4.3 Pi efflux detoxifies rhizosphere Al3+ and immobilization of Al with P in the epidermis confers Al tolerance in XZ29
It was suggested that a constitutive efflux (Pellet, Papernik, & Kochian, 1996) or leakage (Pettersson & Strid, 1989) of P from roots contributed to Al-tolerance in wheat. However, those reports on Pi efflux at tissue level were not verified by subsequent investigations (Tang, Garvin, Kochian, Sorrells, & Carver, 2002). In this study, we found Pi efflux/leakage occurred from root apices, a primary target of Al toxicity (Ryan, Shaff, & Kochian, 1992; Ryan, Ditomaso, & Kochian 1993), and mostly from the elongation zone (Figure 7c). The high cytosolic Pi availability in XZ29 provides sufficient stored Pi for Pi efflux/leakage. We propose that this rapid efflux/leakage of Pi could bind with Al3+, reducing Al uptake into the cytosol (Figure 9). This is partially supported by the fact that XZ29 accumulated less Al in the epidermis at the elongation zone than XZ9 (Figure 7a,b). However, the candidate transporters or ion channels responsible for the Pi efflux/leakage need be further studied.
Immobilization of Al by P in cell walls was previously linked with Al tolerance in buckwheat (Fagopyrum esculentum ) (Zheng et al., 2005), although other more established mechanisms contribute to the high Al tolerance of this species (Ma, Zheng, Matsumoto, & Hiradate, 1997; Ma et al., 2001). In our study, we found the co-localization of Al and P in the inner epidermal layer (cell wall and apoplast) of root mature zone in XZ29, strongly suggesting the formation of Al-P complex (Figure 8). It is reported that binding Al with vital components of cell wall, such as pectin and semicellulose, reduced the flexibility of cell wall, leading to the rupturing of the rhizodermis and outer cortex and inhibition of the root elongation (Kopittke, Menzies, Wang, & Blamey, 2016). Immobilization Al with large quantity of avaliable P may reduce the binding Al with pectin and semicellulose, thereby alleviating the Al stress on the structure and function of the cell wall in XZ29.