Witches' brooms in Siberian stone pine as somatic mutations and initial genetic material for breeding of nut-bearing and ornamental cultivars

Abstract

For the raising of the Siberian stone pine (Pinus sibirica Du Tour) nut-bearing and ornamental cultivars, the most important traits are a dense crown, slow growth and precocity. Generative mutations of this kind are eliminated by natural selection, therefore, somatic mutation rsearch is important. Among somatic mutations, the most promising one is the so-called 'witches' brooms' (WB) where crown fragments demonstrate slowed growth and intensive branching. WB occasionally occurs in native populations. According to phytopathology textbooks, WB are caused by various pathogen species (viruses, mycoplasmas, fungi). The WB of this kind are characterized by a sickly look, full suppression of reproductive functions, a short life and a nidus pattern of distribution. There are also WB of different types: with a high vitality and fertility, a long life and sporadic distribution. They occur very rarely (about 1 per 10 000 trees) across the species' range. We investigated 18 trees with WB of this type. The size of WB ranged from 0.3 to 30 m, age varied from 30 to 300 years. Male cones were absent in all WB. Female cone initiation was normal if the WB was located in the top part of a crown. Scions from WB and a normal crown (NC) of the same tree were grafted on identical rootstocks. On average, the height of 7-year-old WB grafts (WBG) was 2 times lower, and the stem diameter was 2 times higher than in the NC grafts (NCG). It was achieved due to the fundamental differences in the shoot system morphogenesis. Here are three principal differences in decreasing order of importance: (i) WBGs were characterized by the absence or near absence of apical dominance. The NCG had no more than 3 orders of branching, and the length of the 1st order axis was on average 5 times larger than the axis of the 3rd order. The WBG had 6-7 orders of branching, and the length of shoots of 5-6th orders averaged 80-90% of the length of the first orders. (ii) At an identical shoot length, the number of lateral buds in the WBG was 3-4 times more. This superiority was achieved in several ways: a greater shoot number per node, the presence of one or even two additional nodes (summer shoot) on many annual shoots, the formation of lateral long shoots outside any nodes (instead of short shoots). (iii) The number of stem units, their length, and annual shoot length in the WBG were 1.5-2 times less compared with the NCG. The first seed cones were initiated in both variants in the year of grafting. However, by the end of 5-7 year period of supervision, the WBG surpassed the NCG in the number of cones by 5-10 times. WB and NC have also been tested on seed progeny (WBP and NCP). In the WBP halfsib families, a clear tendency of splitting into two classes was observed: (1) normal seedlings, statistically indistinguishable from the homogenous NCPs and (2) seedlings with growth essentially slowed down and very intensive branching. The ratio of the two classes was about 1:1. Therefore, the studied WB are a dominant somatic mutation. The principles of using of these mutations in breeding programs are discussed.