Simple explanation about pollen pistil interaction

1. RECOGNITION AND REJECTION
MECHANISM OF POLLEN BY PISTIL
AND PROTEINS INNVOLVED IN IT
By
S.Lakshmi
1st M.Sc, Department of Botany,
Queen Mary`s College

2. INTRODUCTION
 Over the past decade or so, there has been significant progress towards elucidating
the molecular events occurring during pollination in flowering plants.
 This process involves a series of complex cellular interactions that culminates in the
fusion between male and female gametes.
 The process of pollination in flowering plant is of indirect type. It is referred to as
siphonogamy.
 Siphonogamy is a condition in plants in which pollen tubes are developed for the
transfer of the male cells to the eggs.
 The egg is well protected in such a way the sperm should travel via. Stigma, style and
ovary to reach the egg inside ovule.

4. STEPS INVOLVED
 Adhesion of pollen to the stigma
 Pollen hydration
 Pollen germination and initial growth on the stigma surface
 Pollen tube growth through the style and pollen tube guidance
 Sperm entering the ovule
 Pollen Fusion with the egg
 Control of pollen viability by incompatibility responses
 Self incompatibility (SI)
 Interspecific incompatibility responses

6. POLLEN ADEHSION:-
 Stigma is the site for receiving pollen grain
 Stigma is of two types
 Dry stigma- no exudates secretion
 Wet stigma- exudates secreted- promotes more adhesion of pollen
 The adhesion of pollen grain with dry stigma is under polygenic control.
 In the Brassicaceae it is known that the pollen coat or tryphine contains components
involved in mediating cell–cell interactions between pollen and stigma
 SLG and SLR1 are known to interact with Pollen Coat Protein (PCPs) invitro
 Pollen Coat Protein are small and cystine rich.
 PCP A1 and PCP A2 interact with SLG 1and SLR 1 invitro thus confirming some role of
this protein in pollen adhesion.

8. Pollen Hydration
 Long chain lipids acts as signals to stimulate pollen hydration
 Cer and Pop1 mutants having defect in pollen hydration have defects in lipid
biosynthesis
 The role of exudates in pollen hydration is best exemplified by Nicotiana where pollen
grains adhere to the top of the papillae above the level of exudates failed to hydrate
making it apparent that exudates are candidly involved in pollen hydration
 There are some other molecules other than exudates and lipids involved in this process
 Mutation in coat protein i.e. Loss of Oleosin domain GRP17 delays pollen hydration
 Aquaporins- Water channel stigmatic proteins, which in addition to their role in self
incompatibility also governs the process of pollen hydration

10. POLLEN GERMINATION AND POLLEN
TUBE GROWTH IN STIGMA
 Pollen immerse and produces tube which starts growing in the stigma and then
reach the style
 The pollen tube penetration method varies for different species. E.g. For
Arabidopsis the pollen tube develops between cuticle and cell wall until it reaches
ovary
 The lipids (triglycerides, hydrophobic) in exudates direct the pollen tube by
controlling the water flow.
 The hydrophobic nature of tube establish a gradient of water in the direction of
penetration guiding the tube.
 In this way pollen tube enters into stigma

12. POLLEN TUBE GROWTH IN STYLE
 Style are of 3 types based on morphology
 Solid – Dicots- central specialised elongated transmitting cells
 Semi-solid
 Open or hollow- Monocots- hollow canal and layer of glandular cells which secretes some mucilaginous
substances
 Pollen tube grow through intercellular spaces of transmitting tissue with biochemical features
 Extracellular matrix in transmitting tissue has proteins such as
 Arabinogalactan proteins- pollen tube adhesion, guidance and nutrition
 Proline rich glycoproteins
 Extensin like proteins
 TTS (Trans. Tissue specific protein)- Shows maximum gradient of glycosylation near the ovarian end of the
style.
 Thus pollen tube grows along with gradient increase in TTS glycosylation. Further the tube de- glycosylate
the TTS for release of nutrition for its growth.
 Other proteins in pollen tube growth are NaTTS, PELP (Pollen specific Extensin like Protein)
 In hollow style, a cystine rich adhesin SA protein are implicated for pollen tube adhesion, growth and
guidance

14. POLLEN TUBE ENTER INTO THE OVULE
 There are certain chemotrophic factors from the viable ovule which attracts the
pollen tube growth towards them
 Pollen tube enters the ovule through 3 ways
 Porogamy- entry via micropyle
 Chalazogamy- entry via Chalaza
 Mesogamy- entry via integuments
 Pollen tube always enters the ovule through micropyle
 Synergids receives the pollen tube and degenerates before the arrival of tube
 After this pollen tube ruptures receiving the male gametes for double fertilization
(embryo and endosperm)

18. SEXUAL INCOMPATIBILITY
 When a pistil with a functional female gamete fails to set seeds after pollination by
viable and fertile pollen grains which can otherwise bring about fertilization in
other pistil, then they are said to be sexually incompatible.
 Sexual incompatibility is of two types
 Inter specific incompatibility –
Pollen grains from other species are rejected because they are too dissimilar from the
recipient. It is polygenic.
 Intra-specific incompatibility / Self incompatibility-
Pollen grains originated from the donor plant of same species is rejected because it is too
similar, because of originating from genetically same or closely related plant.

20. SELF INCOMPATIBILITY
 It is controlled by single multi allele S locus with gene encoding pollen component
and stylar component
 The interaction of this S-gene pollen and stylar proteins constitute the recognition
and rejection reaction
 Self incompatibility is of two types
 Gametophytic incompatibility- Determined by the genotype of pollen or gametophyte
itself.
 Sporophytic incompatibility- Determined by the genotype of the sporophytic plant from
which the pollen is derived.

21. GAMETOPHYTIC INCOMPATIBILITY
 Both compatible and incompatible pollen are allowed to produce pollen tube
 But the incompatible pollen tube growth is arrested in style when it reaches one-
third by stylar protein- S linked glycoprotein with RNAs activity (S- RNAses)
 The S- RNAses enter the pollen tube and its interaction with the pollen S- protein
determine whether to degrade pollen RNA or not.
 Incompatible- S- RNAses degrades and stops the pollen tube growth
 Compatible- S- protein of pollen supresses RNA activity and promote pollen tube
growth.

23. SPOROPHYTIC INCOMPATIBILITY
 Stigma is the site of rejection
 The incompatible pollen are not allowed or incapable of producing pollen tube
 If the tube is produced, the malformed pollen tube are unable to penetrate the
stigma which develop callose deposition in the stigmatic papillae
 Proteins involved in sporophytic incompatibility are
 Pollen Coat Protein (PCO) such as SCR
 S-stigmatic proteins such as SRK (S- receptor kinase) and SLG (S locus glycoprotein)

26. SPOROPHYTIC INCOMPATIBILITY
 Following pollination the pollen coat protein SCR frost to form a layer between
Pollen coat and Stigma
 If SCR of pollen and SRK of recipient stigma are encoded with same S- haplotide
the two are incompatible
 The inner domain of SRK phosphorylates the ARC1 which initiates a cascade of
inter cellular signalling which regulate activity of aquaporins in the stigma papillae
to limit availability of water for the incompatible pollen for its germination and
growth