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Salicylic Acid: A Regulator of Plant Growth and Development

Neha Sharma1, Vivek Sharma2, Vasudha Sharma3, and Renu Bhardwaj1

1 Department of Botanical and Environmental Sciences, Guru Nanak Dev University, Amritsar, India

2 Agricultural and Biological Engineering Department, University of Florida, Gainesville, FL, USA

3 Department of Soil, Water, and Climate, University of Minnesota, Saint Paul, MN, USA

Introduction

In plants, the phytohormones act as endogenous signals, both spatially and temporally, regulating a number of physiological functions. The cross talk between various phytohormones helps the plant to withstand biotic and abiotic stresses. This cross talk of plant hormones has evolved into a complex network within the plants, thus helping the plants having a balanced reaction to developmental and environmental stimuli (Sharma et al. 2018, 2019a; Koo et al. 2020). Salicylic acid (SA) or ortho-hydroxybenzoic acid is a member of the group of plant phenolics with a seven-carbon (C) skeleton. A study of reproductive structures and leaves of 34 plant species confirmed that SA is ubiquitously distributed in plant kingdom (Raskin et al. 1990). The name SA is from Salix (Latin word) as it was found to be an active constituent of willow tree bark (Salix sp.) which was used extensively to cure fever and aches (Khan et al. 2015).

The biosynthesis of SA in plants involves the isochorismate synthase (ICS) pathway and phenylalanine ammonia-lyase (PAL) pathway (Janda et al. 2014). The ICS pathway was first discovered in Pseudomonas species and the PmsCEAB gene cluster was found to play the key role in the synthesis of SA. The conversion of chorismate to isochorismate (IC) is catalyzed by PmsC gene and then isochorismate pyruvatelyase encoded by the PmsB gene converts IC to SA making SA synthesis from chorismate a two-step process (Mercado-Blanco et al. 2001; Lefevere et al. 2020). In the PAL pathway, the key enzyme is chorismate mutase (CM) which catalyzes the process of converting CM to prephenate. Prephenate gets converted to phenylalanine (Phe), which in turn is converted to trans-cinnamic acid (tCA) by the enzyme PAL. The next step involves the catalyzing of the conversion of tCA to benzoic acid (BA) by abnormal inflorescence meristem1 (AIM1), which is a multifunctional protein (MFP) family member (Rylott et al. 2006; Arent et al. 2010). The last step in the PAL pathway is the conversion of BA to SA which is presumed to be catalyzed by benzoic acid hydroxylase (Lefevere et al. 2020).

The ICS as well as PAL pathways to synthesize SA start from chorismate, and the importance of both ICS and PAL varies in different species of plants, as not all enzymes which catalyze various reactions in these pathways have been found in all plants. The ICS pathway plays an important role in SA biosynthesis in Arabidopsis, and PAL has been found to be more important in rice, while in soybeans, both pathways contribute equally (Silverman et al. 1995; Duan et al. 2014).

In plants, SA plays a significant part in the growth, development, and in the protection from biotic and abiotic stresses (Khan et al. 2015; Sharma et al. 2019b, 2020; Prakash et al. 2021) (Figure 1.1). The role of SA in defense mechanisms of plant was established during the last 30 years and before that it was recognized as an unimportant secondary plant metabolite. Since 1979, when White (1979) reported the role of SA in tobacco plants' disease resistance, numerous findings showed the role of SA as an important regulatory substance in plants (Chen et al. 2009). Studies have shown that in plants, SA plays a vital part in disease resistance, DNA damage/repair, seed germination, fruit yield, and thermogenesis (Dempsey and Klessig 2017). Increased levels of SA are seen in the presence of an infection, and if supplied exogenously, SA strengthens the plant defense system (Lefevere et al. 2020). In this review, we have focused on the role of SA in plants as a regulator of growth and development and providing resistance against various stresses.

Figure 1.1 Schematization of the role of salicylic acid in plants.

Source: Based on Khan et al. 2015; Sharma et al. 2019b, Sharma et al. 2020; Prakash et al. 2021.

