Rosetted siblings in F₂ of a cross in pomegranate (Punica granatum L.) can be useful model for rosetting investigations

Abstract

Environmentally and developmentally controlled rosette formation is welldocumented in certain crop species. But this report describes a geneticallyinduced rosetting observed in pomegranate (Punica granatum L.).Rosette genotypes were detected at a frequency of 0.12 in the F₂ familiesof cv. Ganesh and a recessive rosette mutant clone of cv. Kabul Yellow. Therosetted seedlings had leaves almost touching the ground at emergence, whichwere closely set on a thick, compressed stem with narrow inter-nodes andinactive subapical meristem. They were similar in some certain respects tothe naturally occurring rosetting noticed in lettuce, spinach and lisianthus.The occurrence of rosette type was attributed to a recessive mutant gene ofKabul Yellow and was ascribed the gene symbol rg (rosette growth) whichwas found to alter a host of morphological traits in rosette siblings innursery and field. The foliage colour, pink of Ganesh and yellow of KabulYellow, served as visual marker in distinguishing segregating progeny intopink/yellow normal and rosette types in the nursery. A monogenic segregationin F₂ for pink and yellow was evident both in normal and rosette siblings.When a single gene mutation for rosetting was considered a large portion ofthe rosette progeny in F₂ was found missing. This has been explained on thebasis of two conditional lethal genes operating in rosette individuals.Moreover, the F₂ and BC₁ seeds had very low seed fertility indicatingthat F₁ possibly carried heavy load of lethal genes. On transplantation,the rosette mutants started to grow tall and erect, like spinach, but had acompact habit with restricted branching and exhibited early leaf senescence,leaving a crown of bushy growth at the tip of almost naked, brittle shoots.Rosettes differed significantly from normal siblings for all the morphologicalcharacteristics studied. The normal progenies flowered and set fruits within13 months, the rosette ones almost failed to do so even after growing for 38months, although they showed relatively loose rosetting than in the nursery.However, one seedling produced 3 small flowers with rudimentary ovary.Moreover, it was observed that the rosette plants had a tendency to regressto normal state, since 9 out of the 31 rosette progeny produced axilary shootsthat had normal stem and leaf attributes. The individuals with both normal androsette shoots should serve the same purpose in research work, as do perfect isogeniclines. Based on the frequency at which the normal shoots appeared the role ofcryptic transposable elements (TEs) was suspected rather than back mutation.Rosette siblings with altered morphology reported here can be secured inabundant number along with the contrasting normal siblings by raising theF₂, which provide a good opportunity to gain an insight in the control ofvarious plant developmental processes. Besides, it can be viewed as a valuablemodel system for fundamental research on physiological, biochemical andmolecular genetical bases of rosetting in crop plants.