High Temperature Inhibits The Formation Of Apical Hooks In Arabidopsis Thaliana And HLS1 Promotes The Mechanism Of Thermal Morphogenesis

Temperature is an important environmental factor which affects the growth and development of plants.Unable to move,plants are reactive to changes in diurnal temperature and seasonal temperature.High temperature changes a series of morphological structures of plants,known as thermomorphogenesis.It is well known that high temperature promotes cell elongation and flowering time in light-grown Arabidopsis thaliana.However,the effect of high temperature on dark-grown etiolated seedlings is not clear.Phytohormone ethylene stimulates the exaggerated hook formation in Arabidopsis etiolated seedlings.It has been reported that other phytohormones like jasmonate or gibberellins could inhibit or coordinate exaggerated hook formation respectively.However,whether environmental factors,like temperature,participate in this process is unknown.In our study,we focused on high temperature(28°C)regulates apical hook development in Arabidopsis etiolated seedlings in darkness.We found that in darkness,high temperature inhibits the ethylene-triggered exaggerated hook formation in Wildtype Arabidopsis,reduces the hook curvatures and promotes the apical hook opened.And high temperature also reduces the hook curvatures in the constitutively ethylene responsive mutant ctr1-1.But high temperature does not disturb the stability and transcriptional activity of the central transcription factor EIN3,suggesting high temperature acts downsteam of ethylene signaling.The main reason for hook formation is the asymmetric distribution in the inner and outer sides of the hypocotyl tip of the seedlings.We found that high temperature reduces the natural auxin levels in the hook regions and interferes their asymmetric distributions.We took transcriptome sequencing analysis and found that high temperature decreases the transcriptional activity of the auxin-responsive genes SAURs(SAUR25/SAUR29/SAUR53/SAUR69).In addition,we took Gene Ontology functional analysis and found that functional category “response to auxin” was enriched in the down-regulated genes.To explore the mechanisms for reducing auxin accumulation,we detected the transcriptional activity of several genes encoding auxin biosynthesis related enzymes and found that the transcription of most YUCCA genes were down-regulated under high temperature treatment.In our study,we show that the currently reported plant thermo-sensory components in light-grown plant(Phytochromes-PIF4 modules)are not sufficient for thermo-sensing in etiolated seedlings.We speculated that plants sense the elevated ambient temperature through a distinct mechanism in darkness.According to the above research,we show that high temperature inhibits the ethylene-triggered exaggerated hook formation in Arabidopsis etiolated seedlings.High temperature reduces the auxin activity in hook regions to antagonize ethylene-induced exaggerated apical hook formation via decreasing the transcriptional activity of the auxin biosynthesis related genes YUCCAs.HOOKLESS1(HLS1)is a key regulated factor in the apical hook development of etiolated seedlings,which encodes a putative N-acetyltransferase.We found that hls1 mutants exhibit defected hook phenotypes in the prensence of ethylene or not as well as the high temperature treatment or not.Meanwhile,we also observed thermomorphogenesis-related phenotypes of hls1 mutants in white light and found that HLS1 plays an important role in promoting the thermomorphogenesis in Arabidopsis.We further took transcriptome sequencing analysis and found that approximately half of high temperature-regulated genes were dependent on HLS1.And 87% of HLS1-dependent genes were also dependent on PIF4.In conclusion,HLS1 is a key regulated factor in apical hook development and also promotes the thermomorphogenesis in light-grown Arabidopsis seedlings.In this study,We also show that HLS1 regulates the changes of the differential transcriptome expression patterns and this process is also dependent on PIF4.