Soybean leaf age influences the infection process of Phakopsora pachyrhizi

Soybean yield has been drastically reduced by rust, caused by Phakopsora pachyrhizi, worldwide. This study investigated the response of soybean leaf age in the same plant against P. pachyrhizi infection by examining the outcome of physiological parameters (leaf gas exchange and fluorescence of chlorophyll a ) linked to biochemical (concentrations of carbohydrates, malondialdehyde, hydrogen peroxide, superoxide anion radical, phenolics, lignin, and photosynthetic pigments as well as activities of antioxidant enzymes), and molecular (defense-related genes expression) variables. Plants were inoculated with P. pachyrhizi and three leaves per plant, from base to top [name as older (L1) and younger (L2 and L3)] were used. Rust severity was higher on older leaves compared to younger leaves. On older leaves, the photosynthetic apparatus was impaired, sucrose, starch, phenolics, and lignin concentrations were higher, and the antioxidant metabolism (great SOD, APX, and CAT activities) was more robust. Younger leaves showed reduced rust severity resulting in less damage to their photosynthetic apparatus, lower malondialdehyde and hydrogen peroxide production, and great concentrations of photosynthetic pigments, fructose, and glucose. Up-regulation of PAL1.1, PAL3.1, CHIA1, LOX7, PR-1A, ICS1, JAR1, and SuSy occurred on older leaves compared to younger leaves. By contrast, PAL2.1, PR-10, and ETR1 were up-regulated on younger leaves than older ones. In short, the resistance of soybean leaves against P. pachyrhizi infection was influenced by their age. It seems plausible that the susceptibility of older leaves on the lower plant canopy of plants to rust may result from an allocation of photoassimilates to younger leaves.