Abstract
A key role of boron in plants is to cross-link the cell wall pectic polysaccharide rhamnogalacturonan-II (RG-II) through borate diester linkages. Phenylboronic acid (PBA) can form the same reversible ester bonds but cannot cross-link two molecules, so can be used as an antagonist to study the function of boron. This study aimed to evaluate the effect of PBA on apple (Malus domestica) pollen tube growth and the underlying regulatory mechanism. We observed that PBA caused an inhibition of pollen germination, tube growth and led to pollen tube morphological abnormalities. Fluorescent labeling, coupled with a scanning ion-selective electrode technique, revealed that PBA induced an increase in extracellular Ca2+ influx, thereby elevating the cytosolic Ca2+ concentration [Ca2+]c and disrupting the [Ca2+]c gradient, which is critical for pollen tube growth. Moreover the organization of actin filaments was severely perturbed by the PBA treatment. Immunolocalization studies and fluorescent labeling, together with Fourier-transform infrared analysis (FTIR) suggested that PBA caused an increase in the abundance of callose, de-esterified pectins and arabinogalactan proteins (AGPs) at the tip. However, it had no effect on the deposition of the wall polymers cellulose. These effects are similar to those of boron deficiency in roots and other organs, indicating that PBA can induce boron deficiency symptoms. The results provide new insights into the roles of boron in pollen tube development, which likely include regulating [Ca2+]c and the formation of the actin cytoskeleton, in addition to the synthesis and assembly of cell wall components.
Highlights
Boron is known to be essential for plant growth as it functions as a crosslinking molecule in the cell wall and participates in other processes, such as maintenance of plasma membrane function, and in several metabolic pathways [1,2]
As we mentioned in introduction, addition of Phenylboronic acid (PBA) could disrupt boron diester cross-links by forming monoester linkages at positions normally occupied by boron
We found that PBA treatments reduced the pollen germination rate, leading to a retardation of pollen tube growth, which has been noted as a boron deficiency symptom [20]
Summary
Boron is known to be essential for plant growth as it functions as a crosslinking molecule in the cell wall and participates in other processes, such as maintenance of plasma membrane function, and in several metabolic pathways [1,2]. 90% of cellular boron is localized in the cell wall [3]. Borate contains two pairs of hydroxyl moieties that can form reversible diester bonds with molecules containing cis-diols in a favorable conformation, so it has a cross-. Effects of Phenylboronic Acid on Pollen Tube Growth
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