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Phloem is the vascular tissue responsible for the transport of sugars from source tissues (ex. photosynthetic leaf cells) to sink tissues (ex. non-photosynthetic root cells or developing flowers). Other molecules such as proteins and mRNAs are also transported throughout the plant via phloem.
Phloem is composed of several cell types including sclerenchyma, parenchyma, sieve elements and companion cells. The sieve element and companion cell are found closely associated with each other in what is referred to as the sieve element/companion cell complex. One or more companion cells may be associated with a single sieve element.
The so called “sieve element” may be more specifically referred to as a sieve tube member (angiosperms) or sieve cell (gymnosperms and ferns). The sieve cells of gymnosperms lack a sieve plate and instead have sieve pores throughout the cell wall which allow flow between adjacent cells. The sieve tube members found in flowering plants are generally wider than sieve cells and have sieve plates connecting the ends of adjacent cells. These sieve plates are areas with many pores through which adjacent cells are connected by a continuous cytoplasm.
Phloem cell types
Blue: Sieve element- conducting element of the phloem
Yellow: Companion Cell- "life support" cell for the sieve element
Red: Fibers- made of sclerenchyma cells and provides structural support for the plant
Green: Parenchyma- acts as packing material between other cell types and helps transfer materials to the SE/CC complex
Sieve Element/Companion Cell Development
Not a great deal is known about the genetic mechanisms involved in the specification of phloem cells during differentiation. What is known is that ALTERED PHLOEM DEVELOPMENT (APL), a MYB-transcription factor, plays a part in inhibiting xylem cells and promoting the formation of phloem cells. Mutations in APL give rise to plants with cells showing xylem characteristics where phloem cells should be. Over-expressing APL inhibits the formation of xylem. Cytokinin signaling is also required to maintain cell identities other than xylem and is therefore important for the formation of phloem. VAHOX1, a homeobox gene from tomato, shows phloem specific expression during secondary growth and is therefore a candidate gene playing a role in phloem specification from the vascular cambium.
It is known that the sieve element (SE) and companion cell (CC) arise from an unequal division of a common “phloem mother cell.” This mother cell may be found in the procambium in the case of primary phloem or in the vascular cambium in secondary phloem.
The SE then undergoes a “partial programmed cell death.” This highly selective degradation of cellular organalles eliminates the vacuole, cytoskeleton, ribosomes, Golgi bodies and nucleus. The endoplasmic reticulum becomes modified to form the sieve endoplasmic reticulum (SER) which lacks ribosomes. The plasma membrane survives the degradation process as does the SER, mitochondria (although they may become swollen), P-proteins, and plastids. These few remaining organelles take a parietal position along the edge of the SE. This emptying of the SE is essential to allow the unimpeded flow of water, signal proteins, mRNA, and photoassimilates which travel through the SE. During SE maturation, the cell walls connecting adjacent SEs become modified to form sieve plates. These sieve plates are modified cell walls with plasma membrane lined pores which allow the phloem stream to pass from one SE to the next. The plasmodesmata, which symplastically connect the SE to the CC, become modified to form the pore-plasmodesma (PPUs). These PPUs are branched tunnels on the CC end and converge to form a single tunnel on the SE end. The PPUs play an integral role in maintaining the SE in a partially dead state by connecting it to the CC. The CC remains in a fully intact state and plays a life support role by channeling necessary biomolecules from the fully functioning CC to the SE.