Developmental Responses To Elevated Ambient Temperature And Alternative Splicing In Poplar

Plants exhibit a range of different growth rates are known to be regulated both by geneticfactors and by environmental conditions. Each plant species or even each cultivar has its ownoptimal growth temperature, which indicates a genetic factor or a regulating system is involved.In many plants, elevated ambient temperature induces heat stress and a variety of physiologicalconsequences including a reduced vegetative growth rate, but the mechanism remains largelyunclear. Like many other experimental plant organisms, a bunch of poplar species, as modeltree species, are propagated and characterized in a controlled experimental condition of20°Cto25°C. As an area of poplar plantation, Beijing bears its average temperature between15°C to25°C during the active growth season from April to October. With the concern of relatively lowconditional experimental temperature, as well as the low average temperature in poplarplantation area, and the impact of global warming on tree growth, effects of a higher ambienttemperature on tree growth need to be investigated.In order to avoid various influences of uncontrolled environmental factors, a set ofchambers were employed to conduct the elevated day-time ambient temperature treatment of aclone of hybrid poplar saplings84K (Populus alba×P. glandulossa). Both treated and controlplants were analyzed phenotypically and physiologically, and the first two internodes includingapical meristematic tissue and the fifth internode undergoing active secondary tissuedifferentiation were separately harvested for transcriptome sequencing and analysis.Long term (5weeks) moderately elevated day-time ambient temperature (37°C) hasprofound effects both on architecture and on growth rate in poplar. In contrast to elevatedtemperature treatment of many plant species that leads to reduced vegetative growth rate,moderately elevated day-time ambient temperature obviously increased both longitudinalgrowth rate and lateral growth rate by three weeks treatment. In addition to increasedvegetative growth rate, saplings acclimated to moderately elevated day-time ambient temperature were subjected severe architecture changes compared to control plants. Leafelevation angle (defined as the angle between the leaf rachis, at its insertion point to the stem,and the horizontal plane) increased, and both leaf length and leaf width reduced in treatedsaplings. Close observation indicated the reduced leaf size was due to reduced cell populationrather than altered cell size in leaves. The content of cellulose and lignin were reduced afterheat treatment, which might be related with the reduction of secondary cell wall biosynthesisrelated type II cellulose synthase. Despite reduced cell population and size, leaves of treatedsaplings exhibited moderate increase of both net photosynthesis and transpiration. In order tofurther reveal molecular mechanisms involved, a genome-wide transcriptome analysis wasperformed and a set of transcripts and transcription factors was found to be dramaticallychanged during this process. Hormone signal related different expressed genes were classifiedduring the heat treatment. The different expressed gene involved in auxin signal is larger thanthat in other hormone signals, which imply that the poplar developmental phenotype might berelated with the auxin signal pathway.Moreover, alternative splicing events in poplar were investigated during the developmentstage and heat treatment. As reported in Arabidopsis and rice, the dominant alternative splicingevents in poplar is intron retention. The ratio of alternative splicing events is differ greatly indifferent tissues and different developmental stages, suggesting that alternative splicinginvolved in the regulation of tissues differetiation and plant development.