Effects of 1-aminocyclopropane-1-carboxylate and paclobutrazol on the endogenous hormones of two contrasting rice varieties under submergence stress

Phytohormone play important roles in regulating developmental processes and signaling networks involved in plant responses to submergence stress. We studied the growth, leaf endogenous hormonal levels and related genes of two contrasting rice genotypes (IR64 and IR64-Sub1) pretreated with 1-aminocyclopropane-1-carboxylate (ACC) and paclobutrazol (PB) under submergence conditions. Submergence promoted underwater shoot elongation, chlorophyll degradation and expression of OsCIPK15, altered the leaf endogenous hormone levels, accelerated the yellowing and senescence of leaves, and decreased shoot dry weight and the plant survival rate. IR64 was more sensitive to submergence stress, showing larger increases of leaf GAs, ABA, IAA contents and the expression of OsCPS1, OsGA20ox1 and OsIAA11, which in IR64-Sub1 were significantly decreased with prolonged stress. Furthermore, IR64 showed decreased ZR content, cZOGT1 expression, GA/ABA and ZR/IAA, while IR64-Sub1 showed opposite trends. Shoot elongation, leaf yellowing and senescence, GAs and IAA contents and the expression of OsCPS1, OsGA20ox1 and OsIAA11 in submerged rice leaves were enhanced by ACC application, whereas PB treatment showed opposite effects, and the effects on underwater elongation of PB and Sub1 were clearly superimposable. Compared with GA/ABA, ZR/IAA was not only suitable for non-Sub1 genotypes, but also more accurate in reflecting the submergence characteristics in IR64-Sub1. These findings improve our understanding of the intricate web of connections between plant hormones that regulate physiological responses to stress.