Photorespiration is a complex biological process in plants, often seen as an inefficient byproduct of photosynthesis. While photosynthesis fixes carbon dioxide to produce energy, photorespiration consumes oxygen and releases carbon dioxide. Despite its seemingly wasteful nature, understanding this pathway helps explain how plants adapt to various environmental conditions and offers insights into improving agricultural productivity.
The Photorespiration Process
Photorespiration fundamentally begins with the enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). This enzyme can react with either carbon dioxide or oxygen. When RuBisCO binds with carbon dioxide, it leads to the Calvin cycle, which produces sugars. However, when it binds with oxygen, photorespiration is initiated, producing a two-carbon molecule called phosphoglycolate, which cannot be directly used in the Calvin cycle, along with a three-carbon molecule, 3-PGA.
To salvage carbon from phosphoglycolate, plants engage in a multi-step process that spans three different organelles: the chloroplast, peroxisome, and mitochondrion. First, within the chloroplast, phosphoglycolate is converted into glycolate. This glycolate then moves to the peroxisome, where it is converted to glyoxylate and subsequently to the amino acid glycine.
Next, two molecules of glycine travel from the peroxisome to the mitochondrion. Here, these two glycine molecules are converted into one molecule of serine, releasing a molecule of carbon dioxide and ammonia. Finally, serine returns to the peroxisome, where it is converted to glycerate, which then re-enters the chloroplast to be phosphorylated into 3-PGA, allowing some of the carbon to re-enter the Calvin cycle. This entire pathway consumes energy in the form of ATP and reducing power (NADPH), making it an energetically costly process for the plant.
Factors Triggering Photorespiration
Photorespiration is initiated primarily by environmental conditions that favor RuBisCO’s oxygen-binding activity over its carbon dioxide-binding function. A key factor is a high concentration of oxygen relative to carbon dioxide within the plant’s leaves. During active photosynthesis, oxygen is produced, and if stomata (leaf pores) are closed, this oxygen can accumulate, increasing the likelihood of RuBisCO binding to it.
Elevated temperatures also significantly promote photorespiration. As temperatures rise, the solubility of carbon dioxide in the plant’s cells decreases more rapidly than that of oxygen. Additionally, RuBisCO’s affinity for oxygen increases at higher temperatures, making it more prone to react with oxygen rather than carbon dioxide.
Drought conditions commonly trigger photorespiration as well. When plants experience water scarcity, they close their stomata to conserve water, which limits the uptake of fresh carbon dioxide from the atmosphere. This stomatal closure leads to a reduction in internal carbon dioxide levels while oxygen, produced by photosynthesis, continues to accumulate, creating an environment inside the leaf that favors RuBisCO’s oxygenase activity.
Implications for Plants and Evolutionary Responses
Photorespiration carries a significant energy cost for plants, consuming ATP and NADPH that could otherwise be used for carbon fixation in photosynthesis. This process can reduce the efficiency of photosynthesis by as much as 25% in C3 plants, leading to a loss of already-fixed carbon as carbon dioxide, diverting resources away from growth and sugar production.
Despite the energetic cost, photorespiration may offer some protective benefits to plants, particularly under stressful conditions. It can act as an energy sink, helping to dissipate excess light energy when carbon dioxide fixation is limited, which prevents damage to the photosynthetic machinery. This protective role is especially relevant during periods of drought or high light intensity, where a plant might otherwise suffer from photo-oxidative damage.
Some plants have developed strategies to minimize photorespiration, leading to the emergence of C4 and CAM (Crassulacean Acid Metabolism) photosynthetic pathways. C4 plants, such as corn and sugarcane, spatially separate initial carbon dioxide fixation from the Calvin cycle. They use an enzyme with a higher affinity for carbon dioxide to concentrate it in specialized bundle sheath cells, thereby reducing RuBisCO’s exposure to oxygen.
CAM plants, like cacti and pineapples, employ a temporal separation strategy. They open their stomata and fix carbon dioxide at night when temperatures are cooler and humidity is higher, storing the carbon as organic acids. During the day, with stomata closed to conserve water, these stored organic acids release carbon dioxide to the Calvin cycle, maintaining high carbon dioxide concentrations around RuBisCO and thus minimizing photorespiration.
Modifying Photorespiration for Agriculture
Research focuses on engineering photorespiration in C3 crops like wheat, rice, and soybeans to improve their yield and resource use efficiency, with the potential to significantly increase crop productivity, especially under conditions of increasing temperature and drought. Scientists are exploring various biotechnological approaches, including optimizing the native photorespiratory pathway or introducing alternative, more efficient pathways.
One promising strategy involves engineering more efficient RuBisCO enzymes that have a higher specificity for carbon dioxide over oxygen. Another approach is to introduce synthetic photorespiratory bypasses that can more quickly and efficiently recycle the byproducts of RuBisCO’s oxygenase activity. Early studies in model plants like tobacco have shown that such modifications can lead to substantial increases in biomass and crop yields, sometimes by as much as 40%. These advancements could help address global food security challenges by making crops more resilient to changing environmental conditions.