Abstract
BackgroundThe Fertilization-related kinases (FRK) form a class that belongs to the MEKK subfamily of plant MAPKKKs. It was recently shown that FRK class kinases expanded during angiosperm evolution, reaching their maximum numbers in the lineage leading to solanaceous species and culminating in the Solanum genus where they account for more than 40% of the total MEKKs. The first members studied, ScFRK1 and ScFRK2 were shown to play a pivotal role in gametophyte development in the wild potato species Solanum chacoense.ResultsScFRK3 is also involved in gametophyte development. ScFRK3 is expressed in developing pollen and young ovules, reaching its highest level immediately after meiosis and during the mitosis steps in both gametophytes. Hence, three independent lines of ScFRK3 RNAi mutant plants showed decreased number of seeds per fruit. We also observed an important number of degenerated embryo sac in mature ovary. Analysis of ovule development showed that most embryo sac did not enter mitosis I in ScFRK3 RNAi mutant plants. Severe lethality was also observed during male gametophyte development, pollen being arrested before mitosis I, as observed in the female gametophyte. Obvious defects in vegetative organs were not observed, emphasizing the reproductive roles of the FRK class kinases. To isolate MAP kinases acting downstream of ScFRK3, a de novo S. chacoense transcriptome from male and female reproductive organs was assembled. Of the five ScMKKs and 16 ScMPKs retrieved, only the ScMKK3 interacted with ScFRK3, while only the ScMPK13 interacted with ScMKK3, leading to an apparent single three-tiered canonical MAP kinase cascade combination involving ScFRK3-ScMKK3-ScMPK13.ConclusionsThe ScFRK3 MAPKKK is involved in a signaling cascade that regulates both male and female gamete development, and most probably act upstream of ScMKK3 and ScMPK13.
Highlights
The Fertilization-related kinases (FRK) form a class that belongs to the Mitogen-Activated Protein Kinase Kinase Kinase (MEKK) subfamily of plant MAPKKKs
Solanum chacoense Fertilization-Related Kinase 3 (ScFRK3), a MEKK related to the Arabidopsis MAPKKK19, 20 and 21 From the combined S. chacoense ovule and pollen tube transcriptome, 21 different MEKKs were isolated with six of them (ScFRK1–6) belonging to the FRK class [36]
In fully characterized solanaceous genomes, like Solanum tuberosum and Solanum lycopersicum, their total number (15 and 17 FRKs, respectively) account for more than 40% of the whole MEKKs subfamily
Summary
The Fertilization-related kinases (FRK) form a class that belongs to the MEKK subfamily of plant MAPKKKs. The first pollen mitosis (PMI) occurs after maturation of the haploid microspore, producing two different cells inside the pollen grain: a large central cell, called the vegetative cell, and a smaller one, the generative cell [3]. Once pollen grains land on the stigma of a suitable flower of the same species, the vegetative cell generates a pollen tube that will elongate through the style toward the ovules. In solanaceous species like Solanum chacoense, the second pollen mitosis (PMII) occurs within the generative cell during tube elongation to form two identical sperm cells [3]. When the pollen tube reaches the micropyle of the ovule, the two sperm cells are released from the pollen tube to fertilize the egg cell and the central cell, leading to double fertilization giving rise to the diploid embryo and the triploid endosperm [4]
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