MOTHER OF FT AND TFL1 regulates seed germination and fertility relevant to the brassinosteroid signaling pathway

Abstract

Brassinosteroids (BRs) are a family of plant steroid hormones that play diverse roles in many aspects of plant growth and development. For example, BRs promote seed germination by counteracting the inhibitory effect of ABA and regulate plant reproductive development, thus affecting seed yield. We have recently reported that MOTHER OF FT AND TFL1 (MFT) regulates seed germination through a negative feedback loop modulating ABA signaling in Arabidopsis. Here, we show that MFT function is also relevant to the BR signaling pathway. In mft loss-of-function mutants, the application of BR could not fully antagonize the inhibitory effect of exogenous ABA on seed germination, suggesting that BR promotes seed germination against ABA partly through MFT. In addition, mft enhances the low-fertility phenotype of det2 in which BR biosynthesis is blocked. This phenotype, together with the observation that MFT is expressed in gametophytes and developing seeds, suggests that MFT and BR play redundant roles in regulating fertility. Therefore, these results suggest that MFT affects seed germination and fertility relevant to the BR signaling pathway.

Plant hormones exert profound effects on many fundamental processes during plant growth and development. With respect to seed development and germination, it has long been known that abscisic acid (ABA) and gibberellin (GA) are two major types of phytohormones that play antagonistic roles in regulating these events. Not until recently, another group of phytohormones, namely brassinosteroids (BRs), has also been found to counteract the inhibitory effect of ABA on seed germination.^1,2 In addition, BR has been suggested to act in parallel with GA to promote cell elongation and germination.^1,3,4

BRs are a class of polyhydroxysteroids that are found in a wide variety of plant species.⁵ They can be detected in almost every plant tissue, with the highest abundance in the pollen and seeds.⁶ The most active component in the family of BRs is 24-epibrassinolide (BL), which is capable of activating BR signaling.⁶ In Arabidopsis, when the early steps of BR biosynthesis are blocked, the resulting defects include reduced male fertility under normal growth conditions^7,8 and decreased germination percentage in the presence of exogenous ABA.¹ Thus, BR plays an indispensible role in the control of seed development and also contributes to the regulation of seed germination.

We have previously reported that MOTHER OF FT AND TFL1 (MFT) responds to both ABA and GA signals to regulate seed germination.⁹ Here we show that MFT functions in regulating seed germination and fertility, which is also relevant to the BR signaling pathway. Thus, MFT seems to function specifically in seeds in response to various phytohormones.

BL Effect in Counteracting the Inhibitory Effect of ABA on Seed Germination is Compromised in mft-2

It has been reported that BL treatment is able to rescue the low germination phenotype of both GA-biosynthetic and GA-insensitive mutants, and that the germination of BR-biosynthetic and BR-insensitive mutants are more strongly inhibited by ABA than that of wild-type seeds.¹ Thus, like GA, BR can also antagonize ABA to promote seed germination. Since we have shown that MFT responds to both ABA and GA signals to regulate seed germination,⁹ we further investigated whether MFT is also involved in the crosstalk between ABA and BR. Interestingly, we found that in the presence of ABA, the ability of BL to elevate the germination rate was much reduced in mft-2 loss-of-function mutants (Fig. 1), indicating that the antagonistic effect of BR against ABA in seed germination is partly mediated through MFT.

Figure 1.

BR antagonizes ABA to promote seed germination partly through MFT. Germination phenotypes of wild-type and mft-2 seeds treated with 10 µM ABA plus different concentrations of BL are compared. Error bars denote SD. BL, 24-epibrassinolide.

mft-2 Enhances the Phenotypes of det2-1

We further crossed mft-2 with a BR-biosynthetic mutant det2-1 to examine the role of MFT in the absence of endogenous BR. It is known that DET2 acts at the second step of BR biosynthesis and that mutations in DET2 lead to many defects in plant growth and development, such as reduced male fertility and short stature.^7,8 We found that under normal growth conditions, det2-1 mft-2 double mutant showed much more severe phenotypes than det2-1 single mutant in terms of fertility and stature (Fig. 2A), whereas mft-2 had no obvious phenotypes at all (data not shown). This demonstrates that MFT is involved in regulating fertility and stature when BR biosynthesis is blocked, implying that MFT and BR play redundant roles in regulating certain developmental processes.

Figure 2.

MFT is involved in regulating fertility in the absence of BR. (A) Phenotypes of two-month-old det2-1 and det2-1 mft-2 mutants under normal growth conditions. (B) GUS staining pattern of a MFT(P2)-GUS open flower. (C) GUS staining pattern of a MFT(P2)-GUS mature ovule. (D) In situ localization of MFT in wild-type developing seeds at the torpedo stage. Scale bars, 100 µm. Longitudinal sections hybridized with the antisense and sense probes are compared.

MFT is Expressed in Gametophytes and Developing Seeds

The low fertility phenotype of det2-1 mft-2 prompted us to check the expression of MFT during gametogenesis and embryogenesis. GUS staining of MFT(P2)-GUS plants⁹ showed that MFT was highly expressed in mature pollen grains in stamens (Fig. 2B) and in the endosperm of mature ovules (Fig. 2C). We have recently shown that MFT is highly expressed in developing seeds by quantitative real-time PCR,⁹ but the precise localization of MFT in developing seeds has not been examined. We thus performed in situ hybridization to examine MFT expression in developing siliques and revealed that MFT was expressed predominantly in the outer integument of the seed coat, and also in the endosperm and embryo (Fig. 2D). Although mft mutants did not exhibit obvious defects in fertility under normal growth conditions, the low-fertility phenotype of det2-1 mft-2 and the expression pattern of MFT suggest that MFT is involved in regulating fertilization particularly when BR biosynthesis is blocked.

Conclusions

In plant development, hormonal cross-talk forms a complex signaling network in which the downstream physiological events are well coordinated. Such coordination involves a variety of genes that integrate signals at different levels. We show in this addendum to our recent report⁹ that MFT also plays a role in the cross-talk between ABA and BR in seed germination. In addition, we further show that MFT promotes fertility when BR biosynthesis is blocked. These findings reinforce the notion that MFT may play a protective role in reducing the damage to plant development caused by environmental stress conditions.

Addendum to: Xi W, Liu C, Hou X, Yu H. MOTHER OF FT and TFL1 regulates seed germination through a negative feedback loop modulating ABA signaling in Arabidopsis. Plant Cell. 2010;22:1733–1748. doi: 10.1105/tpc.109.073072.