CLONING A PUTATIVE SELF-INCOMPATIBILITY GENE FROM THE POLLEN OF THE GRASS PHALARIS-COERULESCENS

Phalaris coerulescens, gametophytic self-incompatibility is controlled by two unlinked genes: S and Z. A probable S gene has now been isolat ed and sequenced, This represents a novel self-incompatibility gene is olated from pollen in the multilocus system of a monocotyledonous plan t. The gene is similar to 3 kb long, split by five introns, and exclus ively expressed in the mature pollen. The deduced amino acid sequences from the S-1, S-2, and part of the S-4 alleles showed that the protei n has a variable N terminus and a conserved C terminus. The sequence o f a complete mutant at the S locus indicated that mutations in the con served C terminus, a thioredoxin-like region, led to loss of function. We propose that the gene has two distinct sections, a variable N term inus determining allele specificity and a conserved C terminus with th e catalytic function. The gene structure and its deduced protein seque nces strongly suggest that this monocotyledon has developed a self-inc ompatibility system entirety different from those operating in the dic otyledons. The possible interactions between S and Z genes in both pol len and stigma are discussed.