Date Published: April 19, 2019
Publisher: Public Library of Science
Author(s): Xiuli Yu, Hui Liu, Na Sang, Yunfei Li, Tingting Zhang, Jie Sun, Xianzhong Huang, Rosalyn B. Angeles-Shim.
Plant phosphatidylethanolamine-binding protein (PEBP) is comprised of three clades: FLOWERING LOCUS T (FT), TERMINAL FLOWER1 (TFL1) and MOTHER OF FT AND TFL1 (MFT). FT/TFL1-like clades regulate identities of the determinate and indeterminate meristems, and ultimately affect flowering time and plant architecture. MFT is generally considered to be the ancestor of FT/TFL1, but its function is not well understood. Here, two MFT homoeologous gene pairs in Gossypium hirsutum, GhMFT1-A/D and GhMFT2-A/D, were identified by genome-wide identification of MFT-like genes. Detailed expression analysis revealed that GhMFT1 and GhMFT2 homoeologous genes were predominately expressed in ovules, and their expression increased remarkably during ovule development but decreased quickly during seed germination. Expressions of GhMFT1 and GhMFT2 homoeologous genes in germinating seeds were upregulated in response to abscisic acid (ABA), and their expressions also responded to gibberellin (GA). In addition, ectopic overexpression of GhMFT1 and GhMFT2 in Arabidopsis inhibited seed germination at the early stage. Gene transcription analysis showed that ABA metabolism genes ABA-INSENSITIVE3 (ABI3) and ABI5, GA signal transduction pathway genes REPRESSOR OF ga1-3 (RGA) and RGA-LIKE2 (RGL2) were all upregulated in the 35S:GhMFT1 and 35S:GhMFT2 transgenic Arabidopsis seeds. GhMFT1 and GhMFT2 localize in the cytoplasm and nucleus, and both interact with a cotton bZIP transcription factor GhFD, suggesting that both of GhMFT1, 2 have similar intracellular regulation mechanisms. Taken together, the results suggest that GhMFT1 and GhMFT2 may act redundantly and differentially in the regulation of seed germination.
In the life cycle of higher plants, the transition from vegetative to reproductive growth (floral transition) is controlled by endogenous cues and external cues [1–3]. Previous studies on the molecular mechanisms of flowering initiation in Arabidopsis thaliana have uncovered six major pathways to orchestrate flowering time, such as photoperiod, vernalization, ambient temperature, gibberellin (GA), autonomous and age pathways . Another transition in a flowering plant is from embryonic growth to germination growth . Seed dormancy can form a soil seed bank and open a protective mechanism for the seed in an unfavorable environment . Seed germination plays an essential part during the growth of the descendants and it is a prerequisite for crop yield . Breaking dormancy is affected by internal cues and external cues , which are primarily controlled by two major antagonistic phytohormones, abscisic acid (ABA) and GA . It is well known that ABA functions in developmental processes of many plants like seed germination and dormancy. ABA inhibits seed germination by hampering water absorption as well as the rupture of seed coat and endosperm [10, 11]. The extensive molecular genetics analyses reveal that ABA-INSENSITIVE3 (ABI3) and ABI5 are the essential regulators in regulating ABA to boost seed dormancy and suppress seed germination [12–15]. GA can counteract the effects of ABA and promote seed germination, plant growth and early flowering. During seed germination, GA accumulation is accompanied by a decrease of ABA content, indicating that GA and ABA play opposite roles in seed germination [16, 17]. GA3-oxidase (GA3OX) is a pivotal enzyme controlling GA biosynthesis, which is encoded by four genes GA3OX1, GA3OX2, GA3OX3 and GA3OX4. The ga3ox1 ga3ox2 double mutant exhibits serious defects in seed germination, indicating that GA3OX1 and GA3OX2 are crucial genes regulating seed germination [18, 19]. DELLA proteins, consisting of REPRESSOR OF ga1-3 (RGA), GA-INSENSITIVE (GAI), and RGA-LIKE1-3 (RGL1-3) [20–24], act as repressors in regulating GA signaling, among which RGA and RGL2 play roles in inhibiting seed germination by mediating the interaction between GA and ABA [14, 22, 25].
Genome-wide analysis identified two MFT homoeologous gene pairs in G. hirsutum, namely, GhMFT1-A/D and GhMFT2-A/D. Expression of GhMFT1 and GhMFT2 homoeologs remarkably increased during ovule development but quickly decreased during seed germination. Their expression levels were significantly affected by ABA. Ectopic overexpression of GhMFT1 and GhMFT2 in Arabidopsis repressed seed germination at the early stage. Moreover, the expression levels of AtABI3, AtABI5, AtRGA and AtRGL2 were obviously upregulated in transgenic Arabidopsis seeds. Both GhMFT1 and GhMFT2 interact with a bZIP transcription factor GhFD. Taken together, we speculate that GhMFT1 and GhMFT2 may act redundantly in the regulation of seed germination.