The Arabidopsis KINβγ Subunit of the SnRK1 Complex Regulates Pollen Hydration on the Stigma by Mediating the Level of Reactive Oxygen Species in Pollen

Pollen–stigma interactions are essential for pollen germination. The highly regulated process of pollen germination includes pollen adhesion, hydration, and germination on the stigma. However, the internal signaling of pollen that regulates pollen–stigma interactions is poorly understood. KINβγ is a plant-specific subunit of the SNF1-related protein kinase 1 complex which plays important roles in the regulation of plant development. Here, we showed that KINβγ was a cytoplasm- and nucleus-localized protein in the vegetative cells of pollen grains in Arabidopsis. The pollen of the Arabidopsis kinβγ mutant could not germinate on stigma, although it germinated normally in vitro. Further analysis revealed the hydration of kinβγ mutant pollen on the stigma was compromised. However, adding water to the stigma promoted the germination of the mutant pollen in vivo, suggesting that the compromised hydration of the mutant pollen led to its defective germination. In kinβγ mutant pollen, the structure of the mitochondria and peroxisomes was destroyed, and their numbers were significantly reduced compared with those in the wild type. Furthermore, we found that the kinβγ mutant exhibited reduced levels of reactive oxygen species (ROS) in pollen. The addition of H2O2 in vitro partially compensated for the reduced water absorption of the mutant pollen, and reducing ROS levels in pollen by overexpressing Arabidopsis CATALASE 3 resulted in compromised hydration of pollen on the stigma. These results indicate that Arabidopsis KINβγ is critical for the regulation of ROS levels by mediating the biogenesis of mitochondria and peroxisomes in pollen, which is required for pollen–stigma interactions during pollination.花粉与柱头的相互作用对于花粉萌发至关重要。花粉萌发是一个高度受调控的过程，涵盖花粉黏附、水合作用以及在柱头上的萌发步骤。然而，目前对于调控花粉-柱头相互作用的花粉内部信号通路仍知之甚少。KINβγ是SNF1相关蛋白激酶1复合物（SNF1-related protein kinase 1 complex）的植物特异性亚基，该复合物在植物发育调控中发挥重要作用。本研究表明，KINβγ是拟南芥花粉营养细胞中定位于细胞质与细胞核的蛋白质。尽管拟南芥kinβγ突变体的花粉在体外可正常萌发，但无法在柱头上完成萌发。进一步分析显示，kinβγ突变体花粉在柱头上的水合作用受到损害。然而，向柱头施加水分可在体内促进突变体花粉的萌发，这表明突变体花粉的水合作用受损是其萌发异常的原因。在kinβγ突变体花粉中，线粒体与过氧化物酶体的结构遭到破坏，且其数量相较于野生型显著减少。此外，本研究发现kinβγ突变体花粉中的活性氧（reactive oxygen species, ROS）水平降低。在体外添加过氧化氢（H₂O₂）可部分弥补突变体花粉的吸水能力缺陷；而通过过表达拟南芥过氧化氢酶3（CATALASE 3）降低花粉中的ROS水平，则会导致花粉在柱头上的水合作用受损。以上结果表明，拟南芥KINβγ通过介导花粉中线粒体与过氧化物酶体的生物发生，对ROS水平进行调控，这一过程对于授粉过程中的花粉-柱头相互作用至关重要。