You left too soon, leaving in shock and pain. We, the students you raised, continuing your research path, embedded in the biotech industry and academia, your spirit of science pulses within us, miss you.
Just yesterday my doctoral student Daphna finished preparing our joint article for submission. We thought about how we can continue to consult with you, continue to use your expertise to reveal the wonders of nature. And Hagar, another doctoral student, is already working on the report you were supposed to read, as a member of her accompanying committee... and I haven't even mentioned Ido, the doctoral student who was a graduate student with you, who told me that a few days ago you submitted the article summarizing his work with you. He told how You were happy and optimistic.
But it seems that no one owes you more than I do. Young and inexperienced I arrived at your doorstep almost twenty years ago. I asked to taste the taste of scientific research, and you opened your door with a wide smile; you gave me everything I needed to succeed. You taught, instructed, isisted on each and every comma. The laboratory you established was an example and model of a vibrant and busy research team. We worked together with pleasure, researched diligently. You demanded from us almost as much as you demanded from yourself. Every successful experiment you wanted us to check again. Every test that was successful, you wanted another person to verify. We worked day and night, following you, captivated by your magic and trusting in the high standrad you set for us. Silent witnesses are the articles you left behind. Each one of them is a work of art, a science built to perfection with an internal logic, which leads the reader from a question to an answer, and from an answer to a new question, and so on. You wrote your articles like a real legal case; Fastened and without leaving any shadow of doubt. You were a world-renowned pioneer in understanding that oxidation plays a central role in transmitting signals that influence plant response to light and stress factors. Over the years you have had many discoveries of phenomena and processes in plant physiology and biochemistry.
I remember being excited before meeting you. I studied the material well and memorized every detail so that I could clearly explain to you the results of the last experiment. An alluring aroma of coffee draws me into your office. Years before the capsule craze, you had the best coffee machine, with the freshest beans. At the door, your mountain bike, the last word in the field, the hobby you devoted yourself to. Just a moment and I'm already in the office... Jazz sounds surround me. You open files on the Macintosh computer, and you already say "look how the graphs are displayed, there are no such things on a PC, only on Apple"... and then dive into the science. How are redox signals transmitted in the chloroplast? Would you believe this is possible in a biological solution? After all, this requires highly specific proteins... which can transmit important signals in the control of the repair mechanisms of photosynthesis... so you went on and on, with infectious enthusiasm.
In the longer conversations, you also shared more personal experiences with us. One day, for example, you told me about your unique diving style, which is not very deep, certainly not the fastest. The beauty, as you said, lies precisely in the stillness, in the lack of movement. You described how you slowly descend and settle in the heart of the reef, then sit quietly, uniting with the coral and fish around you, as if you were one of them. Or then, as you explained, you see nature in all its glory... Indeed, your inner peace, ability to observe, wisdom, originality, were the qualities that stood out in your personality as a researcher as well. These were the secret of your magic when we followed you.
In the research team we were good and cohesive friends. In the laboratory we worked together, supporting and helping each other. And after work we would meet on fun days, go to the beach together or meet at a restaurant. And you knew how to appreciate good food... I remember picking cherries in the Golan Heights. With your tall stature, with your sturdy body, you reached the top of the tree. You gently bend it for us, and while you hold it, we come closer to gather the sweet fruits.
Avihai, this is how we will always remember you, the benevolent giant, who cannot be moved, a pioneering and resourceful researcher, in whose school we grew up, to marvel at the nature of plants and decipher their secrets.
Your student always,
As I write this, I find it hard to believe that its been over a year since Avihai left left us so suddenly, and over a quarter century years since we first met at the International Congress of Plant Molecular Biology in Amsterdam.
