Phloem Protein: Structure, Classifications and Functions

P Proteins (Phloem Proteins) are a category of proteins found in the sap of the sieve tubes of the phloem of Angiospermic plants. P-proteins were also as called ‘slime bodies’ of ‘slime’ in the old literature. P proteins are usually found in the phloem of dicot plants.

P Protein

Characteristics of P Protein
Ø  P proteins 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.

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.

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