Michurin’s legacy to biological science.
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2011-03
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Today, Ivan Michurin is remembered mostly for the discredited Michurinian genetics. However, this great
horticulturist and Russia’s ‘Luther Burbank’, in the course of more than 60 years in plant breeding, not
only produced more than 300 strains of horticultural plants but also elaborated many theoretical principles
and practical methods. Over the past several decades, however, Michurin’s name has become inextricably
linked to that of his most notorious disciple – TD Lysenko. Of late, Michurin’s name has been slowly
reappearing in scientific literature (Flegr 2002; Ivanyi 2003; Liu 2006; Liu et al. 2010). The main reason is
that it has been shown that some epigenetic changes can be passed on to the offspring in ways that appear
to violate Mendelian genetics. In addition, it has been shown that the plant genome is remarkably unstable,
capable of undergoing change and generating variability during very early stages of development, and
there is increasing evidence in support of Michurin’s methods of mentor grafting and mentor pollination.
The basic principle of Michurin’s operations was the change in heredity induced by environmental
changes in the early development stages of plants. He proved by experiments that the acclimatization of
plants is indeed possible, but ‘only by planting the seeds’. He made great use of grafting as a means of
influencing and improving immature plants, and showed that plants could be altered by grafting in a
sufficiently early phase of development. He emphasized repeatedly that young plant organisms were
highly susceptible to the influence of environmental conditions. The young organism resulting from the
cross of the initial parental pairs is distinguished by its destabilized heredity and hence possesses great
plasticity. It should be noted that Michurin’s assumption is not only consistent with Darwin’s and
Burbank’s ideas but also supported by recent findings. Throughout his career, Darwin consistently linked
the cause of variation to changes in the environment. Recently, it has been shown that adaptive phenotypic
plasticity is generally expressed for plants exposed to an environmental perturbation during very early
stages of development (Amzallag 2004). Several other instances have also been documented in which the
genome does alter in response to the environment, and this change usually occurs in the early stage of
development (Durrent 1962; Cullis 2005). It should be noted that Michurin’s idea was supported by
Konrad Lorenz, who later proposed that imprinting occurs in ‘critical periods’, which are limited and
severely restricted to the animal’s very early life (cited in Tzschentke and Plagemann 2006). Now it is
known that nutrition in early life might influence adult phenotypes through DNA methylation. On
observing agouti mice born to mothers that were fed diets containing different amounts of methyl donors,
one will see mice with coats of different shades of colour, from yellow to brown to almost black (Vercelli
2004). Cropley et al. (2006) reported the effects of specific timing of maternal dietary methyl
supplementation on the coat colour of the offspring, and found that maternal supplementation only during
mid-gestation substantially affected offspring coat colour, and this effect is inherited by the subsequent
generation.
Michurin’s most important contribution, in Vavilov’s opinion, was that he promoted the idea of
remote hybridization, he made very original attempts to produce new species of plants by crossbreeding
them with other species and he demonstrated both in theory and in practice the correctness
of that method (Popovsky 1984). Michurin was one of the first investigators in the history of plant
breeding to use not only inter-specific but also inter-generic hybridization between such taxonomically
remote species, and obtained dozens of valuable plant varieties. To cross species of plants that were least
closely related, it is necessary to find ways and means to overcome the resistance to crossing. One of the
important barriers to wide hybridization is rejection of pollen by a foreign style. Either the pollen is unable to germinate or the pollen tube is inhibited in the pistil before it reaches the egg. A thorough
study of pollination biology in fruit plants enabled Michurin to introduce the method of mentor
pollination to breeding, thus overcoming the difficulties in wide hybridization. Mentor pollination is the
use of a small amount of pollen that is highly compatible with the seed parent, mixed with a large
amount of pollen from the intended pollen parent to increase the chances of success in wide
hybridization. Since this method was discovered by Michurin, it has been investigated by breeders as a
possible tool to overcome incompatibility barriers both in self- and inter-specific incompatibility (Stettler
1968; Pandey 1977; Knox et al. 1987; Gaget et al. 1989; Wenslaff and Lyrene 2000). For example,
obtaining blackthorn×plum hybrids was greatly facilitated by using a pollen mixture of different plum
varieties (as compared with pollination by each plum variety separately). The cross sand cherry×plum
only succeeded when plum pollen was mixed with myrobalan pollen (Yenikeyev 1965). In blueberry
wide hybridization, when Vaccinium elliottii ‘Oleno’ was the seed parent, no hybrids were produced
unless mentor pollen was utilized (Wenslaff and Lyrene 2000).