Avihai Danon was a giant in many ways. His physical size, his huge smile and hands, juxtaposed with his uncompromising devotion to scientific excellence, were a unique combination. We shared a common interest in plant responses to light and in understanding protein translation and stability. Avihai took these subjects to a biochemical mechanistic level that often far exceeded my own ability to follow! His depth of questioning and of experimental exploration were unparalleled. His loyalty to "the data", and his original interpretations were not always immediately accepted. Our meetings were often joyfully spent reanalyzing his lab's data, looking for holes in the logic. Avihai was not one to acquiesce to dogma, and his findings and models have withstood the test of time. Over time, his work in defining the oxidative and reductive pathways of regulatory proteins has proven critical to our understanding of the intricate regulation of redox signaling in mediating plant responses to changing environmental conditions. I think he would take great satisfaction in knowing that this past year his research has now yielded a patent where this research is applied for increasing plant and algae growth and yield. In this, his basic research takes him back to his days as a farmer, and Avihai's legacy lives on not in ly in the lab, but hopefully also in the field.
Prof. Daniel A. Chamovitz
Avihai was highly driven to understand how plants adapt to changing environments. His motivation was to unravel the intrinsic regulatory factors that allow plants to achieve homeostasis through the coordination of photosynthetic activity and its downstream metabolism under dynamic light conditions. While much research in this field aimed at understanding adaptation to extreme light conditions, Avihai recognized that the plant’s goal of attaining homeostasis is not limited to harsh or extreme conditions. Rather, homeostasis equips organisms with internal stability under everyday conditions, which provides the capacity to cope with impending future events. Accordingly, his experiments were almost always conducted under non-stress conditions.
Specifically, his main interest throughout the years was to understand redox regulation, i.e., the regulation of enzyme activity by post-translational reduction-oxidation modifications. This field emerged in the 1960s and 1970s when Bob Buchanan and his coworkers discovered that enzymes involved in the Calvin cycle, the metabolic pathway responsible for carbon assimilation, are regulated by this kind of post-translational modification1,2. It was found that light energy triggers a cascade of electron transfer that culminates in the breakdown of disulfide bonds via reduction reactions. When the proteins are reduced, they become active, allowing light availability to be communicated to carbon assimilation reactions. Although this mode of reductive activation has been extensively studied since its discovery, little was known about the turn-off signals - the oxidative pathway responsible for the reformation of disulfide bonds and shut-down of the reduction-activation effect. As a result, an integral piece of the puzzle was missing because the off signal is just as important for the function of a regulatory mechanism as the on signal.
The formation of disulfide bonds can be triggered by hydrogen peroxide, a partially reduced form of oxygen with the potential, when accumulated to high levels in the cells, to harm essential biological molecules. In order to prevent cellular damage, hydrogen peroxide and other forms of reactive oxygen species (ROS) are tightly controlled by the antioxidant machinery within the cell. While many studies have been conducted to determine how plants detoxify ROS under stress conditions, Avihai asked the opposite question, how ROS serve as signaling molecules in the presence of such efficient antioxidant systems? In other words, while most research focused on how ROS overproduction is combatted by antioxidant activity, Avihai believed that the intense reductive activity requires oxidative feedback. This original perspective led him to discover a novel molecular pathway.
As a biochemist at heart3–5, Avihai was motivated to discover the molecular pathway involved in transmitting oxidative signals to proteins. He understood that this pathway is likely highly sensitive to low hydrogen peroxide levels, allowing it to escape the intense antioxidant system. Thus, he searched the plant genomes for proteins that are capable of catalyzing disulfide bond formation, and identified and characterized a group of proteins that he termed AtACHTs6. These proteins belong to the thioredoxin (Trx) protein family, which has been found to be involved in the reductive activation pathway. But, unlike most chloroplast Trx proteins, this new family has a relatively high redox midpoint potential, rendering it likely to catalyze the opposite reaction, i.e., the creation of disulfide bonds.
