Abstract
Boron is an important micronutrient for plants. However, boron is also toxic to cells at high concentrations, although the mechanism of this toxicity is not known. This study aimed to evaluate the effect of boron toxicity on Malus domestica pollen tube growth and its possible regulatory pathway. Our results showed that a high concentration of boron inhibited pollen germination and tube growth and led to the morphological abnormality of pollen tubes. Fluorescent labeling coupled with a scanning ion-selective electrode technique detected that boron toxicity could decrease [Ca2+]c and induce the disappearance of the [Ca2+]c gradient, which are critical for pollen tube polar growth. Actin filaments were therefore altered by boron toxicity. Immuno-localization and fluorescence labeling, together with fourier-transform infrared analysis, suggested that boron toxicity influenced the accumulation and distribution of callose, de-esterified pectins, esterified pectins, and arabinogalactan proteins in pollen tubes. All of the above results provide new insights into the regulatory role of boron in pollen tube development. In summary, boron likely plays a structural and regulatory role in relation to [Ca2+]c, actin cytoskeleton and cell wall components and thus regulates Malus domestica pollen germination and tube polar growth.
Highlights
Boron is an essential micronutrient for the normal development of higher plants (Blevins and Lukaszewski, 1998)
Our results revealed that boron toxicity could interrupt the calcium gradient at the tip of a pollen tube and block its polar growth, likely via disturbing the actin organization and disturbing the cell wall material directional transportation and cell wall construction
The average growth rate of pollen tubes treated with 0.2% boric acid was distinctly slower than for the control: FIGURE 1 | Morphology of Malus domestica pollen tube under boron toxicity. (A) Morphology of a control pollen tube with slender diameter and straight shape. (B) Morphology of a pollen tube treated with 0.2% boric acid, showing an abnormal tube. (C) Abnormal pollen tubes showing the twisted morphology and swollen tip, respectively. (D) Fluorescein diacetate (FDA) fluorescence indicated viability of the swollen pollen tube
Summary
Boron is an essential micronutrient for the normal development of higher plants (Blevins and Lukaszewski, 1998). Boron is known to affect the mechanical properties of the cell wall (Dumont et al, 2014). There is a narrow range of favorable boron concentrations for plant development. Abnormal levels of boron can be toxic or can trigger deficiency symptoms (Pérez-Castro et al, 2012). The optimum boron level for one species can be either toxic or insufficient for other species (Blevins and Lukaszewski, 1998). Boron toxicity is an important agricultural problem that limits crop productivity (Nable et al, 1997) and attracts increase interest. Boron toxicity has been shown to affect several developmental or biochemical processes in plants (Sakamoto et al, 2011), including
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