Hormonal responses of rice to organ-targeted cold stress

Rice is a plant species sensitive to cold stress, which renders seriously its cultivation. Responses to cold stress (5°C, 24 hours) in whole plants, or targeted to rice leaves or roots were followed at the hormonome, transcriptome, proteome, and sugar levels, to find organ-specific responses and processes affected by cold acclimation. Targeted stresses caused proteomic changes mainly in the unexposed organs. An increase in abscisic acid (ABA) was accompanied by a decrease in jasmonic acid (JA) (in roots and non-stressed leaves) and vice versa (JA increased in stressed leaves). Both hormones promote the cold tolerance of plants. In this way, JA could indirectly reduce negative effects of cold on photosynthesis in leaves, while ABA dominates in roots (stimulation of protective substances, especially dehydrins, control of water regime). The decrease in cytokinins trans-zeatin and dihydrozeatin in crowns correlated with stress-induced growth suppression. Leaf-targeted cold stress impaired photosynthesis and decreased sugar levels, diminishing their transport, which correlated with an increase in salicylic acid, which is a signal for sugar unloading from phloem into roots. Root-targeted stress suppressed cytokinin biosynthesis and upward transport, and promoted sugar accumulation in leaves. Acclimation per se activated the transcriptome and proteome response to cold –ABA, JA and ethylene were upregulated, moderately suppressing plant growth. Pre-acclimated plants showed less profound hormonal changes than directly stressed plants, positively affecting levels of growth-related phytohormones in the unexposed organs (cytokinins in roots of leaf-stressed plants; auxins in leaves of root-stressed plants).