Elsevier

Plant Science

Volume 288, November 2019, 110242
Plant Science

Petal abscission in roses is associated with the activation of a truncated version of the animal PDCD4 homologue, RbPCD1

https://doi.org/10.1016/j.plantsci.2019.110242Get rights and content

Highlights

  • RbPCD1 encodes a truncated version of the animal PDCD4 gene.

  • It is up-regulated during ethylene-induced and natural abscission.

  • RbPCD1 expression is activated by ethylene in all tissues.

  • It acts as a transcriptional repressor and localized to the nucleus and cytoplasm.

  • RbPCD1 expression affects normal growth processes of the plant.

Abstract

Abscission is a developmental process that leads to shedding of organs not needed by the plant. Apart from wall hydrolysis, the cells of the abscission zone (AZ) are also believed to undergo programmed cell death (PCD). We show that ethylene-induced petal abscission in Rosa bourboniana is accompanied with the activation of RbPCD1 (PROGRAMMED CELL DEATH LIKE 1) encoding a protein of 78 amino acids. Its expression increases during natural and ethylene-induced petal abscission. Its transcription in most tissues is up-regulated by ethylene. RbPCD1 shows similarity to the N-terminal domain of animal PDCD4 (PROGRAMMED CELL DEATH PROTEIN 4) proteins that are activated during apoptosis and function as transcriptional and translational repressors. RbPCD1 resides in the nucleus and cytoplasm and acts as a transcriptional repressor. Constitutive expression of RbPCD1 in transgenic Arabidopsis is seedling lethal. Heat-induced expression of RbPCD1 under the soybean heat-shock promoter affects leaf function, inflorescence development, silique formation, seed yield and reduces survival. Nuclear localization of RbPCD1 is necessary for manifestation of its effects. RbPCD1 may be necessary to mediate some of the ethylene-induced changes during abscission and senescence in specific tissues.

Introduction

Abscission is a process that results in shedding of various plant parts (leaves, petals, stamens, flowers, fruits) when these are no longer needed. Cell separation occurs due to AZ-specific activation of various cell wall-modifying genes and proteins such as polygalacturonases [[1], [2], [3]], beta-1,4-glucanases [[4], [5], [6]], pectate lyases [7], xyloglucan endotransglucosylase/hydrolases [8,9], expansins [10,11] etc.

The process of separation of the entire organ proceeds within the abscission zone, a small defined zone of cells that are smaller, cytoplasmically denser and morphologically and physiologically distinct from neighbouring cells [[12], [13], [14], [15], [16]]. The AZ cells possess differential sensitivity to hormones like ethylene and auxin and are able to uniquely respond to abscission cues through regulation of a specific set of genes within the abscission zone [[17], [18], [19], [20], [21], [22]].

Since abscission zone cells ultimately undergo cell wall disassembly and cell separation during the process, there has been speculation as to whether AZ cells undergo programmed cell death as observed during senescence. Previous observations have shown indications of PCD during the abscission processes in different plants. In Pelargonium, petal abscission was accompanied with protoplast shrinkage and cell wall degradation in AZ cells [23] while in Chamelaucium uncinatum, cells immediately adjacent to the AZ lost viability [24]. In Prunus, leaf abscission was associated with cell wall degradation and release of cytoplasm followed by cell death [25]. One of the most prominent features of PCD namely DNA degradation, chromatin condensation, loss of cell viability and change in nuclear structure was observed during tomato pedicel abscission [26]. Nucleases such as the LX ribonuclease associated with ethylene responses, senescence and programmed cell death [[27], [28], [29]] were also found to be involved in abscission since antisense LX tomato plants showed delayed abscission of leaves [30]. Similarly, the BIFUNCTIONAL ENDONUCLEASE gene (BFN1), active in the leaf and the fruit abscission zones of both Arabidopsis and tomato, was found to be associated with senescence in Arabidopsis [31] and developmental PCD in tomato [32]. Transcriptomic studies during abscission in apple fruitlets [20] and soybean, tomato and Arabidopsis [33] have identified genes associated with PCD, autophagy and those encoding cysteine proteases and clp proteases.

We are interested in studying the regulation of petal abscission in ethylene-sensitive fragrant roses and the various changes occurring during progression of ethylene-mediated petal abscission [[7], [8], [9],11,22,34]. We have previously shown that rapid transcription of RbCP1 during petal abscission and the expression of a 37 kDa cysteine protease (encoded by RbCP1) resulted in decrease in total protein content during abscission in rose [34] suggesting that petal AZ cells may undergo PCD. In this study, we have identified RbPCD1 as an abscission up-regulated gene, encoding a novel truncated version of the conserved animal PDCD4 proteins that is seedling lethal when expressed in transgenic Arabidopsis. Its ability to suppress transcription and its up-regulation by ethylene in different tissues suggests that it may actively participate to mediate ethylene responses in processes like abscission and senescence.

Section snippets

Plant material, growth and treatments

Flowers of the ethylene-sensitive, early-abscising fragrant rose (Rosa bourboniana var. Gruss an Teplitz) were used to isolate the gene and to examine gene expression. Treatment of flowers of R. bourboniana (ethylene-sensitive, early-abscising) and R. hybrida (var. Opening Night, less ethylene-sensitive, late-abscising) with and without ethylene was carried out essentially as described [7,11]. The petal abscission zone tissue was the 2 mm2 tissue of the petal base in contact with the thalamus [7

Isolation and sequence analysis of RbPCD1

To obtain an insight into the genes that are expressed differentially in the AZ, an analysis of the abscission zone transcriptome was carried out using mRNA differential display. DDRT-PCR was performed as described [49] with minor modifications that included the use of γ32P-ATP for end-labeling of anchor primers to get uniform band intensity, instead of α32P-dCTP at the DDRT-PCR step [37]. One of the genes identified by mRNA differential screening of abscission up-regulated RNAs [50], RbPCD1,

Discussion

Organ abscission is strongly responsive to ethylene and often accompanied with PCD-like features [23,24,30,32,56,57]. The clearest evidence for PCD, however, came from studies in tomato flower and leaf AZ tissues wherein a disruption of nuclear shape and structure was observed in the later stages of abscission, indicating chromatin disorganization [26].

We had previously shown that rose petal abscission was accompanied with the expression of an ethylene-inducible cysteine protease gene, RbCP1 [34

Acknowledgements

We are grateful to Prof. Shucai Wang (Northeast Normal Univ, Changchun, China) for effector and reporter vectors for protoplast transfection study and to Dr Ute Hoecker (Univ of Cologne, Germany) for the gift of the pRTL.2 vector. We thank Dr Sadaf Khan (a former post doc in our lab) and Dr PA Shirke (Dept of Plant Physiology, CSIR-NBRI) for help with the PAM-Imaging. We are grateful to Mr Ram Awadh for taking care of the rose plants. PS, APS, SKT and VK were supported by the Council of

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    1

    Current address: National Institute for Plant Genome Research, New Delhi-110067, India.

    2

    Current address: National Centre for Natural Products Research, School of Pharmacy, University of Mississippi, MS 38677, USA.

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