Question

What are the functions of p-protein and callose protein in the conduction of food from the phloem?

Answer 1

P-protein also called Phloem protein is present in large amounts in the sap- conducting sieve elements of the phloem in angiosperms plants. Callose is a plant polysaccharide that is composed of glucose residues, linked through Beta-1,3-linkages with some Beta-1,6-branches.

Complete answer:
Phloem, a living vascular complex tissue that transports the soluble organic compounds (sugars) from the source to the sink i.e., from photosynthetic leaf cells to the non-photosynthetic root cells or flowers. Its two main cells are sieve tubes and companion cells and it is generally located outside the xylem. It uses turgor pressure and energy in the form of ATP to actively transport these sugar molecules from source to sink. The Pressure flow hypothesis explains the movement in the phloem.
P-proteins (Phloem Proteins) is a proteinaceous structure present in many angiosperms that contain granular, fibrillar, tubular, or crystalline structures. Its earliest stages of formation are closely related to dictyosome cisternae, dictyosome vesicles, and/or spiny vesicles. They are later transformed into structurally different components. They act as ‘damage control’ or ‘puncture repair substance’ by accumulating as granules in the damaged portion of the phloem. They prevent the loss of food from the phloem by forming an aggregation in the form of a clot in the sieve plate. Their aggregation is called ‘slime plugs’.
Callose is produced due to the glucan synthase-like gene (GLS) by the callose synthase located in the plasma membrane. In the conditions of stress and damage, it acts as a temporary cell wall. It is laid down at plasmodesmata and acts as a plant defense mechanism by acting like a typical plant response during stress. During wounding in the phloem, the callose protein is deposited at the sieve plate to stop the translocation of sugars.
P- proteins and callose proteins deposit as granules in the sieve tube element of the phloem, forming a blockage in the translocation of sugar when an injury or wound is observed in the phloem.

Note: The pressure or Mass flow hypothesis explains the movement in the phloem. In the photosynthetic leaves of the plant, the presence of sugars creates a high concentration of sugar leading to the formation of a diffusion gradient by which water flows in the leaves resulting in the formation of turgor or hydrostatic pressure. This pressure is the driving force for the translocation of sugars in the phloem.