Cotyledons, the seed leaves that emerge first from a germinating seed, are usually short-lived structures. In most dicotyledonous crops they wither and drop off within days of germination, their job of fueling early growth quickly done. Castor bean is different. It belongs to a group of epigeal species whose cotyledons persist, turn green, and become fully functional photosynthetic organs. This persistence makes them a critical window of opportunity: if the seedling can establish photoautotrophic capacity in its cotyledons, it gains a foundation for survival that transient cotyledons simply cannot provide.

The research team, led by Jingyao Tian, Yingnan Wang, Xiaoxia Deng, Yueming Li and Jixiang Lin, set out to determine what happens to this delicate process when salt stress enters the picture. Salinity is among the most serious environmental constraints on crop production worldwide, imposing both osmotic stress, which makes it harder for roots to take up water, and ionic stress, in which sodium and chloride ions accumulate to toxic levels. Understanding how salt affects the earliest stages of photosynthetic development could therefore have far-reaching implications for growing oilseed crops in saline soils.

The experiment was elegantly simple in design. Castor bean seedlings were exposed to a salt solution of 150 millimolar sodium chloride, a concentration that represents meaningful agricultural stress, while being brought into the light. The researchers then sampled the cotyledons at five time points—0, 2, 6, 12 and 24 hours of illumination—to track, in fine detail, how the greening process unfolded under saline versus normal conditions. This time-course approach allowed them to capture the dynamics of de-etiolation as it happened, rather than merely comparing endpoints.

The results showed that salt stress significantly delayed cotyledon de-etiolation. The mechanism of this delay was traced to disruptions in the assembly of thylakoid membrane protein complexes, the intricate molecular machinery embedded within chloroplast membranes that captures light energy and converts it into chemical energy. Chloroplast ultrastructural development was also impaired, meaning the internal architecture of these organelles—the stacked membranes and fluid-filled spaces where photosynthesis takes place—failed to develop normally under salt exposure during the early hours of light exposure.

Yet the story did not end with damage and delay. As de-etiolation progressed beyond the first twelve hours, the cotyledons mounted a remarkable selective recovery. Rather than attempting to repair everything at once, the seedlings appeared to prioritize the protection of photosystem II integrity, the light-harvesting complex that splits water molecules and provides the electrons that drive the entire photosynthetic electron transport chain. Photosystem II is particularly vulnerable to stress because of its complex architecture and its role in generating reactive oxygen species when damaged, making its protection a logical first line of defense.

This prioritization manifested in several coordinated ways. The cotyledons enhanced their absorption of potassium ions, a strategy that helps maintain ionic homeostasis by counterbalancing the sodium that floods in under saline conditions. Pigment contents rose as well, with elevated levels of both chlorophyll and carotenoids. Chlorophyll is the primary light-capturing pigment, while carotenoids serve a protective role, dissipating excess light energy as heat before it can cause oxidative damage. Together, these increases augmented the photoprotective capacity of the cotyledons at precisely the moment when their developing photosynthetic apparatus was most at risk.

At the protein level, the researchers documented a sophisticated reallocation of resources across the thylakoid membrane. The cotyledons increased the turnover of two critical reaction center proteins: PsaC, the core protein of photosystem I, and PsbA, better known as the D1 protein of photosystem II. The D1 protein is famous among plant biologists as the most rapidly turned-over protein in the chloroplast, constantly damaged by light and continuously replaced. Under salt stress, the castor bean cotyledons accelerated this repair cycle while simultaneously enhancing electron transport mediated by the cytochrome b6f complex, the molecular bridge that shuttles electrons between the two photosystems. The net effect was a photosynthetic apparatus that prioritized photosystem II functional integrity and basal metabolism, accepting reduced photosystem I capacity as the price of keeping the core light reactions running.

The broader significance of these findings extends beyond basic plant physiology. The authors argue that persistent cotyledons of Ricinus communis are not merely transient organs during early germination but serve as a growth buffer and recovery foundation for seedlings under salt stress. By coordinating the modulation of thylakoid membrane protein complexes during de-etiolation, the cotyledons act as the critical window during which the seedling establishes its photoautotrophic capacity—the ability to feed itself through photosynthesis. A seedling that successfully navigates this window, even under salinity, emerges with a far better chance of long-term establishment than one whose cotyledons fail during this period.

