High temperatures modified plant responses to abiotic stress situations

Abstract

Plants growing in the field are affected by several adverse environmental conditions at the same time. The simultaneity of abiotic factors affecting plants creates a new stress situation different from the individual ones. Global warming is increasing earth surface temperature, and this is accompanied by other environmental changes (soil degradation, increase of drought periods, changes in precipitation pattern). All these changes will affect the arable lands around the world and crop production will be reduced unless new cultivars capable to face the environmental changes are developed. Therefore, the main objective of this PhD is to study the impact of different abiotic stresses in combination with high temperatures on plant physiology. To achieve this objective, the present work is divided into four chapters. In Chapter 1, it is demonstrated that Carrizo citrange is more tolerant to drought and heat stress combination than Cleopatra mandarin. Our work reveals that the higher activation of the enzymes involved in the antioxidant ROS detoxification system of Carrizo prevents cells from the oxidative damage derived from this stress combination. It is showed that the activity of SOD in Carrizo is higher than in Cleopatra at basal levels and under stress combination. Furthermore, gene expression and enzymatic activity of APX and CAT are strongly induced under stress combination in Carrizo, while in Cleopatra this induction is much lower. In Chapter 2, it is showed that under drought and heat stress combination the accumulation of key proteins for plant acclimation to drought or heat stress, such as APX1 and the chaperone HSP101, is higher in Carrizo than in Cleopatra, which may be involved in the higher tolerance of the former to the adverse situation. Chapter 3 aims to further study the effect of abiotic stress conditions in combination with high temperatures in citrus plants. For this purpose, Carrizo citrange plants were subjected to wounding or salinity at high temperatures. Results indicate that high temperatures modify Carrizo responses to wounding and salinity. Under wounding and heat stress, stomatal aperture in plants is lower than under wounding at low temperatures. In addition, Jasmonates accumulation pattern changed. While wounding caused a higher accumulation of JA, stress combination caused a lower increase of JA levels and a significant rise of OPDA content. Salt and heat stress combination caused more severe damage on Carrizo plants than salt stress at low temperatures. Due to the stress combination, toxic chloride ions highly accumulated in Carrizo leaves. Salt and heat stress combination induced high accumulations of ABA and JA and resulted in a specific transcriptomic, stomatal and transpiration response different from that caused by individual stresses. Finally, in Chapter 4, to deeply investigate physiological, hormonal and molecular responses of plants to high light intensity and high temperatures, Arabidopsis thaliana plants were subjected to a combination of these stresses. JA and JA-Ile levels increased specifically under stress combination whereas no change was observed under individual high light or heat stress. This hormonal increase after stress combination together with the upregulation of JA-responsive genes and the lower tolerance of the JA-deficient mutant aos to stress combination indicated that JA is a key hormone for plant acclimation to a combination of high light and heat stress. In addition, it was observed that Arabidopsis plants affected by high light and heat stress were unable to restore PSII activity after stress, likely because damage caused in D1 protein exceeded its repairment and reassembly under the simultaneous stress conditions. Therefore, this work has focus in studying physiological, hormonal and transcriptomic responses to different stress situations in citrus and model plants to obtain a global view of plant mechanisms to tolerate complex abiotic stress situations.