Floral organ abscission is regulated by a positive feedback loop

Abstract

Abscission is the process by which plants shed unwanted organs, either as part of a natural developmental program or in response to environmental stimuli. Studies in Arabidopsis thaliana have elucidated a number of the genetic components that regulate abscission of floral organs, including a pair of related receptor-like protein kinases, HAESA and HAESA-like 2 (HAE/HSL2) that regulate a MAP kinase cascade that is required for abscission. HAE is transcriptionally up-regulated in the floral abscission zone just before cell separation. Here, we identify AGAMOUS-like 15 (AGL15; a MADS-domain transcription factor) as a putative regulator of HAE expression. Overexpression of AGL15 results in decreased expression of HAE as well as a delayed abscission phenotype. Chromatin immunoprecipitation experiments indicate that AGL15 binds the HAE promoter in floral receptacles. AGL15 is then differentially phosphorylated through development in floral receptacles in a MITOGEN-ACTIVATED PROTEIN KINASE KINASE 4/5-dependent manner. MAP kinase phosphorylation of AGL15 is necessary for full HAE expression, thus completing a positive feedback loop controlling HAE expression. Together, the network components in this positive feedback loop constitute an emergent property that regulates the large dynamic range of gene expression (27-fold increase in HAE) observed in flowers when the abscission program is initiated. This study helps define the mechanisms and regulatory networks involved in a receptor-mediated signaling pathway that controls floral organ abscission.A bscission is the process that plants use to shed unwanted organs. Various plants can abscise leaves, fruits, and flowers. One of the most noticeable abscission events occurs when deciduous trees and shrubs shed their leaves in the fall. Plants can abscise organs as part of a developmental program or in an inducible manner in response to stimuli like abiotic or biotic stress. For example, tomatoes can abscise leaves and flowers in response to drought or insect feeding (1-3). In order for abscission to occur, a layer of small and cytoplasmically dense cells, known as an abscission zone, must be laid down during development at the boundary between the organ to be abscised and the body of the plant (4).Arabidopsis thaliana has been used to elucidate a number of the molecular and genetic components that regulate abscission of floral organs. In Arabidopsis, floral organs abscise shortly after pollination, which corresponds to stage-16 flowers or approximately floral position 4-6, where anthesis is defined as position 1 and older flowers are defined as increasing numerical positions ( Fig. 1 A and B) (5). A number of genetic components regulating this abscission process have been described thoroughly in recent reviews (6, 7). In brief, a pair of related receptor-like protein kinases, HAESA and HAESA-like 2 (HAE/HSL2), are required for floral abscission (Fig. 1C) and are thought to be triggered by a peptide derived from INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) (8, 9). A mitogen-act...

Core Mechanisms Regulating Developmentally Timed and Environmentally Triggered Abscission

“…Single mutants of HAE are identical to wild-type plants in terms of floral organ abscission but are clearly defective in drought-triggered leaf abscission. To date, no mutations have been discovered that make floral organ abscission occur before pollination, although the double mutant agl15 agl18 and IDA overexpressors abscise at an earlier floral position (Stenvik et al, 2006;Patharkar and Walker, 2015). Perhaps the more quantitative nature of leaf abscission will allow the discovery of mutations that cause more leaves to fall off when a plant experiences drought.…”

“…However, since we found ABA is not essential for abscission in Arabidopsis, it is more likely ABA acts as an additive factor rather than an initiator. HAE coexpressed genes have overrepresented AtMYC2 binding sites in their promoters (Patharkar and Walker, 2015). AtMYC2 is a basic helixloop-helix transcription factor that can activate transcription of ABA-responsive genes (Abe et al, 2003).…”

“…AtMYC2 is a basic helixloop-helix transcription factor that can activate transcription of ABA-responsive genes (Abe et al, 2003). Another possible way HAE transcription could be activated in abscission zones of wilted cauline leaves is by activation of the MAPK cascade that leads to HAE expression through inactivation of the transcriptional repressor AGL15 (Patharkar and Walker, 2015;Patharkar et al, 2016).…”

“…RNA isolation, reverse transcription, and quantitative PCR were performed as described previously (Patharkar and Walker, 2015), except Hot Start Taq (New England Biolabs) was used in place of Platinum Taq. Primers for HAE and the reference gene At5g46630 were described previously (Patharkar and Walker, 2015). Forward 59-GAGTAGTCCTTGTGTAGCGG-39 and reverse 59-TCTTA-GAAGGAGCAGAAGGAG-39 were used as primers for IDA quantification.…”

“…Briefly, a molecular pathway has been described in which a peptide derived from INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) is thought to trigger a pair of redundant receptor-like protein kinases, HAESA (HAE) and HAESA-LIKE2 (HSL2). A mitogenactivated protein kinase (MAPK) pathway, consisting of MAPK KINASE4 (MKK4)/MKK5 and MAPK3 (MPK3)/MPK6, positioned genetically downstream of HAE/HSL2, phosphorylates a MADS-domain transcription factor, AGAMOUS-LIKE15 (AGL15), which results in a transcriptional increase of HAE, thereby creating a positive feedback loop Stenvik et al, 2008;Patharkar and Walker, 2015). NEVERSHED (NEV), an ADP-ribosylation factorGTPase-activating protein, is thought to traffic HAE/HSL2 and other receptor-like protein kinases to the correct subcellular location and is also required for abscission (Liljegren et al, 2009;Burr et al, 2011;Liljegren, 2012).…”