Plant cell organelles that perform photosynthesis and store starch

Plant cells with visible chloroplasts

Oh, you want to talk about plastids. Fine. These rather industrious membrane-bound organelles are consistently found nestled within the cells of plants , algae , and, for those with a discerning eye for biological minutiae, certain other eukaryotic organisms. Don’t mistake them for mere cellular components; plastids are, in essence, domesticated endosymbiotic cyanobacteria that, at some point approximately 1.5 billion years ago, decided to integrate rather than freeload, fundamentally reshaping the trajectory of life on Earth. [1] It’s a classic tale of assimilation, truly.

Among the various specialized forms these organelles assume, you’ll encounter the ubiquitous chloroplasts , the verdant engines of photosynthesis . Then there are the more aesthetically inclined chromoplasts , dedicated to the sophisticated synthesis and meticulous storage of various pigments, because even cells appreciate a vibrant hue. Not to be overlooked are the leucoplasts , a collection of non-pigmented plastids, some of which possess the remarkable capacity to differentiate into other specialized variants as cellular needs dictate. And, for the truly specialized, or perhaps one might say, the evolutionarily adventurous, there are the apicoplasts , non-photosynthetic relics found within the notoriously parasitic apicomplexa , derived from a secondary endosymbiosis event. They’ve clearly moved beyond the quaint practice of sunbathing.

The foundational primary endosymbiosis event, a pivotal moment in cellular evolution, is estimated to have transpired roughly 1.5 billion years ago. This monumental integration occurred within the lineage of the Archaeplastida —a clade encompassing all land plants , red algae , green algae , and the somewhat less celebrated glaucophytes . The initial symbiont was likely a cyanobiont , a type of symbiotic cyanobacteria with genetic ties to the genus Gloeomargarita . [2] [3] As if one such profound event weren’t enough, another distinct primary endosymbiosis occurred much later, approximately 140 to 90 million years ago, involving the photosynthetic plastids found in Paulinella amoeboids . This particular event saw the integration of cyanobacteria from the genera Prochlorococcus and Synechococcus , collectively known as the “PS-clade.” [4] [5] The evolutionary narrative doesn’t stop there, of course. Secondary and tertiary endosymbiosis events have subsequently unfolded across a vast array of organisms, leading to an even greater diversity of plastid forms and functions. Some organisms have even developed the somewhat opportunistic, if not outright thieving, capacity to sequester ingested plastids from their prey—a transient process elegantly termed kleptoplasty .

It was A. F. W. Schimper [6] [a] who, with commendable clarity, first provided a formal name, detailed description, and precise definition for these organelles. Plastids, as we now understand them, harbor their own distinct double-stranded DNA molecule. For a considerable time, this DNA was presumed to be exclusively circular, mirroring the circular chromosome characteristic of prokaryotic cells. However, recent findings suggest that this assumption might be, shall we say, a touch simplistic, with evidence pointing towards a more nuanced, potentially even "..a linear shape" in certain contexts. These organelles are not just energy converters; they are sophisticated biochemical factories, serving as crucial sites for the manufacturing and storage of pigments and a host of other vital chemical compounds indispensable to the autotrophic eukaryotes they inhabit. Many plastids contain biological pigments , such as those directly involved in photosynthesis or those that simply dictate a cell’s color, adding a splash of visual information to the cellular landscape. Even in organisms that have, perhaps regrettably, forfeited their photosynthetic capabilities, plastids remain highly useful, particularly for the synthesis of complex molecules like the isoprenoids . [8]

In land plants

Plastid types

Leucoplasts in plant cells.

Chloroplasts, proplastids, and differentiation

In the realm of land plants , the plastids that deign to contain chlorophyll are the primary architects of photosynthesis . They masterfully convert external sunlight energy into internal chemical energy, simultaneously drawing carbon from the Earth’s atmosphere and, in a rather generous gesture, enriching it with the very oxygen that sustains most complex life. These are, as you might have surmised, the chloroplasts , easily identifiable by their characteristic green hue (see top graphic).

