A tomato heat-tolerant mutant shows improved pollen fertility and fruit-setting under long-term ambient high temperature

https://doi.org/10.1016/j.envexpbot.2020.104150Get rights and content

Highlights

  • 15 heat tolerant tomato lines screening from over 4000 EMS Micro-Tom mutant populations displayed in two types of fruit-setting: parthenocarpic and seeded fruits.

  • Long-term heat stress (HS) 35° C increased the deform flowers and damaged 50 % pollen viability.

  • HT7 mutant produced more viable pollens than WT in HS.

  • HT7 mutant produced normal fruits containing seeds, while, WT could not produce seeded fruits in HS.

  • HT7 mutant highly expressed heat shock transcription factor (SlHsfA1b) and heat shock protein (SlHsp101).

Abstract

Heat stress (HS) is a major problem of tomato production worldwide, as it reduces fruit setting due to the adverse effects on pollen development and fertility. In this study, we isolated heat-tolerant (HT) mutants providing improved fruit-setting under long-term ambient high temperature by testing over 4000 lines from the Micro-Tom tomato mutant collection. The HT mutants were categorized into two types, namely those that produced parthenocarpic fruit, and those that produced fruit with seeds. Among the HT mutants, HT7 plants produced fruit with seeds, had a higher fruit number and seeded-fruit yield, and the total pollen number and viability were much higher under HS conditions than those of the WT under both control and HS conditions. HT7 also succeeded at fertilization even under HS conditions due to higher pollen viability than the WT. In addition, HS-related genes, such as SIHsfA1b3 and Hsp101, were more highly expressed in HT7 than in WT. These results suggest that HT7 could be a valuable genetic resource for elucidating heat tolerance mechanisms and a breeding material for improving heat-tolerant fruit-setting in tomato.

Introduction

Tomato (Solanum lycopersicum) is an undeniably important vegetable crop species in terms of fresh and industrial products. It is also well known as an incredible source of rich nutrient components, such as vitamin C, β-carotene and lycopene, that positively impact human health (Bergougnoux, 2014). However, the yield and quality of tomato are adversely affected by various biotic and abiotic stresses.

Heat stress (HS) is one such abiotic stress that causes multiple negative effects on plant morphology, physiology, biochemistry and molecular pathways at all vegetative and reproductive stages. In tomato cultivation, HS markedly affects reproduction and fertilization, leading to their failure and a decrease in the quantity and quality of harvested fruit (Craufurd et al., 1998; Vara Prasad et al., 1999, 2000; Sato et al., 2000). According to the research of the Intergovernmental Panel on Climate Change (IPCC), the global surface temperature will increase 0.3 °C during the next decade (Abdrabo and Adger, 2014; Jones et al., 1999). Therefore, HS on food production will be an even more serious problem. To cope with HS consequence, many scientists and breeders have tried to solve the problems in tomato production arising from HS. Although some tomato varieties have developed improved thermotolerance, their quality and fruit yield are not sufficient (Bita and Gerats, 2013). Thus, it is necessary to create novel genetic resources for heat-tolerant (HT) tomato cultivars with high and stable fruit production.

In general, when the ambient temperatures of plants are 10–15 °C higher than their optimum temperatures for cultivation, such temperature conditions are defined as constituting HS. When tomato plants are cultivated under temperature conditions above 35 °C, they are drastically damaged at various stages of development, including seed germination, vegetative and reproductive growth, and fruit setting (Wahid et al., 2007). There are several reports on the ability of pollen to form in tomato plants subjected to HS. The seeded fruit weight and starch content in pollen grains have been compared in various cultivars grown under 31/25 °C, 32/26 °C, and 28/22 °C conditions; the quality and quantity of pollen grains in HT cultivars are hardly affected compared to those in heat-sensitive cultivars (Firon et al., 2006). Abdul-baki evaluated the heat tolerance of several agricultural tomato cultivars and wild species by comparing the fruit setting, seed numbers and yield in a greenhouse under high-temperature conditions. The results showed that HS increased the abscission of flowers and decreased fruit setting and yield (Abdul-Baki and Stommel, 1995). Moreover, there have been several reports on tomato and other plant species regarding the factors that control pollination under HS. Flowers exposed to HS showed negative effects at various developmental stages, such as the inhibition of pollen release from anthers by the failure of dehiscence, stigma exposure due to decreased anther length, and pistil hyperplasia (Takeoka et al., 1991; Porch and Jahn, 2001; Sato et al., 2000, 2006) Thus, we should consider these several factors to be strategies to improve tomato fruit production under high-temperature conditions.

The dwarf tomato cultivar Micro-Tom is a promising experimental material for elucidating the molecular mechanisms of important breeding traits in tomato because this cultivar has unique characteristics, such as a small size and short life cycle, and it can be easily crossed with other tomato cultivars and genetically transformed (Sun et al., 2006). Additionally, whole-genome sequence data of Micro-Tom are available through the ‘TOMATOMICS’ database (Kobayashi et al., 2014). Moreover, a comprehensive Micro-Tom mutant population generated with ethyl methanesulfonate (EMS) treatment is available for the isolation of mutants for important breeding traits (Saito et al., 2011; Shikata et al., 2016).

As the first step to improve heat tolerance in tomato, using the Micro-Tom mutant population, we tried to isolate mutants showing higher fruit setting ability than that of the WT when exposed to HS. These mutants were named tomato heat-tolerant mutants (HTs). Eventually, we identified two types of HT mutants: one showed parthenocarpic fruit setting, and the other showed fruit setting with seeds. In this study, among the various HT mutants, we describe the detailed phenotypes of the HT7 mutant that showed fruit setting with seeds under HS conditions and discuss the potential mechanism of heat tolerance.

