On the complex nature of unifoliata locus and discovery of tightly linked gene causing multipistillate “umbelliferous” floral structure in pea (Pisum sativum)
Sharma, B. Div. of Gen., Indian Agric. Res. Inst.
New Delhi, India
The unifoliata mutation of pea was first reported by Eriksson(1). Lamprecht (2) placed the gene Uni in linkage group (LG) III. The classical unifoliata mutation produces a single sessile leaflet of larger than usual size. Lamprecht (4) called it a “complex” mutation of homologous genes, also found in Phaseolus vulgaris and Ph. multiflorus (3). The epithet “complex” was given to this locus primarily because, besides converting a compound petiolate tendrilled leaf into simple, sessile and tendril-less, the unifoliata mutation also changes floral structure drastically. The flowers are, as usual, borne on a single peduncle in the leaf axil. However, each peduncle carries a large globule of dichotomously branched cluster of totally sterile flowers, each producing a small ovary. Many of these ovaries are incomplete in development, showing a clear transition from an open leafy structure to fully closed pistil. They are frequently without ovules, especially when the ovaries are underdeveloped and open. As many as 64 pistils were counted in a single globose cluster. A somewhat similar mutation was induced in Tradescantia and called “umbelliferous” (7). Such mutations were also called “clover type” (9) and symbol Umb was used to identify the genes producing such phenotype in recessive condition.
A mutation called “tendrilled acacia” (10, 11, 12) turned out to be a new allele of the gene Uni (5, 6). Sarala and Sharma (8) also showed tac (uni) to be linked with Apu and St on LG III. However, this mutation does not produce globular structure of sterile multipistillate flowers. The flowers are always borne singly; a few of these are open (not papillionaceous) and partially sterile. In totality, a tac plant can be called fertile. Another mutation with a different type of apical leaflet was reported by Sharma and Kumar (14). Thus, two fertile mutations (alleles) of the uni locus, tacB and tacS, are already known.
Another recessive unifoliate mutation has been induced in variety Large Podded G-20, which carries a single large sized leaflet on a short petiole. Its leaflet resembles the large terminal leaflet of tacB mutation. The petiole is neither thickened at the base nor tapering towards the end as is characteristic of a normal compound leaf. The phenotype gives an impression as if this unifoliate structure represents only the terminal part of the leaf and the petiole in this case is the subterminal segment of leaf rachis in which a tacB mutation occasionally produces tendrils (hence called tendrilled acacia) or remains without tendrils, and then the leaf has a long barren gap between the last pair of lateral leaflets and the large apical one.
On the basis of the above analysis it is concluded that this new mutation is another allele at the Uni locus and hence tentatively called “petiolate unifoliate” with gene symbol unipet awaiting experimental confirmation through mapping analysis. However, the unipet mutation being an allele of the Uni gene is supported by the fact that the unipet plants always produce globose sterile inflorescence of the so-called umbellate type as it happens in the well known unifoliata mutation of Lamprecht.
Another interesting fact was revealed by these mutation studies: several mutations with “umbelliferous” flowers were induced by fast neutrons, ethylene amine and diethyl sulphate without altering leaf structure, which remained tendrilled compound with the usual pairs of lateral leaflets. The “umbelliferous” inflorescence is known to accompany unifoliata mutations, and the unipet mutation isolated in the present study also produces “umbelliferous” inflorescence. However, because “umbelliferous” inflorescence can also be induced without affecting leaf morphology, it is clear that this floral trait is controlled by a genetic element other than the Uni locus as proposed by Lamprecht even without having a fertile mutation like tacB and “umbelliferous” mutations with wild type leaves.
The mutation affecting floral structure is named, following the precedent in other plants, as umbelliferous and designated by gene symbol Umb, which in recessive condition produces globose “umbelliferous” floral structure.
The history of the Uni locus in Pisum sativum shows that it comprises two tightly linked genes—Uni and Umb—which are frequently mutated together as a result of a large deletion in the uni-umb region, spanning over the two independent functional units. The evidence available at present in support of this hypothesis is that both uni (tac) and umb mutations can be induced as individual events without mutually affecting their specific target organs, i.e., leaf and inflorescence, respectively.
It was proposed earlier (13) that the lateral and apical tendrils in Pisum differ in the genetic control during their differentiation. The tac alleles invariably eliminate the lateral tendrils (with leaky expression) and convert the apical tendril into a normal sized leaflet but have no influence on the lateral leaflets. The discovery of unipet mutation supports this hypothesis and also demonstrates that the uni mutation actually eliminates the lateral leaflets and tendrils while modifying the apical tendril into a normal leaflet. This results into placement of a nearly sessile leaflet on the main stem. The tacB and tacS alleles allow formation of lateral leaflets while suppressing lateral tendrils. The tacpet mutation totally eliminates lateral leaflets and tendrils while retaining the subterminal part of the leaf rachis, occupied by the lateral tendrils in wild type, and simultaneously converts the terminal tendril into leaflet. Finally, the Lamprecht’s unifoliata eliminates the leaf rachis completely, leaving behind a sessile leaflet which is actually the terminal tendril modified into leaflet and located right on the main stem.
Under the above scheme of gene expression and differential effect of the Uni alleles on various components of leaf, it would be appropriate to redesignate the old unifoliata mutation as uniL (superscript L for Lamprecht).
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13. Sharma, B. 1981. Pulse Crops Newslett. 1(1): 56-57.
14. Sharma, B. and Kumar, S. 1981. Pulse Crops Newslett. 1(3): 27-28.