Phloem – Structure and Function

Phloem is a type of vascular tissue in plants responsible for transporting organic nutrients, especially sugars, from the parts of the plant where they are made (sources) to where they are needed or stored (sinks). It is one of the two types of vascular tissue found in vascular plants, the other being xylem. It forms a complex network of cells that help to move nutrients throughout the plant. In this article, we will learn about phloem structure, phloem function, mechanism of transport by plants, and significance of phloem in plant physiology.

What is Phloem?

Definition of Phloem: Phloem is a type of vascular tissue responsible for transporting organic materials produced during photosynthesis, called photosynthates, from the leaves to other parts of the plant body, including the stem and the roots.

The phloem is one of the two conducting tissues in plants. It is made of four components: sieve tubes, companion cells, phloem parenchyma, and phloem fibers. All these components except the phloem fibers are living. Phloem helps in the translocation of food or photosynthates from the source (the site where they are produced) to sink.

The phloem runs parallel to the xylem in the stem and is continued throughout the stem’s length. Depending on its origin, phloem can be categorized into two types: Primary phloem and secondary phloem. Primary phloem arises from the procambium and includes protophloem and metaphloem. The secondary phloem arises from the vascular cambium.

Structure of Phloem

A phloem is composed of four types of cells. These components are made up of three types of cells : conducting cells, supporting cells, and parenchyma cells. The component of phloem are discussed below.

Sieve Elements

Sieve elements comprise two types of cells: sieve tube elements in angiosperms and sieve cells in gymnosperms. Sieve elements are the principal conducting cells. They are elongated cells with thin walls and lack a nucleus at maturity. Sieve elements are connected end-to-end via sieve plates, which contain pores that allow for the movement of sap between adjacent cells. Connected sieve elements form the sieve tube with a central pore through which nutrients pass. Sieve tubes are found only in angiosperms. In gymnosperms, sieve cells that do not have sieve plates are the primary conducting tissue.

Phloem Parenchyma Cells

Phloem has two types of parenchyma cells that are usually undifferentiated and have storage functions.

• Companion cells: These are connected to sieve elements by plasmodesmata and play a vital role in supporting their metabolic functions. They are typically smaller but have dense cytoplasm and numerous organelles, including mitochondria, ribosomes, and endoplasmic reticulum.
• Albunimous cells: These cells have similar functions to companion cells but are associated with sieve cells and are found only in gymnosperms.

Phloem Sclerenchyma Cells

These are the supporting tissues in the phloem responsible for structure and rigidity. It consists of two types of cells:

• Fibers: Elongated, lignified cells that provide mechanical support to the phloem tissue.
• Sclereids: These are irregular cells that provide rigidity to the phloem system in plants.

Phloem Function

The function of the phloem can be summarized as follows:

• Organic nutrient transport, primarily sugars, is synthesized in photosynthetic tissues (source) to various parts of the plant (sink) for utilization, growth, and storage.
• Translocation of signaling molecules like hormones such as auxins, cytokinins, gibberellins, and other signaling molecules regulates various physiological processes, including growth, flowering, and stress responses.
• Maintenance of source sink regulation by allocating resources based on different plant parts’ metabolic demands and developmental needs.
• Structural support to the plant by providing rigidity.
• Promote defense against herbivores by producing hard tissues that are usually difficult to chew.

Types of Phloem

There are two types of phloem based on their origin

Primary Phloem

Primary Phloem is formed during the primary growth of plant tissues. The primary phloem is formed either during embryonic development from the procambium or in mature stems and roots from the vascular cambium. The primary phloem comprises components like sieve tubes, companion cells, and parenchyma cells. Usually, the primary phloem is located at the periphery of stems and roots.

Secondary Phloem

Secondary phloem forms during plant secondary growth by the vascular cambium, contributing to stem and root girth. It undergoes structural modifications and develops new cell types like fibers, rays, and cell wall compositions.

Mechanism of Transport by Phloem

Phloem transportation consists of three processes: Loading, Translocating, and Unloading phloem.

Loading of Phloem

There are two types of phloem loading mechanisms:

• Active phloem loading: It is also known as the sucrose-H+ cotransporter mechanism. In this mechanism, sucrose produced in the leaves diffuses out of photosynthetic tissue into the apoplast. Next, H+ ions are actively pushed through the plasma membrane of companion cells and into the apoplast by a carrier protein that uses ATP. Then, finally, co-transportation of the H+ ion and sucrose occurs in the companion cells, from where it moves into the sieve element.
• Passive phloem loading: Where organic solutes move freely from mesophyll cells through symplast to sieve tubes of phloem element via companion cells through plasmodesmata.

Translocation

After phloem loading, sucrose is translocated from the source to the consumption end or sink organs.

