A leucine-rich repeat receptor-like protein kinase (LRPKm1) gene is induced in Malus x domestica by Venturia inaequalis infection and salicylic acid treatment

A cDNA clone encoding a leucine-rich repeat (LRR) receptor-like protein kin ase (LRPKm1) of Malus x domestica cv. Florina has been isolated using as a heterologous probe a cloned gene encoding a polygalacturonase-inhibiting pr otein (PGIP) of Phaseolus vulgaris L. A genomic clone containing the 5'-reg ulatory region and a 5' portion of the open reading frame of the LRPKm1 gen e has also been isolated. An open reading frame of 2997 nt (999 amino acids ) was present in the cDNA clone, encoding a receptor-like protein comprisin g a 21 amino acid signal peptide for secretion, a leucine zipper, 23 LRRs, a putative membrane-spanning region and a serine/threonine protein kinase d omain. LRPKm1 shows homology to the A. thaliana receptor-like protein kinas e RLK5 and, to a minor extent, to PGIP. The LRPKm1 region from +5 to +600 e xhibits an alternative reading frame that encodes a product corresponding t o a proline-rich protein fragment homologous to several hydroxyproline-rich proteins. Southern blot analysis showed that LRPKm1 belongs to a multigene family and that there is length polymorphism of the hybridizing restrictio n fragments among different M. x domestica cultivars. Northern blot analysi s was carried out on mRNA extracted from infected leaves of either cv. Flor ina (resistant to Venturia inaequalis) or cv. Golden Delicious (susceptible to V. inaequalis), and from tissues treated with salicylic acid. A 3500 bp transcript hybridizing at high stringency with the LRPKm1 cDNA accumulated in response to infection or salicylic acid treatment. Transcript accumulat ion was more intense in the incompatible interaction than in the compatible one. The possible involvement of this receptor-like protein kinase in resi stance of apple to phytopathogenic fungi is discussed.