ABA-Mediated Inhibition of Germination Is Related to the Inhibition of Genes Encoding Cell-Wall Biosynthetic and Architecture: Modifying Enzymes and Structural Proteins in Medicago truncatula Embryo Axis

Abstract

Radicle emergence and reserves mobilization are two distinct programmes that are thought to control germination. Both programs are influenced by abscissic acid (ABA) but how this hormone controls seed germination is still poorly known. Phenotypic and microscopic observations of the embryo axis of Medicago truncatula during germination in mitotic inhibition condition triggered by 10 microM oryzalin showed that cell division was not required to allow radicle emergence. A suppressive subtractive hybridization showed that more than 10% of up-regulated genes in the embryo axis encoded proteins related to cell-wall biosynthesis. The expression of alpha-expansins, pectin-esterase, xylogucan-endotransglycosidase, cellulose synthase, and extensins was monitored in the embryo axis of seeds germinated on water, constant and transitory ABA. These genes were overexpressed before completion of germination in the control and strongly inhibited by ABA. The expression was re-established in the ABA transitory-treatment after the seeds were transferred back on water and proceeded to germination. This proves these genes as contributors to the completion of germination and strengthen the idea that cell-wall loosening and remodeling in relation to cell expansion in the embryo axis is a determinant feature in germination. Our results also showed that ABA controls germination through the control of radicle emergence, namely by inhibiting cell-wall loosening and expansion.

Disruption of Abscisic Acid Signaling Constitutively Activates Arabidopsis Resistance to the Necrotrophic FungusPlectosphaerella cucumerina

“…described cell wall alterations found in the tomato ABAdeficient sitiens mutant (Asselbergh et al, 2007) and with the hypothesized modulation of cell wall architecture by ABA signaling (Gimeno-Gilles et al, 2009). The relevance of the cell wall in directly mediating the resistance of Arabidopsis to different pathogens, including PcBMM, has been reported previously (Hernández-Blanco et al, 2007;Denoux et al, 2008;Sánchez-Rodríguez et al, 2009;Ramírez et al, 2011;Delgado-Cerezo et al, 2012).…”

Rapid endocytosis is triggered upon imbibition in Arabidopsis seeds

“…36 In embryo axes of non-dormant Medicago truncatula seeds germinating on water, the expression of genes involved in cell wall biosynthesis: xyloglucan endotransglucosylase (XET), cellulose synthase, arabinogalactan protein, two extensins and PPRD (protein with proline-rich domain) increased throughout germination. 37 More importantly, genes known to play a key role in cell-wall loosening and expansion, such as expansins, 1-3 glucanase and XET, were significantly expressed in the embryos as early as 5 or 15 h of imbibition, namely before germination completion and radicle extension. 37 It has been proposed that the endocytic delivery of cell wall pectins represents a ready-to-use reservoir of building blocks for rapid cell wall formation during cytokinesis 38 or cell expansion 39 and emerges to be critical for the polarity of pollen tubes.…”

“…37 More importantly, genes known to play a key role in cell-wall loosening and expansion, such as expansins, 1-3 glucanase and XET, were significantly expressed in the embryos as early as 5 or 15 h of imbibition, namely before germination completion and radicle extension. 37 It has been proposed that the endocytic delivery of cell wall pectins represents a ready-to-use reservoir of building blocks for rapid cell wall formation during cytokinesis 38 or cell expansion 39 and emerges to be critical for the polarity of pollen tubes. 40 So, the internalized RG-II observed here could operate as an intracellular pectin reservoir for the rapid cell expansion that proceeds upon imbibition.…”

Overexpression of AtMYB52 Confers ABA Hypersensitivity and Drought Tolerance

“…Thus, our observations raise an interesting possibility that ABA hypersensitivity of the AtMYB52 OX lines may be associated with changes in cell wall architecture. It has been reported that ABA inhibits seed germination by inhibiting cell-wall loosening and expansion (Gimeno-Gilles et al, 2009). On the other hand, cell wall arabinan is essential for guard cell function (Jones et al, 2003; and seedling growth (Gomez et al, 2009).…”