Inter-specific crosses that are normally difficult or impossible to obtain can also be facilitated by
previous grafting between the intended sexual partners, which Michurin termed ‘preliminary
vegetative approximation’. The method of ‘preliminary vegetative approximation’ consists in the
following: cuttings of 1-year-old hybrid seedlings are grafted onto a branch of the crown of a mature
tree of a different species or genus. The grafted cuttings continue to develop in the course of the next
5 or 6 years under the constant influence of the tree to which they have been grafted, and by virtue of
this they partially change their structure. When the first flowers appear on the grafted cuttings, they
may be pollinated by the pollen collected from the flowers of the tree on which the cuttings grew. In a
report published in 1925, Michurin described his innovation in the following words: ‘I use this
vegetative change as an auxiliary means to approximating two different plant species so as to obtain a
sexual hybrid by crossing them in the future. This is the secret of my success in obtaining
interspecific plant hybrids, such as crosses between the apricot and the plum, sweet and sour
cherry…’. This method has been applied not only to fruit trees but also to annual plants. It is well
known that wheat and rye are very difficult to hybridize. Hall (1954) found that only 2% to 3% of
wheat flowers pollinated by rye produced seeds. However, of the 2897 wheat flowers borne by plants,
which, as embryos, had been grafted onto rye endosperm, 400 (14%) gave seeds when pollinated by rye.
Nirk (1959) reported the successful crossing of Lycopersicon esculentum with L. peruvianum by
employing the reciprocal grafting of parent plants prior to hybridization. This method has also been used
in wide hybridization of other plants (Evans and Jones 1964; Rao and Ram 1971).
Although Darwin was the first to put forward the concept of graft hybridization, he did not know
under what conditions graft hybridization could be effected. One of Michurin’s most notable
achievements was his mentor grafting method, with which Darwin’s ‘unknown conditions’ of
producing graft hybrids could be revealed. This method involves the following: by grafting several
scions taken from old strains of fruit trees onto the lower branches of a young seedling’s crown, the
young seedling acquires properties that it earlier lacked, these properties being transmitted to it
through the grafted twigs of the old strain. He emphasized repeatedly that this method could be
employed effectively only on young seedlings and not on old and long-established varieties. By this
method Michurin produced or improved a number of new good strains. Later, Michurin’s mentor
grafting method was improved and applied to annual plants. In the last decades, several independent
groups of scientists repeatedly showed that mentor grafting was a simple and effective means of
plant breeding, and that graft-induced variant characteristics were stable and inheritable (Taller et al.
1998; Liu 2006). It should be noted that as early as 1933, Michurin deduced that the stock genes could
be transferred into the scion and incorporated with the scion genes and vice versa (Michurin 1949).
Recently, Stegemann and Bock (2009) demonstrated that plant grafting could result in the exchange of
genetic information via either large DNA pieces or an entire plastid genome. This observation provides
a nice confirmation of horizontal gene transfer between the stock and the scion. It should be mentioned
that it was Michurin’s work on graft hybridization that led Hasek to make the crucial decision to
choose chicken embryo as his experimental model, which turned out to be crucial for the discovery of
immunological tolerance (Ivanyi 2003). Michurin’s work on graft hybridization also inspired animal
breeders to investigate the effects of blood transfusion on hereditary traits, which was also known as
animal vegetative hybridization (Liu 2008).
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Liu Yongsheng, Wang Guangyin, Li Xiuju. Michurin’s legacy to biological science. Journal of Biosciences. 2011 Mar; 36(1): 13-16.