Reactive oxygen species and nitric oxide induce senescence of rudimentary leaves and the expression profiles of the related genes in Litchi chinensis

Abstract

Litchi is one of the most important subtropical evergreen fruit trees in southern Asia. Previous studies indicated that high-temperature conditions encourage growth of rudimentary leaves in panicles and suppress flowering. We have demonstrated that methyl viologen dichloride hydrate (MV) and sodium nitroprusside (SNP) promoted flowering in litchi partially by inhibiting the growth of rudimentary leaves via reactive oxygen species (ROS) and nitric oxide (NO). In the present study, we examined the microstructure and ultrastructure, programmed cell death (PCD) ratio, nuclei morphology of the rudimentary leaves, and the expression of senescence-related genes after the treatment with ROS or NO. The results showed that chromatins of the ROS- or NO-treated cells in the rudimentary leaves were condensed. Fusion of the cytoplasm-digesting vesicles with the vacuole and degradation of cytoplasm forming scattered debris were found in those of the treated cells. Treatment with ROS or NO increased the cell PCD ratio. Morphology of the nuclei stained by propidium iodide (PI) showed that nuclei shape became irregular after the ROS or NO treatment. Further, the expression levels of LcRboh, LcMC-1-like, and LcPirin were higher in the ROS- and NO-treated rudimentary leaves than those in the control ones, suggesting that these genes may be involved in the ROS and NO-induced senescence and abscission of the rudimentary leaves in litchi. Our results suggested that ROS and NO play an important role in inducing the senescence of the rudimentary leaves, and ROS- and NO-induced PCD may be involved in the regulation of the rudimentary leaf growth in litchi.

Climate change: tricking the trees

Warmer winter temperatures suppress flowering of litchi trees, threatening fruit production, but researchers have found another way to trigger litchi trees to reproduce. In litchi, spring buds contain both flower and leaf primordia, and cold cues the plants to shed the young leaves, allowing the flower primordia to develop. With recent warmer winters, the leaves are crowding out the flowers. Biyan Zhou at South China Agricultural University investigated whether spraying with compounds that make the trees produce stress-signaling molecules, such as nitrous oxide, would mimic the effects of a cold winter. Following treatment, the trees shed their rudimentary leaves and flowers developed. Microscopic examination of buds showed that the leaf shedding process looked similar to cold-triggered shedding. This study offers a method to induce litchis to flower and fruit, even in a changing climate.