Plant Responses to Heat Stress

• Heat stress affects crop development, growth, and productivity.
• Plants have adaptive responses for either avoidance or tolerance of heat stress.
• The plant responds to heat, which can be molecular, biochemical, cellular, physiological, and morphological, and is being used by scientists to develop new cultivars for the future.

The increase in temperature due to climate change is threatening plant growth and productivity. The heat triggers protective molecular, biochemical, cellular, physiological, and morphological plant responses to heat stress. Scientists are intensively studying these plant responses to heat stress to maintain crop productivity in the future. Though a complete understanding of heat stress response is still missing, find out what we know.

Figure 1: “Some of the effects triggered by heat stress on plants,” dos Santos et al. (2022). (Image credits: https://doi.org/10.3390/stresses2010009)

Why Plant Responses to Heat Stress Are Important

An 0.2°C rise in global temperatures is expected every decade. The prediction has scientists worried, as heat stress can adversely affect all organisms, including plants that suffer more since they are sessile and cannot escape high temperatures. Consequently, changes in seed germination, growth, phenology, photosynthesis, transpiration, flowering, grain filling, sensitivity to pests, carbohydrate metabolism, and increased oxidate stress and senescence could reduce biomass accumulation and affect yield; see Figure 1.

To develop new crop cultivars that remain high-yielding, scientists must know how plants respond and adapt to heat stress for trait selection.

Plant growth and yield depend on many molecular and biochemical reactions, physiological processes, and cellular and morphological features sensitive to temperature. Plant responses will also occur at these levels to protect the plant from heat stress. The plant’s response to heat stress varies based on the duration and extent of high-temperature exposure and the specific type of plant.

Plant responses to heat stress start from early sensing and are followed by signal transmission. In the long term, plant adaptations include phenological and morphological changes. The short-term plant adaptive mechanisms to heat stress, which interest crop scientists in developing new cultivars, can broadly be classified as avoidance or tolerance and involve one or more plant responses

Avoidance Mechanisms: Plants avoid heat stress through short-term avoidance mechanisms that are mainly morphological, physiological, and biochemical changes. It can include leaf orientation, larger xylem vessels, cooling through transpiration, early maturation, and membrane lipid compositions. See Figure 2.

Tolerance Mechanisms: Heat tolerance or thermotolerance is the plant’s ability to grow and yield even under high temperatures. Tolerance is highly specific and can vary between species, individuals, and even organs and tissues within a plant.

Most tolerance mechanisms are biochemical, molecular, and physiological and involve stress proteins, antioxidants, ion transporters, osmoprotectants, transcriptional control, and signaling cascades. The rationale is that plant survival depends on detecting heat stress and generating and transmitting the signal to initiate biochemical and physiological changes. Heat also alters gene expression and the synthesis of many plant metabolites to increase tolerance. See Figure 2.

Figure 2: “Different adaptation mechanisms of plants to high temperature. A: Avoidance, T: Tolerance,” Hasanuzzaman et al. (2013). (Image credits: doi: 10.3390/ijms14059643)

The various ways some typical responses help plants adapt to heat stress and how they can be used to develop new crop cultivars are discussed.

Molecular Responses

Heat stress leads to oxidative stress by interfering with redox homeostasis, which increases ROS production. Heat also harmed the ROS elimination processes. Excess ROS alters gene expression, enzyme inactivation, membrane decomposition, and molecule oxidation.

In response to stress, plants produce enzymes that are antioxidants and scavenge the reactive oxygen species (ROS), such as superoxide, hydroxyl radicals, and hydrogen peroxide. Besides the enzymes such as superoxide dismutases, catalase, monodehydroascorbate reductase, and glutathione reductase, ascorbate, glutathione, flavonols, and α-tocopherol also act as antioxidants. Increasing these antioxidants can be a way to boost heat tolerance,

Heat-tolerant crops can also be developed by manipulating the rubisco activase and enzymes that neutralize ROS in photosynthesis.

Epigenetics is another pathway for developing heat tolerance. A typical plant response to prevent potential heat stress damage is the synthesis of protective proteins controlled by heat stress transcription factors (HSFs). MiRNAs and transcriptional factors are other genes expressed in the stress regulatory circuit. For example, heat-tolerant maize and sweet potato have been developed using transgenic approaches.

Biochemical Responses

Many enzymes in adequate concentrations directly improve heat tolerance. High temperatures stimulate the biosynthesis of enzymes that act against stress, like auxins, ethylene, salicylic acid, jasmonate, brassinosteroids, and cytokinin. The biosynthesis occurs by producing transcription factors that activate stress response genes.

