Elsevier

Gene

Volume 597, 15 January 2017, Pages 30-39
Gene

Research paper
Distinct gene networks drive differential response to abrupt or gradual water deficit in potato

https://doi.org/10.1016/j.gene.2016.10.024Get rights and content

Highlights

  • Transcriptomic changes under abrupt or gradual exposure to water stress were studied in potato cell cultures.

  • Water stress adaptation requires a deeper transcriptomic reprogramming than abrupt stress.

  • Modulation of ethylene signalling network as a unique trait of potato cell adaptation.

  • Stress responsive genes identified in potato cells were differentially expressed in vivo as well.

Abstract

Water-limiting conditions affect dramatically plant growth and development and, ultimately, yield of potato plants (Solanum tuberosum L.). Therefore, understanding the mechanisms underlying the response to water deficit is of paramount interest to obtain drought tolerant potato varieties. Herein, potato 10 K cDNA array slides were used to profile transcriptomic changes of two potato cell populations under abrupt (shocked cells) or gradual exposure (adapted cells) to polyethylene glycol (PEG)-mediated water stress. Data analysis identified > 1000 differentially expressed genes (DEGs) in our experimental conditions. Noteworthy, our microarray study also suggests that distinct gene networks underlie the cellular response to shock or gradual water stress. On the basis of our experimental findings, it is possible to speculate that DEGs identified in shocked cells participate in early protective and sensing mechanisms to environmental insults, while the genes whose expression was modulated in adapted cells are directly involved in the acquisition of a new cellular homeostasis to cope with water stress conditions.

To validate microarray data obtained for potato cells, the expression analysis of 21 selected genes of interest was performed by Real-Time Quantitative Reverse Transcription PCR (qRT-PCR). Intriguingly, the expression levels of these transcripts in 4-week old potato plants exposed to long-term water-deficit. qRT-PCR analysis showed that several genes were regulated similarly in potato cells cultures and tissues exposed to drought, thus confirming the efficacy of our simple experimental system to capture important genes involved in osmotic stress response. Highlighting the differences in gene expression between shock-like and adaptive response, our findings could contribute to the discussion on the biological function of distinct gene networks involved in the response to abrupt and gradual adaptation to water deficit.

Introduction

Water limiting conditions drastically affect plant growth and production. In particular, water deficit, which causes also osmotic stress, severely impairs plant development through the imposition of molecular and physiological changes that, in case of severe and durable water deficit, lead to disorganization of cell membranes, metabolic toxicity and inhibition of photosynthesis (Zhu, 2001). However, even non tolerant plants respond and might adapt to drought by modulating the expression of several genes (Seki et al., 2003, Shinozaki et al., 2003, Urano et al., 2010, Yamaguchi-Shinozaki and Shinozaki, 2005). Usually, stress-related genes govern important cell mechanisms such as protein metabolism, transcriptional and post-transcriptional regulation, signalling and stress perception, ion homeostasis, detoxification and damage repair (Ambrosone et al., 2012, Bray, 2002, Rensink et al., 2005, Shinozaki et al., 2003). However, the survival or death of plants experiencing water deficit is known to depend not only on the plant genetic background, developmental stage and morphology, but also on the severity and duration of the stress event (Leone et al., 1994a). Therefore, while gradual and continuous imposition of water deficit can produce metabolic and molecular modifications that allow a new cellular homeostasis and active growth even in not optimal environmental conditions, a short exposure to severe water stress may activate genes involved in the prevention/repair of cellular damage, as well as genes involved in the stress perception/signalling function (Ambrosone et al., 2011, Leone et al., 1994b).

Many varieties of S. tuberosum, the most commonly consumed potato, are very sensitive to water-limiting conditions, which reduce marketable yield (Deblonde and Ledent, 2001, Weisz et al., 1994). Potato is the most important non-grain food crop, and given the increased occurrence of drought and its severity, great efforts need be addressed to understand genetic mechanisms controlling potato plant adaptation to water deficit. In that respect, recently published outcomes from the potato genome sequencing project have provided important genomic information allowing dissecting complex genetic traits of agronomical interest (The Potato Genome Sequencing Consortium, 2011).

Over the past two decades, we deeply investigated the physiological and molecular responses of potato cell cultures exposed to osmotic stresses, demonstrating that they respond in different ways if exposed abruptly or gradually to water deficit elicited by PEG addition into the culture medium (Leone et al., 1994a). Our previous studies indicated that water stress, when imposed gradually to potato cells, is compatible with active cellular growth, primarily due to restored normal protein synthesis (Leone et al., 1994b), proline and polyamine accumulation (Scaramagli et al., 2000) and changes in membrane composition and fluidity (Leone et al., 1996).