Beyond their role as photosynthetic powerhouses, other plastids are engaged in the intricate synthesis of fatty acids and terpenes . These compounds can either be directly utilized for energy production or serve as essential raw materials for the construction of myriad other complex molecules. For instance, the plastids residing in epidermal cells are responsible for fabricating the constituent components of the plant cuticle —that protective outer layer—including its [epicuticular wax]. This entire process often begins with palmitic acid , which itself is initially synthesized within the chloroplasts of the underlying mesophyll tissue . Furthermore, plastids are adept at storing diverse cellular reserves, including vital starches , fats , and proteins . [9]

Every plastid traces its lineage back to proplastids (sometimes referred to as proplasts [10] ), which are typically found in the meristematic regions of a plant, those zones of active growth and division. These foundational proplastids , along with nascent chloroplasts , commonly propagate through binary fission . Interestingly, even more mature chloroplasts retain this capacity for division, ensuring the continued proliferation of these essential organelles.

The undifferentiated nature of plant proplastids allows them to undergo remarkable differentiation , transforming into various specialized forms tailored to specific cellular functions (refer to the top graphic for a visual representation). They are capable of developing into any of the following distinct variants: [11]

• Chloroplasts : The quintessential green plastids, primarily responsible for the execution of photosynthesis .
• Etioplasts : These are essentially precursor chloroplasts , typically found in plants grown in darkness. They await light exposure to mature and initiate photosynthetic activity.
• Chromoplasts : As their name suggests, these are the brightly colored plastids, specializing in the synthesis and storage of various pigments, contributing to the vibrant hues of flowers and fruits.
• Gerontoplasts : These plastids assume control during plant senescence , orchestrating the systematic dismantling of the photosynthetic apparatus as a plant ages and prepares for dormancy or death.
• Leucoplasts : These are colorless plastids, primarily involved in the synthesis of monoterpenes and other non-pigmented compounds.

From the broad category of leucoplasts emerge even more finely specialized plastids, each with a distinct storage function:

• The aleuroplasts , for instance.
• Amyloplasts : These are dedicated to storing substantial quantities of starch . Beyond mere storage, they play a critical role in detecting gravity , a function essential for maintaining geotropism , ensuring that roots grow downwards and shoots upwards.
• Elaioplasts : Specializing in the accumulation and storage of fats and oils.
• Proteinoplasts : These plastids are involved in the storage and, in some cases, modification of protein .
• Or the less commonly known Tannosomes : These are responsible for synthesizing and producing tannins and various polyphenols , compounds often involved in plant defense and coloration.

The plasticity of these organelles is quite remarkable; depending on their evolving morphology and the specific functional demands of the cell, plastids possess the inherent ability to both differentiate and redifferentiate between these various forms, adapting to the dynamic needs of the plant.

Plastomes and Chloroplast DNA/ RNA; plastid DNA and plastid nucleoids

Each plastid, in its quiet efficiency, meticulously maintains multiple copies of its own distinct genetic blueprint, a unique genome referred to as a plastome (a portmanteau derived from ‘plastid genome’). In the case of a chlorophyll plastid, or chloroplast , this is more specifically termed a ‘chloroplast genome’ or, perhaps more commonly, ‘chloroplast DNA’. [12] [13] The precise number of genome copies within each plastid is not static; it exhibits considerable variability. In rapidly dividing, youthful cells, which might only contain a handful of plastids, the copy number can soar to 1000 or even more. Conversely, in mature cells, which typically house numerous plastids, this number can drop to 100 or fewer.

A plastome is typically a comprehensive genome that encodes crucial transfer ribonucleic acids (tRNA s) and ribosomal ribonucleic acids (rRNAs ). It also holds the genetic instructions for proteins directly involved in photosynthesis and the intricate processes of plastid gene transcription and translation . However, it’s vital to recognize that these plastid-encoded proteins constitute only a meager fraction of the total protein complement necessary to construct and maintain any given type of plastid. The vast majority of plastid proteins are, in fact, encoded by nuclear genes residing within the plant cell’s nucleus. Consequently, the expression of both nuclear and plastid genes is meticulously co-regulated, a sophisticated coordination act ensuring the harmonious development and differentiation of plastids.

Many plastids, particularly those indispensable for photosynthesis , are characterized by numerous internal membrane layers, forming an elaborate internal architecture. Within this complex environment, plastid DNA doesn’t just float freely; it exists as intricate protein-DNA complexes. These complexes are strategically associated with localized regions within the plastid’s inner envelope membrane and are aptly named ‘plastid nucleoids ’. Unlike the highly organized nucleus of a eukaryotic cell, a plastid nucleoid is notably not enveloped by its own nuclear membrane. Each nucleoid region may contain a surprising density of genetic material, often holding more than 10 copies of the plastid DNA .

