Incompatibility in Plants

In this article we will discuss about:- 1. Recognition and Rejection of Incompatibility 2. Biological Significance of Incompatibility.

Recognition and Rejection of Incompatibility:

The rejection of ‘self’ in self-incompatibility system implies the existence of a recognition event. In fact pollen-stigma interaction is an interesting example of cell recognition in flowering plants. Both self-and cross-pollinations are the outcome of such interactions and generally manifest into self-and cross-incompatibility mechanisms.

The receptive surface of the stigma may be encountered with both self and foreign pollen and the occurrence of recognition system permits the selection of the compatible one. Following compatible pollination, the accepted pollen grain germinates to produce a pollen tube which penetrates the cuticle of the stigma papillae, passes through the transmitting tissue of the style, enters the embryo sac where the male gametes may fuse with the egg cell and primary endosperm nucleus, respectively.

It is interesting to note that physical, mechanical and physiological mechanisms have evolved to preserve the species as a breeding group and also to promote outcrossing within the species. Discriminations are apparently made at several levels: interfamilial, intergeneric, interspecific and intraspecific. Interfamily crosses are rarely successful, since differences in genome homology prevent zygote development.

Also intergeneric crosses are generally precluded but are successful in a few reported cases e.g. Festuca x Lolium or Raphanus x Brassica. However, interspecific crosses are usually more successful than intergeneric ones. In fact, interspecific crosses are more compatible except where specific recognition or S gene operates to prevent inbreeding.

The success of a pollination implies a fine degree of reciprocity and coadaptation between the pollen and the female organs and ‘mismatch’ at any stage during sequential events of fertilization produces an effective barrier excluding delivery of the male gametes.

It is highly important and relevant to understand the structure of pollen wall and that of the stigma papillae which establish the initial contact and their chemical composition in order to understand the physico-biochemical processes underlying the recognition rejection mechanism(s).

Pollen wall surface is intricately patterned and such patterning is often species specific or group specific. It consists of an outer patterned layer, the exine, mode of sporopollenin and an inner pecto-cellulosic layer, the intine. The cavities in the exine are filled with protein, glycoproteins and lipids derived from the tapetal cells of the anther just prior to pollen maturation.

In contrast, the intine proteins are products of the haploid pollen grain. The mature pollen is apparently coated with a thin osmiophilic layer, the chemical nature of which is not known. The presence of tryphirie coating which makes the initial contact with stigma papillae upon pollination has also been reported.

In flowering plants two stigma types are found. These are dry and wet stigma types. The latter stigmas are characterized by the presence of copious fluid on its surface. The wall of the stigma papillae shows a cuticle with discontinuities and below this is present pacto-cellulosic wall. This in turn is separated from the cytoplasm by plasmalemma.

In the cytoplasm are present microbodies in which recognition proteins are synthesized. These are transferred to the outer surface of the cuticle through plasmalemma, pecto-cellulosic wall and discontinuities in the cuticle and form pellicle over it. This layer could be the site of recognition reactions involved in incompatibility response(s).

The physio-chemical studies conducted by Mattsson et al (1974) on surfaces of dry stigma papillae of Silene, Brassica and Raphanus revealed the presence of an external proteinaceous pellicle overlying the cutinized layer of wall of stigma papillae and it is functionally important in the capture and hydration of the pollen. This constitutes the first step of recognition reactions.

The observations showed the binding of the emitted proteins to the pellicle immediately after the initial contact, indicating that this is the primary recognition site. Events following pollination in the wet type of stigmas e.g. in Petunia, Nicotiana and Prenus are relatively less understood. In a recent paper Paul (1978) have suggested that stigma receptivity is signalled by an accumulation of exudate on the stigma surface.

They studied the breeding behaviour of T₅₂an autosterile and T₅₁ an autofertile lines of Vicia faba and concluded that appearance of exudate before the anther dehiscence with increased quantity before and after flower opening in T₅₂ indicates this species to be self- pollinated. In autosterile line, the probability of out crossing might be expected to increase due to quantity of exudate.

