P – Protein (Phloem Protein)
(Structure, Classification and Functions of Phloem Proteins)
What are P Proteins?
Ø P Proteins (Phloem Proteins) are a category of proteins found in the sap of the sieve tubes of the phloem of Angiospermic plants.
Ø P-proteins were also as called ‘slime bodies’ of ‘slime’ in the old literature.
Ø P proteins are usually found in the phloem of dicot plants.
Ø They are very rarely reported in monocots.
Ø P proteins are completely absent in the phloem of Pteridophytes and Gymnosperms.
Ø P proteins occur in different forms in the different developmental stages of sieve tubes.
Ø P proteins can exist in the sieve tubes as tubular, globular, fibrillar, granular and crystalline forms.
Ø P proteins are highly polar molecules and they can form gel like substance in the presence of water.
Ø In a completely mature sieve tube element, the P proteins are located mainly in the peripheral portion of the cytoplasm (near the inner side of the cell wall).
Ø PP 1 (P protein 1) and PP 2 are the two P proteins of Cucurbits whose structure and mechanism of action were described in detail.
Ø Both PP 1 and PP 2 are synthesized in the companion cell and transported to the sieve tube element
Ø PP 1 is present in all developmental stages of the sieve tubes.
Ø PP 2 is produced only after the complete maturation of the sieve tubes.
Ø Dictyosomes are reported to be involved in the synthesis of P proteins
Application in Research: P proteins can be used as markers to investigate long distance transport of molecules in plants.
Classification of P proteins
Ø On the basis of final organization in the sieve tubes, the P proteins have been divided into two categories.
(A). Non-dispersive P proteins
(B). Dispersive P proteins
(A). Non-Dispersive P Proteins:
Ø Non-dispersive P proteins are formed during the early developmental stage of the sieve tubes.
Ø Once the sieve tube element is matured, the non-dispersive P proteins are converted into large visible globular bodies and they stay inside the sieve tubes.
Ø About 10% of Dicot plants of Angiosperms possess non-dispersive P proteins.
Ø Forisomes observed in the phloem of leguminous plants are modified non-dispersive P proteins.
Ø Forisomes can quickly change their low size conformation to large size conformation.
Ø The large size forisomes can block or prevent or delay the phloem transport.
(B). Dispersive P Proteins
Ø Similar to non-dispersive P proteins, the dispersive P proteins are also formed during the early developmental stages of the sieve tubes.
Ø But unlike the non-dispersive type, the dispersive P proteins are converted to long fine filaments and dispersed in the lumen of the sieve tubes.
Ø About 90% of the dicots of Angiosperms possess dispersive P proteins.
What are the Functions of P Proteins?
Ø The exact function of P proteins is unknown.
Ø In plants, P proteins acts as ‘puncture repair substance’ of the phloem
Ø When the phloem is get injured, the P proteins first migrate near to the sieve area of the sieve tubes and they accumulate there as granules.
Ø The P protein granules along with a callose (a carbohydrate) can block the sieve tubes.
Ø The aggregation of P protein are called slime plugs
Ø They can plug the sieve tube during phloem injuries or abscission of organs.
Ø Thus they prevent the loss of food materials through the wound.