Consider the journey from an undifferentiated proplastid to a specialized plastid: a proplastid typically harbors a solitary nucleoid region, usually situated near its center. As this proplastid embarks on its developmental or differentiating path, the developing plastid acquires multiple nucleoids , which then migrate and become localized at the periphery of the organelle, firmly bound to the inner envelope membrane. Throughout this entire process of differentiation —from proplastids to chloroplasts , and indeed when plastids transition from one specialized type to another—these nucleoids undergo significant transformations in their morphology, overall size, and precise location within the organelle. This dynamic remodeling of plastid nucleoids is largely believed to be orchestrated by meticulous modifications to both the abundance and the specific composition of the proteins associated with the nucleoids themselves.

In typical plant cells , one might observe the occasional formation of long, slender protuberances, rather charmingly termed stromules . These delicate extensions emanate from the main body of the plastid, reaching out into the surrounding cell cytosol , sometimes even forming intricate interconnections between several plastids. This network allows for the efficient movement of proteins and smaller molecules both within and through these stromules . Curiously, in a laboratory setting, most cultured cells—which tend to be considerably larger than their natural counterparts—exhibit a propensity for producing exceptionally long and abundant stromules that often extend all the way to the cell periphery, a testament to their inherent adaptability or perhaps just a quirk of artificial environments.

A rather intriguing piece of evidence emerged in 2014, suggesting the potential, albeit controversial, loss of the plastid genome in Rafflesia lagascae, a non-photosynthetic parasitic flowering plant, and in Polytomella , a genus of non-photosynthetic green algae . Despite extensive and diligent searches for plastid genes in both of these taxons , no definitive results were yielded. However, the conclusive assertion that their plastomes are entirely absent remains a subject of ongoing scientific debate. [14] Some scientists, with a healthy dose of skepticism, argue that a complete loss of the plastid genome is highly improbable. Even these non-photosynthetic plastids, they contend, typically retain essential genes required to complete various crucial biosynthetic pathways , including, for example, the intricate process of heme biosynthesis. [14] [15]

Even in the hypothetical scenario of a complete plastid genome loss within Rafflesiaceae , the plastids themselves are still present, albeit as “shells” devoid of DNA content. [16] This structural persistence, despite genetic emptiness, bears an unsettling resemblance to the enigmatic hydrogenosomes found in various organisms, raising further questions about the minimal requirements for organelle maintenance.

In algae and protists

The diversity of plastids extends significantly beyond land plants, encompassing a fascinating array of types within algae and protists :

• Chloroplasts : These are, of course, found in green algae (the evolutionary progenitors of plants) and in other organisms that have, through various evolutionary pathways, derived their genomes from green algae.
• Muroplasts : Also known by the more descriptive terms cyanoplasts or cyanelles, these are the distinctive plastids of glaucophyte algae. They bear a striking resemblance to plant chloroplasts , with one critical difference: they possess a peptidoglycan cell wall that is remarkably similar in composition to that found in bacteria , a clear echo of their ancient cyanobacterial origins.
• Rhodoplasts : These are the vibrant red plastids characteristic of red algae . Their unique pigmentation allows them to photosynthesize efficiently even at extraordinary marine depths, reportedly down to 268 meters. [11] A key distinction between the chloroplasts of plants and rhodoplasts lies in their starch metabolism: plant chloroplasts synthesize and store starch as granules within the plastids. In contrast, red algae synthesize floridean starch , but this is stored outside the plastids, specifically in the cytosol . [17]
• Secondary and tertiary plastids : These represent a more complex evolutionary history, originating from subsequent endosymbiosis events involving the engulfment of either green algae or red algae by other eukaryotic cells.
• Leucoplast : In the context of algae , this term is broadly applied to all unpigmented plastids. Their precise functions, however, can diverge significantly from those of the leucoplasts found in plants.
• Apicoplast : These are the non-photosynthetic plastids found in the Apicomplexa , a phylum of obligate parasitic organisms. Their presence is a result of secondary endosymbiosis , a testament to a photosynthetic past now repurposed for parasitic survival.

