Pollen-Expressed and Pollen-Specific Genes | Palynology

In this article we will discuss about the pollen-expressed and pollen specific genes.

It is recognized that there are two different sets of genes required for pollen development, one set for the development of pollen grains prior to anthesis, and the other for pollen germination and tube growth. Several reports are accessible aimed at examining the genes expressed during pollen development.

In maize, about 24,000 different m-RNAs are reported in mature pollen grains, compared to 31,000 different m-RNAs in cells of shoot tissues. In Tradescantia about 20,000 different genes (m-RNAs) are expressed in pollen as compared to about 30,000 genes in vegetative shoots.

In tobacco anthers, about 26,000 active genes are reported. So it is evident that a large number of genes remain active in the morphologically unfussy male gametophyte (pollen) compared to the multifaceted vegetative tissue.

Most of the genes expressed in pollen are cellular “housekeeping genes” and about 65% of these genes are also expressed in vegetative tissues. It is estimated that only 10-30% of genes expressed specifically in pollen which has been evaluated by screening pollen- specific clones in pollen cDNA library.

The pollen-expressed genes can be categorized broadly into two groups: early genes and late genes. Early genes (e.g., actin gene) are concerned with pollen developmental processes, while late genes (e.g., Zm 13 gene of maize, Lat 52 gene of tomato) encode proteins linked with pollen maturation and/or germination and pollen tube growth. But this division is to some extent artificial and cannot describe all pollen-expressed genes, since several pollen-specific genes show transitional or incessant expression during pollen development.

As these genes are required during entire pollen developmental stage, so they might be housekeeping in function. It is evident that the genes required for germination and pollen tube growth are already there at the time pollen is released from the anther and this provides sustaining evidence that the main function of late genes are to produce proteins necessary for germination and tube growth.

Table 3.7 shows an exhaustive list of pollen specific genes which have been cloned and at least partially characterized.

The pollen-specific genes are broadly categorized into three groups based on their sequence homology to other known proteins:

i) Genes that have sequence homology to wall degrading enzymes,

ii) Genes that have sequence homology to cytoskeletal proteins, and

iii) Genes that have sequence homology to pollen allergens.

Some other types of pollen-specific genes are also identified.

1. Expression of Genes with Homology to Wall Degrading Enzymes:

The maize gene (Zm 58), the tobacco gene (G 10) and two tomato clones expressed in tomato anthers (Lat 56) and pollen (Lat 59) demonstrate a similarity in their amino acid sequence to pectate lyases (Table-3.7), especially those known from Erwinia (a bacterial plant pathogen).

This similarity can be explained in the light of comparisons of the invasive growth of pollen tubes within the female tissue to a pathogen-host interaction. Pectate lyase is concerned with wall synthesis and cell wall degradation of stylar tissue. It is interesting to note that a style expressed gene also demonstrates homology to pectate lyase.

Several cDNA clones corresponding to polygalacturonase are isolated from pollen of maize (PGE W2247), Oenothera organensis (P₂ family), tobacco (Npgl), Brassica napus (Sta 44.4) (Table-3.7). In Brassica napus, a cDNA is also isolated that shows sequence homology with pectin esterase (Bp 19).

2. Expression of Genes with Homology to Cytoskeletal Proteins:

Several proteins encoded by a large number of pollen – expressed genes are the cytoskeleton – related proteins. These include actin (Tac 25) expressed in tobacco; α-tubulin (TUA 1) expressed in Arabidopsis; P-tubulin (tubl, tub 4 and tub 5) expressed in maize; profilin (Zm PRO 1,2,3) in maize. These cytoskeletal genes are also found to be necessary for pollen tube growth.

It is interesting to note that the incidence of actin and myosin in higher plants has also been recognized in pollen tubes of Lily and Amaryllis, pollen of Luffa, tobacco and maize.

In tobacco 25-30 actin genes are reported of which only one is pollen specific (Tac 25), while the others show pollen-preferential expression. It is reported that amino acid composition and circular dichroism spectrum of actin of maize pollen are identical to those of human muscle actin and the pollen F-actin can trigger the activity of the muscle myosin ATPase sevenfold.

