ALTERED MERISTEM PROGRAM1 has conflicting effects on the tolerance to heat shock and symptom development after Pseudomonas syringae infection

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Highlights

  • A novel amp1-32 mutant shows aberrant growth and development phenotypes.

  • The amp1-32 mutant tolerates heat treatment, but shows severe symptom development after P. syringae infection.

  • Mutation of the AMP1 causes transcriptional reprogramming in Arabidopsis.

Abstract

An Arabidopsis thaliana ALTERED MERISTEM PROGRAM1 (AtAMP1), which encodes a putative glutamate carboxypeptidase, not only controls shoot apical meristem development, but also is involved in tolerance response to abiotic stresses. Here, we introduce a novel mutant; named amp1-32 that is a phenocopier to previously isolated different amp1 mutant alleles. Interestingly, tiny leaves were continuously developed at the bottom of pre-emerged leaves in the amp1-32. The amp1-32 mutant was less sensitive to heat shock treatment lasting for 3 h, whereas disease symptoms were severely developed in the mutant after Pseudomonas syringae infection. The mRNA levels of 171 genes were significantly altered in the mutant, as compared to wild-type plants. The transcription of genes involved in hormone signaling, post-embryonic development, and shoot development were up-regulated in the amp1-32 mutant, whereas expression of genes related to responsiveness to pathogens and (in)organic matters, were decreased in the mutant. Taken together, perturbation of CK- and ABA-related events by AMP1 mutation caused aberrant development phenotype and conflicting responses against abiotic and biotic stresses in Arabidopsis.

Introduction

As sessile living organisms, plants must maintain optimal energy allocation between growth/development, and immunity/tolerance under inevitable environmental changes. Plant hormones, such as cytokinins (CK), auxin, salicylic acid (SA), and abscisic acid (ABA), are mainly responsible for the energy allocation by regulating the expression of hormone-responsive genes [1], [2]. Generally, phytohormones important for growth and development have antagonistic effects on plant immunity, and vice versa [3], [4], [5]. So far, a number of genes associated with the crosstalk between hormones have been molecularly identified [3], [6], [7].

Over the last few decades, several research groups have identified Arabidopsis thaliana altered meristem program1 (amp1) mutants, also named constitutive morphogenesis2 (cop2), hauptling (hpt), and primordia timing (pt) [8], [9], [10], [11], [12], [13], [14]. The amp1/cop2/hpt/pt mutants had an increased level of CK, which results in several aberrant developmental features, such as high rate of leaf initiation, abnormal number of cotyledon, and altered flowering time. The pleiotropic effect of AMP1 was due to a negative role of AMP1, repressing cell division during embryogenesis [12], [15]. In addition, the amp1 mutant was not only hypersensitive to exogenous ABA treatment, but they also accumulated a higher amount of ABA than wild-type plants under abiotic stress conditions, which implies involvement of AMP1 in ABA-related signaling, as well as in CK-dependent events [16], [17], [18].

The AMP1 gene, which encodes a putative glutamate carboxylpeptidase, is transcribed in all tissues of Arabidopsis in a CK- and ABA-independent manner [17], [19]. Interestingly, exogenous treatment of ABA triggered AMP1 protein degradation [17]. In contrast, the protein stability of AMP1 has still remained elusive in CK-treated plants. However, this result does strongly suggest that the AMP1 protein must be excluded to activate CK and/or ABA-related signaling in Arabidopsis. Indeed, several genes, such as a cyclin CycD3, β-amylase, and a pyruvate decarboxylase, whose expression is tightly associated with CK signaling, were strongly expressed in amp1 mutant [14], [19]. In addition, a mutation of AMP1 gene led to increased transcription of ABA-responsive genes in Arabidopsis [16], [17].

MicroRNAs (miRNAs) regulate gene expression via the degradation of their target mRNAs, and translational inhibition [20]. Compared with the cleavage of target mRNAs, a molecular mechanism for miRNA-mediated translational repression is poorly clarified [21], [22], [23]. Fascinatingly, AMP1 protein and its paralog LIKE AMP1 (LAMP1), together with miRNA, interrupted the association of target mRNA with the ER membrane-bound polysome to repress protein biosynthesis [23]. It was also reported that the AMP1 mutation did not affect transcription of target genes for certain miRNAs in plants [21], [23], [24]. In agreement with these results, microarray analysis using 7000 unique genes revealed that only 4 genes were strongly expressed in the amp1-1/pt mutant alleles, as compared with wild type [19].

Here, we describe a novel amp1 mutant allele, called amp1-32. Like the other amp1 mutant alleles previously described, nonsense mutation of the AMP1 had pleiotropic effects on multiple traits of Arabidopsis in a photoperiod-independent manner. The amp1-32 mutant was less sensitive than wild type against heat shock stress, but showed severe symptom development in response to Pseudomonas syringae infection. The mutation, however, did not affect the multiplication of the bacterial population in the infected leaves. Finally, whole transcriptome analysis of amp1-32 mutant revealed that the transcription of genes involved in plant development, hormone signaling, and immunity, was controlled by AMP1.

Section snippets

Plants and pathogens

Arabidopsis ecotype Columbia-0 (Col-0), Landsberg erecta (Ler-0), and amp1-32 plants (Col-0 background), grew in potting mixture (Dongbu Farm Hannong, Korea), in an environmentally controlled growth chamber (21 ± 1 °C, 50–60% relative humidity, 12 h day and 12 h night). Seeds of wild type, and amp1-32 mutant were sequentially sterilized with 70% ethanol and 50% bleach solution, and kept at 4 °C for 2–3 days until sowing. Plants were watered twice per day during the whole growth period.

A

A novel amp1-32 mutant shows aberrant growth and development phenotypes

During bulk multiplication of Arabidopsis T-DNA insertion mutant collection, we found a mutant plant with abnormal leaf development patterns, as compared with wild type plants. Since the mutant phenotype did not link to the T-DNA insertion mutation (data not shown), the mutant plants were crossed with Arabidopsis ecotype Ler-0 in order to pinpoint a causal mutation. Morphological phenotypes of the F1 hybrids resembled those of the Col-0 and Ler-0 plants, indicating that the mutation was

Conflict of interest

The authors declare no conflict of interest.

Acknowledgements

This research was supported by the Junior Principle Investigators Program (PJ009789) from the Rural Development Administration of Korea, to HWJ.

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    These authors equally contributed in this paper.

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