Low soil pH modulates ethylene biosynthesis and germination response of Stylosanthes humilis seeds

ABSTRACT

The tropical forage legume Stylosanthes humilis is naturally distributed in the acidic soils of the tropics. However, data concerning the role of low soil pH in the control of S. humilis seed germination remains limited. Recently, we have demonstrated that acidic soil triggers increased ethylene production during germination of S. humilis seeds, concomitantly with higher root penetration into the soil. Our finding points an important role of low soil pH as a signal allowing penetration of root in the soil through interaction with the ethylene signalling pathway. Herein, we discuss how low soil pH induces changes on seed hormonal physiology.

Ethylene is involved in several plant growth and developmental processes, including senescence, flowering, fruit ripening and seed germination.¹ Moreover, ethylene is a key factor that regulates many of the response of plants to environmental stress², suggesting that ethylene concentration has a physiological role in mediating developmental adaptation. In this context, ethylene is required for germination of Stylosanthes humilis seeds, a tropical forage legume known to exhibit good persistence and productivity in acid soils.^3,4 As seed germination strategy is an important factor determining survival and maintenance of plant population, ethylene would play an important role in the successful germination of S. humilis seeds under acidic conditions. However, until now, there has been a lack of comparative data detailing the involvement of low pH soil and ethylene during seed germination.

In our original research article⁵ we showed that low soil pH increased ethylene biosynthesis during germination of S. humilis seed, facilitating root penetration into the soil. Soils with pH < 5.0 effectively increases 1-aminocyclopropane-1-carboxylic acid (ACC) concentration and the subsequent biosynthesis of ethylene by the enzymes ACC synthase (ACS) and ACC oxidase (ACO) (Figure 1). In plants, ACS has been reported as a major point of regulation in the ethylene biosynthesis pathway.⁶ Moreover, ACO activity is generally not a limiting step for ethylene production in most plant tissues.⁶ In this respect, ethylene biosynthesis in S. humilis seeds is limited by ACO activity.³ However, important questions remain about the regulation of ACS and ACO enzymes during seed germination and seedling growth under acidic soil. For example: does the putative signal triggered by low pH soil affect the regulation of the transcript levels of ACS and ACO genes and/or activation of those proteins? ACS genes are transcriptionally induced in response to drought, shade, flooding, salinity, heavy metal and osmotic stress.^7-12 Thus, it seems reasonable to suggest low soil pH as another player in the list of signals that trigger ethylene biosynthesis.

Figure 1.

A working model for the integrated regulation of ethylene production and germination of S. humilis seeds under the acidic conditions of tropical soils. The illustration shows the progression of S. humilis seed from sowing to final seed germination. The seed begins to produce ethylene following sowing at the rate dependent of soil pH. Hence, soil with pH < 5.0 acts as a strong signal that regulates ethylene biosynthesis by an increase in 1-aminocyclopropane-1-carboxylic acid (ACC) synthase activity, which leads to elevated concentration of cellular ACC. Low soil pH-induce ethylene production also appears to depend on the ACC oxidase activity (ACO). Once the radicle has emerged, elongation growth occurs at a rate determined by ethylene and indole-3-acetic acid (IAA) concentrations. Interestingly the ethylene-induced production of auxin appears to be associated with tryptophan (Trp) concentration and the subsequent synthesis of IAA via tryptophan aminotransferase (TA), TA-related (TARs) and flavin-containing monooxygenase family proteins (YUCCA).¹³ Thus downward growth in response to low soil pH (< 5.0) facilities the radicle tip penetrating into the soil. Additionally, the ethylene signaling pathway most likely regulates upward hypocotyl growth. A high pH (> 5.0) condition should not produce the same effect. SAM, S-adenosyl methionine. 3-IPA, indole-3-pyruvic acid.

Acidic condition-induced root penetration into the soil is also dependent on the synthesis of the auxin indole-3-acetic acid (IAA).⁵ Exactly how low pH soil induces auxin biosynthesis is still unclear, but it may involve alterations in ethylene concentration. Of note, our results revealed changes in tryptophan concentrations in root of seedlings grown in pH 5.8 soil compared with pH 3.9 soil.⁵ In this context, ethylene concentration could trigger a generation of IAA by modifying the activity of tryptophan aminotransferase (TA)/TA-related (TARs) converting tryptophan to indole-3-pyruvic acid (3-IPA), while flavin-containing monooxygenase family proteins (YUCCA) convert 3-IPA to IAA (Figure 1).¹³ Interestingly, ethylene and IAA accumulation under soil with pH < 5.0 decreases radicle elongation, allowing root penetration into the soil. By contrast, soil with pH > 5.0 is associated with a lower ethylene and IAA production and impaired root penetration into the soil (Figure 1). Thus, the biosynthesis and signaling of ethylene and auxin in S. humilis seeds may constitute points of convergence that offer the necessary flexibility for generating differential response of initial root growth within a given soil pH condition.

The effect of low soil pH was not limited to the increase of root penetration; it also modified growth of hypocotyl of S. humilis seedlings. Soil with pH < 5.0 reduced hypocotyl length in comparison with seedlings grown in soil with pH > 5.0 (Figure 1). ACC concentrations were increased by the reduction of soil pH in hypocotyl.⁵ Moreover, soil pH did not influence the concentration of IAA in hypocotyl.⁵ It is also interesting that the ACC concentrations were lower in hypocotyls than in roots of S. humils seedlings grown in soil with pH < 5.0.⁵ Presumably, ethylene concentrations that occur in hypocotyl under acidic conditions are not enough to maintain the curvature of apical hook for a long period of time. These findings are interesting as they suggest that control of initial upward hypocotyl growth in acidic environments could be mediated by ethylene signalling. It is known that carbon and nitrogen metabolism are subject to hormonal regulation,¹⁴ raising the question of how primary metabolism is regulated and what function it serves to seed germination under acidic conditions.

In summary, germination behavior of S. humilis seeds in response to ethylene suggest a general adaptive mechanism to low soil pH. Stylosanthes species with higher capacity for ethylene biosynthesis are also well adapted to saline environment.¹⁵ Therefore, as seeds of Stylosanthes have different abilities to synthesize ethylene, one question for the future is: what is the role of ethylene in the natural distribution of Stylosanthes species in the acidic conditions of tropical soils?

Abbreviations

ACC

1-aminocyclopropane-1-carboxylic acid

ACS

ACC synthase

ACO

ACC oxidase

IAA

indole-3-acetic acid

SAM

S-adenosyl methionine

Trp

tryptophan

TA

tryptophan aminotransferase

TARs

tryptophan aminotransferase-related

YUCCA

flavin-containing monooxygenase family proteins

3-IPA

indole-3-pyruvic acid

Acknowledgments

Financial support from the Brazilian founding agencies including National Council for Scientific and Technological Development (CNPq), the Agency for the Support and Evaluation of Graduate Education (CAPES) and the Foundation for Research Assistance of the Minas Gerais State (FAPEMIG) is gratefully acknowledged.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.