Seeds - Part 2 - Phytohormones and Biostimulants

Understanding seeds

Plants species have evolved an array of hormones and signalling molecules which create a complex network of cues for responding to many different stimuli. From growth regulation and stress responses, to nutrient acquisition and microbial interaction, hormones are involved in almost all aspects of plant life (Bari et al 2009, Rubio et al 2009, Davis 2010, Foo et al 2019). The complexity of these chemical signals is further compounded by intricate interplay, dose dependant responses, crosstalk, and feedback loops (Jang et al 2020, Peres et al 2019, El-Esawi 2017, Boutte et al 2020).

Starting with germination, through to seed development, and finally the breaking of dormancy, each developmental stage of a plant’s life, including stress responses, are entwined with hormonal signalling as well as environmental cues. Both germination and seed dormancy are influenced by plant hormones (Graeber et al 2012). After imbibition, germination progresses with systematic utilization of proteins through mobilization by the enzymatic breakdown of preserves, for example by carboxypeptidase and aminopeptidase. Up to 74 proteins have been identified as undergoing alteration during germination in the model species Arabidopsis thaliana (Gallardo et al 2001). This is part of a large, coordinated network of molecular changes in the seed, which also involve the redistribution and ratio changes of hormones such as gibberellins and abscisic acid (Graeber et al 2010, Ali-Rachedi et al 2004, Frinklestien et al 2008). Additionally, other signalling mechanisms such as nitrogen species can also enhance the germination process – nitrous oxide, for example, can activate the breakdown of starch to sugar for energy release in wheat seeds (Zhang et al 2009, Zhang et al 2005).

Phytohormones

Gibberellins and abscisic acid are the two primary regulators of seed dormancy and germination (Miransari and smith 2014, Shu et al 2016). Gibberellins are a well-known germination enhancer, as they are key in the breaking of dormancy – however, they do not fully control seed dormancy (Bewley 1997, Miransari et al 2009). One of the key roles of gibberellins is the inhibition of the hormone abscisic acid, which is known to prevent germination by inhibiting the cell cycle (Toyomasu et al 1994, Atia et al 2009). As the seed matures, and builds towards dormancy, the metabolic rate starts to decrease, moisture levels drop, and abscisic acid levels are increased (Matill et al 2008). Balancing these two hormones will be one of the key elements in determining germination success versus the dormancy state (White et al 2000).

Another phytohormone, ethylene, can be important in germination success as amongst other things it plays a role in the elongation of the radicle in concert with gibberellins. Ethylene, however, has a much wider influence on the plant and acts upon many different stages for many different purposes, probably best known for its influence on fruit ripening (Nath et al 2006). Ethylene is particularly important in stress responses where it is upregulated and can inhibit growth by redirecting the plants energy sources towards dealing with the stress. The production of ethylene is increased during germination although it is unclear if it is a result of the progression of germination, or whether it is directing germination (Matilla 2000, Pennazio and Roggero 1991, Zapata et al 2004).

Biostimulants

Biostimulants can come in two main forms:

 1) Plant-derived biostimulants, which are natural and usually comprised of plant/seaweed extracts which contain humic acids, amino acids, microbes, and polypeptides (Posmyk et al 2016, Kauffman et al 2007, Cavani et al 2006).

2) Cultured microbes which are used to populate the roots/plant tissues which enhance nutrient availability, or can prime the plants for pathogen attacks (Remy et al., 1994; Smith and Gianinazzi-Pearson, 1988).

More information on this second type of biostimulant will be included in the follow-up article. For the remainder of this article, we will focus on plant-derived biostimulants. Animal-derived biostimulants are perhaps more historically used, but will not be discussed here.

There are numerous types of plant-derived extracts that are beneficial to plant growth when applied at different developmental stages (Zulfiqar et al 2019). However, there is less information of those that can be used for the purposes of enhancing germination. A study from back in 2001 showed various vegetables were responsive to the treatment of humic acid and biozyme which enhanced their germination rate under various saline conditions (Yildirim et al 2001).

This treatment contained plant-derived hormones such as cytokinins and auxin precursors amongst other enzymes and amino acids. Hydrolysates – defined as any product for protein hydrolysis, or in other words, partly broken-down proteins, have been shown as highly effective at enhancing early plant life. A sunflower-derived hydrolysate has shown auxin-like activity when applied to maize (Ugolini et al 2014). In another study, soy-flour based hydrolysates were used successfully to enhance uniformity and vigour in tomato and broccoli, although it did not notably alter germination rate. (Amirkhani et al 2017).

Conclusions

Taken together, there are many ways which one could look to improve the quality of seeds and seedlings, and the recent surge in technologies of plant-derived products has provided further insights into which players are influencing which part of the developmental process. The PharmaSeeds research and development department are investigating the best ways to enhance seed and plant health to drive more predictable outcomes.  In the next part of the series, we look more closely at the use of beneficial microbes as a way of enhancing plant vigour.

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