Fruit cracking has long been a topic of great concern for growers and researchers of litchi (Litchi chinensis Sonn.). To understand the molecular mechanisms underlying fruit cracking, high-throughput RNA sequencing (RNA-Seq) was first used for de novo assembly and characterization of the transcriptome of cracking pericarp of litchi. Comparative transcriptomic analyses were performed on non-cracking and cracking fruits. A total of approximately 26 million and 29 million high quality reads were obtained from the two groups of samples, and were assembled into 46,641 unigenes with an average length of 993 bp. These unigenes can be useful resources for future molecular studies of the pericarp in litchi. Furthermore, four genes (LcAQP, 1; LcPIP, 1; LcNIP, 1; LcSIP, 1) involved in water transport, five genes (LcKS, 2; LcGA2ox, 2; LcGID1, 1) involved in GA metabolism, 21 genes (LcCYP707A, 2; LcGT, 9; Lcβ-Glu, 6; LcPP2C, 2; LcABI1, 1; LcABI5, 1) involved in ABA metabolism, 13 genes (LcTPC, 1; Ca²⁺/H⁺ exchanger, 3; Ca²⁺-ATPase, 4; LcCDPK, 2; LcCBL, 3) involved in Ca transport and 24 genes (LcPG, 5; LcEG, 1; LcPE, 3; LcEXP, 5; Lcβ-Gal, 9; LcXET, 1) involved in cell wall metabolism were identified as genes that are differentially expressed in cracked fruits compared to non-cracked fruits. Our results open new doors to further understand the molecular mechanisms behind fruit cracking in litchi and other fruits, especially Sapindaceae plants.

Auxin, a vital plant hormone, regulates a variety of physiological and developmental processes. It is involved in fruit abscission through transcriptional regulation of many auxin-related genes, including early auxin responsive genes (i.e., auxin/indole-3-acetic acid (AUX/IAA), Gretchen Hagen3 (GH3) and small auxin upregulated (SAUR)) and auxin response factors (ARF), which have been well characterized in many plants. In this study, totally five auxin-related genes, including one AUX/IAA (LcAUX/IAA1), one GH3 (LcGH3.1), one SAUR (LcSAUR1) and two ARFs (LcARF1 and LcARF2), were isolated and characterized from litchi fruit. LcAUX/IAA1, LcGH3.1, LcSAUR1, LcARF1 and LcARF2 contain open reading frames (ORFs) encoding polypeptides of 203, 613, 142, 792 and 832 amino acids, respectively, with their corresponding molecular weights of 22.67, 69.20, 11.40, 88.20 and 93.16 kDa. Expression of these genes was investigated under the treatment of girdling plus defoliation which aggravated litchi fruitlet abscission due to the blockage of carbohydrates transport and the reduction of endogenous IAA content. Results showed that transcript levels of LcAUX/IAA1, LcGH3.1 and LcSAUR1 mRNAs were increased after the treatment in abscission zone (AZ) and other tissues, in contrast to the decreasing accumulation of LcARF1 mRNA, suggesting that LcAUX/IAA1, LcSAUR1 and LcARF1 may play more important roles in abscission. Our results provide new insight into the process of fruitlet abscission induced by carbohydrate stress and broaden our understanding of the auxin signal transduction pathway in this process at the molecular level.

Sucrose phosphate synthase (SPS), sucrose synthase (SuSy), and neutral invertase (NI) are the three enzymes involved in sucrose synthesis and cleavage in higher plants. However, the key enzyme(s) responsible for sucrose accumulation is/are inconsistent in different species or different varieties of the same species. The regulation of sucrose accumulation in Tainung 11 pineapple (Ananas comosus) has not been reported to date. In this study, the sucrose content and the activities and patterns of transcript levels of the three enzymes were analysed in the flesh of Tainung 11 pineapple during fruit development. From 20 to 100 days after anthesis (DAA), the activities and transcript levels of SPS and SuSy gradually increased. Meanwhile, the NI activity and transcript level dramatically decreased from 40 to 100 DAA. The sucrose accumulation increased from 20 to 100 DAA. These findings suggested that AcSPS, SPS, AcSuSy, and SuSy were responsible for sucrose synthesis, but AcNI and NI were responsible for sucrose cleavage in Tainung 11 pineapple. The subcellular localisations of AcSPS, AcSuSy, and AcNI were first identified. Both AcSPS and AcSuSy were found in the cytosol, while AcNI was localised in the vacuole.

Sucrose plays a crucial role in plant growth and development. Sucrose phosphate synthase (SPS, EC 2.4.1.14) is a key rate-limiting enzyme in the sucrose metabolism of plants. To date, the genome-wide identification, characterisation, and expression profile analysis of SPS gene families have been reported in many species but not in pineapple (Ananas comosus). In this study, five SPS genes (AcSPS1–5), which contained sucrose synthase, glucosyltransferase, and SPP-like conserved domains, were identified. The phylogenetic tree indicated that these AcSPS genes were divided into three clades, i.e. A, B, and C. The spatiotemporal expression pattern of AcSPS gene families were first examined in pineapple. Among all tissues, AcSPS2 had the highest expression in flesh. Meanwhile, AcSPS2 expression was significantly upregulated during fruit development. Furthermore, AcSPS2 expression, SPS activity, and sucrose content consistently increased with fruit development. These results may indicate that AcSPS2 played a major role in sucrose metabolism during fruit development. AcSPS5 was strongly expressed in the peduncle, pericarp, and core. On the basis of its abundant expression, AcSPS5 was speculated to play an important role in plant growth.