Unloading of Phloem

Phloem unloading occurs similarly to phloem loading, either by symplast or apoplast. When sugar arrives at the receiving end, it is unloaded from the filter tube into the cells or sink. There are three types of phloem unloading mechanisms:

• Sieve element unloading: Simple diffusion of materials outside the sieve tube into the sink.
• Short-distance transport: also known as post-sieve element transfer, sugars are transported out of the sieve cells and into the sink cells.
• Storage and metabolism: Food is metabolized or stored in the skin cells as fruits.

Regulation of Phloem Transport

Phloem transport, like any other physiological process in plants, is regulated by many factors. These include:

Mass Flow Hypothesis

The Pressure flow mechanism, also known as the mass flow hypothesis, assumes that sugars synthesized in photosynthetic tissues (sources) are actively transported into the phloem sieve tubes, creating a high concentration of solutes. This osmotic gradient drives the movement of water into the phloem, resulting in increased pressure. Thus, The pressure gradient propels the flow of phloem sap from source to sink regions, where sugars are unloaded and utilized or stored.

Hormonal Control

Various hormones like auxin and cytokinin drive phloem transport and act as signals that drive the flow of nutrients from one part of the plant to another based on environmental and metabolic cues.

Expression of Proton-Coupled Sugar Transporters (SUTs)

Phloem loading and unloading into the tissues is also regulated by a particular type of transporter present in the cell membrane of phloem tissues, which facilitates the uptake of sugars into the phloem cells, thereby regulating their transport.

Factors Affecting Phloem Transport

There are several factors that affect phloem transport in plants. These include:

• Concentration Gradient: The concentration of solutes, particularly sugars, in source and sink tissues creates osmotic gradients that drive phloem sap flow.
• Pressure Gradient: Pressure differences in turgor pressure between source and sink tissues facilitate the movement of phloem sap.
• Metabolic Activity: Metabolic processes, such as photosynthesis, respiration, and synthesis of organic compounds, generate the energy and substrates required for phloem transport.
• Environmental Factors: External conditions such as temperature, light, humidity, and water availability can modulate phloem transport by affecting plant metabolism and physiological responses.
• Hormonal Regulation: Plant hormones, including auxins, cytokinins, gibberellins, and abscisic acid, regulate phloem transport by modulating gene expression, ion uptake, and cell differentiation.

Significance of Phloem in Plant Physiology

Phloem tissue in plants regulates transport and has several other significant functions. These include:

• Nutrient Transport: Phloem transports sugars, amino acids, hormones, and other metabolites synthesized in photosynthetic tissues to non-photosynthetic plant tissues.
• Resource Allocation: Phloem distributes resources within the plant, ensuring all plant parts can efficiently utilize resources.
• Long-Distance Signaling: Phloem sap contains signaling molecules such as hormones, RNA, and proteins that can act as long-distance communication signals.
• Defense and Stress Responses: Phloem sap contains defense compounds such as phenolics, alkaloids, and secondary metabolites that can confer resistance against pathogens, herbivores, and environmental stresses.
• Regulation of Growth and Development: Phloem-derived signals and hormones are critical in regulating plant growth, development, and adaptive responses to changing environmental conditions.

Conclusion – Phloem

In summary, the phloem performs essential functions in plants. Plant growth and development are significantly supported by the phloem. Phloem cells include sieve elements, companion cells, and parenchyma cells. Organic substances like sugars, amino acids, and hormones are transported by the phloem from source organs like leaves to sink organs like roots, fruits, and developing tissues in a process known as translocation. Through the actions of partner cells that preserve the integrity and functionality of sieve components, this transport takes place via pressure flow processes.

FAQs on Phloem

What are the Main Components of Phloem?

The main components of the Phloem are the conducting sieve elements: the sieve tube and sieve cells, the parenchyma cells, and the supporting sclerenchyma cells.

Why is Phloem Called Living Tissue?

Phloem is called living tissue because they have a nucleus and all essential organelles for sustaining life and is also supported by companion cells. On the other hand, the xylem is dead tissue.

What is the Function of Phloem?

The function of the Phloem is to transport food and other organic materials like hormones from the source to a sink in the plant body.

What are the Types of Cells of Phloem?

The types of cells of the Phloem are the conducting cells called sieve elements, the parenchyma cells, and the supporting sclerenchyma cells.

What are the Four Elements of the Phloem?

The four elements of the Phloem are sieve elements, companion cells, fibers, and sclerieds.

What is the Structure of Phloem?

The structure of phloem consists of four main types of cells: sieve tube elements, companion cells, phloem fibers, and parenchyma cells.

Where is Phloem Located?

Phloem is located within the vascular tissue system of plants, alongside xylem.

What is Phloem and Xylem?

Phloem tissue in plants are responsible for transporting sugars and other organic nutrients from leaves to other parts of the plant, while xylem transports water and minerals from roots to other plant tissues.

What is Phloem Parenchyma?

Phloem parenchyma is the specialized cells within the phloem tissue of plants that provide metabolic support and store nutrients for the plant.

How is Phloem Different from ylem?

Phloem transports organic nutrients like sugars and amino acids from leaves to other parts of the plant, while xylem transports water and minerals from roots to other plant tissues.