Section snippets

Plant materials and screening conditions for heat-tolerant mutants

The tomato (Solanum lycopersicum) dwarf cultivar Micro-Tom was used in all experiments. The mutant population of Micro-Tom, which includes over 4000 EMS mutant lines, and the WT (TOMJPF0001), which were conserved and distributed by the National BioResource Project (NBRP) tomato mutants archive, were used.

For the isolation of HT mutant lines, seeds were cultivated in a greenhouse at the University of Tsukuba and Central Japan Rail Company from 2011−2014. The seeds were sown in 5 cm diameter pots

Screening for HT mutants from an EMS mutant population

The screening for HT mutant lines from over 4000 Micro-Tom mutants generated by EMS treatment is summarized in Fig. S2a. We carried out the primary screening in a greenhouse in the summer of 2013 and isolated 91 candidate mutant lines from mutant collections under high-temperature conditions based on fruit setting. Subsequently, we evaluated those mutant lines in a greenhouse in the summer of 2014 from May to September. In the second screening, we identified 15 HT mutant lines with improved

Discussion

The purpose of this study was to isolate and characterize mutant lines with improved fruit-setting under high temperature conditions from a comprehensive Micro-Tom tomato mutant population. The isolated HT lines were categorized into two types based on their fruit setting patterns, namely those that showed parthenocarpic fruits, including lines HT1, HT4, HT10, and HT15, and those that showed fruits with seeds due to high pollen viability, including line HT7. We placed emphasis on characterizing

Conclusions

High temperature is one of the consequences of global warming, which extremely affects crop production and typically decreases the growth and development of tomato plants. The heat-tolerant mutant tomato HT7 line positively responded to long-term exposure to heat stress by producing more viable pollen and maintaining normal flower structure for self-pollination. Hence, HT7 can be a prominent breeding material for enhancing heat tolerance of commercial tomato cultivars, which may contribute to a

Author contributions statement

KH, SF, SF, and TH performed the experiments of Fig. S1, S2, S3 and S4. DP conducted experiments of Fig. S5, S6, S7, all tables and all Figs from 1 to 8. DP and KH prepared the manuscript. KH, YS and HE supervised and proofreading the manuscript. All authors reviewed the manuscript.

Declaration of Competing Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The tomato seeds (TOMJP00001) and EMS mutant populations were provided by the University of Tsukuba, Gene Research Center, through the National Bio-Resource Project (NBRP) of the Japan Agency for Research and Development (AMED), Japan. The 1st and 2nd screenings were performed in collaboration with Central Japan Railway Company, Komaki, Aichi, Japan.

We are very grateful to Y. Fujimori, N. Inage, R. Masuda, M. Miyamoto, and M. Yamaguchi for their skilled technical assistance. We also gratefully

References (51)

  • A.A. Abdul-Baki et al.

    Pollen viability and fruit set of tomato genotypes under optimum-and high-temperture regimes

    HortScience

    (1995)
  • R. Arias et al.

    Correlation of lycopene measured by HPLC with the L*, a*, b* color readings of a hydroponic tomato and the relationship of maturity with color and lycopene content

    J. Agric. Food Chem.

    (2000)
  • C.E. Bita

    Heat Stress Tolerance Responses in Developing Tomato Anthers. PhD Thesis

    (2016)
  • C.E. Bita et al.

    Plant tolerance to high temperature in a changing environment: scientific fundamentals and production of heat stress-tolerant crops

    Front. Plant Sci.

    (2013)
  • J.J. Burke et al.

    Enhancement of reproductive heat tolerance in plants

    PLoS One

    (2015)
  • S.W. Choi et al.

    Evaluation of internal control genes for quantitative realtime PCR analyses for studying fruit development of dwarf tomato cultivar ‘Micro-Tom

    Plant Biotechnol

    (2018)
  • K. Chusreeaeom et al.

    Regulatory change in cell division activity and genetic mapping of a tomato (Solanum lycopersicum L.) elongated-fruit mutant

    Plant Biotechnol.

    (2014)
  • Ciheam

    Identité et qualité des produits alimentaires méditerranéens

    Mediterra

    (2007)
  • P.Q. Craufurd et al.

    Heat tolerance in cowpea: effect of timing and duration of heat stress

    Ann. uppl. Bid

    (1998)
  • G. De Martino et al.

    Functional analyses of two tomato APETALA3 genes demonstrate diversification in their roles in regulating floral development

    Plant Cell

    (2006)
  • P.K. Hsu et al.

    Two phloem nitrate transporters, NRT1.11 and NRT1.12, are important for redistributing xylem-borne nitrate to enhance plant growth

    Plant Physiol.

    (2013)
  • L.J. Huang et al.

    CaHSP16.4, a small heat shock protein gene in pepper, is involved in heat and drought tolerance

    Protoplasma

    (2019)
  • P.D. Jones et al.

    Surface air temperature and its changes over the past 150 years

    Rev. Geophys.

    (1999)
  • M. Kobayashi et al.

    Genome-wide analysis of intraspecific dna polymorphism in “micro-tom”, a model cultivar of tomato (Solanum lycopersicum)

    Plant Cell Physiol.

    (2014)
  • E.M. Kramer et al.

    Molecular evolution of genes controlling petal and stamen development: duplication and divergence within the APETALA3 and PISTILLATA MADS-Box gene lineages

    Genetics.

    (1998)
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