There are series of biochemical events occurring after the deposition of the pollen grain on the stigma surface and these ultimately lead to either acceptance or rejection of pollen grain.

In the following some points for non-acceptance of pollen grains are mentioned:

i. The first recognition event following pollination is the hydration of compatible pollen in which foreign pollen remains dry and is not recognised.

ii. After hydration the exine proteins are immediately released through the surface of microspores and this signals the initiation of pollen germination and tube growth. This constitutes the recognition event.

iii. This step comprises activation of cuticle hydrolysing enzyme (e.g. cutinase, etc.). Usually, this system is defective in SSI and thus results in poor tube growth with little or no tube penetration.

iv. The pollen tube continues to grow through the style in compatible-mating but in GSI system the pollen tube growth, usually stops within the style.

v. The time of discharge of gametes may be hindred and thus fertilization is prevented.

Clearly, recognition is mediated by surface secreted determinants which are possibly proteins including lectins, cell wall components such as arabino-galactin-proteins, arabino-xylans, and allergens. Knox (1976) reported the binding of the determinants of the stigma surface to both concanavaline A and β-glycosyl artificial carbohydrate antigen in Gladiolus gandavensis.

FITC con A is bound specifically to mature stigma papillae indicating the presence of surface acceptors for this lectin. The specificity of binding is demonstrated if we preincubate it with 0.25 M—methyl D- glucoside. This is also true of Helianthus annuus. The binding of con A to acceptor glycoprotein on stigma surface has also been demonstrated by fluorescent labelling techniques and with binding of 125 I-labelled con A to stigma surface.

The presence of nonspecific esterases in both the pollen wall emission and on the stigma surface suggests that these enzymes may be involved in the active cutinase complex which is essential for the breakdown of the cuticle to allow pollen tube entry.

The presence of lectin con A on the stigma surface effectively blocked cutinase activation that is although comparative pollen germinated and produced long tubes, these failed to penetrate the cuticle.

Surface determinants including those exhibiting esterase activity were removed by treatment of the stigmas with sodium deoxycholate. After such treatment, no germination of compatible pollen was detected, indicating removal of receptors.

In Helianthus annuus, Vithange and Knox (1977) also observed the deposition of the 1,3 β-glucan, callose in both pollen, pollen tube and interacting stigma papillae soon after mating with self-pollen and with pollen from other genera of composite.

The genetic studies following the discovery of S-gene and its multiple alleles provide a meaningful clue to the understanding of the physiological barriers. The nature of the S-gene remains the subject of much speculation. Many different hypothesis have been advanced to explain the expression of the S-gene.

The antigen, antibody theory of East assumed several years ago (1929) that L-allele specific ‘antigen’ from the pollen combine with S-allele specific ‘antibody’ from the stigma to produce inhibitory complexes. This theory led Lewis (1960) to propose that pollen and stigma factors could be sterically complementary.

Linskens (1965) expressed some doubts on East’s hypothesis and pointed out that antigen antibody reactions are specific to the animal kingdom and cannot be obtained directly from plant extracts without the preparation of animal antisera (Fig. 23-4).

He suggested that S-allele in the style codes via antibody synthesizing unit (ASU) which when in excess inhibits its own production (cross pollination) or combines with a protein in the pollen tube to form an inhibiting xy complex (self-pollination).

Another possible explanation for the inhibition of the pollen tube growth arises from the dimer hypothesis of Lewis (1964). It is assumed that following self-pollination, the polypeptides in the pollen tube coded by specificity part of S- allele of pollen dimerize on the surface of the pollen tube with identical polypeptides in the style coded by specificity part of S-allele of style to form a dimer represser.

This presumably switches off one or several loci responsible for pollen metabolism and thus pollen tube growth. Lewis considers that only a tetramer is physiologically active in producing incompability and proposed that identical polypeptides first dimerise in the pollen and in the style and after self-pollination tetramerizes with the possible aid of glucose to form a tetramer regulator which acts to inhibit the production of growth substances or functions directly as a growth repressor.