Salicylic Acid and Plant Growth

SA plays an important role in plant growth, along with other phytohormones, and its effects on growth, when applied exogenously, is affected by the species of the plant and its stage of development as well as its concentration (Vicente and Plasencia 2011). It has been reported that more than 1?mM of SA is considered a high concentration and has negative effects (Koo et al. 2020). Barley and maize seeds did not show any germination when imbibed in >3?mM of SA (Guan and Scandalios 1995; Xie et al. 2007). On the contrary, when maize seeds were imbibed in ~0.3?mM- ~0.9?mM of SA, an increased germination speed and enhanced shoot length were recorded (Sallam and Ibrahim 2015). SA (aqueous solution), when applied to soybean shoots in the form of spray, increased the shoot and root growth significantly. Although, photosynthetic rate was not found to have any significant effect with this treatment (Gutiérrez-Coronado et al. 1998). In soybean, wheat, maize, and chamomile, SA has been found to stimulate growth. An increased growth of ~20 and 45% in the shoots and roots, respectively, was observed in soybean plants when treated with 10?nM, 100?µM, and up to 10?mM of SA. In wheat seedlings, development of larger ears and enhancement of cell division was observed in the shoot apical meristems, with a treatment of 50?µM SA. In chamomile plants, 50?µM SA stimulated the growth and an opposite effect was observed at a concentration of 250?µM SA (Gutiérrez-Coronado et al. 1998; Shakirova et al. 2003; Gunes et al. 2007; Kovácik et al. 2009). In apple, strawberry, and mango plants, fruit setting was enhanced with SA treatment (Shaaban et al. 2011; Kazemi 2013; Ngullie et al. 2014).

The relationship between SA, reactive oxygen species (ROS), and mitogen-activated protein kinase (MAPK) cascades has been found to be very important in regulating plant growth. Zhang and Klessig (1997) found that Arabidopsis MPK6 is an orthologue of tobacco SA-induced protein kinase (SIPK), and has been suggested to have an important role in growth and development (Bush and Krysan 2007; Wang et al. 2007, 2008). It has been reported that in regulation of cell growth, MAPK cascades act as mediators between phytohormones, SA, and ROS signaling (Foreman et al. 2003; Potocký et al. 2007).

SA also plays an important role in regulating flowering. Lee and Skoog (1965) indicated its flower-inducing effects for the first time. 4?µM SA was reported to promote flower bud formation from callus of tobacco. Then, Cleland and Ajami (1974) reported the isolation and identification of SA in aphid honeydew as the substantial factor for flower induction in short-day plant Xanthium strumarum. Later, SA's role in Impatiens balsamina, Oncidium (orchid species), Pisita stratiotes L., and Arabidopsis thaliana as a stimulatory factor on flowering was demonstrated. In thermogenic plants, the inflorescences were found to have high levels of endogenous SA (Raskin et al. 1990), while in non-thermogenic plants, SA levels were found to increase twofold in tobacco and fivefold in Arabidopsis leaves at the time of initiation or transition toward flowering (Yalpani et al. 1993; Abreu and Munné-Bosch 2009). Similarly, Arabidopsis plants, which were SA deficient (NahG, sid1/eds5, and sid2) exhibited a phenotype having late flowering (Martínez et al. 2004). In sunflower, it was discovered that the transcription factor HAHB10 (belonging to HD-Zip II family) plays a role in responding to biotic stress and inducing flowering and it was observed that treatment with SA induces the HAHB10 expression (Dezar et al. 2011).

However, the possibility of endogenous regulation by SA alone in case of flowering was weakened as there was not much difference in the levels of SA in aphid honeydew from flowering as well as vegetative plant parts. Thus, it was established that SA promotes and regulates flowering in conjugation with other plant growth regulators (Raskin 1992).

Salicylic Acid and Photosynthesis

In plants, photosynthesis is considered to be a very delicate physiological process. Heat stress can harm the photosynthetic apparatus, and plants have developed protective mechanisms like dissipating the excess excitation energy, utilizing heat shock proteins and plant growth regulators. SA has been reported to play a significant part during abiotic stresses to plants (Wang et al. 2010; Kohli et al. 2017, 2018). SA application enhanced the photosynthetic capacity in barley and spring wheat plants which were exposed to salt and drought stress (El-Tayeb 2005; Arfan et al. 2007). In tobacco and Arabidopsis, when SA was applied exogenously, it improved the heat...