This selective recovery mechanism offers new insights into the adaptive functions of persistent cotyledons and provides a theoretical basis for improving seedling establishment in castor bean and other salt-tolerant oilseed crops grown in saline soils. As agricultural pressure pushes cultivation onto marginal, salt-affected land, understanding how the earliest stages of a plant’s life can be buffered against salinity becomes increasingly valuable. The castor bean’s strategy—triage at the molecular level, protecting what matters most first—may well inform future breeding and biotechnological approaches aimed at engineering crops that can green successfully even when the ground beneath them is salt-laden.

Subject of Research: Thylakoid protein adaptation in castor bean cotyledons during de-etiolation under salt stress

Article Title: Functional persistence of cotyledons under salinity: thylakoid protein adaptation during de-etiolation in Ricinus communis

Article References: Tian, J., Wang, Y., Deng, X., Li, Y., & Lin, J. (2026). Functional persistence of cotyledons under salinity: thylakoid protein adaptation during de-etiolation in Ricinus communis. BMC Plant Biology. https://doi.org/10.1186/s12870-026-10030-w

Image Credits: AI Generated

DOI: 10.1186/s12870-026-10030-w

Keywords: Ricinus communis, cotyledons, de-etiolation, salinity stress, thylakoid membrane proteins, photosystem II, chloroplast development, potassium homeostasis, D1 protein turnover, cytochrome b6f, seedling establishment, oilseed crops

Fruit peel color is far more than a cosmetic attribute. In peach, it is determined by the dynamic balance between two pigment systems: chlorophyll, which gives young fruit their green hue, and anthocyanins, the red-purple pigments responsible for the blush that develops as fruit mature. While the regulatory networks that control anthocyanin biosynthesis have been extensively mapped over the past two decades, the mechanisms governing chlorophyll accumulation and peel greening have remained comparatively obscure. This asymmetry has left a significant hole in the understanding of how fruit quality traits are wired at the genetic level, because the green-to-red transition affects not only appearance but also photosynthetic capacity, nutrient content, and consumer perception of freshness.

To close that gap, the research team began with a systematic survey of the peach genome. They identified nine members of the ARR-B family of transcription factors, a group of proteins related to the two-component signaling systems that plants use to relay hormonal and environmental signals. Among these nine candidates, one gene stood out: PpAPRR2, a pseudo-response regulator whose expression was predominantly active during early fruit development. Critically, the timing of its expression showed a strong positive correlation with chlorophyll levels in the peel, suggesting that the gene might be driving the greening process rather than merely riding along with it.

The first step in characterizing the protein was to determine where it operates inside the cell. Subcellular localization assays showed that PpAPRR2 accumulates in the nucleus, the expected address for a transcription factor that binds DNA and regulates other genes. This nuclear localization is consistent with the protein’s role in the ARR-B family, whose members typically function as downstream effectors in phosphorelay signaling cascades, translating signals from cytokinin and other pathways into changes in gene expression. With the protein confirmed as a nuclear player, the team moved on to the decisive question: what happens to fruit when the gene’s activity is manipulated?

The answer came from virus-induced gene silencing, or VIGS, a technique that uses an engineered viral vector to instruct the plant’s own machinery to suppress a target gene. When the researchers silenced PpAPRR2 in developing peach fruit, the effect was unmistakable. Chlorophyll content dropped, and the expression of photosynthesis-related genes declined in parallel. The silenced fruit lost their vibrant green coloration, demonstrating that PpAPRR2 is not a passive marker of greening but an active promoter of it. Without the gene’s function, the chlorophyll biosynthesis machinery in the peel sputters, and the fruit’s photosynthetic apparatus fails to develop properly during the critical early phase of growth.