Some transcription factors could also depress enzyme biosynthesis; for example, the NAC transcription factor reduces salicylic acid in tobacco, making plants more susceptible to heat.

Understanding whether genetic control of specific hormone biosynthesis pathways is positive or inhibitory allows precise genetic modification to increase heat tolerance.

Physiological Responses

Several primary physiological functions are affected by heat stress. Photosynthesis is optimum between 20-30°C. Over these temperatures, the photosynthetic rate drops due to many reasons. The photosynthesis apparatus is damaged, and the activity of crucial enzymes like ATP synthases is reduced. Higher temperatures increase photorespiration as the RuBisCO enzyme’s affinity for carbon dioxide decreases, thereby reducing carbon fixation.

Closure of stomata to reduce transpiration is another reason for the reduction in photosynthesis and biomass accumulation due to heat stress. Many research projects focus on lowering transpiration to increase photosynthesis in heat stress.

Plants have evolved photosynthesis C4 and CAM pathways adapted to extreme heat. However, most plants, including crops, are C3 plants, and the photosynthetic structure and mechanisms are unsuitable for functioning in hot climates. The C4 and C3 pathways are studied to improve plant productivity during heat stress.

Scientists must also consider other mechanisms that occur during heat stress. These do not increase tolerance or avoidance but are related to general plant response to stress. Extreme heat can accelerate the senescence of leaves or other plant parts, inhibiting optimum plant growth. Also, plants alter carbon allocation during heat stress to prioritize survival over biomass accumulation by converting starch to sugar, energy, and other metabolites for damage control.

Devices like the CI-340 photosynthesis tool allow scientists to study photosynthesis, transpiration, stomatal conductance, and chlorophyll fluorescence to maximize plant efficiency and productivity.

Cellular Mechanisms

To protect themselves from heat, plants change several cellular structures. For example, the cell wall becomes stiff, and there is a rise in wall polysaccharides and lignin content. The reduction in starch content in leaves reduces the mesophyll cell size and alters internal membrane organization.

The change in cell wall composition of lipids, polysaccharides, lignin, hemicellulose, and pectin levels can be measured by spectroscopy by CI-710s SpectraVue Leaf Spectrometer from CID Bio Science Inc.

Morphological Responses

The consequences of changes at the molecular, biochemical, and physiological levels can be seen in changes to plant morphology.

Crucial traits like leaf expansion are reduced by hampering plant growth, adversely affecting biomass accumulation.

However, plants have also developed several morphological adaptations to heat stress, such as increasing the root system. Many transformations occurring in leaves to reduce evapotranspiration, such as folding or curling leaves, leaf area reduction, and leaf thinning, are heat avoidance traits that scientists can select in new cultivars.

Scientists can measure leaf area changes with the CI-202 Portable Laser Leaf Area Meter and CI-203 Handheld Laser Leaf Area Meter.

Changes to crop canopy and Leaf Area Index can be estimated by CI-110 Plant Canopy Imager.

Future-proofing Crops

Plant responses to heat stress vary due to climatic zones, seasons, daily rhythms, species, and crop developmental stages. Therefore, despite intensive research, a complete understanding of plant responses to heat stress remains elusive due to lacunae in physiological and biochemical information. However, molecular approaches are already being used to develop heat tolerance in new cultivars. High-temperature stress and plant responses will likely remain a primary crop production concern, and precise, non-destructive scientific tools will ease researchers’ jobs in finding solutions.

Sources

Albertos, P., Duendar, G., Schenk, P., et al. (2022): The transcription factor BES1 interacts with HSFA1 to promote heat stress resistance of plants. EMBO Journal. DOI: 10.15252/embj.2021108664

dos Santos, T.B., Ribas, A.F., de Souza, S.G.H., et al. (2022). Physiological Responses to Drought, Salinity, and Heat Stress in Plants: A Review. Stresses, 2, 113-135. https://doi.org/10.3390/stresses2010009

Hasanuzzaman, M., Nahar, K., Alam, M. M., Roychowdhury, R., & Fujita, M. (2013). Physiological, biochemical, and molecular mechanisms of heat stress tolerance in plants. International journal of molecular sciences, 14(5), 9643–9684. https://doi.org/10.3390/ijms14059643

Saini, N., Nikalje, G.C., Zargar, S.M. et al. (2022). Molecular insights into sensing, regulation, and improving heat tolerance in plants. Plant Cell Rep 41, 799–813. https://doi.org/10.1007/s00299-021-02793-3