Citrus is the prime fruit crop cultivated worldwide, and well known for nutritional, organoleptic and nutraceutical related properties. Citrus fruits are rich source of several secondary metabolites (SM), like flavonoids, ascorbic acid, polyphenols, carotenoids, terpenoids, and limonoids. Citrus have evolved a multitude of defense mechanisms to enhance their resilience against biotic and abiotic stresses, with SM playing a critically significant role in plant survival, defence, adaptability, and reproduction. Among all biotic stresses, Huanglongbing (HLB, citrus greening) is the major constraint to limits citrus production. HLB is a destructive disease caused by Candidatus Liberibacter spp. HLB has been found in nearly all cultivated varieties which causes the losses by affecting fruit development, quality, tree vigor, and production. This review gives brief highlights on these compounds and discussed their function in citrus defence mechanism, role in fruit maturation, metabolic responses against Huanglongbing (HLB), and hormone signalling pathways. Majority of the amino acids, flavanone, terpenes, flavone, flavonoids, aldehyde, volatile compounds, and monoterpenes in HLB-tolerant cultivars were much higher than susceptible. Various metabolic studies indicated that specific metabolites play a crucial role in limiting the spread and multiplication of the pathogens. Additionally, these metabolic signatures have the potential to be developed as markers for tolerance against HLB. Advanced tools like CRISPR/Cas9, metabolomics, and synthetic biology should be employed to validate the candidate genes involved in enhancing the production of compounds associated with HLB tolerance. The identified genes or SM could serve as valuable targets for citrus breeding, providing long-term solutions to combat HLB disease.

The prevalence of rough lemon rootstock in India is presently threatened by the spread of Phytophthora root rot. Developing an alternative Phytophthora-resistant rootstock that also withstands other abiotic constraints is a priority for a sustainable citrus industry in India. Complementary progenitors can be found in citrus germplasm to combine the desired traits, particularly between Citrus and Poncirus, a closely related genus. The production of hybrids accumulating the desired dominant traits, irrespective of the heterozygosity level of the parents, is an effective way to develop new candidate rootstocks. Rough lemon was hybridized with Swingle citrumelo, X-639 citrandarin, and GouTou Cheng. A reliable method was developed for greater recovery of hybrid seedlings from these crosses on the basis of morphology and simple sequence repeat (SSR) markers. The greatest proportion of hybrid seedlings resulted from embryo rescue at 95 days after pollination (DAP) (48.5% in rough lemon × X-639 citrandarin and 56.7% in rough lemon × Swingle citrumelo) on the basis of multifoliate leaves. The proportion of hybrid seedlings decreased with increase in embryo development stage after 95 DAP until the last stage of observation (125 DAP). Significantly higher proportion of multifoliate seedlings was recovered by in vitro culture compared with in vivo germination from the mature seeds (26.9% vs. 5.2%, respectively). Swingle and X-639 citrandarin are not homozygous for the multifoliate allele from Poncirus, and GouTou Cheng lacks the multifoliate allele; thus, unifoliate hybrids were selected from all crosses on the basis of SSR marker genotypes.

Effect of ten rootstocks on flowering and pollen viability of Kinnow mandarin