Drought tolerant maize cultivar accumulates putrescine in roots

Highlights

• Drought stress led to increase in H2O2 content and higher activity of antioxidant enzymes in roots of maize cultivars.

• Putrescine was accumulated in roots of maize cultivars in response to drought stress.

• Transcription of genes involving in putrescine metabolism (ZmPAO 1–6, ZmODC, ZmSPDS) were affected by drought stress.

• Genotypic variation in antioxidant response and putrescine metabolism was observed in roots of maize cultivars under drought

Abstract

We compared putrescine metabolism in roots of tolerant maize cultivar (Karoon) to a susceptible cultivar (260) under drought stress. The experiment was conducted in growth chamber under controlled condition in 80 pots. Drought stress led to significant increase in root growth indices, H2O2 content and activity of antioxidant enzymes (superoxide dismutase, catalase, and peroxidase). The qPCR analysis revealed that expression of maize polyamine oxidase (ZmPAO) genes including ZmPAO1, ZmPAO2, ZmPAO4 and ZmPAO6 as well as ornithine decarboxylase (ZmODC) significantly increased in root of both cultivars in response to drought. However, up-regulation of ZmPAO3 and ZmPAO5 was only observed in root of tolerant cultivar. Consistently, drought caused increased polyamine oxidase activity in the tolerant cultivar. Transcription of spermidine synthase (ZmSPDS) was down-regulated by drought stress. Transcript levels of ZmPOA, ZmODC and ZmSPDS were more affected by drought in tolerant cultivar compared to susceptible cultivar. Increase in putrescine content under drought stress was more prominent in roots of tolerant cultivar, compared to susceptible cultivar. We attribute the accumulation of putrescine to increased rate of putrescine synthesis, due to higher expression of ZmPAOs and ZmODC and decreased rate of its depletion for biosynthesis of higher polyamines due to down-regulation of ZmSPDS.

Introduction

The present study focused on putrescine accumulation and underlying mechanisms in maize cultivars differing in response to drought stress. Contrasting these cultivars will provide the better understanding of the importance and the extend of putrescine metabolism role in drought tolerance in root of maize. As one of the most widely distributed crops, maize production is frequently constrained by drought, which poses a serious threat to sustainable maize production (Batool et al., 2014; Wang et al., 2019). To cope with drought stress, plants employ wide range of response mechanisms at molecular, cellular and whole plant levels, including stomatal closure, overproduction of abscisic acid (ABA), enhanced activity of antioxidant systems, as well as accumulation of various metabolites (soluble sugars, free amino acids and polyamines) (Ghotbi-Ravandi et al. 2014, 2016; Seki et al., 2007; Zhou and Yu, 2010).

Polyamines (PAs) including putrescine, spermidine and spermine are low molecular weight aliphatic polycations found in all living organisms (Sequera-Mutiozabal et al., 2016). Polyamines play a significant role in several physiological processes in plants including embryogenesis, cell division, flowering, aging, as well as stress response (Liu et al., 2015; Sagor et al., 2016). Accumulation of PAs improves tolerance mechanisms of plants under variety of environmental stresses (An et al., 2012; Bibi and Osterhuis, 2010; Zhou and Yu, 2010). Polyamines are involved in scavenging free radicals, modulation of activity of ion channels and stabilization of membranes, DNA and RNA structure and protein conformation under drought stress (Groppa and Benavides, 2008). It has been well characterized that stress-tolerant plants have a great capacity to increase biosynthesis of polyamines, especially putrescine in response to abiotic stresses compared to susceptible plants (Hussain et al., 2011). Accumulation of putrescine and enhanced rate of putrescine/(spermidine and spermine) under stress has been observed in several plant species (An et al., 2012; Shu et al., 2012; Zhang et al., 2020). Increase in endogenous putrescine content was reported in leaves of tomato under cold and biotic stress (Tsaniklidis et al., 2020), rice under osmotic stress (Chen and Kao, 1993), wheat under cadmium stress (Groppa et al., 2007), and in yellow lupine under drought stress (Juzon et al., 2017). The putrescine accumulation in plants exposed to drought stress protects plants from oxidative injury by maintaining cell pH and ion homeostasis and enhancing antioxidant system, etc. (Paradi et al., 2003; Talaat and Shawky, 2013).

Putrescine is synthesized either indirectly from arginine or directly from ornithine by the action of arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) enzymes, respectively (Tiburcio et al., 1997). Furthermore, putrescine can be re-synthesized through the back-conversion pathway of polyamines spermine to spermidine and spermidine to putrescine, which is carried out by the polyamine oxidase enzyme (PAO) (Sequera-Mutiozabal et al., 2016). On the other hand, putrescine can be oxidized by enzyme diamine oxidase (DAO) or converted into spermidine by the action of spermidine synthase enzyme (SPDS) (Gill and Tuteja, 2010). These enzymatic pathways control the putrescine homeostasis in plant cells.

There is extensive literature on polyamines metabolism in response to environmental stresses. However, most of these studies have focused on either protective mechanism of polyamines in shoot tissues (as protectant of photosynthetic apparatus) or roles of exogenous polyamines treatment in induction of protective mechanisms in plants. Far fewer studies have focused on below-ground responses of polyamines to unfavorable environmental conditions. Our experiment design contrasts two maize genotypes to provide a better understanding of the importance of putrescine metabolism in drought stress tolerance. The aim of the present study was 1) to quantify the impacts of drought on growth, enzymatic antioxidant system, putrescine content and expression patterns of related genes in maize roots, and 2) to contrast the differential responses of tolerant and susceptible maize cultivars to drought stress.