As the climate continues to change over the coming decades, conditions for crop growth will become more challenging. Heat stress is already a major obstacle in the way of increased crop growth; accounting for 42%, 31% and 50% of maize, wheat and rice yield losses, respectively. The damaging effects of heat stress on crop yields will only increase, as the average global temperature continues to rise, and seasonal periods of extremely high temperatures become more common. Therefore, in the face of an ever-warming climate, the production of high-yielding crop varieties, that are tolerant to heat stress, is of paramount importance.
Heat stress has a detrimental effect on many physiological and biochemical processes in plants, chief amongst which is photosynthesis; the reaction whereby plants use energy in sunlight to convert atmospheric carbon dioxide into sugars, which are used to fuel the plant’s growth and development. This reaction is conducted by the enzyme RuBisCO, which is not only the most abundant protein in plant leaves, but also the most abundant protein on Earth. However, despite being found in all photosynthetic organisms, RuBisCO has a number of inadequacies which make it very susceptible to inefficiency and inactivation under heat stress.
Firstly, as well as being able to covert carbon dioxide into sugars via photosynthesis, RuBisCO is also able to use atmospheric oxygen in a competitor reaction known as photorespiration, whereby no growth-promoting sugars are made. This is a major problem at high temperatures, as the effective concentration of carbon dioxide in the atmosphere decreases rapidly as temperature increases (due to changes in its solubility); meaning photorespiration occurs at a faster rate than photosynthesis, and fewer sugars are produced to fuel plant growth. Secondly, RuBisCO’s enzymatic activity is largely dependent on activation by other proteins, namely RuBisCO activase (Rca). The activase enzyme is able to remodel RuBisCO’s protein structure, which facilitates the release of inhibitory molecules that bind to RuBisCO and prevent photosynthesis from occurring. However, the remodelling ability of RuBisCO activase decreases as temperature increases, whilst the production of inhibitory molecules that bind to RuBisCO increases with temperature; a nightmare scenario for photosynthesis. All of this means that, under high temperatures, there are less sugars being produced to fuel growth and the production of grains, seeds or fruits - essentially leading to reduced crop yields.
Recent work from a team at Lancaster University focussed on the aforementioned RuBisCO activase protein in wheat. Their work built on previous findings that wheat produces three different RuBisCO activase proteins which differ in stability under heat stress, as the team aimed to determine whether the abundance of the most thermostable protein (Rca1β) would accumulate in wheat leaves, in response to heat stress. They found that exposing wheat to heat stress led to a reduced rate of photosynthesis, resulting in reduced grain yield. However, they also found that the thermostable RuBisCO activase protein (Rca1β) was three times more abundant in leaves, during and after heat stress and that the activation state of RuBisCO after 5 days of heat stress was no different than that observed under normal conditions; suggesting the decreased rate of photosynthesis was not due to a lack of RuBisCO activation. This means that plants may synthesize their most thermostable versions of RuBisCO activase proteins in response to heat stress, as these proteins are able to activate RuBisCO, allowing some sugar production via photosynthesis despite the increased temperature. In fact, the research team suggest that similar mechanisms of preferentially synthesizing thermostable versions of RuBisCO activase proteins under heat stress may be widespread amongst many plant species. Although this has not been extensively studied, it does suggest that plants have an inherent ability to maintain some RuBisCO activity under heat stress, via the preferential synthesis of different versions of key regulatory proteins.
The results of this work present a novel way in which crops may be made to be more tolerant to heat stress; furthering our current understanding of stress responses and adaptation, whilst taking us one step closer towards more resilient crop varieties. However, these crop varieties will require multiple tools which make them resilient to the changing climate. This requires an integrated approach to crop improvement; marrying the results from multiple research papers, such as this one, to better equip crops for the harsh conditions that will inevitably meet them in the coming years. The development of such varieties is, no doubt, years away from fruition, however research such as this is paving the way towards these varieties becoming reality.