## 1 Introduction  
Aging has always been a topic discussed by many scholars, and the free radical theory is one of the main theories to explain it [1]. Under normal circumstances, reactive oxygen species (ROS) in cells are always in the dynamic balance of production and elimination. It is involved in various life activities including gene transcription and protein synthesis. Long term exposure to the stimulus will rapidly increase the ROS content to the threshold that the antioxidant enzyme system cannot independently clear, and the oxidative stress reaction of the body begins to occur. Excessive free radicals begin to attack important biological macromolecules (nucleic acids, proteins, etc.) in cells, causing the loss of cell function and the decline of human body function, and eventually leading to the aging of the body [2, 3]. Hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)), the primary form of reactive oxygen species (ROS), is a by-product of cell metabolism which is involved in cell redox signaling. Excess H\({}_{2}\)O\({}_{2}\) leads to lipid peroxidation and affects cellular senescence and apoptosis. H\({}_{2}\)O\({}_{2}\) induces cellular damage and is one of the most widely used in vitro models of oxidative stress [4]. As the first defense barrier of human body, skin is always threatened by oxidative damage. Oxidative stress can lead to the increase of intracellular matrix metalloproteinase-ases (MMPs) and the degradation of extracellular matrix (ECM) [5, 6]. ECM is made up of a variety of matrix proteins that play an important role in cellular metabolism. The disruption of ECM homeostasis and its protease dysfunction can lead to loss of skin intercellular lipids and disintegration of dermal support structures, and ECM homeostasis can be sustained by degradation and synthetic pathways. Decreased levels of proteoglycans in the ECM result in a reduction of the skin's ability to retain water and moisture, and a disruption of the normal structure and function of the skin. Collagen, a protein in the ECM, is essential for maintaining the physicochemical and mechanical properties of the skin. MMPs are a key group of proteases that disrupt the ECM [7; 8; 9]. Elevated levels of MMP expression trigger collagen degradation, disruption of antioxidant enzymes and expression of skin aging-related proteins, leading to skin aging, including loss of elasticity, wrinkles, dryness and hyperpigmentation [10]. Thus, it is thought that by promoting collagen synthesis in the skin and reducing MMP activity, the skin ageing process can be improved. Other studies have shown that phosphatidylinositol kinase (PI3K) plays an important regulatory role in collagen gene expression [11]. In addition, several downstream targets of PI3K, such as protein kinase B (AKT) are associated with collagen regulation, suggesting that the PI3K/AKT signaling pathway is closely related to skin aging due to oxidative stress [12]. Coix is a cereal plant of the family Gramineae which is widely grown in Asian countries. Coix seed is the mature kernel of _Coix lachryma-jobi_ L., which is rich in various active nutrients, including proteins, amino acids, dietary fibre, fatty acids, polysaccharides and many vitamins, making it a dual-use crop for medicinal purposes [13]. It has potential for immunomodulation, reduction of cellular inflammation caused by lipoprotein oxidation, regulation of blood lipids, anti-obesity, antioxidant and allergic reaction inhibition [14; 15; 16], and has also been used in cardiovascular, diabetic, fatty liver and anti-cancer treatments [17; 18; 19]. Microbial fermentation is a green and environmentally friendly method of biotechnological transformation. Studies have reported that the fermentation broth of Coix seed with _Lactobacillus plantarum_ has significantly higher nutritional activity components such as fatty acids, soluble dietary fibre and organic acids than unfermented Coix seed, which can promote the stability and prolong the storage time of Coix seed [20].