Changes in gene expression during development and thermogenesis in Arum

Abstract

Thermogenesis in Arum species is induced by salicylic acid (SA) and caused by activation of the alternative respiration pathway and the alternative oxidase (AOX), resulting in heat production. The enzymes phosphoenolpyruvate carboxylase (PEPCase) and NAD-dependent malic enzyme (NAD-ME) also show dramatic increases in activity during thermogenesis and are essential for heat generation. In this current study, we characterized the timing and localization of changes in levels of AOX, NAD-ME, and PEPCase polypeptide accumulation, and changes in Ppc and Me mRNA accumulation, in various Arum tissues during prethermogenic development and during thermogenesis. In addition, changes in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) gene expression were analysed at the level of rbcL protein and mRNA accumulation. AOX, PEPCase, and NAD-ME all increased only in clubs during development, increasing 5–6-fold by the latest prethermogenic stage and remained at this level. The induction of thermogenesis did not cause any changes in levels of AOX, indicating that SA does not affect levels of the enzyme itself, but instead must act to stimulate activity. Reported increases in NAD-ME activity during club development correlated closely with mRNA and protein accumulation, whereas PEPCase activity appears to be determined by post-transcriptional and post-translational processes. Interestingly, Rubisco protein and mRNA were found in relatively abundant amounts in clubs during early developmental stages, and disappeared rapidly as the thermogenic enzymes began to increase in abundance. The induction of thermogenesis by a synthetic inducer of plant pathogen resistance, 2,6-dichloroisonicotinic acid, as well as the involvement of SA and AOX in both processes, reinforces evidence for a link between mechanisms controlling disease resistance in all plants and thermogenic induction in Arum.