscisic acid; Pfr = Phytochrome

produce necessary C2H4 for breaking dormancy. Such seeds do not germinate until biosynthesis of C2H4 starts.

The requirement of C₂H₄ for breaking dormancy of such seeds has been verified by the use of C₂H₄ biosynthesis inhibitors and C₂H₄ releasing compounds (Matilla, 2000;

Kepczynski et al., 2003). The involvement of C₂H₄ in dormancy breakage is also reported by Brono and Taylor (1975) and Rock and Quatrano (1995). However, its effectiveness for dormancy breakage is dependent on its biological active concentration. Its biological active concentration decides whether it can break dormancy or not (Machabée and Saini, 1991; Matilla, 2000). Literature on use of exogenous C2H4 for dormancy breakage points out that the most of dormant seeds do not germinate unless seeds get 0.1 to 200 µL C₂H₄ for breaking seed dormancy (Zimmerman and Hitchcock, 1935; Gallardo et at., 1992, 1994; Carmona and Murdoch, 1995).

The use of C2H4 for breaking seed dormancy is not a new approach. In past, exogenous C2H4 was also reported to break dormancy and speed up germination rate in cocklebur (Xanthium pennsylvanicum), red root pigweed (Amaranthus retroflexus) (Egley, 1980; Schonbeck and Egley, 1981), aged Striga lutea (Egley and Dale, 1970) and aged Brassica napus seeds. Esashi et al. (1990) also reported that exogenous C2H4

boosted the germination percentage even more than anaerobiosis. Correspondingly, Schonbeck and Egley (1981) also described that exogenous C2H4 improved seed germination of red root pigweed seed but the effect was found temperature dependent.

In 1975, Esashi and Katoh (1975) pointed out C₂H₄ binding essential for seeds of many species to disrupt dormancy or stimulate germination. Adkins and Ross (1981) also confirmed this point. Due to active role of C₂H₄ in plant physiology, the researchers developed ethephon (2-chloroethylphosphonic acid) to promote C₂H₄ role in agriculture.

Numerous studies have exposed notable enhancements in germination rate and seedling vigor on the application of ethephon (Ketring and Melouk, 1982; Black et al., 1986;

Esashi et al., 1990; Corbineau and Come, 1995). Its application improved Echinacea angustifolia seeds germination up to 90% (Feghahati and Reese, 1994). It was also found to break Echinacea seed dormancy. Sari et al. (2001) and Qu et al. (2004) noticed an increase in germination rates of E. angustifolia and E. pallida seeds on treating with ethephon. They informed that exogenous C2H4 substituted chilling technique for breaking E. angustifolia and E. pallida seeds dormancy. Currently, Kepczynski and Sznigirappi (2013) described that ethephon treated dormant seeds geminated earlier than untreated seeds.

It is believed that use of exogenous C2H4 is essential for seed breakage since recently reported work on exogenous C2H4 have also explored C2H4 role as signaling agent during germination. In recent past, Baskin et al. (2003) discovered that C₂H₄ signals whether water is available or not for seed germination. Their work also suggests that the phenomenon of germination of Schoenoplectus hallii seeds only in some wet years is connected with C₂H₄ signaling for water availability. Qu et al. (2004) also reported signaling role of C₂H₄ for the presence or absence of light during germination. Seeds of Echinacea angustifolia DC and Echinacea pallida (Nutt.) treated with 1 mM ethephon showed increase in germination and seedling growth compared to untreated seeds in light and dark. Their findings also showed that ethephon promoted E. angustifolia and E.

pallida seed germination even in darkness due to C2H4 signaling mechanism. Although, seeds undergo many physiological changes for successful crop stand, but the classical triple response plays a vital role in this sense. Arshad and Frankenberger (2002) studied the reason of classical triple response in etiolated pea seedlings and finally concluded that this phenomenon was just because of methionine dependent C2H4.

Exogenous C2H4 efficiently enhanced germination of Cicer arietinum seeds (Gallardo et al., 1994), okra seeds (Kashif et al., 2008), tomato seeds (Siddiq, 2012) and other vegetable seeds (Yaseen et al., 2012). All studies suggest an increase in seed germination and seedling vigor due to exogenous C2H4. However, these effects depend on C2H4 exposure time, application method, C2H4 concentration and C2H4 source.

Furthermore, these effects also differ with variations in environmental conditions and type of genotype (Schonbeck and Egley, 1981). Overall, it can be concluded that C₂H₄ is crucial for breaking dormancy of seeds (Gallardo et al., 1994).