Abstract
Walnut, an important non-wood product forest tree, has free root hairs in orchards. Root hairs are specialized cells originating from the root epidermis that are regulated by plant hormones, such as auxins. This study was conducted to evaluate the effect and mechanism of phosphorus stress on root hair growth of walnut (Juglans regia L.) seedings by auxin (IAA) biosynthesis and transport. Both low phosphorus (LP) and no phosphorus stresses (NP) heavily decreased plant height, leaf number, total root length, root surface, shoot and root biomass, and root nutrient contents. The LP treatment significantly increased root hair growth, accompanied with up-regulation of the positive regulation root hair growth gene JrCPC and down-regulation of the negative regulation root hair growth gene JrTTG1, while the NP treatment had opposite effects. The root IAA level, IAAO activities, IAA transport genes (JrAUX1, JrLAX1, and JrPIN1), and the biosynthesis genes (JrTAA1 and JrTAR1) were increased by the LP treatment, while the NP treatment decreased all of them. Interestingly, the auxin biosynthesis gene CsYUCCA1 was not affected, which suggested that P mainly affects root hair growth of walnut by regulating auxin transport, and then affects root nutrient absorption and plant growth.
Highlights
Plant roots have a high degree of phenotypic plasticity, and their morphology is affected by various factors, including soil nutrients, such as phosphorus (P) (Wu et al, 2013)
The auxin biosynthesis gene CsYUCCA1 was not affected, which suggested that P mainly affects root hair growth of walnut by regulating auxin transport, and affects root nutrient absorption and plant growth
Most part of phosphorus in soils of orchard is immobile and unavailable, and many fruit trees often suffer from phosphorus stress which affects the biomass accumulation of fruit trees, the yield and quality of fruit
Summary
Plant roots have a high degree of phenotypic plasticity, and their morphology is affected by various factors, including soil nutrients, such as phosphorus (P) (Wu et al, 2013). P is one of the necessary macronutrients for plant growth and metabolism, in photosynthesis, energy accumulation, and respiration (Balyan et al, 2016). 80 % of soil P is in organic forms (Jungk, 2001), or is fixed by soil organic matter, causing P immobility and deficiency in plants and soils (Zheng et al, 2015). P deficiency or stress is widespread in the plant kingdom (Bargaz et al, 2013). Plants have evolved at least two pathways to respond to P-deficient stress, the direct uptake pathway from the rhizosphere by root epidermal cells and root hairs, and the indirect uptake pathway via fungi (Richardson et al, 2009).
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