Self-incompatibility (SI) is a genetic mechanism that restricts inbreeding in flowering plants. In the nightshade family (Solanaceae) SI is controlled by a single multiallelic S locus. Pollen rejection in this system requires the interaction of two S locus products: a stylar (S)-RNase and its pollen counterpart (pollen S). pollen S has not yet been cloned. Our understanding of how this gene functions comes from studies of plants with mutations that affect the pollen but not the stylar SI response (pollen-part mutations). These mutations are frequently associated with duplicated S alleles, but the absence of an obvious additional allele in some plants suggests pollen S can also be deleted. We studied Nicotiana alata plants with an additional S allele and show that duplication causes a pollen-part mutation in several different genetic backgrounds. Inheritance of the duplication was consistent with a competitive interaction model in which any two nonmatching S alleles cause a breakdown of SI when present in the same pollen grain. We also examined plants with presumed deletions of pollen S and found that they instead have duplications that included pollen S but not the S-RNase gene. This finding is consistent with a bipartite structure for the S locus. The absence of pollen S deletions in this study and perhaps other studies suggests that pollen S might be required for pollen viability, possibly because its product acts as an S-RNase inhibitor.S elf-incompatibility (SI) in many plant families is controlled by a multiallelic S locus that enables a style to reject any pollen expressing the same S allelic specificity as itself (1). In the Solanaceae, the family that includes tobacco, tomato, and petunia, SI is described as gametophytic because the allelic specificity of each pollen grain is determined by its own haploid genotype. The S locus in this family encodes a secreted extracellular RNase [stylar (S)-RNase] that accumulates in the style (2). Recognizing which S allele each pollen grain expresses is thought to require an interaction between the S-RNase and an unknown product(s) of a second S locus gene called pollen S (3, 4).As part of a strategy to identify pollen S, we isolated Nicotiana alata plants with gamma ray induced mutations that specifically affect the SI phenotype of pollen but not the SI phenotype of the style (5). Such plants are called pollen-part mutants (PPMs). Because ionizing radiation can cause either the deletion of part of a chromosome or chromosomal aberrations such as translocations, inversions, and fragments (6), the mutations in PPMs are likely to be complex because they can arise through one of a few different types of lesion.Among the PPMs described so far, the most frequent types of lesion are either translocations or small ''centric'' fragments (short extra chromosomes) that carry a duplicated copy of an S allele (5, 7-10). Breakdown of the pollen SI response in these plants occurs because of a ''competitive interaction'' that enables pollen with two different S alleles (but not two i...
In vitro transcripts of full-length cDNA clones of the Johnsongrass strain of Johnsongrass mosaic potyvirus (JGMV-Jg) were infectious on maize and sorghum when inoculated by mechanical or by biolistic bombardment. Two of the cDNA clones with spontaneous mutations in the coat protein were not infectious. Sequence differences between infectious and non-infectious transcripts revealed that alteration of inferred amino sequences, near or in the N-terminus of the coat protein, profoundly affected the infectivity of transcripts. Transcripts of chimeric full-length cDNA of JGMV-Jg, containing coat protein sequences from the Krish-infecting strain of JGMV, were infectious in Krish resistant sorghums.
A distinct feature of eukaryotic genomes is the presence of gene families. The polygalacturonase (PG) (EC3.2.1.15) gene family is one of the largest gene families in plants. PG is a pectin-digesting enzyme with a glycoside hydrolase 28 domain. It is involved in numerous plant developmental processes. The evolutionary processes accounting for the functional divergence and the specialized functions of PGs in land plants are unclear. Here, phylogenetic and gene structure analysis of PG genes in algae and land plants revealed that land plant PG genes resulted from differential intron gain and loss, with the latter event predominating. PG genes in land plants contained 15 homologous intron blocks and 13 novel intron blocks. Intron position and phase were not conserved between PGs of algae and land plants but conserved among PG genes of land plants from moss to vascular plants, indicating that the current introns in the PGs in land plants appeared after the split between unicellular algae and multicelluar land plants. These findings demonstrate that the functional divergence and differentiation of PGs in land plants is attributable to intronic loss. Moreover, they underscore the importance of intron gain and loss in genomic adaptation to selective pressure.
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