NEVERSHED and INFLORESCENCE DEFICIENT IN ABSCISSION are differentially required for cell expansion and cell separation during floral organ abscission in Arabidopsis thaliana

Abstract

Floral organ shedding is a cell separation event preceded by cell-wall loosening and generally accompanied by cell expansion. Mutations in NEVERSHED (NEV) or INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) block floral organ abscission in Arabidopsis thaliana. NEV encodes an ADP-ribosylation factor GTPase-activating protein, and cells of nev mutant flowers display membrane-trafficking defects. IDA encodes a secreted peptide that signals through the receptor-like kinases HAESA (HAE) and HAESA-LIKE2 (HSL2). Analyses of single and double mutants revealed unique features of the nev and ida phenotypes. Cell-wall loosening was delayed in ida flowers. In contrast, nev and nev ida mutants displayed ectopic enlargement of abscission zone (AZ) cells, indicating that cell expansion alone is not sufficient to trigger organ loss. These results suggest that NEV initially prevents precocious cell expansion but is later integral for cell separation. IDA is involved primarily in the final cell separation step. A mutation in KNOTTED-LIKE FROM ARABIDOPSIS THALIANA1 (KNAT1), a suppressor of the ida mutant, could not rescue the abscission defects of nev mutant flowers, indicating that NEV-dependent activity downstream of KNAT1 is required. Transcriptional profiling of mutant AZs identified gene clusters regulated by IDA-HAE/HSL2. Several genes were more strongly downregulated in nev-7 compared with ida and hae hsl2 mutants, consistent with the rapid inhibition of organ loosening in nev mutants, and the overlapping roles of NEV and IDA in cell separation. A model of the crosstalk between the IDA signalling pathway and NEV-mediated membrane traffic during floral organ abscission is presented.

Core Mechanisms Regulating Developmentally Timed and Environmentally Triggered Abscission

“…A lot of recent attention has been given to activation of cell wall-modifying enzymes that hydrolyze the middle lamella of the abscission zone as the implied mechanism of cell separation (Niederhuth et al, 2013a(Niederhuth et al, , 2013bLiu et al, 2013). Our findings strongly suggest that while drought triggers abscission, water is required for abscission to occur.…”

Four shades of detachment: Regulation of floral organ abscission

“…Interestingly, a recent comparative study indicated that the role of IDA in cell separation (Phase 3) and organ separation (Phase 4) could differ based on the strength of the allele and the ecotype backgrounds. 44 Petal break-strength measurements uncovered that the alleles of ida-1 (C24) and ida-2 (Col) behaved differently in cell separation (Phase 3) and organ separation (Phase 4). 44 Incomplete organ separation in the ida-1 mutant (compared to ida-2) indicates that cell wall loosening (or cell wall disassembly) alone (Phase 3) may be insufficient to fully separate the organs from the parent plant.…”

“…44 Petal break-strength measurements uncovered that the alleles of ida-1 (C24) and ida-2 (Col) behaved differently in cell separation (Phase 3) and organ separation (Phase 4). 44 Incomplete organ separation in the ida-1 mutant (compared to ida-2) indicates that cell wall loosening (or cell wall disassembly) alone (Phase 3) may be insufficient to fully separate the organs from the parent plant. This study also indicates that successful organ separation requires a coordination of events that is absent from the ida-1 mutants, or simply implicates that the expression of IDA-like genes in floral AZs is higher in Col than in C24 that may not require the substantially different signaling cascades existing in Col than C24.…”

“…8 Whereas nev mutants display distinct uncontrolled cell expansion late in abscission, the AZ cell morphology early in abscission is similar to ida mutants and appears to be normal. 44 …”

“…49 Collectively, both parallel and convergent signaling pathways mediated by NEV-and IDA-HAE/HSL2 modules contribute to floral organ abscission. [9][10][11]44,49 Based on the petal break-strength profiles and the collective observations of uncontrolled cell enlargement and expansion in nev mutants, the role of NEV-mediated intracellular trafficking is required for cell separation (Phase 3), and for organ separation and cell expansion (Phase 4) (Fig. 1B(iii)).…”