Li, Xiaoqing and Davies, Bill and Forde, Brian (2016) Plant root development and hormone signalling during drought stress. PhD thesis, Lancaster University.
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Abstract
The plant root system is crucial for plant survival, growth and development, and it plays an important role in plant resistance to drought stress. Drought is one of the primary factors that restrict plant growth and yield, and its threat to crop yields will increase along with the growing food demand by the population of a world experiencing a changing climate. In response to drought in plants, various hormones are vital regulators, because they are able to manipulate plant development and in some cases minimise the adverse impact of drought. Therefore, understanding how the plant root system will adapt to a soil drying challenge is crucial. Of particular importance is the plant response to a non-lethal drought stress, which is often encountered in the field. Elucidation of the mechanisms underlying such responses, including hormonal regulations, may help crop scientists improve the plant performance under drought. A six-day progressive soil drying pot experiment was designed to examine the synchronisation of physiological responses in maize (Zea mays L.) roots and leaves during soil drying. It was found that maize roots showed earlier responses to soil drying than leaves in changing growth rates, water potentials and hormone levels. Root growth was stimulated at soil water content of 25−32% (ca. 41% in well-watered pots), while both root growth and leaf elongation were inhibited when soil water content was below 20%. Root abscisic acid (ABA) level gradually increased when soil water content was lower than 32% during soil drying. The stimulation and inhibition of root growth during soil drying may be regulated by root ABA, depending on the degree of the concentration increase. The ethylene release rates from leaves and roots were inhibited during soil drying, which occurred later than the increase in ABA levels. In a subsequent root phenotyping study on 14 maize genotypes, significant genetic variation was observed in root angle and size (root length, surface area and dry weight), and in the plasticity of these traits under mild and severe drought stress. Genotypes with a steeper root angle under well-watered conditions tended to display more promotion or less inhibition in root size under drought. Further analysis showed that combined traits of maize root angle, its plasticity and the root size plasticity under drought may be a better predictor for maize drought resistance than a single one of these traits. Moreover, root angle was found positively related to the leaf and root ABA levels and negatively related to the root tZ (a cytokinin) level under well-watered conditions. In another study on the crosstalk of drought-related hormones using the model plant Arabidopsis thaliana L., the biphasic responses of root elongation to ABA were confirmed, i.e. low external ABA concentrations stimulated root growth while high ABA concentrations inhibited it. Furthermore, ethylene and auxin were found to be involved in these responses. The inhibitory effect of high ABA levels on root growth was reduced or even eliminated when Arabidopsis was chemically treated to inhibit the ethylene biosynthesis or signalling, or to block auxin influx carriers. This was confirmed using mutants with blocked ethylene or auxin signalling, or a defect in the auxin influx carrier AUX1. On the other hand, the stimulatory effect of low ABA levels on root growth was lost when Arabidopsis seedlings were chemically treated to inhibit the auxin efflux carriers, and in mutants with blocked auxin signalling or with a defect in the PIN2/EIR1 auxin efflux carrier. These results indicate that ABA regulates root growth through two distinct pathways. The inhibitory effect that operates at high ABA concentrations is via an ethylene-dependent pathway and requires auxin signalling and auxin influx through AUX1. The stimulatory effect that operates at low ABA concentrations is via an ethylene-independent pathway and also requires auxin signalling and auxin efflux through PIN2/EIR1. This research contributes to our understanding of the responses of plant root system to different degrees of non-lethal drought stress, and it highlights the importance of root traits that may be important to plant drought resistance. The potential involvement of hormones (ABA, ethylene, auxin and cytokinin) in these processes is clarified. The knowledge gained may be integrated in novel crop management strategies to plan irrigation and help in the development of drought resistant crop varieties.