Lewis’s model was taken up by Ascher (1966) who established a link between the dimer hypothesis of SI and the operator regulator system of Jacob and Monad (1961) and made provision for three different sites of genetic activity in pollen.

Firstly, a low velocity operon, which controls germination and early pollen tube growth and which may eventually be switched off when the pollen metabolites are exhausted (Fig. 23-5).

Secondly, a high velocity operon, responsible for pollen tube growth through the style tissue and is switched on by stylar metabolites.

Thirdly, a regulator containing necessary information for coding a specific monomer in the pollen tube.

In the style Ascher considers only 2 sites of genetic activity namely the two regulators; which each code for a specific monomer. When the monomer in the pollen tube is identical to one of the monomers in the style, a dimer repressor is formed which switches off the high velocity operon in the pollen and consequently prevents the pollen tube growth to proceed.

Though this is highly speculative, nevertheless it is an attractive model which is consistent with most of the facts known to date on the structure of S- locus. However, there is still a need to confirm this modulation by direct chemical evidence.

Somewhat different features have been considered by Van der Donk (1975) in a model which he proposed and was based on his biochemical studies regarding synthesis of RNA and proteins in Petunia styles (Fig. 23-6).

According to him, the recognition reaction is not based on the interaction of identical S-polypeptides as suggested by Lewis (1965) but upon matching of different substances in pollen and styles, respectively produced by the pollen part and the styler part of the S-locus. Styler gene activity is blocked by the pollen specific polypeptides and consequently growth of pollen tube ceases as its resources are consumed.

Heslop-Harrison (1975) proposed a model (Fig. 23-7) for sporophytic SI and concluded that interaction of ‘exine’ and ‘pellicle’ recognition factors determine the acceptance or rejection of the pollen grain. The most convincing evidence for similarity of recognition molecules has come from the immunochemical studies of pollen and stigma in SI systems. (Fig. 23-7).

Pollen antigens claimed to reflect S- gene specificity have been detected in two systems, Oenothera organensis by Lewis (1967) and Petunia hybrida by Linskens (1960). The antigenic nature of the S-gene products in the female stigma has been investigated by Linskens (1960) and Nasrallah (1970).

The binding of the pollen antigens to the female stigma surface has been shown by immunofluorescence in both compatible and SI genotypes of the grass Phalaris. Recently Clarke (1977) have attempted to identify factors involved in pollen-stigma recognition using immunochemical methods.

Gel immuno-diffusion experiments revealed that stigma surface determinants in this system were partially identical with determinants of other tissues such as corm, petal of the same plant. It may be that variation in the degree of identity of this component in pollen and stigma controls positive recognition in the pollen stigma systems.

The recent immunological studies confirm the suggestions of Lewis, Ascher, and Linskens that the recognition genes in SI plants may specify identical products in stigma and pollen. It is, however, quite possible that some of the antigens are isozymes of hydrolytic enzymes common to both sites.

Biological Significance of Incompatibility:

Nature regulates a balance between inbreeding and outbreeding by enforcing incompatibility. Continued selfing of the plants causes homozygosity whereas (Fig. 23-8) occurrence of interspecific incompatibility causes reproductive isolation. In this manner a bottle-neck is created for the free flow of genes between the populations of a species.

Sexual incompatibility may be a serious drawback for further plant improvement programme. Homozygous individuals have low survival value but are extremely useful to conduct basic genetic studies and undertake plant breeding programme.

Homozygous individuals can be obtained in several ways. The conventional methods were laborious and time consuming whereas pollen and protoplast culture technique are the chief methods to obtain haploids.

Interspecific incompatibility also precludes wide hybridization and thus prevents formation of new varieties having desirable characters. For the past several years different methods have been evolved, tested and employed to overcome intra- and interspecific incompatibility.