Let's shape again: the concerted molecular action that builds the pollen tube

The pollen tube is being subjected to control by a complex network of communication that regulates its shape and the misfunction of a single component causes specific deformations. In flowering plants, the pollen tube is a tubular extension of the pollen grain required for successful sexual reproduction. Indeed, maintaining the unique shape of the pollen tube is essential for the pollen tube to approach the embryo sac. Many processes and molecules (such as GTPase activity, phosphoinositides, Ca²⁺ gradient, distribution of reactive oxygen species and nitric oxide, nonuniform pH values, organization of the cytoskeleton, balance between exocytosis and endocytosis, and cell wall structure) play key and coordinated roles in maintaining the cylindrical shape of pollen tubes. In addition, the above factors must also interact with each other so that the cell shape is maintained while the pollen tube follows chemical signals in the pistil that guide it to the embryo sac. Any intrinsic changes (such as erroneous signals) or extrinsic changes (such as environmental stresses) can affect the above factors and thus fertilization by altering the tube morphology. In this review, the processes and molecules that enable the development and maintenance of the unique shape of pollen tubes in angiosperms are presented emphasizing their interaction with specific tube shape. Thus, the purpose of the review is to investigate whether specific deformations in pollen tubes can help us to better understand the mechanism underlying pollen tube shape.

The role of reactive oxygen species in pollen germination in Picea pungens (blue spruce)

Endogenous ROS, including those produced by NADPH oxidase, are required for spruce pollen germination and regulate membrane potential in pollen tubes; [Formula: see text] and H ₂ O ₂ are unevenly distributed along the tube. Recently, the key role of reactive oxygen species (ROS) in plant reproduction has been decisively demonstrated for angiosperms. This paper is dedicated to the involvement of ROS in pollen germination of gymnosperms, which remained largely unknown. We found that ROS are secreted from pollen grains of blue spruce during the early stage of activation. The localization of different ROS in pollen tube initials and pollen tubes demonstrated the accumulation of H₂O₂ in pollen tube apex. Colocalization with mitochondria-derived [Formula: see text] showed that H₂O₂ is produced in mitochondria and amyloplasts in addition to its apical gradient in the cytosol. The necessity of intracellular ROS and, particularly, [Formula: see text] for pollen germination was demonstrated using different antioxidants. ·OH and extracellular ROS, on the contrary, were found to be not necessary for germination. Exogenous hydrogen peroxide did not affect the germination efficiency but accelerated pollen tube growth in a concentration-dependent manner. The optical measurements of membrane potential showed that in spruce pollen tubes there is a gradient which is controlled by H⁺-ATPase, potassium- and calcium-permeable channels, anion channels and ROS, as demonstrated by inhibitory analysis. An important role of NADPH oxidase in the regulation of ROS balance in particular, and in germination in general, has been demonstrated by inhibiting the enzyme, which leads to the reduction in ROS release, depolarization of pollen tube plasma membrane, and blocking of pollen germination.

Tip-localized Ca2+ -permeable channels control pollen tube growth via kinase-dependent R- and S-type anion channel regulation

Pollen tubes (PTs) are characterized by having tip-focused cytosolic calcium ion (Ca2+ ) concentration ([Ca2+ ]cyt ) gradients, which are believed to control PT growth. However, the mechanisms by which the apical [Ca2+ ]cyt orchestrates PT growth are not well understood. Here, we aimed to identify these mechanisms by combining reverse genetics, cell biology, electrophysiology, and live-cell Ca2+ and anion imaging. We triggered Ca2+ -channel activation by applying hyperpolarizing voltage pulses and observed that the evoked [Ca2+ ]cyt increases were paralleled by high anion channel activity and a decrease in the cytosolic anion concentration at the PT tip. We confirmed a functional correlation between these patterns by showing that inhibition of Ca2+ -permeable channels eliminated the [Ca2+ ]cyt increase, resulting in the abrogation of anion channel activity via Ca2+ -dependent protein kinases (CPKs). Functional characterization of CPK and anion-channel mutants revealed a CPK2/20/6-dependent activation of SLAH3 and ALMT12/13/14 anion channels. The impaired growth phenotypes of anion channel and CPK mutants support the physiological significance of a kinase- and Ca2+ -dependent pathway to control PT growth via anion channel activation. Other than unveiling this functional link, our membrane hyperpolarization method allows for unprecedented manipulation of the [Ca2+ ]cyt gradient or oscillations in the PT tips and opens an array of opportunities for channel screenings.