OsAP79 induced the reduction of IAA accumulation in the root thereby inhibiting root elongation
Plant APs are involved in precursor protein processing, protein degradation, PCD, leaf senescence, stress resistance, and disease resistance senescence (Chen et al ., 2015). Nevertheless, no insect resistance function has been reported until now. Here, we found that the overexpression of OsAP79 could significantly increase rice resistance against sucking insects by reducing BPH weight gain and honeydew excretion (Figure 2e and 2f). Our findings improve the understanding of the functions of the AP family. Plant phytohormones play an important role in growth and immunity. For instance, auxin, a negative regulator of plant immunity, has long been known to actively modify plant defense (Navarro et al ., 2006). This is done by reducing the rice auxin content via enhanced disease resistance to fungal and bacterial pathogens by constitutively expressingOsGH3.8 (Ding et al ., 2008) and OsGH3.1 (Domingoet al ., 2009). OE lines of OsAP79 had a significantly low IAA content, which caused enhanced BPH resistance; the opposite result was detected in the knockout lines. Our results indicate thatOsAP79 positively confers BPH resistance by negatively regulating IAA content (Figure 4f).
Auxin is an essential hormone that regulates the development and growth of plants. In this study, OE lines of OsAP79 presented shorter roots than NIP due to decreased IAA content, and knockout lines showed the opposite result (Figure 4b, 4d, and 4f). This is another indicator of a positive correlation between root length and IAA content. The OE lines of OsGH3s also had reduced content of free IAA, and presented differences in morphological phenotypes, such as dwarfism, as well as fewer lateral or crown roots (Zhang et al ., 2009, Fuet al ., 2011, Du et al ., 2012). Moreover, the apple geneMsGH3.5 functions as an auxin response gene that restricts plant shoot and root development (Zhao et al ., 2020). Therefore, we propose that IAA is an important regulatory hormone ofOsAP79 in BPH resistance and root length.