After characterizing the biochemical properties of this new protein family, Avihai sought to determine whether these newly identified proteins can catalyze the oxidation reaction within the cells. In a set of biochemical and physiological experiments, Avihai and his group characterized a novel regulatory pathway in which electrons are transferred from target proteins to hydrogen peroxide through the activity of members of the AtACHT family and of 2-Cy peroxiredoxin, a highly sensitive peroxidase. Thus, filling in the missing piece of the redox regulation puzzle7,8. Avihai suggested that the two opposing regulatory pathways, i.e., reduction-activation and oxidation-inhibition, coordinate to provide plants with a homeostasis control mechanism that allows them to efficiently adapt to sudden changes in light intensity8. This homeostatic control mechanism was later demonstrated to be involved in fine-tuning of various metabolic pathways in chloroplasts, such as regulation of electron transfer reactions7, cyclic electron flow9,10, chlororespiration11 and starch metabolism8.
The understanding of the involvement of the redox homeostasis control mechanism in determining starch biosynthesis suggests a new biotechnological approach for increasing starch content in plants. As it turned out, oxidative activity lowers the rate at which starch is synthesized throughout the day. According to a newspaper article covering Avihai’s work on redox regulation of starch biosynthesis, “it's as if the plant is pressing on both the gas (reductive activity) and brake (oxidative activity) pedals simultaneously" 12. As a result, mutating key components of the oxidative pathways yielded plants with significantly higher starch levels8. Realizing the potential of this discovery, Avihai sought to exploit these findings to increase crop plant productivity, and a newly established biotech company is currently employing these findings to produce highly productive crop plants.
Close examination of Avihai's work uncovers his independent thinking and remarkable ability to formulate coherent hypotheses and then design programs to prove them, even if it required many years of intensive research. In many aspects, Avihai's studies were ahead of their time. I am confident that the seeds he sowed during his scientific career will sprout and bear sweet fruits, that will contribute to worldwide efforts to increase crop production and ensure food security.
- Buchanan, B. B. & Balmer, Y. Redox regulation: a broadening horizon. Annu. Rev. Plant Biol. 56, 187–220 (2005).
- Buchanan, B. B. The birth of redox regulation. Mol. Plant 7, 1–3 (2014).
- Danon, A. Redox reactions of regulatory proteins: do kinetics promote specificity? Trends Biochem. Sci. 27, 197–203 (2002).
- Wittenberg, G. & Danon, A. Disulfide bond formation in chloroplasts: Formation of disulfide bonds in signaling chloroplast proteins. Plant Sci. 175, 459–466 (2008).
- Peled-Zehavi, H., Avital, S. & Danon, A. B. T.-M. I. R. S. Methods of redox signaling by plant thioredoxins. in Methods In Redox Signaling 251–256 (Mary Ann Liebert Inc., 2010).
- Dangoor, I., Peled-Zehavi, H., Levitan, A., Pasand, O. & Danon, A. A small family of chloroplast atypical thioredoxins . Plant Physiol. 149, 1240–1250 (2009).
- Dangoor, I., Peled-Zehavi, H., Wittenberg, G. & Danon, A. A chloroplast light-regulated oxidative sensor for moderate light intensity in Arabidopsis. Plant Cell 24, 1894–906 (2012).
- Eliyahu, E., Rog, I., Inbal, D. & Danon, A. ACHT4-driven oxidation of APS1 attenuates starch synthesis under low light intensity in Arabidopsis plants. Proc. Natl. Acad. Sci. 112, 12876 LP – 12881 (2015).
- Wolf, B.-C. et al. Redox regulation of PGRL1 at the onset of low light intensity. Plant J. 103, 715–725 (2020).
- Chaturvedi, A. K., Dym, O. & Fluhr, R. PGRL1A redox states alleviate photoinhibition in Arabidopsis during step changes in light intensity. bioRxiv 2022.06.07.492398 (2022) doi:1101/2022.06.07.492398.
- Rog, I., Chaturvedi, A. K., Tiwari, V. & Danon, A. Low light-regulated intramolecular disulfide fine-tunes the role of PTOX in Arabidopsis. Plant J. 109, 585–597 (2022).
Dr. Shilo Rosenwasser