The anatomy of walnut staminate inflorescences from the time of initiation until budbreak has been studied by several investigators (Li et al., 2011; Luza and Polito, 1988). The consensus among these studies is that the development of a walnut staminate inflorescence requires ≈1 year from the time of initiation to maturation. However, the differentiation course varies by cultivar and climate (Polito and Katherine, 1997; Zhang et al., 1995). Marker characteristics should be linked genetically to the process of differentiation to ensure stability in the face of climatic variation. Attempting to project a marker event may be required. Markers that are easily duplicated and measured can be used to track events during histogenesis and enable comparisons of different studies. Morphological features that can be used as indicators of developmental stage have been identified in various plants. For example, flower bud development in Cyclocarya paliurus can be defined based on floret size, anther color, and characteristics of the stigma (Fu et al., 2011), whereas the emergence of a globular embryo is a reliable indicator of the beginning of tissue differentiation in pineapple [Ananas comosus (He et al., 2012)]. Evaluating development based on morphology is easy, but the relationship between the morphology and physiology of walnut catkins has not been established.
Auxin is an essential hormone that has been implicated in many aspects of plant growth and development (Woodward and Barrel, 2005), and the relationship between auxin and flower formation has been extensively studied (Aloni et al., 2006; Cheng and Zhao, 2007; Jiménez, 2005; Marleen and Eva, 2012). Auxin is considered to be a key regulator of floral development, but the exact role of auxin in this process has only recently come to be well understood (Chandler, 2011). Valentina et al. (2008) demonstrated that auxin synthesized in anthers coordinates anther dehiscence and pollen maturation, whereas auxin transport contributes to the independent regulation of preanthesis filament elongation; hence, auxin is an important physiological regulator of staminate flower differentiation. Using immunologic techniques, auxin can be detected in situ in plant tissues. Indeed, immunohistochemical localization studies have been performed in maize [Zea mays (Vysotskaya et al., 2007)], arabidopsis [Arabidopsis thaliana (Aloni et al., 2003)], the shoot apices of strawberry [Fragaria ×ananassa (Hou and Huang, 2005)], and hybrid poplar [Populus alba × (P. davidiana × P. simonii) × P. tomentosa (Dong et al., 2012)].
Previous attempts have been made to establish the relationships among the morphological characteristics, physiology, and histogenesis of staminate flowers in walnut (Vladimra and Sladky, 1971; Yates and Sparks, 1992). The application of exogenous hormones could transform the staminate primordia into vegetative buds. These findings comprise a first step toward elucidating the mechanism of walnut staminate flowering and the role of hormones in walnut catkin formation. Additionally, they have created a foundation for studying the regulation of staminate flowering in walnut. The results of this study also have significance for dioecious species, in which male and female individuals produce unisexual flowers and are characterized by sexual dimorphism (Cristiana Moliterni et al., 2004).
In this present study, an anti-indole acetic acid (IAA) monoclonal antibody was used to detect and localize endogenous IAA during walnut flower differentiation using an immunochemical approach. We provide a substantial base for the further research on the mechanism of IAA action during staminate flower bud differentiation.
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