Functional Characterization of Tomato Phytochromes in Response to Heat and Drought Stresses

概要

Environmental changes have become one of the most negative factors affecting plant growth and development. In recent years, heat and drought stresses are predominantly spread worldwide and threaten agriculture on a large scale by causing a significant reduction in plant growth and yield depending on the growth stage and severity of the stress. Tomato (Solanum lycopersicum L.) is one of the most widespread vegetable crops worldwide due to its high consumption either in fresh or processed use. This high consumption is because of its delicious taste and its content of the valuable components that are important for the human diet. In addition to its usage for plant research as a model plant which is characterized by containing fruits. Despite all that, tomato is sensitive to most environmental stresses such as heat and drought stresses.

In plant, light control plant growth and development in all growth stages. Plants absorb light by their photoreceptors; the most characterized ones are phytochromes (PHYs) which absorb red and far-red light. It is well documented that plant growth and development (from seed germination to flowering) can be controlled by PHYs. In addition, they regulate both biotic and abiotic stresses, such as high and low temperatures, salinity, drought, toxic metals, ultraviolet-B light, and herbivory by changing a wide range of biochemical and molecular responses. Tomato has five PHYs (PHYA, PHYB1, PHYB2, PHYE, and PHYF) in its genome. The most popular types are PHYA, B1, and B2. So, this study aimed to characterize the role and the function of tomato PHYA, PHYB1, and PHYB2 in response to high temperature and drought stresses. Tomato phyA mutant and phyB1B2 double mutant cv. 'Moneymaker' was used in this study.

Under heat stress (HS), tomato phyA and phyB1B2 mutants exhibited a heat tolerance during the seed germination, vegetative growth stage, and after a short exposure time of HS during the flowering phase. While by the long period of HS during the flowering phase and under the fruiting stage, phyA and phyB1B2 showed a similar response as the WT which showed a sensitive response at all studied phases. The most obvious stage to show phy mutants tolerance was the vegetative growth stage. The WT leaves were severely damaged when plants were grown at 37 ºC, but the phyA and phyB1B2 leaves were healthy. Furthermore, the heat tolerance of phy mutants during the vegetative growth stage was also observed under greenhouse (GH) conditions during the summer. At this stage, both phy mutants achieved a high degree of membrane stability and enhanced water saving by regulating stomatal closure and decreasing stomatal number. Also, both phy mutants upregulated the expression level of important heat- responsive genes, including heat shock transcription factor (HSF) genes, related to heat tolerance specially HSFA2 and HSFB1. Furthermore, the phyA mutant enhanced the proline production, in addition to the lower malondialdehyde (MDA) accumulation. These results clarified the response of tomato phyA and phyB1B2 mutants to HS during the vegetative growth stage. During the flowering stage when the WT, phyA, and phyB1B2 plants were exposed to hot summer temperature in the GH, the phyB1B2 produced fertile pollens for more time under HS followed by the phyA and the lowest was the WT. Additionally, the phyA inhibited the formation of flower abnormalities and stigma exertion phenomena compared to the WT. Suggesting that both phy mutants improved the heat tolerance of tomato flowers compared to the WT.

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