The plastid found in photosynthetic Paulinella species is often referred to as a ‘cyanelle’ or chromatophore, diligently serving its role in photosynthesis . [18] [19] What makes this particular plastid noteworthy is its origin from a much more recent endosymbiotic event, estimated to have occurred within the last 140–90 million years. This represents a rare and distinct second primary endosymbiosis of cyanobacteria , entirely independent of the ancient event that gave rise to the Archaeplastida . [20] [21]

It’s worth noting that certain plastid types, specifically etioplasts , amyloplasts , and chromoplasts , are considered plant-specific and are generally not observed in algae . [citation needed ] Furthermore, plastids in algae and hornworts can also exhibit structural differences from typical plant plastids, notably by containing specialized structures known as pyrenoids . [22]

Inheritance

In the grand scheme of biological reproduction, most plants exhibit a rather strict uniparental inheritance pattern for their plastids, meaning these organelles are typically passed down from only one parent. Generally speaking, angiosperms (flowering plants) primarily inherit their plastids from the female gamete . In contrast, many gymnosperms (conifers and their relatives) often receive their plastids from the male pollen . Similarly, algae also adhere to this uniparental inheritance, ensuring that the plastid DNA from the other parent is, for all intents and purposes, completely lost from the lineage.

Under normal circumstances, particularly in intraspecific crossings—those resulting in typical hybrids within the same species—the inheritance of plastid DNA appears to be rigorously uniparental, almost exclusively stemming from the female line. However, when it comes to interspecific hybridizations, the inheritance pattern becomes, shall we say, considerably more erratic and less predictable. While plastids are still predominantly inherited from the female parent in these interspecific crosses, there exists a notable body of reports detailing instances where hybrids of flowering plants have demonstrably inherited plastids from the male parent. Furthermore, approximately 20% of angiosperms , including commercially significant species like alfalfa (Medicago sativa), naturally exhibit a biparental inheritance pattern for their plastids, making them a fascinating exception to the rule. [23]

DNA damage and repair

The delicate DNA within the plastids of maize seedlings is not immune to the ravages of time and environment; it demonstrably accumulates increasing amounts of damage as the seedlings progress through their developmental stages. [24] This damage is predominantly attributed to the pervasive oxidative environments generated by photo-oxidative reactions and the energetic, yet potentially destructive, processes of photosynthetic and respiratory electron transfer . While some of these damaged DNA molecules are meticulously repaired by cellular machinery, any DNA that harbors unrepaired damage is, rather unsentimentally, degraded into non-functional fragments, ensuring cellular integrity.

Crucially, the essential DNA repair proteins are not encoded by the plastid’s own genome ; instead, they are encoded by the cell’s nuclear genome . Once synthesized, these proteins are then painstakingly translocated into the plastids, where they perform their vital duty of maintaining genome stability and integrity by diligently repairing the plastid’s DNA . [25] A compelling example of this intricate cooperation can be observed in the chloroplasts of the moss Physcomitrella patens . Here, a protein involved in DNA mismatch repair (Msh1) actively interacts with other proteins crucial for recombinational repair (RecA and RecG). This collaborative effort is fundamental to preserving the stability of the plastid genome , a testament to the complex, multi-layered strategies cells employ to safeguard their genetic information. [26]

Origin

Plastids, as we’ve already established with a sigh of cosmic weariness, are widely considered to be the venerable descendants of ancient endosymbiotic cyanobacteria . The primary endosymbiotic event that initiated the lineage of the Archaeplastida is hypothesized to have occurred a staggering 1.5 billion years ago [27] . This singular, transformative event bestowed upon early eukaryotes the remarkable capacity to perform oxygenic photosynthesis , forever altering the atmospheric composition and paving the way for the evolution of complex life. [28] Since that primordial integration, three distinct evolutionary lineages have emerged within the Archaeplastida , each characterized by uniquely named plastids: the familiar chloroplasts in green algae and plants , the vibrant rhodoplasts in red algae , and the distinctive muroplasts found in the glaucophytes . These plastids are not merely differentiated by name; they exhibit significant variations in their specific pigmentation and their intricate ultrastructure. For instance, chloroplasts in plants and green algae have, over evolutionary time, shed all their phycobilisomes —the efficient light harvesting complexes that are still present in cyanobacteria , red algae , and glaucophytes . Instead, plant chloroplasts developed the complex internal structures of stroma and grana thylakoids . The glaucocystophycean plastid , in stark contrast to chloroplasts and rhodoplasts , still retains the discernible vestiges of its ancestral cyanobacterial cell wall , a living fossil of its prokaryotic past. All of these primary plastids, irrespective of their specific lineage, are consistently enveloped by two membranes, a tell-tale sign of their ancient endosymbiotic origin.

