**1. Introduction**

Plants are subjected to various biotic (insects, parasites, nematodes, weeds, bacteria, fungi, viruses, etc.) and abiotic stress factors in their natural environment due to the stationary lifestyle. A major part of the abiotic stresses is caused by factors related to the physical and chemical composition of the soil, while the rest may be related to climate properties such as cold and heat, UV exposure, and light intensity. Among these, heat stress is particularly important since all the anabolism and catabolism reactions require particular cardinal temperatures for enzyme activities. Average surface temperature of the earth increased roughly 1°C since the beginning

of pre-industrial era 120–140 years ago. In local terms, it may seem insignificant; however, globally accumulated heat has vast effects. Various independent research groups measure and calculate global average surface temperatures through absolute temperature observations and temperature anomalies from different locations [1]. According to the Annual Global Climate Report 2021 statistics of The National Oceanic and Atmospheric Administration (NOAA), 2021 was the sixth warmest recorded year of the earth since 1880 by the rate of 0.84°C higher than average of twentieth century. It was also the 45th consecutive year in which the average global temperature surpassed the average of twentieth century, which means it was never colder than average since 1977. Each year in the last decade takes place among the top ten warmest years. The average temperature increase per decade was in range of 0.08°C since 1880; however, the rate was increased 2.25-fold to rate of 0.18°C since 1981. Worse than this rate, the earth is expected to warm roughly 1.5°C within the next two decades [2]. Moreover, major crops in tropical and subtropical regions present 2.5–16% yield losses for every 1°C increase in seasonal temperatures. Global temperature rises also lead to reduction in land and sea, more frequent heavy rains, increase on habitat ranges of some plants and animals and decrease on some others, regionally [3].

Heat stress changes diverse molecular pathways and causes physiological and morphological alterations. Various stages of plant development such as germination, seedling emergence, tillering, floral initiation, pollination, fertilization, and consequently yield and grain quality are in range of heat effects. There are multiple factors which may divert the heat effects to a more dramatic or mild direction. Length, abruptness, and magnitude of heat are the major factors along with relatively minor factors as soil moisture and atmospheric CO2 concentrations [4]. Anther and pollen development stages are considered as the most heat vulnerable stages; however, exposure during earlier stages may also lead to inadequate germination through reduced root and shoot growth. Heat exposure after the germination stage reduces green leaf area and the number of tillers per plant to ease the effects through reducing exposure surface. Prolonged exposure after anthesis may lead to flower abortion. Heat stress after flowering stage is referred as terminal heat stress which effects early meiosis to tetrad stages of pollen production and utterly reduce grain number, filling, and maturity. Terminal heat stress does not only reduce quantitative traits but also reduce qualitative traits such as dry matter accumulation and grain quality. Developmental stage-specific treatments and breeding strategies against various heat regimes are still under investigation [5, 6].

Physiological functions are mediated through enzymatic processes in all living organisms. Even though, there are thermophile organisms in lower evolutionary branches as archaea, bacteria, and fungus which all have resilient enzyme systems. Any deviations over the optimum temperature hamper enzymatic processes of plants. Photosynthesis is one of the most vital but fragile metabolic processes which is severely affected by heat stress. Some heat acclimation adaptations including reduced spiky leaf shape, altered leaf orientation, rolled leaves or small surface hairs, thick waxy cuticle, and stomatal crypts are present in high heat climate plants to reduce drastic effects; however, crop plants do not possess most of these structures. Symptoms as lower stomatal conductance, reduced CO2 assimilation, and water loss utilize non-photorespiratory processes. Heat stress also directly alters enzyme and protein structure and cell membrane permeability leading to photochemical modifications in chloroplasts, damage on thylakoid membrane, and reduction of soluble

#### *Transgenic Plants in Heat Stress Adaptation: Present Achievements and Prospects DOI: http://dx.doi.org/10.5772/intechopen.111791*

proteins as Rubisco and Rubisco binding proteins. Damaged chloroplasts cripple the photosynthetic capacity of plants and lead to leaf senescence, while disturbed thylakoid membranes elevate cellular reactive oxygen species levels. Respiration is crucial for leaf surface cooling in trade-off water loss. Leaf water potential and transpiration pull is a driving force for nutritional uptake and transport of photosynthesis assimilates from leaves to the grains. Heat stress also disturbs nitrate and ammonium assimilation. Factors as decreased root mass, surface area, and/or a decrease in nutrient uptake per unit root or direct heat damage to roots are plausible for nutrient acquisition decrease. Uptake of most of the nutritional elements is mediated by specific influx or efflux protein activity. Therefore, reduced proteins per unit root rate directly affect the mineral content [7, 8].

Heat stress causes all the above-mentioned damage through direct (primary) and indirect (secondary) effects at different levels. Weakening and damaging bio membrane integrity, altering fluidity, leading to electrolyte leakage, denaturing and misfolding proteins are among the most deleterious direct damage effects. Indirect effects can be listed as oxidative stress, methylglyoxal (MG) stress, and osmotic stress. Therefore, plant heat stress tolerance and adaptation mechanisms include heat shock proteins (HSPs), antioxidant systems, osmolytes, fortification of membrane lipids, and MG detoxification, in general. The present chapter will summarize the current knowledge on heat stress tolerance/adaptation approaches and will discuss transgenic approach contribution to these mechanisms with the emphasis on prospects.
