High temperatures change the perspective: Integrating hormonal responses in citrus plants under co-occurring abiotic stress conditions

Plants growing in the field are subjected to multiple stress factors acting simultaneously. Abnormally high temperatures are expected to affect wild plants and crops in the next years due to global warming. In this work, we have studied physiological, hormonal and molecular responses of the citrus rootstock, Carrizo citrange (Poncirus trifoliata L. Raf. x Citrus sinensis L. Osb.) subjected to wounding or high salinity occurring individually or in combination with heat stress. According to our results, combination of high salinity and heat stress aggravated the negative effects of salt intoxication in Carrizo. The high transpiration rate caused by high temperatures counteracted physiological responses of plants to salt stress and increased Cl- intake in leaves. In addition, 12-oxo-phytodienoic acid accumulated specifically under combination of wounding and heat stress, whereas at low temperatures, wounded plants accumulated jasmonic acid (JA) and JA-isoleucine (JA-Ile). Moreover, an antagonism between salicylic acid (SA) and JA was observed, and wounded plants subjected to high temperatures did not accumulate JA nor JA-Ile whereas SA levels increased (via isochorismate synthase biosynthetic pathway). Wounded plants did not accumulate abscisic acid (ABA) but its catabolite phaseic acid. This could act as a signal for the upregulation of (ABA)-RESPONSIVE ELEMENT (ABRE)-BINDING TRANSCRIPTION FACTOR 2 (CsAREB2) and RESPONSIVE TO DISSECATION 22 (CsRD22) in an ABA-independent way. This work uncovers some mechanisms that explain Carrizo citrange tolerance to high temperatures together with different hormonal signals in response to specific stresses. It is suggested that co-occurring abiotic stress conditions can modify (either enhance or reduce) the hormonal response to modulate specific responses.