Regulation of floral organ abscission in Arabidopsis thaliana

Abstract

Abscission is a developmental program that results in the active shedding of infected or nonfunctional organs from a plant body. Here, we establish a signaling pathway that controls abscission in Arabidopsis thaliana from ligand, to receptors, to downstream effectors. Loss of function mutations in Inflorescence Deficient in Abscission (IDA), which encodes a predicted secreted small protein, the receptor-like protein kinases HAESA (HAE) and HAESA-like 2 (HSL2), the Mitogen-Activated Protein Kinase Kinase 4 (MKK4) and MKK5, and a dominant-negative form of Mitogen-Activated Protein Kinase 6 (MPK6) in a mpk3 mutant background all have abscission-defective phenotypes. Conversely, expression of constitutively active MKKs rescues the abscission-defective phenotype of hae hsl2 and ida plants. Additionally, in hae hsl2 and ida plants, MAP kinase activity is reduced in the receptacle, the part of the stem that holds the floral organs. Plants overexpressing IDA in a hae hsl2 background have abscission defects, indicating HAE and HSL2 are epistatic to IDA. Taken together, these results suggest that the sequential action of IDA, HAE and HSL2, and a MAP kinase cascade regulates the programmed separation of cells in the abscission zone.protein phosphorylation ͉ signal transduction A bscission is a physiological process that involves the programmed separation of entire organs, such as leaves, petals, flowers, and fruit. Abscission allows plants to discard nonfunctional or infected organs, and promotes dispersion of progeny. At the cellular level, abscission is the hydrolysis of the middle lamella of an anatomically specialized cell layer, the abscission zone (AZ), by cell wall-modifying and hydrolyzing enzymes. Thus, abscission requires both the formation of the AZ early in the development of a plant organ and the subsequent activation of the cell separation response (1-4).Studies using Arabidopsis thaliana have implicated the involvement of several different genes in the control of abscission including potential signal molecules, receptors and other gene products (4). HAESA (HAE), one of the first Arabidopsis receptor-like protein kinases (RLK) identified, is expressed in floral organ AZs and antisense experiments show a reduction in the level of HAE protein is correlated with the degree of defective floral organ abscission. Expression of HAE is not altered in etr1-1 (an ethylene-insensitive mutation), implying an ethyleneindependent role in abscission (5). Inflorescence Deficient in Abscission (IDA) encodes a small protein with an N-terminal signal peptide. Analysis of ida mutant plants indicates IDA regulates floral organ abscission through an ethylene insensitive pathway (6). Overexpression of IDA results in early abscission and the production of a white substance in the floral AZs. The main components of the white substance are arabinose and galactose (7).Here, we report that components of a MAPK signaling cascade also have roles in the regulation of abscission. A MAPK cascade is a regulatory module with three protein kina...

PEPR2 Is a Second Receptor for the Pep1 and Pep2 Peptides and Contributes to Defense Responses inArabidopsis

“…CLAVATA1 (CLV1), BAM1 (for barely any meristem 1), BAM2, and BAM3 are required for meristem function (Clark, et al, 1997;DeYoung, et al, 2006). HAESA (Jinn et al, 2000) and HSL2 (for HAESA like 2) regulate floral organ abscission (Cho et al, 2008;Stenvik et al, 2008). PXY/TDR is a receptor of TDIF (for tracheary element differentiation inhibitory factor) (Fisher and Turner, 2007;Hirakawa et al, 2008).…”

“…It is possible that the induction of defense-related genes and enhancement of basal resistance by Pep1 occurs through activating the same MAPK cascade after perception by PEPR1 and PEPR2. There is precedence for MAPK signaling downstream of LRR RLKs from studies of the roles of HAESA and HAESA LIKE2, which also belong to LRR XI subfamily (Cho et al, 2008;Stenvik et al, 2008).…”

CAST AWAY, a Membrane-Associated Receptor-Like Kinase, Inhibits Organ Abscission in Arabidopsis

“…Since NEV, IDA, HAE, and HSL2 each regulates the cell separation stage of organ separation (Butenko et al, 2003;Cho et al, 2008;Stenvik et al, 2008;Liljegren et al, 2009), we tested whether disruption of CST activity might also suppress the ida and hae hsl2 mutant phenotypes. We found that mutations in CST do not rescue the shedding defects of ida or hae hsl2 flowers (Fig.…”

“…The primers and restriction enzyme for genotyping these alleles are described in Supplemental Table S2. Mutant alleles described previously include hae-1, hsl2-1, ida-2, nev-2, nev-3, and nev-6 (Cho et al, 2008;Stenvik et al, 2008;Liljegren et al, 2009).…”

“…The HAESA (HAE) and HAESA-LIKE2 (HSL2) LRR-RLKs redundantly activate a mitogen-activated protein kinase signaling cascade that leads to cell separation and release of the outer floral organs (Jinn et al, 2000;Cho et al, 2008). The predicted signaling ligand for HAE/ HSL2 is INFLORESCENCE DEFICIENT IN ABSCIS-SION (IDA), a small, secreted peptide (Butenko et al, 2003;Cho et al, 2008;Stenvik et al, 2008). Two inhibitors of organ separation that may directly regulate HAE/HSL2 are the EVERSHED (EVR) and SOMATIC EMBRYOGENESIS RECEPTOR-LIKE KINASE1 (SERK1) LRR-RLKs Lewis et al, 2010).…”