The S-locus receptor kinase gene in a self-incompatible Brassica napus line encodes a functional serine/threonine kinase.

Abstract

An S-receptor kinase (SRK) cDNA, SRK-910, from the active S-locus in a self-incompatible Brassica napus W1 line has been isolated and characterized. The SRK-910 gene is predominantly expressed in pistils and segregates with the W1 self-incompatibility phenotype in an Fz population derived from a cross between the self-incompatible W1 line and a self-compatible Westar line. Analysis of the predicted amino acid sequence demonstrated that the extracellular receptor domain is highly homologous to S-locus glycoproteins, whereas the cytoplasmic kinase domain contains conserved amino acids present in serinehhreonine kinases. An SRK-910 kinase protein fusion was produced in Escherichia coli and found to contain kinase activity. Phosphoamino acid analysis confirmed that only serine and threonine residues were phosphorylated. Thus, the SRK-910 gene encodes a functional serinelthreonine receptor kinase.

Secondary Evolution of a Self-Incompatibility Locus in the Brassicaceae Genus Leavenworthia

“…It was shown previously that the SRK gene is more highly expressed in stigmas [44],[46] and that the SCR gene is expressed in anthers [13],[44] in Brassica and Arabidopsis, which is concordant with their respective roles in the SI mechanism. In Leavenworthia, LaLal2 expression was detected at similar levels in leaves, roots, and anthers and at higher levels in stigmas at the different stages of flower development (Figure 7A).…”

Natural antisense transcripts of the S locus receptor kinase gene and related sequences in Brassica oleracea

“…2D) suggest that these transcripts may have a speci®c function in this organ. SRK has previously been shown to be transcribed in anthers using RNA gel blots and double-stranded probes which would not have distinguished between sense and antisense transcripts (Stein et al 1991;Goring and Rothstein 1992;Delorme et al 1995). Interestingly, in an S 3 homozygous line, an SRK probe detected a 2.8-kb transcript in anthers which diered in size from the four transcripts found in mature stigmas (Delorme et al 1995).…”

“…However, it is possible that these transcripts interfere with the expression of sense SRK transcripts in other tissues, where they are more abundant than sense transcripts. It has been known for some time that SRK is tightly regulated, SRK mRNA being detectable only in mature stigmas and, at a much lower abundance, in anthers (Stein et al 1991;Goring and Rothstein 1992;Nasrallah et al 1994b;Delorme et al 1995) but not in other tissues. This suggests that it may be deleterious for the plant to express SRK in other tissues, perhaps because it would interfere with the functioning of other receptor kinases of the S gene family (Pastuglia et al 1997).…”

“…Rejection of self-pollen occurs when the same S allele is expressed both in pollen and stigma. SRK is predicted to encode a membranespanning receptor kinase (Stein et al 1991;Delorme et al 1995) and there is genetic evidence that this gene is essential for the self-incompatibility response (Goring et al 1993;Nasrallah et al 1994a). SRK is expressed only in the stigma and anthers of the¯ower, mRNA accumulating to higher levels in the stigma than in the anther.…”

Molecular genetics, physiology and biology of self-incompatibility in Brassicaceae

“…37–40) Interestingly, in the case of B. napus , the S -locus region was found to have originated from the introgressed B. rapa genome and not from B. oleracea . 31,37) …”