Aluminium-induced reduction of plant growth in alfalfa (Medicago sativa) is mediated by interrupting auxin transport and accumulation in roots.

Shengyin Wang,Peng Zhou,Yuan An,Ge Wang,Xiaoyan Ren,Bingru Huang

doi:10.1038/srep30079

Abstract

The objective of this study was to investigate Al3+-induced IAA transport, distribution, and the relation of these two processes to Al3+-inhibition of root growth in alfalfa. Alfalfa seedlings with or without apical buds were exposed to 0 or 100 μM AlCl3 and were foliar sprayed with water or 6 mg L−1 IAA. Aluminium stress resulted in disordered arrangement of cells, deformed cell shapes, altered cell structure, and a shorter length of the meristematic zone in root tips. Aluminium stress significantly decreased the IAA concentration in apical buds and root tips. The distribution of IAA fluorescence signals in root tips was disturbed, and the IAA transportation from shoot base to root tip was inhibited. The highest intensity of fluorescence signals was detected in the apical meristematic zone. Exogenous application of IAA markedly alleviated the Al3+-induced inhibition of root growth by increasing IAA accumulation and recovering the damaged cell structure in root tips. In addition, Al3+ stress up-regulated expression of AUX1 and PIN2 genes. These results indicate that Al3+-induced reduction of root growth could be associated with the inhibitions of IAA synthesis in apical buds and IAA transportation in roots, as well as the imbalance of IAA distribution in root tips.

Highlights

The objective of this study was to investigate Al3+-induced indole acetic acid (IAA) transport, distribution, and the relation of these two processes to Al3+-inhibition of root growth in alfalfa
When alfalfa seedlings were exposed to varying concentrations of AlCl3 (0, 50, 100, 200 or 300 μM) for 48 h, the inhibition of roots elongation was positively dependent on AlCl3 concentrations, in which the higher decline rate of relative root growth was in the Al3+ concentration of 100 μM (Supplemental Fig. 1)
When alfalfa was exposed to Al3+-stressed conditions, IAA concentrations in apical buds and root tips were significantly decreased; the IAA concentrations in root tips in both -Al and Al treatments decreased to a very low level after the apical buds were removed

Summary

Introduction

The objective of this study was to investigate Al3+-induced IAA transport, distribution, and the relation of these two processes to Al3+-inhibition of root growth in alfalfa. Al3+ stress upregulated expression of AUX1 and PIN2 genes These results indicate that Al3+-induced reduction of root growth could be associated with the inhibitions of IAA synthesis in apical buds and IAA transportation in roots, as well as the imbalance of IAA distribution in root tips. Al3+ stress induces callose production in plasmodesmata, which may block auxin or indole acetic acid (IAA) transport through the symplast and result in a Al3+-induced inhibition of root cell elongation[7,8]. Our previous study showed that IAA concentrations in apical buds and root tips of Al3+-stressed seedlings decreased in a short period (1–3 d) of Al3+ stress[20], which may reduce the growth of alfalfa seedlings under Al3+ stress. We aimed to further investigate the following questions: (1) What was the reason of IAA accumulation decrease in the Al3+-stressed root tips? (2) What was the effect of Al3+ stress on cell structure and IAA distribution in root tips? (3) Was there any effect of IAA on the cell structure formation of the Al3+-stressed root tips?

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