Although one can argue that a cellular system may not fully represent the complex in vivo plant behaviour to a changeable environment, investigating plant stress response at cellular level allows to identify genes mostly involved in cell stress perception and transduction, as well as in controlling and maintaining cellular homeostasis. Worth of note, several lines of evidence indicate that the experimental conditions used to impose plant water stress frequently do not reflect what really happens in soil. For instance, sudden exposure to severe water stress occur rarely in the field, while more often plants need to adapt to natural fluctuation of environmental conditions which make the physiological response much more complex than a simple experimental binary condition (stress vs control). Overall, this suggests that transcriptional changes conferring tolerance to severe water stress may not sustain plant growth under gradual adaptation to water limiting conditions (Claeys and Inzé, 2013, Claeys et al., 2014, Clauw et al., 2015). As consequence, the comprehension of these distinct dynamics is really appealing for genetic improvement programs conceived to select drought-resistant varieties (Blum, 2005).

With this respect, comparative transcription profiles of the response to abrupt or gradual water stress conditions may provide additional information about genes regulated in severe water stress that might belong to early responsive genes, providing initial protection and amplification of primary osmotic/water stress signals, and about genes whose expression is changed during gradual adaptation that might be involved in tolerance to stress conditions.

Because of the complexity of the overall plant response to drought stress, in recent years global transcriptome analyses have replaced the gene-by-gene approaches in different plant species to identify genes whose expression is regulated in response to water stress (Gullì et al., 2015, Iovieno et al., 2016, Kreps et al., 2002, Li et al., 2016, Roche et al., 2008, Seki et al., 2002, Xue et al., 2008) or to other abiotic stresses since it allows the simultaneous analysis of a great number of genes (Legay et al., 2009).

This study was aimed to capture and compare transcriptional profile changes in potato cells exposed to abrupt (shock) or gradual imposition (adaptation) to water stress by using a potato 10 K cDNA array slides containing approximately 10,000 potato cDNA clones. By exploiting an alternative route towards the identification of water stress responsive genes, we provided evidences that distinct gene networks are regulated in potato cell exposed to abrupt or gradual adaptation to water stress. Furthermore, to verify whether genes involved in the response of the whole plant to stress can be identified by an in vitro simplified cellular system, a selected number of stress responsive genes in potato cells was monitored in potato plants subjected to a mild drought-stress treatment. In conclusion, we demonstrated that potato cells respond to water deficit according to the intensity and the duration of water limiting conditions providing unique molecular signatures for potato stress adaptation in vitro and in vivo.

Section snippets

Cell cultures, plant growth and stress treatments

Potato cell cultures were obtained from leaf callus of potato plants and subcultured every 7 days. Cells were maintained in a modified Murashige and Skoog medium in a rotary shaker at 28 °C in the dark as described in Leone et al., 1994a. Gradual acclimation to low water potential was achieved by transferring cells to nutrient medium containing increasing concentrations of PEG 8000 (5, 10, 15 and 20% w/v). Cells were maintained at each PEG concentration for at least 2 subculture cycles and then

Potato transcriptome in response to water stress

In order to dissect global gene expression modifications occurring during gradual plant adaptation with respect to abrupt water stress, large-scale expression analyses were conducted by microarray technology in three potato cell populations (Fig. S1): i) cell cultures maintained in optimal growth conditions (control); ii) cell cultures, exposed abruptly to hyperosmotic medium containing PEG (20% w/v) for 24 h (shocked cells); iii) cell suspensions cultured under long-term water stress conditions

Discussion

Osmotic stress induces deep alterations of plant biochemical and physiological processes with obvious and well known consequences on crop productivity (Araus et al., 2002). The activation of stress-responsive mechanisms depend strictly on the duration and intensity of stress as well as on plant capability to trigger cellular responses in short and long-term and consequently to limit damage and preserve the cell structures (e.g. in shock conditions) and to restore a new homeostatic balance (e.g.

Conflict of interest

The authors declare that they have no competing interests.

Aknowledgements

This work was funded by a grant from the Italian Ministry of Research (MIUR, PON02_00395_3215002 ‘GenHort’). We express our thanks to Gaetano Guarino, Rosario Nocerino and Roberta Nurcato for excellent technical assistance.

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    Present Address: Instituto de Ciencias de Materiales de Aragón (ICMA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain.

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