## 4 Discussion
It implies that the regulation of root hairs under PAW treatment involves sophisticated genetic networks, not simply a single pathway. Moreover, this result indicates the increased root hair number induced by PAW, which results in securing more space for root epidermal cells to come into contact with nutrients per unit of root cell. These differences may lead to improved growth because the root hairs are in response to the availability of NO3\({}^{-}\) which has an important role in nitrogen (N) source nutrient acquisition when plants are exposed to nitrate-rich soils. It is known that nitrate also can act as signaling molecules that can control downstream gene expression in plants. Previously, the change in the root architecture in plants by the nitrate concentration was presented as an example of plasticity in the developmental process in response to the environment [36; 47]. For example, the increment of nitrate concentration of 0.1 to 1 mM KNO3 was known to induce root hair and lateral root formation [48; 49; 50]. In contrast, high nitrate concentration suppressed the root formations [47; 48; 51]. At the molecular level, we confirmed the effect of the PAW treatment by checking the expression of marker genes related to root development. One of the key genes of root hair initiation and root hair elongations, _COBL9_, was induced in the PAW5 and suppressed in the PAW19 treatment condition. This expression result confirmed our finding of the different phenotypes in root hair number and root length between the Col-0 and _cob19-1_ mutants which were affected by the _COBL9_ expression (Tables 2 and 4, Figure 6). and our PAW generated from SDBD mainly affects the nitrate concentration. In plants, auxin has a major role in root hair development, and the auxin transporter proteins, _AUX1_ and _LAX3_, regulate lateral root formation [35; 50]. Short root hair is due to mutations of _AUX1_ genes indicating _AUX1_ controls elongation of the root hair [52]. We checked the expression of two auxin transport genes, _AUX1_ and _LAX3_, which were induced in the PAW5 treatment condition; however, the expression was suppressed in the PAW19 treatment condition only in _AUX1_. This expression pattern of the _AUX1_ gene is in line with a previous report that _AUX1_ predominantly regulates root hair elongation whereas _LAX3_ mostly regulates the lateral root formation [35]. Furthermore, we confirmed the expression of the _OBP4_, _XTH9_ and _XTH17_ genes that are known to regulate root hair initiation and elongation and are sensitive to the nitrate concentration in the media [50; 53]. In detail, _OBP4_ acts as a negative regulator of root growth, while _XTH9_ and _XTH17_ act as a positive regulator of root growth through cell expansion. The expression showed that these genes were affected by the PAW treatments. All the PAW treatments suppressed the expression of _OBP4_. Although the expressions of _XTH9_ and _XTH17_ were suppressed in the PAW19 treatment condition, the maximal observed effect was observed on the root development by the PAW5 treatment compared to all the other PAW generation times (PAW7, 12, 19 and 40). Therefore, the PAW5 condition affected the root growth by positively regulating the expression of _XTH9_ and _XTH17_ which may mainly be caused by the root cell expansion. Moreover, as showed in the Figure 5 and Table 5, the cell expansion of PAW19 was severely reduced, thus correlating with lower expression of _XTH_ genes. It is suggested that PAW-mediated root architecture regulates the root developmental genes in _Arabidopsis_, and it is at least involved in the root auxin transporter signaling system. Taken together, the molecular mechanism of the PAW treatment increases the nitrate concentration in PAW which induces the activation of the nitrate-responsive and cell expansion genes leading to the induction of the root hair density and root length. However, the mechanism we explored may still be affected by other characters such as a change in the osmolarity of the medium.