The characterisation of tapetum-specific cDNAs isolated from a Lilium henryi L. meiocyte subtractive cDNA library

Differential screening of a meiocyte subtractive cDNA library from Lilium henryi L. has identified a group of 16 anther-specific partial cDNAs. Three of these sequences, LHM2, LHM6 and LHM7 have been further characterised. Hybridisation in situ with antisense riboprobes of LHM2, LHM6, and LHM7 gives a strong, clear signal which, contrary to expectations, is localised to the tapetal layer surrounding the meiocytes and not the meiocytes themselves. Developmental slot blots demonstrate that mRNAs corresponding to the three LHM cDNAs are transcribed from prophase of meiosis I to the uninucleate microspore stage, while Northern analysis reveals these tapetally expressed cDNAs to correspond with transcripts of some 500 bp. Although LHM2 is less abundant than LHM6 and LHM7, the pattern of developmental expression, and the size range of the transcripts suggests that all three cDNAs may be related. The deduced polypeptide products of LHM6 and LHM7 share 71% identity over a conserved region of 38 residues. Inverse polymerase chain reaction was used to obtain the full sequence for LHM7. Its deduced protein sequence has a signal peptide indicating it may be secreted; the cleaved protein has a molecular weight of 8.9 kDa. Furthermore, the LHM7 protein has significant levels of homology with tapetally expressed proteins from Arabidopsis thaliana, Antirrhinum majus and Lycopersicon esculentum. All contain a highly conserved pattern of cysteine residues present in seed and non-specific lipid transfer proteins. The function of this gene product is discussed in the perspective of current models of another development.