This study investigated the effects of high humidity on the growth and photosynthetic and physiology traits of tomato plants under high temperature stress (HT). The results showed that high humidity effectively alleviated the limitation of HT on plant growth and increased the root-to-shoot ratio. In addition, high humidity also increased the chlorophyll content, net photosynthetic rate, and maximum photochemical quantum yield of PSII in tomato seedlings under HT stress, but declined the stomatal limitation value. Moreover, JIP-test showed that increasing air humidity improved the quantum yields and efficiencies of HT-stressed tomato plants and increased the size of functional antenna, while reduced the activity of a portion of reaction centers. Besides, high humidity increased the activity of antioxidant enzymes, but decreased the content of malondialdehyde and hydrogen peroxide in HT-stressed tomato plants. Therefore, high humidity improved the growth and alleviated photoinhibition and oxidative stress of tomato seedlings under heat stress.

Phosphorus plays an important role in plant growth and development, and is an important limiting factor for crop yield. Although previous studies have shown that 6-phosphogluconate dehydrogenase (6PGDH) plays an important role in plant resistance to adversity, its response to low phosphorus (P) stress remains unknown. In this study, we reported the cloning and characterization of a cytosolic 6PGDH gene, Gm6PGDH1, which enhanced the tolerance to phosphate (Pi) starvation by improving root system development and modifying the antioxidant system in transgenic plants. Gm6PGDH1 was highly expressed in the root at full bloom stage, and strongly induced by Pi starvation. The results from intact soybean composite plant and soybean plant, both containing a Gm6PGDH1-overexpressing construct, showed that Gm6PGDH1 was involved in root system development, and subsequently affected P uptake under Pi-deficient conditions. Meanwhile, the accumulation of reactive oxygen species (ROS) in the root tip of transgenic soybean was reduced, and the activity of ROS-scavenging enzymes was enhanced compared with those of the wild type under Pi-deficient conditions. Interestingly, we found that the overexpression of Gm6PGDH1 weakened the response of several other important Pi-answer genes to Pi starvation, such as some purple acid phosphatases (PAPs) and redox-related genes. In addition, the results from a virus-induced gene silencing (VIGS) indicated that Gm6PGDH1 might have functional redundancy in soybean, and the results from a heterogeneous transformation system showed that overexpressing Gm6PGDH1 also enhanced tolerance to Pi starvation in transgenic Arabidopsis. Together, these results suggested the great potential of Gm6PGDH1 in crop breeding for low Pi tolerance.