First Insights into the Role of Polyamines in Biotic Stress Resistance Induced by Potassium Phosphite in Potato

Abstract

Phosphites (Phi) are inorganic salts widely used to protect plants from pathogens, especially oomycetes, on a wide range of horticultural crops. They were studied at the molecular level in different pathosystems; however, their mode of action is not completely understood. In this work, we investigate the participation of polyamine (PA) metabolism in the defence mechanism induced by Phi. To this aim, potato plants were treated with potassium phosphite (KPhi), and 3 days later, leaves were inoculated with Phytophthora infestans (P. infestans). The content of free PAs was analysed and showed that KPhi induced the accumulation of spermine (Spm), spermidine (Spd) and putrescine (Put). However, when treated leaves were detached and inoculated with P. infestans, their accumulation was even higher. In addition, the expression of genes implicated in PA synthesis and degradation was measured. Most of the genes involved in PA biosynthesis showed no changes in their expression upon KPhi treatment, except for those involved in Spm synthesis: Spm synthase and S-adenosyl-L-methionine decarboxylase (SPMS and SAMDC, respectively). On the other hand, the catabolism genes: diamine oxidase (DAO), and polyamine oxidases PAO1, PAO2 and PAO5, but not PAO4, decreased their expression after inoculation with P. infestans, and this was reverted by KPhi pretreatment. To investigate this effect, assays were carried out using a PA oxidase inhibitor showing that defence responses induced by KPhi do not require products of PA catabolism. Additionally, we analysed in silico phytohormone and stress-responsive cis-elements in PAO gene promoters, which might explain, at least in part, the different behaviour of PAO4.