Silencing experiments reveal what a gene does, but overexpression reveals what it can do, and for that the team turned to a heterologous system. They introduced the peach PpAPRR2 gene into tomato, a species with well-established transformation protocols and a fruit whose pigmentation has been studied intensively. The result was striking: transgenic tomato fruit carrying the peach gene developed a darker green color than their wild-type counterparts, accumulated more chlorophyll, and showed increased expression of genes involved in chlorophyll biosynthesis. In effect, a peach gene was able to reprogram the pigment economy of a distantly related fruit, evidence that the regulatory mechanism is conserved across species and that PpAPRR2 is a portable tool for enhancing greening.

Transcriptome analysis of the overexpressing tomato fruits added a genome-wide dimension to the picture. Sequencing of the messenger RNA population confirmed that photosynthetic pathways were promoted in the transgenic fruit, while genes involved in chlorophyll degradation were suppressed. This dual action, boosting construction of the photosynthetic apparatus while simultaneously slowing its dismantling, provides a mechanistic explanation for the elevated chlorophyll levels observed. It also suggests that PpAPRR2 does not operate in isolation but coordinates a broad transcriptional program that touches both the assembly and the maintenance of chloroplasts, the organelles where chlorophyll resides and photosynthesis takes place.

The study does not exist in a vacuum. APRR2-family genes have repeatedly surfaced in genetic analyses of fruit color across the plant kingdom. In tomato, an APRR2-like gene was linked to pigment accumulation through network inference analysis, and the celebrated Uniform ripening locus was shown to encode a Golden 2-like transcription factor regulating fruit chloroplast development. In cucumber, melon, watermelon, pepper, zucchini, and wax gourd, APRR2 orthologs or related pseudo-response regulators have been associated with immature fruit rind color, peel pigmentation, and chloroplast biogenesis. The peach work extends this emerging pattern to a stone fruit, a lineage that diverged from the tomato and cucurbit lineages tens of millions of years ago, and it strengthens the case that APRR2-type regulators represent an ancient and conserved module for controlling green fruit pigmentation.

The broader context of chloroplast regulation is also instructive. GLK transcription factors, such as the PpGLK1 gene previously characterized in peach by some of the same researchers, are known to coordinate expression of the photosynthetic apparatus in Arabidopsis and crops alike. Cytokinin signaling, which flows through genuine response regulators rather than pseudo-response regulators, is another established driver of chloroplast development. PpAPRR2 sits at an intriguing intersection of these systems: as a pseudo-response regulator, it belongs to the two-component signaling family, yet its demonstrated role in directly promoting chlorophyll biosynthesis genes suggests it functions as a transcriptional activator in its own right. Disentangling how PpAPRR2 interacts with GLK factors, hormonal signals, and the circadian clock, to which pseudo-response regulators are intimately tied, will be a central question for future work.

For growers and consumers, the practical implications are tangible. Peel color influences marketability, and greener immature fruit often signals better photosynthetic performance and, in some crops, higher nutritional value. Because PpAPRR2 overexpression enhanced greening in a heterologous species, the gene could serve as a target for genome editing or transgenic approaches aimed at tuning chlorophyll levels in peach and beyond. The authors also deposited their raw sequence data in a public archive, making the resource available to the wider community. As the genetic control of fruit coloration comes into sharper focus, regulators like PpAPRR2 are transforming what was once a purely descriptive trait, peel color, into an engineerable parameter of fruit quality, one transcription factor at a time.

Subject of Research: Regulation of chlorophyll biosynthesis and chloroplast development in peach fruit by the PpAPRR2 transcription factor

Article Title: PpAPRR2 regulates chlorophyll biosynthesis and chloroplast development during early fruit development in peach

Article References: PpAPRR2 regulates chlorophyll biosynthesis and chloroplast development during early fruit development in peach. (n.d.). https://doi.org/10.1007/s00299-026-04002-5

Image Credits: AI Generated

DOI: 10.1007/s00299-026-04002-5

Keywords: peach, PpAPRR2, chlorophyll, chloroplast development, fruit color, transcription factor, virus-induced gene silencing, tomato overexpression, transcriptome, plant biotechnology, Prunus persica, fruit quality