Plants, ever resourceful in their adaptations, have evolved mechanisms to protect themselves when exposed to fluctuating light conditions. In situations where light intensity exceeds their photosynthetic capacity, the threat of reactive oxygen species causing photoinhibition and hindering photosynthetic efficiency looms. To counter this, chloroplasts have evolved thioredoxin (Trx) proteins, which play a crucial role in regulating redox balance within the photosynthetic apparatus and offer photoprotective functions.
These Trx proteins allow plants to adapt and modulate photosynthesis in response to variations in light intensity. Among these Trx proteins, x- and y-type Trxs have long been recognized for their functional relation, yet their specific roles under fluctuating light conditions remained a mystery.
In a groundbreaking study published in Plant Physiology, researchers from Okayama University and Kyoto Sangyo University in Japan employed Arabidopsis thaliana (Arabidopsis) to investigate Trx regulation during photosynthesis. Their research unveiled a significant discovery: x- and y-type Trxs play a vital role in preventing redox imbalance on the electron-accepting side of photosystem I (PSI), offering a critical photoprotection mechanism under fluctuating light conditions.
The study was led by Assistant Professor Yuki Okegawa from the Institute of Plant Science and Resources at Okayama University, with co-authorship from Dr. Wataru Sakamoto, also affiliated with the Institute of Plant Science and Resources.
Dr. Okegawa shed light on the motivation behind their research, stating, “Arabidopsis contains five types of Trxs, four Trx m, two Trx f, two Trx y, Trx x, and Trx z. Of these, Trx f and Trx m constitute over 90% of chloroplast Trx proteins, while Trx x and Trx y are a minority, comprising less than 10% of Trx proteins. The roles of x- and y-type Trxs have been poorly understood, so we embarked on this study to uncover their significance in photosynthesis and light stress.”
The research team generated mutant Arabidopsis plants, including trx x single mutants, trx y1 trx y2 double mutants, and trx x trx y1 trx y2 triple mutants, and explored the impact on PSI during photosynthesis. Their findings revealed that, under low-light conditions, the electron-accepting side of PSI faced inhibition in trx x and trx x trx y1 trx y2 mutants compared to wild-type plants.
Moreover, these mutants exhibited heightened inhibition on the PSI electron-acceptor side during both low- and high-light phases of fluctuating light. Dr. Okegawa pointed out, “This PSI electron-acceptor side inhibition under fluctuating light resulted in severe PSI photoinhibition in the trx x and trx x trx y1 trx y2 mutants, leading to impaired growth and reduced PSI levels compared to the wild-type plants when measured under these light conditions.”
This breakthrough reveals that Trx x and Trx y play a critical role in mitigating redox imbalances on the PSI electron-acceptor side, thus ensuring the continuation of photosynthesis and preventing photoinhibition. These proteins act as electron sinks during transitions from low to high light, maintaining the oxidized state on the PSI electron-acceptor side, thus preserving the redox balance and serving as a safeguard against sudden changes in light intensity.
The implications of this discovery are far-reaching, potentially offering a key to developing light stress-tolerant crops. Dr. Okegawa and the research team believe that these advancements could revolutionize agriculture by addressing food shortages caused by inadequate crop production through the development of more resilient crops.