3. Pollen-Expressed Proteins that Act as Human Allergens:

A number of allergenic proteins are identified from ragweed (Ambrosia) pollen, of which Amb a I (formerly Allergen E) protein is responsible to cause about 90% of the allergenicity in human being. Amb a I protein is one of the multigene family of very similar proteins namely, Amb a I.1, I.2, I.3, I.4 and Amb a II. Though Amb a II protein shows about 65% homology with Amb a I family, but it demonstrates similar allergenic potential.

All the pollen-specific pectate lyase genes have about 45-48% homology to Amb a I which may share similar epitopes. These genes also demonstrate homology with the partial protein sequence of major allergen (Cry j I) of Cryptomeria japonica (a conifer).

A number of other pollen-expressed allergic proteins are reported from olive tree (Ole e I), maize (Zea m I), rye-grass (Lol p 16), kentucky blue grass (Poa p IX) whose function in pollen is yet to unravel. The two clones, one from rice pollen (PS I) and the other from olive tree pollen (Ole e I) show significant homology to maize gene (Zm 23).

It is known that Ole e I protein is the major allergen of olive tree pollen, hence Zm 13 and PS I class of proteins might thus be potent human allergens. Profilins are the major IgE binding proteins reported from maize (Zm PRO 1,2,3).

It is shown that profilins represent a common class of immuno-reactive agents that are also found in the pollen of birch (Bet V I) and timothy grass having allergenic significance. It is clear that birch pollen allergen, Bet VI shows homology with the major allergens of alder, hazel and hornbeam of the order Fagales.

The function of Bet VI protein in pollen has not yet been established, but it shows homology to plant disease resistance response genes in pea plant. It is postulated that majority of the allergenic proteins present in pollen have important functions in pollen tube growth.

4. Other Pollen-Specific Genes:

The pollen-expressed gene (Lat 52) isolated from tomato pollen shows significant homology with maize gene (Zm 13). These Lat 52 and Zm 13 genes show partial sequence homology to several proteinase inhibitors. It is evident that Zm 13 m-RNA is located in the cytoplasm of vegetative cell as well as within the cytoplasm of the germinating pollen tube. Similar localization is determined for the Lat 52 protein.

It is now established that Lat 52 protein plays a critical role in pollen maturation and/or germination. Genes isolated from Brassica (Bp 10) and tobacco (NTP 303) pollen show homology to ascorbate oxidase, although the exact role of these proteins are still undetermined. One pollen specific gene (SF 3) is isolated from a cDNA library made to mature sunflower pollen.

This gene has zinc-finger domain that match to a so-called LIM motif of several metal binding cystein-rich proteins of animals. This protein is perhaps concerned in the regulation of late pollen gene as several proteins with LIM motif are identified cts regulatory proteins. Several other genes are identified namely, Chi A gene from Petunia, Tpc 44, Tpc 70 from Tradescantia paludosa, Bp 4 (A, B, Qand Oleosin (13) from Brassica napus.

Recently Mayfield (2001) identified two distinct clusters of pollen coat proteins in Arabidopsis thalliana; one cluster encoding six lipases and the second a group of six lipid-binding oleosin genes, including the pollination initiation gene GRP17.

They found that although there are differences within the genes themselves, the clusters are conserved across the ecotype of disparate geographical locations. They also found a large degree of diversity of this coat protein when compared with the coat protein of broccoli, another member of the same family. It is established that this coat protein prevent non-specific pollination.

Conclusion:

Knowledge in pollen chemistry is important because pollen grains are the carriers of male genetic material and also because the nutritional value of pollen depends on the chemical constitution of the grains.

Some pollen grains responsible for respiratory allergic diseases have medicinal significance by virtue of having specific proteins and glycoproteins. Pollen carbohydrate which constitute the major dry matter fraction and occur primarily in the cell walls and as cytoplasmic polysaccharides, vary from species to species.

Amino acid composition in pollen protein also varies with the species. The total free amino acids are usually higher in pollen grains than in leaves or other plant parts. It has been shown that the major pollen proteins are the enzymes. The characterization of proteins in pollen or adhering on the surface as the allergen may give a lead to understand the complete allergic reaction.

The lipid content of pollen yield more energy than other things like carbohydrate and protein. Lipid has a role in metabolism and is involved in production of membranes.

Works on pollen-specific and pollen- expressed genes have not adequately been done. So the biochemists and molecular biologists have a great potentiality to work on such problems.