Plant growth and fertility requires functional interactions between specific PABP and eIF4G gene family members

Abstract

The initiation of protein synthesis requires the involvement of the eukaryotic translation initiation factor (eIF) 4G to promote assembly of the factors needed to recruit a 40S ribosomal subunit to an mRNA. Although many eukaryotes express two eIF4G isoforms that are highly similar, those in plants, referred to as eIF4G and eIFiso4G, are highly divergent in size, sequence, and domain organization. Species of the Brassicaceae and the Cleomaceae also express a divergent eIFiso4G isoform, referred to as eIFiso4G2, not found elsewhere in the plant kingdom. Despite their divergence, eIF4G and eIFiso4G interact with eIF4A, eIF4B, and eIF4E isoforms needed for binding an mRNA. eIF4G and eIFiso4G also interact with the poly(A)-binding protein (PABP) which promotes ribosome recruitment to an mRNA. Increasing the complexity of such an interaction, however, Arabidopsis also expresses three PABP isoforms (PAB2, PAB4, and PAB8) in vegetative and reproductive tissues. In this study, the functional interactions among the eIF4G and the widely-expressed PABP isoforms were examined. Loss of PAB2 or PAB8 in combination with loss of eIF4G or eIFiso4G had little to no effect on growth or fertility whereas pab2 pab8 eif4g or pab2 pab8 eifiso4g1/2 mutants exhibited smaller stature and reduced fertility. Although the pab4 eifiso4g1 mutant grows normally and is fertile, pab4 eif4g or pab4 eifiso4g2 mutants could not be isolated. Even pab4/PAB4 eif4g/eIF4G heterozygous plants exhibited growth defects and low fertility. Mutant co-inheritance analysis in reciprocal crosses with wild-type plants revealed that most ovaries and pollen from pab4/PAB4 eif4g/eIF4G plants were PAB4 eif4g. Similarly, co-inheritance studies with pab4/PAB4 eifiso4g2/eIFiso4G2 plants suggested most ovaries were PAB4 eifiso4g2. These results suggest that a functional interaction between PAB4 and eIF4G and between PAB4 and eIFiso4G2 is required for growth and normal fertility.

Fig 1. Growth characteristics of pab2, pab8, and eif4g combinatorial mutants.

Adult wild-type and mutant plants at the first appearance of the inflorescence.

Fig 2. The effect of pab2, pab8, and eif4g combinatorial mutations on leaf and inflorescence growth.

(A) Leaves from young adult wild-type and mutant plants. (B-D) Flowering wild-type and mutant plants with the inflorescence displayed.

Fig 3. Development of the inflorescence and siliques in pab4/PAB4 eif4g/eIF4G heterozygous plants.

(A) Inflorescence and (B) siliques of wild-type and pab4/PAB4 eif4g/eIF4G plants.

Fig 4. The effect of pab2 or pab8 on the growth of pab4/PAB4 eif4g/eIF4G heterozygous plants.

Flowering wild-type and mutant plants with the inflorescence displayed.

Fig 5. Growth characteristics of pab2, pab8, and eifiso4g combinatorial mutants.

(A) Adult wild-type and mutant plants at the first appearance of the inflorescence in the wild-type. Mutants containing the eifiso4g1/2 mutations are delayed in flowering. (B) Adult eifiso4g1/2 mutant plants at flowering showing the effect of pab2 and/or pab8 mutations on growth and flowering time. The pots shown are the same as those in (A) but at flowering of the eifiso4g1/2 mutant plants.

Fig 6. The effect of pab2, pab8, and eifiso4g combinatorial mutations on leaf size.

(A) Adult leaves of fully flowering wild-type and mutant plants. (B) Leaf series from fully flowering adult eifiso4g1/2 mutant plants showing the effect of pab2 and/or pab8 mutations on the size of every leaf from a representative plant.

Fig 7. The effect of pab2, pab8, and eifiso4g combinatorial mutations on plant stature, flowering, and inflorescence size.

(A) Adult wild-type and eifiso4g isoform mutants showing the effect of pab2/8 mutations on plant and inflorescence growth. (B) Adult eifiso4g1/2 mutant plants showing the effect of pab2 and/or pab8 mutations on plant stature and flowering time.

Fig 8. Development of the inflorescence and siliques in pab4/PAB4 eifiso4g2/eIFiso4G2 heterozygous plants.

(A) Inflorescence and (B) siliques of wild-type and pab4/PAB4 eifiso4g2/eIFiso4G2 plants.