Phytohormone profile and CiFL1 expression in young seedlings of Cichorium intybus L. var sativum exposed to high temperature in relation to vernalization and de-vernalization processes

Highlights

• Cichrium intybus requires vernalization for flowering.

• High temperature paradoxically induce flowering in this species.

• Vernalization inhibited CiFL1 expression while high temperature increased it.

• Impact of high temperature on phytohormonal profile was different for vernalized and non-vernalized plants.

Abstract

Cichorium intybus is a biennial plant species forming a taproot and a leaf rosette during the first year and which requires low temperature during vernalization to flower during the second year of its cycle. Heat stress, however, is known to induce premature flowering during the first year. The present work aims to compare the effect of heat treatment (38 °C during 3 days) and vernalization (4 °C during 6 weeks) on the phytohormonal profile and the expression of two chicory genes (CiFL1 and CiFT) homologous to the Arabidopsis flowering genes FLOWERING LOCUS C (FLC) and FLOWERING LOCUS T (FT). In Arabidopsis, FLC and FT code, respectively, for a repressor and an activator of flowering. Although CiFL1 was inhibited by vernalization, its expression was increased by heat treatment in both vernalized and non-vernalized seedlings while high temperature unexpectedly inhibited FT expression in vernalized seedlings. Vernalization induced a modification in the hormonal profile of C. intybus (cv. Melci) in a sense of a decrease in ethylene production, abscisic acid and total jasmonates content, while the level of salicylic and benzoic acids as well as polyamine spermidine increased. A limited effect on the gibberellins’ profile was observed. The subsequent impact of heat stress on phytohormone content was different in non-vernalized and vernalized seedlings, with a higher abscisic acid and jasmonates and a lower 1-aminocyclopropane-1-carboxylic acid concentration in the former than in the latter. These data are discussed in relation to the putative involvement of phytohormones in stress-induced flowering. It appears that heat stress implies different pathways than vernalization to hasten flowering process.

Introduction

Root chicory (Cichorium intybus L. var sativum) is a biennal plant species which forms a leaf rosette and a taproot during the first growing season and then bolt and flower during the second growing season after vernalization occurred during winter period. The plant stores polysaccharide inulin as a very rich energy source in its taproot during the first growing season and is therefore cultivated as an annual to avoid bolting which reduces the inulin yield. However, early bolting could be observed in field condition during the first year mainly due to vernalization resulting from low temperatures occurring during early spring just after sowing (Dielen et al., 2005).

Although root chicory requires vernalization for flowering initiation (Pimpini and Gianquinto, 1988; Gianquinto, 1997), high temperature could also induce bolting and flowering in this species independently of vernalization (Mathieu et al., 2014, 2018). Paradoxically, high temperature may also have a de-vernalization effect by suppressing flowering in vernalized plants (Périlleux et al., 2013). Hence, high temperature has a dual impact, promoting stress-induced flowering and suppressing it through a de-vernalization process.

The molecular control of flowering induction by vernalization and by high temperatures in root chicory remains poorly understood. In Arabidopsis thaliana, vernalization implies the down-regulation of the gene FLOWERING LOCUS C (FLC) which normally acts as a suppressor of flowering by inhibiting the expression of the flowering integrator genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CO1 (SOC1) (Michaels and Amasino, 1999; Sheldon et al., 1999; Helliwel et al., 2006). The control of flowering by elevated growth temperature in Arabidopsis involves two mechanisms (Capovilla et al., 2015): one is based on the transcription factor PHYTOCHROME INTERACTING FACTOR 4 (PIF4), and the other involves a complex between proteins encoded by FLOWERING LOCUS M (FLM) and SHORT VEGETATIVE PHASE (SVP) (Balasubramanian et al., 2006; Lee et al., 2013; Posé et al., 2013; Kumar et al., 2012; Capovilla et al., 2015). In root chicory, a MADS box sequence similar to FLC and FLM has been identified and named FLC-LIKE 1 (CiFL1) (Périlleux et al., 2013). Vernalization represses CiFL1 expression in root chicory and overexpression of CiFL1 in Arabidopsis delays flowering (Périlleux et al., 2013).

High temperature was recently reported to negatively affect plant growth in chicory and to reduce sugar translocation from leaves to root leading to a decrease in inulin production (Mathieu et al., 2014, 2018). Several phytohormones are involved in the plant response to heat stress. Generally, ethylene, abscisic acid (ABA) and salicylic acid (SA) are rapidly synthesized following heat stress while cytokinins (CKs), auxin and gibberellins (GAs) contents decrease (Larkindale and Huang, 2005). Polyamines (PAs) titers could also increase in plant exposed to high temperature and these compounds are reported to assume key protective roles in response to heat stress (Sagor et al., 2013; Glaubitz et al., 2015).

Hormones, such as CKs and ethylene were suspected to be directly involved in flowering of root chicory (Joseph et al., 1985). In other biennal species, GAs can also substitute for vernalization (Lang, 1957; Mutasa-Göttgens and Hedden, 2009) and promote bolting (Lang, 1957). Depending on the species, ethylene could induce or repress flowering (Abeles, 1973,1992). In the rosette plant spinach, Crèvecoeur et al. (1986, 1988) have shown that there was a burst of ethylene release when plants were induced to flower by long days. In radish and pea seedlings, ethylene release increased gradually during vernalization (Suge, 1977). Jasmonates (JA) (Pedranzani et al., 2003) and SA (Takeno, 2016) are also involved in stress-induced flowering. In Pharbitis nil, auxins and PAs are involved in the induction of flowering by stress (Koshio et al., 2015).

It is still unclear if vernalization process induces a modification in the hormonal profile in Cichorium intybus and if heat-induced de-vernalization also involves a modification in the phytohormonal status. Similarly, the promoting impact of high temperature on flowering may rely on modifications in gene expression, phytohormonal content or both processes but data regarding C. intybus are crucially lacking and the impact of high temperatures on vernalized and non-vernalized plants were never compared. The present study was therefore undertaken in order to compare the impact of high temperature on the expression of CiFL1 and phytohormonal content in vernalized and non-vernalized young seedling.