Cloning and characterisation of genes determining pod morphology in pea

Abstract

Genes expressed in developing pea pods were isolated as cDNAs by differential screening techniques. The cDNAs were characterised by DNA sequencing and expression studies were used to investigate the role of isolated cDNAs in pod development. A clone isolated from a pea {Piswn sativum L.) pod cDNA library was shown to contain the complete coding sequence of a polypeptide with considerable homology to various members of the Rab subfamily of small ras-related GTP- binding proteins. Conserved sequences in the isolated clone include the GTP-buiding site, GDP/GTP hydrolysis domain and C-terminal Cys residues involved in membrane attachment. The high percentage amino acid identity suggests that this cDNA may be the product of a gene, designated Psa-rai?, which is the plant counterpart of Rab7. Rab/Ypt proteins are thought to be involved in intracellular transport from the endoplasmic reticulum to the Golgi apparatus and in vesicular transport. If Psa-ra6 is a functional counterpart of yeast YPT7 (RabT) it should be able to complement a yeast YPT7 mutant. An attempt was made to demonstrate that this was the case. Northern analysis showed invariant expression of Psa-rab in developing pods 'with different phenotypes, indicating an essential function for Psa-rab in developing pods. Hybridisation of the Psa-rab cDNA to pea genomic DNA showed that this protein is probably encoded by a single gene. Nearly isogenic pea lines were selected to investigate the genetic basis for lignification of the pea {Pisum sativum L.) pod endocarp. The development of the pod endocarp in the normal and mutant pea pod phenotypes was examined by histochemical staining and light microscopy. The effect of plant growth regulators on endocarp development was also investigated. A pea pod cDNA library representing poly (A)+ RNA purified from L59 pea pods (genotype, PV; phenotype, lignified endocarp) was differentially screened with total cDNA probes prepared from total pod RNA from L59 and LI390 (genotype, PV; phenotype, no lignification of endocarp) pods 4-6 days after flowering (DAP). Two clones, designated pLP18 and pLP19, were selected for further characterisation on the basis of hybridisation to the L59 cDNA probe, but not the LI390 cDNA probe. Northern blotting was used to show that pLP18 represented a mRNA of 0.95 kb. The predicted polypeptide from the LP18 cDNA encoded a putative blue type I copper protein. The expression pattern of LP 18 mRNA in pods and tissues of the experimental pea lines was determined using RT-PCR quantitation. Hybridisation of the cDNA to pea genomic DNA showed that this protein is probably encoded by a single gene. Clone pLP19 yielded a 1.02 kb cDNA fragment encoding the C-terminal portion of an Hsp70 homologue belonging to a highly conserved family of proteins found in a number of eukaryotic species. Northern analysis of RNA from lignified and unlignified pods showed the presence of differentially expressed LP19 transcripts of varying lengths, which may represent differently processed transcripts. Southern analysis confirmed the presence of a single hybridised band in genomic digests of L59, L58 arid LI390. Several mRNA transcripts of the LP19 gene were isolated which differ in the length of their- 3' untranslated regions. [brace not closed]