Abscisic Acid Promotes Susceptibility to the Rice Leaf Blight Pathogen <i>Xanthomonas oryzae</i> pv <i>oryzae</i> by Suppressing Salicylic Acid-Mediated Defenses

<div><p>The plant hormone abscisic acid (ABA) is involved in a wide variety of plant processes, including the initiation of stress-adaptive responses to various environmental cues. Recently, ABA also emerged as a central factor in the regulation and integration of plant immune responses, although little is known about the underlying mechanisms. Aiming to advance our understanding of ABA-modulated disease resistance, we have analyzed the impact, dynamics and interrelationship of ABA and the classic defense hormone salicylic acid (SA) during progression of rice infection by the leaf blight pathogen <i>Xanthomonas oryzae</i> pv. <i>oryzae</i> (<i>Xoo</i>). Consistent with ABA negatively regulating resistance to <i>Xoo</i>, we found that exogenously administered ABA renders rice hypersusceptible to infection, whereas chemical and genetic disruption of ABA biosynthesis and signaling, respectively, led to enhanced <i>Xoo</i> resistance. In addition, we found successful <i>Xoo</i> infection to be associated with extensive reprogramming of ABA biosynthesis and response genes, suggesting that ABA functions as a virulence factor for <i>Xoo</i>. Interestingly, several lines of evidence indicate that this immune-suppressive effect of ABA is due at least in part to suppression of SA-mediated defenses that normally serve to limit pathogen growth. Resistance induced by the ABA biosynthesis inhibitor fluridone, however, appears to operate in a SA-independent manner and is likely due to induction of non-specific physiological stress. Collectively, our findings favor a scenario whereby virulent <i>Xoo</i> hijacks the rice ABA machinery to cause disease and highlight the importance of ABA and its crosstalk with SA in shaping the outcome of rice-<i>Xoo</i> interactions.</p>