As previously touched upon, the plastid within photosynthetic Paulinella species, often referred to as a ‘cyanelle’ or chromatophore, represents a comparatively recent and entirely separate endosymbiotic event, occurring approximately 90–140 million years ago. [18] [19] This makes it the only other known instance of a primary endosymbiosis event involving cyanobacteria outside of the venerable Archaeplastida . Intriguingly, this particular plastid belongs to the “PS-clade” (comprising cyanobacteria genera like Prochlorococcus and Synechococcus ), which constitutes a distinct sister clade to the cyanobacteria that gave rise to the plastids within the Archaeplastida . [4] [5]

In stark contrast to these primary plastids, which arose directly from the single endosymbiosis of a prokaryotic cyanobacteria , the more intricate ‘complex plastids’ have a more convoluted origin story. They emerged through ‘secondary endosymbiosis ’, a process where a eukaryotic organism, having already acquired a primary plastid, then proceeded to engulf another eukaryotic organism that itself contained a primary plastid. [29] When a eukaryote successfully engulfs either a red or a green alga and manages to retain the algal plastid, that secondary plastid is typically, and rather tellingly, surrounded by more than two membranes—a ghost of its multiple ancestral engulfments. In some instances, these complex plastids may experience a reduction in their metabolic and/or photosynthetic capabilities, adapting to their new host environment. Examples of algae harboring complex plastids derived from the secondary endosymbiosis of a red alga include the heterokonts , haptophytes , cryptomonads , and the majority of dinoflagellates (these are often referred to as rhodoplasts ). Conversely, those that chose to endosymbiose a green alga include the euglenids and chlorarachniophytes (housing chloroplasts ). The Apicomplexa , a phylum of obligate parasitic alveolates that includes the causative agents of devastating diseases such as malaria (Plasmodium spp.) and toxoplasmosis (Toxoplasma gondii ), along with numerous other human and animal afflictions, also harbor a complex plastid. While this organelle has been lost in some apicomplexans (e.g., Cryptosporidium parvum , which causes cryptosporidiosis ), the ‘apicoplast ’ is no longer capable of photosynthesis . Yet, it remains an absolutely essential organelle for the parasite’s survival, making it a highly promising target for the development of novel antiparasitic drugs . A rather poetic twist, considering its photosynthetic ancestry.

And then there are the true opportunists: some dinoflagellates and certain sea slugs , particularly those belonging to the genus Elysia . These organisms engage in a rather audacious practice: they consume algae as food but, instead of fully digesting them, they selectively retain the plastids from their meals. The purpose? To temporarily benefit from the photosynthesis performed by these stolen organelles. After a period of exploitation, the plastids are, with characteristic efficiency, eventually digested as well. This fascinating, if somewhat predatory, process is aptly termed kleptoplasty , a name derived from the Greek kleptes (κλέπτης), meaning “thief.”

Plastid development cycle

An illustration of the stages of inter-conversion in plastids

In 1977, J.M. Whatley, with a clarity that belied the complexity of the subject, proposed a plastid development cycle that challenged earlier, more linear views. Whatley’s insightful model posited that plastid development is far from being a simple, unidirectional progression. Instead, it is a remarkably intricate and cyclic process, allowing for considerable flexibility and interconversion between different plastid forms. At the heart of this cycle are the proplastids , which serve as the fundamental precursor stage for all the more differentiated forms of plastids, as elegantly depicted in the accompanying diagram to the right. [30] This dynamic model underscores the adaptability of plastids, enabling them to respond to changing environmental conditions and cellular demands by transforming from one specialized state to another and, in many cases, back again.

See also

• Mitochondrion – Organelle in eukaryotic cells responsible for respiration
• Cytoskeleton – Network of filamentous proteins that forms the internal framework of cells
• Photosymbiosis – Type of symbiotic relationship

Notes

• ^ Sometimes Ernst Haeckel is credited to coin the term plastid, but his “plastid” includes nucleated cells and anucleated “cytodes” [7] and thus totally different concept from the plastid in modern literature.