Question

What substances plug wounded sieve-tube elements, thereby preventing the leakage of phloem sap? a. X protein and callose b. C protein and callose c. P protein and callose d. P protein and sucrose e. none of the above

Short answer

The correct answer is (c) P protein and callose.

Step-by-step solution

Identify the correct answer

Plant physiology and knowledge about internal transportation of nutrients suggest that wounded sieve-tube elements are usually plugged by a certain type of protein and callose. This effectively helps in preventing the leakage of phloem sap, which is vital for nutrient distribution within the plant. From the given options, we can tell that P protein and Callose pair meet these requirements.

Choose the correct option

Therefore, our correct option is option (c): P protein and callose

Key concepts

Understanding Plant Physiology

Plant physiology encompasses the study of how various parts of a plant function, individually and collectively. One critical aspect of plant physiology that ensures survival is the plant's transport system. In vascular plants, there are two main types of transport tissues: xylem and phloem. The xylem moves water and dissolved minerals absorbed from the soil to different parts of the plant. On the other hand, the phloem is responsible for the transport of substances like sugars, amino acids, and hormones, which are synthesized in one part of the plant and need to be distributed to other parts.

Phloem is composed of several cell types, including sieve-tube elements which are living cells that form a network for distributing the nutrients and are distinctively known for their lack of nucleus. Understanding how plants deal with damage to phloem tissue, such as when sieve-tube elements are wounded, is vital. Such events trigger a quick response to prevent loss of phloem sap, which could be detrimental to the plant's survival.

The Role of Phloem Sap

Phloem sap is the sugar-rich liquid transported through a plant's phloem tissues. This sap is crucial for the growth and development of the plant as it contains the products of photosynthesis, primarily sugars like sucrose, that are necessary for energy and building materials for growth. The phloem sap also contains other organic molecules such as amino acids, hormones, and minerals, and its flow from 'source to sink'—from areas of production such as leaves to areas of storage or growth such as roots, stems, and fruits—supports the plant’s nutritional needs.

Disruption in the flow of phloem sap can have significant consequences on the plant's health. Therefore, when sieve-tube elements are damaged, the plant has a unique defense mechanism that involves the rapid sealing of the affected sieve tubes to prevent the loss of precious sap and to ensure the continuity of nutrient distribution throughout the plant.

P Protein and Callose in Defense

When a plant's sieve-tube elements are wounded, the plant's response mechanism employs P protein and callose to prevent the loss of phloem sap.

P Protein

The P protein, also known as phloem protein, is thought to play a role in the plant's defense and sealing process. Previously considered to be for plugging sieve pores, current views suggest that P proteins rapidly aggregate at the site of injury, potentially to help limit the loss of sap and seal off the damaged area.

Callose

Callose, a large polysaccharide made up of glucose residues, works in tandem with P protein. This substance synthesizes quickly at the site of the phloem injury and strengthens the seal by depositing near the sieve plates—cell wall regions between sieve-tube elements. This deposition forms a barrier that prevents the loss of sap.

Understanding the interplay between P protein and callose is crucial for students studying plant physiology because it highlights the dynamic nature of plant defense mechanisms. The combination of the two substances creates an effective plug at the injury site, enabling the plant to continue functioning normally despite potential stress and damage.